HK1175856B - High-speed liquid crystal polarization modulator - Google Patents

Description

High-speed liquid crystal polarization modulator

Notification of work

2011 LC-TEC Displays AB. Some of the disclosure of this patent document contains material which is subject to copyright protection. As shown in the intellectual property office patent case or record, the owner of the work does not have access to any patent document or patent by any meansBy virtue of the disadvantages of copying copies of the disclosure, all copyrights are not however retained. 37 CFR § 1.71 (d).

Technical Field

The present disclosure relates to a high-speed, liquid crystal polarization modulator (polarization modulator) for time-division multiplexed stereoscopic three-dimensional (3D) applications. More particularly, to a polarization state modulator implemented with first and second liquid crystal devices through which incident light in an input polarization state propagates, and in which the second liquid crystal device compensates for changes in the input polarization state caused by the first liquid crystal device, and a method of driving a liquid crystal cell (liquid crystal cell) to achieve high-speed switching between polarization states.

Background

The polarization modulator can be applied to many different fields such as optical fiber communication, electric welding goggles (welding goggles), and time-sharing multi-task stereoscopic 3D displays. Liquid crystal cells are particularly suitable for modulating the polarization state of light passing therethrough because the liquid crystal material itself is birefringent and the direction of the optical axis of the birefringent material can be controlled by an applied voltage. For some applications, a polarization modulator is used as a polarization switch to switch light from one polarization state to another. To achieve the highest performance in time-multiplexed stereoscopic 3D applications, it is necessary to switch between two orthogonally related (orthogonal) polarization states, such as between right-handed circularly polarized light and left-handed circularly polarized light, or between vertically polarized light and horizontally polarized light.

There are two basic techniques used in time-multiplexed stereoscopic 3D systems, in which left-eye and right-eye images are sequentially rendered into frames (frames) by an imaging device. One of the basic techniques entails the use of active viewing glasses worn by the viewer. Each eyepiece of the active glasses is equipped with a lens assembly comprising a polarization switch located between two polarization films. The active glasses and the imaging device operate synchronously, and each lens assembly alternately passes and blocks sequentially presented images of the eyes of an associated viewer during alternate subframes (subframes) of approximately equal duration, so that the right eye image and the left eye image respectively reach the right eye of the viewer and the left eye of the viewer. Another basic technique entails the use of passive viewing glasses worn by the viewer and the placement of a polarizer (polarizer) and a polarization switch in front of the imaging device. The polarization switch and the imaging device operate synchronously, so that the left eye image and the right eye image are transmitted through a transmission medium and are endowed with different polarization states by the polarization switch. Each eyepiece of the passive glasses is equipped with a lens comprising a polarizing film oriented to analyze the polarization state of incident light carrying left and right eye images to alternately block and pass through them so that the right and left eye images respectively reach the right and left eyes of the viewer. The present disclosure relates to stereoscopic 3D techniques using active or passive viewing glasses.

One of the first polarization modulators using liquid crystals is a Twisted Nematic (TN) cell. The TN cell taught by Helfrich and Schadt, switzerland patent No. CH532261, consists of a positive dielectric anisotropic (positive dielectric anisotropic) liquid crystal material sandwiched between two substrates having light-transmissive electrodes whose surfaces are treated to have the orientation of the axial direction (director) of the liquid crystal material molecules contacting one surface at right angles to the axial direction of the liquid crystal material molecules contacting the other surface. When no voltage is applied, the liquid crystal molecules in the liquid crystal device are uniformly twisted by 90 ° in the axial direction from the inner side surface of the bottom substrate to the inner side surface of the top substrate. Through a "" waveguide "" principle, this has the effect of rotating the linearly polarized incident light by 90 degrees. After applying a voltage to the liquid crystal device, the liquid crystal molecules are oriented axially perpendicular to the substrates, which results in the loss of the twisted liquid crystal molecular axial structure and its ability to rotate linearly polarized incident light. Therefore, the TN cell can be regarded as a polarization switch which rotates the direction of linearly polarized light by 90 DEG when no voltage is applied, and does not rotate the linearly polarized light when a sufficiently strong voltage is applied. One of the problems with using a TN device as a polarization switch is that the transition from a high voltage optical state to a low voltage optical state is too slow for many applications because the restoring torque (torque) for the liquid crystal molecular axis comes only from the elastic force coming from the fixed boundary line formed by the molecular axis contacting the inner surface of the treated electrode. This is referred to as an unpowered transition. On the other hand, the transition from a low voltage optical state to a high voltage optical state can be extremely fast, since the torque to the molecules is now due to the coupling of the applied electric field and the induced dipole moment (dipole moment) of the liquid crystal material. This is a powered transition. Even with the low viscosity, high birefringence liquid crystal materials and liquid crystal display device technologies available today, the transition from the high voltage optical state to the low voltage optical state is still on the order of 2 milliseconds to 3 milliseconds, which is too slow for the latest time division multiplexed stereoscopic 3D applications, where a complete left or right eye image may only be ready in 4 milliseconds or less.

Freiser, U.S. Pat. No. 3,857,629, describes a TN polarized switch in which the switching from the low voltage to the high voltage optical state and from the high voltage to the low voltage optical state is power-supplied and therefore very fast. The switching mechanism uses a special "" dual frequency "" liquid crystal mixture whose sign of dielectric anisotropy changes from positive to negative to increase the driving frequency. Applying a DC or low frequency AC voltage turns the TN apparatus on, while applying a high frequency AC voltage turns the TN apparatus off. However, there are problems with this dual frequency technique. First, since a plurality of regions or patches are formed in the liquid crystal device, this mechanism cannot be uniformly switched over a wide range. Second, the cross-over frequency (cross frequency), the frequency at which the dielectric heterophase property of the liquid crystal changes sign, is extremely temperature dependent, thus limiting the temperature range over which the device can operate successfully. Third, the high frequency drive signal fed to the capacitive load of the liquid crystal device requires significant power, making this system impractical for use on battery-operated, portable devices such as active stereoscopic 3D glasses.

Bos, in U.S. patent No. 4,566,758, describes a liquid crystal polarization switch that operates in the electro-optic mode. The liquid crystal device described by Bos is now known as pi-cell. Such a pi-cell polarization switch can rotate the polarization direction of linearly polarized light by 90 °, but it operates based on a switchable half-wave retarder (retro), rather than the 90 ° "waveguiding" principle of TN displays. The switching of the pi cell mode is faster than that of the TN mode because the internal liquid crystal material flow associated with the switching of the pi cell does not cause a slow "optical rebound". However, the transition from the high voltage optical state to the low voltage optical state is still an unpowered transition, with a response time of about 1 millisecond using existing materials and device technology. Even 1 millisecond response causes image artifacts, brightness impairments, and other artifacts in the latest time-division multiplexed stereoscopic 3D applications.

Clark and Lagerwall describe in us patent No. 4,563,059 a liquid crystal polarization switch based on ferroelectric (ferroelectric) liquid crystal materials, which are a different type of liquid crystal than the aforementioned nematic liquid crystal materials. Such ferroelectric liquid crystals are different from nematic liquid crystals in that the ferroelectric-like liquid crystal molecules themselves are arranged in a layered form. Ferroelectric polarization switches are capable of switching back and forth between two polarization states very quickly because both optical state transitions are power-fed transitions. However, ferroelectric polarization modulators have a number of disadvantages. First, the liquid crystal device needs to have a very thin cell gap (cellgap) on the order of one micron, which makes it difficult to increase the production yield of the ferroelectric liquid crystal device. Secondly, the arrangement of ferroelectric layers is very sensitive to shock and pressure variations, which eliminates many applications requiring manipulation, such as in active stereoscopic 3D glasses worn by the viewer. Third, variations in temperature can also cause misalignment, especially if the temperature is temporarily raised above the crystal array transition temperature.

Other polarization switches use two liquid crystal elements arranged in optical series. Bos, in U.S. patent No. 4,635,051, describes a shutter system comprising first and second variable optical retarders, wherein projections of their optical axes onto the light propagation surfaces of the variable retarders are orthogonal to each other and are disposed between intersecting polarizing plates. It drives the variable delays such that during a first ON or ON time interval, the first variable delay receives a high voltage and the second variable delay receives zero volts, and during a second OFF or OFF time interval, both the first and second variable delays receive the high voltage. As a result, the shutter rapidly becomes ON at the beginning of the first time interval and enters an ON state, and rapidly becomes OFF at the beginning of the second time interval and enters an OFF state. The second time interval is followed by a third time interval of indefinite duration during which both variable delays receive zero volts and relax to their unpowered states. The shutter is in the closed state for a third time interval. This release is relatively slow during the third time interval because it is not powered and must be completed before the shutter can be activated again. This mechanism is not suitable for time-division multiplexed stereoscopic 3D applications, which operate in two time intervals (left image and right image sub-frames) of approximately equal duration.

Bos, in U.S. patent No. 4,719,507, describes an embodiment of a time-multiplexed stereoscopic imaging system comprising a linear polarizer and first and second liquid crystal variable optical retarders whose optical axes are perpendicular to each other. The variable retarders are switched so that during a first image frame the first variable retarder is in a zero-retardance state and the second variable retarder is in a quarter-wave-retardance state, resulting in right-handed circularly polarized light, and during a second image frame the first variable retarder is in a quarter-wave-retardance state and the second variable retarder is in a zero-retardance state, resulting in left-handed circularly polarized light. The second variable retarder does not compensate for the change in the input polarization state of the incident light by the first variable retarder at any one time. During a handoff, one variable delay is enabled while the other variable delay is disabled, and vice versa. One disadvantage of this mechanism is that both transitions comprise relatively slow unpowered transitions, which causes image artifacts, luminance degradation, and other artifacts in the latest time-division multiplexed stereoscopic 3D applications.

Cowan et al, in U.S. patent No. 7,477,206, describe a polarization switch that is driven in a push-pull (push-pull) manner using two liquid crystal variable optical retarders that are capable of switching between zero and quarter-wave retardation, in a manner similar to that described in U.S. patent No. 4,719,507. The same disadvantages of the polarization switch described in U.S. patent No. 4,719,507 apply here.

Robinson and Sharp in U.S. patent No. 7,528,906 describe embodiments of polarization switches using two half-wave pi cells optically connected in series. One embodiment uses two pi cells to create a surface for contacting the axial alignment of molecules by rubbing the surface of the light transmissive electrode in a parallel direction. Which are arranged in such an orientation that the rubbing directions of the two pi cells are at an angle of approximately 43 DEG to each other. Other embodiments use two pi cells with rubbing directions parallel to each other to create one or more intervening passive retardation films. In all cases, the second pi cell does not compensate for the change in input polarization state caused by the first liquid crystal retarder as incident light in an input polarization state propagates through the first and second pi cells. The two liquid crystal devices are driven simultaneously with the same waveform, resulting in a very fast optical response when the two liquid crystal devices switch from a low voltage optical state to a high voltage optical state, since they are both powered transitions, but the simultaneous transition from the high voltage to the low voltage optical state is a non-powered transition, and therefore very slow, reducing the switching performance of the multi-time division stereoscopic 3D application.

And Palmer, U.S. Pat. No. 5,825,441, describe a liquid crystal welding goggle structure comprising two TN components and an intervening polarizing film. At least one TN assembly has a twist angle less than 90 deg. Due to the intervening polarizer, the polarization state of the light entering the second TN assembly is fixed, independent of the change in the input polarization state of the incident light by the first TN assembly, and therefore does not include any compensation. This arrangement gives higher performance for electro-welding applications where extremely high optical density is required over a wide range of viewing angles, but is not suitable for time-division multiplexed stereoscopic 3D applications due to the slow optical response of the unpowered transition.

Disclosure of Invention

An optical polarization state modulator for time-division multiplexed stereoscopic three-dimensional image viewing by an observer without the aforementioned drawbacks. The polarization state modulator receives light in alternating sequence in an input polarization state and carrying first and second perspective images of a scene in different first and second subframes containing updated image portions.

A preferred embodiment of the polarization state modulator comprises first and second liquid crystal devices assembled in an optical train such that polarized light propagating therethrough can undergo a change in polarization state in accordance with voltages applied to the first and second liquid crystal devices. The first and second liquid crystal devices have first and second sets of molecular axes, respectively, and are constructed and oriented such that, upon removal of an applied equal voltage, the molecular axes in the first and second sets relax in coordination with each other and thereby dynamically shift the polarization state change such that a plurality of wavelengths of incident light passing through and exiting the combination of the first and second liquid crystal devices are in the input polarization state.

The driving circuit sends the first and second driving signals to the first and second liquid crystal devices, respectively. The first and second drive signals include lower intensity levels that establish lower intensity molecular axial field states for the first and second liquid crystal devices. The first and second drive signals comprise pulses having lower-to-higher intensity level powered transitions establishing higher intensity molecular axial field states for the first and second liquid crystal devices. The first and second drive signals cooperate with each other during one of the first and second subframes to cause, in the first and second liquid crystal devices, the formation of higher intensity molecular axial field states from which the molecules are axially relaxed during an update image portion of such one subframe, such that the molecules located in the first and second groups are axially shifted by a change in polarization state. The molecular axes of the offset polarization state change impart polarized light to an image passing through the combination of the first and second liquid crystal devices in a first output polarization state equal to the input polarization state. The first and second drive signals cooperate with each other during the other of the first and second subframes to cause, in different first and second liquid crystal devices, the formation of lower and higher intensity molecular axial field states during an update image portion of the other subframe such that molecular axes in the first and second sets are not shifted by a change in polarization state. The molecular axes of the undeflected polarization state change impart polarized light to an image passing through the combination of the first and second liquid crystal devices in a second output polarization state different from the first output polarization state.

One useful characteristic of the two compensating liquid crystal devices is that if the same voltage is applied to both, one liquid crystal device compensates for the change in input polarization state caused by the other liquid crystal device, regardless of the level of the applied voltage. Furthermore, if the applied voltage is changed from one level to another and the liquid crystal material in the liquid crystal device relaxes to the new voltage level, the compensation of the polarization state will continue throughout the relaxation period. This is called dynamic compensation. Thus, if a voltage is applied to the liquid crystal device and then removed, it will continue to compensate throughout the release process without changing the polarization state of the light passing through the combination. Thus, the slow, unpowered transition of the liquid crystal device itself does not cause a change in polarization state. The disclosed drive scheme utilizes the latter characteristic to enable fast switching polarization modulator operation, since the two-crystal device is allowed to reset to a lower voltage polarization state by the slower unpowered transition without any optical change.

The optical polarization state modulator may be incorporated into a stereoscopic 3D system using passive or active viewing glasses.

With respect to a system using passive viewing glasses, an image source and an input polarizer are optically connected to each other. The image source produces first and second perspective images in alternating sequence, with light in an input polarization state and carrying the first and second perspective images exiting the input polarizer to be projected onto a light entrance surface of the optical polarization state modulator. A passive decoder includes first and second viewing devices separated from a light exit surface of the optical polarization state modulator by a transmission medium and configured to receive image-carrying polarized light in first and second output polarization states during different first and second subframes. The first viewing device includes a first polarizer having a first transmission polarization axis oriented to transmit light of a first output polarization state and to block light of a second output polarization state. The second viewing device includes a second polarizer having a second transmission polarization axis oriented to transmit light of the second output polarization state and to block light of the first output polarization state. The passive viewing glasses present the first and second perspective images to the viewer during different first and second sub-frames.

In a system using active viewing glasses, an image source emits light carrying first and second perspective images, travels through a transmission medium, and travels through an input polarizer to produce the light in an input polarization state carrying the first and second perspective images for projection onto respective light entry surfaces of two optical polarization state modulators. Each optical polarization state modulator has an analyzing polarizer optically connected to a light exit surface of the optical polarization state modulator, and an image in one of the first and second output polarization states carries polarized light through the analyzing polarizer to present a corresponding one of the first and second perspective images to a viewer. The input polarizer and the analyzing polarizer of each optical polarization state modulator have an input filter transmission polarization axis and an analyzing filter transmission polarization axis, respectively, in transverse relation to each other.

Other features and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

Drawings

FIG. 1 defines the tilt angle theta and the azimuth angle of a liquid crystal molecular axis n inside a liquid crystal material layer

FIG. 2 shows tilt and azimuthal distributions of first and second liquid crystal devices through which incident light in an input polarization state propagates, and in which the second liquid crystal device compensates for changes in the input polarization state caused by the first liquid crystal device.

FIGS. 3A, 3B, 3C, and 3D show the effect of various drive voltages on output polarization applied to first and second 90 DEG TN liquid crystal devices mounted in a first preferred embodiment, a polarization modulator that can be incorporated into a stereoscopic 3D system using passive or active viewing glasses.

Fig. 4 illustrates a driving method for the first preferred embodiment of fig. 3A, 3B, 3C, and 3D using DC-balanced frame inversion and achieving fast switching between two polarization states.

Fig. 5 illustrates a first alternative driving method for the first preferred embodiment of fig. 3A, 3B, 3C, and 3D, which uses bipolar pulses to achieve DC balance within each sub-frame.

Fig. 6 illustrates a second alternative driving method for the first preferred embodiment of fig. 3A, 3B, 3C, and 3D, which uses over-voltage driving pulses to increase the switching speed.

FIG. 7 illustrates a driving method for active spectacles according to a second preferred embodiment.

FIG. 8 illustrates a driving method for active glasses according to a third preferred embodiment, which includes a blank during the period when the image is being updated.

FIGS. 9A, 9B, 9C, and 9D show the effect of various drive voltages applied to first and second positive ECB liquid crystal devices on output polarization in a fourth preferred embodiment, a polarization modulator that can be incorporated into a stereoscopic 3D system using passive or active viewing glasses.

FIGS. 10A, 10B, 10C, and 10D show measured drive waveforms and optical switching responses for an optical shutter using two positive ECB liquid crystal devices for the fourth embodiment of FIGS. 9A, 9B, 9C, and 9D.

11A, 11B, 11C, and 11D show the effect of various drive voltages applied to first and second liquid crystal π liquid crystal cells on output polarization in a fifth preferred embodiment, a polarization modulator that can be incorporated into a stereoscopic 3D system using passive or active viewing glasses.

FIG. 12 illustrates a driving method using a combination of over-voltage driving and under-voltage driving to improve switching speed.

FIGS. 13A and 13B show a passive stereoscopic 3D viewing system constructed with the polarization modulators of FIGS. 9A, 9B, 9C, and 9D.

FIG. 14 shows simulated optical transmission spectra of clear and light blocking states generated during the first and second sub-frames of the passive stereoscopic 3D viewing system of FIGS. 13A and 13B.

Fig. 15A and 15B show the passive stereoscopic 3D viewing system of fig. 13A and 13B implemented such that the polarization modulator switches between right-handed and left-handed circularly polarized light.

Fig. 16 shows simulated optical transmission spectra of light blocking states of the left eye of the passive stereoscopic 3D viewing system of fig. 15A and 15B, which are constructed using three quarter-wave optical retarders having different wavelength dispersion characteristics.

Fig. 17 shows simulation results of optical transmission of the passive stereoscopic 3D viewing system of fig. 15A and 15B, which is constructed using a quarter-wave optical retarder made of polycarbonate and an ECB element.

Fig. 18 shows an actual measurement of the optical transmission of the passive stereoscopic 3D viewing system of fig. 15A and 15B, constructed as described with reference to fig. 17.

FIG. 19 shows the passive stereoscopic 3D viewing system of FIGS. 15A and 15B implemented with a selective C-compensator to enhance the viewing angle of the perspective images presented to the right and left eyes of the viewer.

FIGS. 20A, 20B, 20C, and 20D show simulated contrast views of the passive stereoscopic 3D viewing system of FIG. 19, where FIGS. 20A and 20B show low contrast viewing angle performance for left and right eyes, respectively, without the selective C-type compensator, and FIGS. 20C and 20D show high contrast viewing angle performance for left and right eyes, respectively, with the selective C-type compensator.

FIG. 21 shows an active stereoscopic 3D viewing system constructed with a polarization modulator similar to that used in the passive system of FIG. 19 and incorporating a C-type compensator in the optical path to improve contrast over a wide range of polar viewing angles.

FIGS. 22A and 22B show simulated contrast plots, by comparison, showing that the range of high contrast viewing angles with minimal ghosting effects can be widened by incorporating a C-type compensator into the optical path as shown in FIG. 21.

FIGS. 23A, 23B, 23C, and 23D show the effect of various drive voltages applied to the first and second liquid crystal VAN cells on output polarization in a sixth preferred embodiment, a polarization modulator that can be incorporated into a stereoscopic 3D system using active or passive viewing glasses.

FIGS. 24A and 24B show the polarization modulators of FIGS. 23A, 23B, 23C, and 23D incorporated into an active stereoscopic 3D viewing system without and with view angle compensation, respectively.

FIGS. 25A and 25B show simulated iso-contrast plots showing, by comparison, the effect of field compensation on extending the range of high contrast viewing angles.

FIG. 26 shows the polarization modulators of FIGS. 23A, 23B, 23CD, and 23D incorporated into a passive stereoscopic 3D viewing system constructed with quarter-wave optical retarders as shown in FIG. 19.

FIGS. 27A and 27B show simulated isometric views of the left and right eye eyepieces, respectively, of the passive glasses of FIG. 26.

Fig. 28 shows a passive stereoscopic viewing system constructed using the polarization modulator of fig. 23A, 23B, 23C, and 23D and passive glasses using different optical compensators for left and right eye glasses.

FIGS. 29A and 29B show simulated isometric views of the separately compensated left and right eye oculars, respectively, of the passive glasses of FIG. 28.

FIG. 30 shows a passive stereoscopic viewing system constructed using the polarization modulator of FIGS. 9A, 9B, 9C, and 9D and passive glasses using different optical compensators for left and right eye glasses.

FIGS. 31A and 31B show simulated isometric views of the separately compensated left and right eye oculars, respectively, of the passive glasses of FIG. 30.

FIG. 32 shows a simulated correlation of normalized birefringence of an ECB liquid crystal mixture and normalized birefringence of a VAN liquid crystal mixture with respect to temperature.

FIG. 33 is a graph showing simulated dependence of phase shift versus voltage at 20 ℃ for an ECB device filled with the ECB liquid crystal mixture of FIG. 32.

FIG. 34 shows four panels V of fixed 180 phase shifts for various operating temperatures of the ECB liquid crystal mixture shown in FIG. 32_HRelative to V_LThe curve of (A) is shown.

FIG. 35 is a simplified functional block diagram of a circuit for adjusting V_HAnd V_LAt a level to maintain the 180 DEG phase shift achieved by the liquid crystal elements of the polarization state modulator over a wide temperature range.

FIG. 36 is a graph showing simulated correlation of phase shift versus voltage at 20 ℃ for a VAN device filled with the VAN liquid crystal mixture of FIG. 32.

FIG. 37 shows four panels V of fixed 180 phase shifts for various operating temperatures of the VAN liquid crystal mixture shown in FIG. 32_HRelative to V_LThe curve of (A) is shown.

Detailed Description

The preferred embodiment is based on first and second liquid crystal devices arranged in optical series and through which incident light in an input polarization state propagates. The second liquid crystal device compensates for the change of the input polarization state by the first liquid crystal device to have a characteristic of exhibiting an unchanged polarization state of all wavelengths in the normal incident light passing through the first and second liquid crystal devices. In this specification, compensation used in first and second liquid crystal devices arranged in optical series and through which polarized light propagates means that, irrespective of the first liquid crystal device changing in any way the input polarization state of light entering the first liquid crystal device, the second liquid crystal device reverses or shifts this change so that the output polarization state exiting the second liquid crystal device is the same as the input polarization state. For the compensation, the first and second liquid crystal devices meet the following conditions: (1) these liquid crystal devices have the same cell gap; (2) these liquid crystal devices are filled with the same liquid crystal material unless optically active dopants (chiral dopants) are added, in which case the dopants have equal but opposite optical rotation; (3) which has no polarization-changing optical member such as a retardation plate or a polarizing plate between the two liquid crystal devices; and (4) the molecular axial field of one of the liquid crystal devices is a 90 DEG rotated mirror image of the molecular axial field of the other of the liquid crystal devices. For the satisfaction of this last condition, the liquid crystal device should have the same voltage applied thereto, or the same applied voltage should be changed to another same applied voltage, and the liquid crystal molecular axial field in the liquid crystal device is dynamically released to a new corresponding equilibrium state. If different voltages are applied to them, the two-crystal device will not compensate.

The liquid crystal molecular axial field indicates the orientation of the local optical axis of the liquid crystal molecules as they change throughout the liquid crystal device. Molecular axial fields in a liquid crystal display are characterized by a set of molecular axes whose orientations vary continuously throughout the device. FIG. 1 shows the molecular axial orientation, or zone axis, expressed as a unit vector n, which may be defined by a tilt angle θ and an azimuthal angleTypically, wherein the tilt angle θ is one of the molecular axial direction and a direction parallel to a substrate containing the liquid crystal material therebetweenOf the plane 10, and azimuth angleIs the angle between a projection 12 of the molecular axis n onto the plane 10 and the X-axis. FIG. 2 is a two-line graph showing an example of tilt angle and azimuth angle distribution curves of a first liquid crystal device (left-hand line graph) and a second liquid crystal device (right-hand line graph), showing how the tilt angle and the azimuth angle vary at respective positions over the entire thickness range (Z-axis) of the liquid crystal devices. These profiles define the molecular axial field of each device. The orientation of the molecular axis at any position Z along the Z axis in the first liquid crystal device can be expressed as a tilt angle theta₁(z) and azimuth angleAnd the orientation of the molecular axis at any position in the second liquid crystal device can be expressed as a tilt angle theta₂(z) and azimuth angle

A mathematical description of the condition (4) for polarization state compensation, i.e. the molecular axial field in the second liquid crystal device is a 90 ° rotated mirror image of one of the molecular axial fields in the first liquid crystal device, can be expressed as two equations:

θ₂(z)=-θ₁(d-z)

where d is the cell gap of the liquid crystal device and z =0 at the liquid crystal device entrance surface and z = d at the liquid crystal device exit surface. For illustrative purposes, the example of FIG. 2 satisfies the above equations, which show tilt angle and azimuthal distribution curves for the first and second liquid crystal devices.

FIGS. 3A, 3B, 3C, and 3D showA first preferred embodiment is a polarization modulator 20 for stereoscopic 3D viewing in conjunction with passive or active viewing glasses, and an image source 22 for generating first (left eye) perspective images and second (right eye) perspective images of a scene in sub-frames of approximately equal duration. FIG. 3A shows an input polarizer 24 on the left, followed by a first TN apparatus 26 and a second TN apparatus 28, incorporated in the optical train and of the conventional 90-degree TN type. The first TN device 26 is constructed by including a liquid crystal material between glass substrates 30, the glass substrates 30 having a light-transmissive electrode layer 32 formed on an upper inner surface thereof. The liquid crystal material comprises electrode surface contact molecular axis 34_cAnd electrode surface not contacting molecular axis 34_n. The second TN device 28 is constructed by including a liquid crystal material between glass substrates 36, the glass substrates 36 having a light-transmissive electrode layer 38 formed on an upper inner surface thereof. The liquid crystal material comprises electrode surface contact molecular axis 40_cAnd electrode surface not contacting molecular axis 40_n. The input polarizer 24 imparts a vertical input polarization state or direction 42 to light propagating from the image source 22 and carrying left and right eye perspective images.

FIG. 3A shows the same low-voltage level driving signal V_LApplied to both TN arrangements 26 and 28, respectively as switches 50 in a display drive circuit 52₁And 50₂As shown. Drive signal V_LBelow the TN threshold voltage or even equal to zero. At this voltage, the surface-untouched molecular axes 34, located within TN apparatus 26 and 28, respectively, from an entry surface 54 to an exit surface 56_nAnd 40_nRotated 90 ° in unison in the Z-direction, and in the TN device 26, the form of rotation is left-handed, and in the TN device 28, right-handed. The TN devices 26 and 28 may each be viewed as rotating 90 DEG in a "" waveguiding "" process of incident light propagating from the image source 22 perpendicular to the input polarization direction 42(0 DEG), with the TN device 26 rotating the perpendicular input polarization direction 42 +90 DEG in a left-handed fashion, and the TN device 28 reversing the previous rotation back to the original 0 DEG perpendicular input polarization direction by rotating it-90 DEG in an opposite right-handed fashionIn the direction 42. The combined TN devices 26 and 28 compensate for each other so that after the incident light passes through, its polarization state remains unchanged, producing an output polarization state or direction 44 that is the same as the input polarization direction 42.

FIG. 3B shows the same high-voltage driving signal V_HApplied to TN apparatus 26 and 28 such that the surfaces are not in contact with molecular axes 34_nAnd 40_nApproximately perpendicular to the boundaries of the liquid crystal device defined by electrode layers 32 and 38, respectively, except in the molecular axis direction 34_CAnd 40_COutside the thin layer. Likewise, the combined TN devices 26 and 28 compensate each other at this voltage.

FIG. 3C shows the driving signal V_HIs removed from TN devices 26 and 28 and converted into a drive signal V_LA snapshot of the molecular axial orientation for a brief period thereafter is taken by the switch position 50 in the display drive circuit 52₁And 50₂As shown. The small arrow 58 in the middle of each TN apparatus 26 and 28 indicates that their respective surfaces are not in contact with the molecular axis 34_nAnd 40_nIs in the process of releasing from the return torsion state. In this case it is dynamically compensated.

FIG. 3D shows that the TN apparatus 26 is driven with the signal V at a high voltage intensity_HIs turned on and the TN apparatus 28 is maintained at V_L. The combination of TN devices 26 and 28 no longer has the effect of compensation because the drive signals applied to TN devices 26 and 28 are different. The polarization state of the first TN arrangement 26 remains unchanged, while the polarization state of the second TN arrangement 28 is rotated by-90 deg.. The combination of TN liquid-crystal devices 26 and 28 thus rotates the polarization state by-90 DEG from the input polarization direction 42 to a horizontal output polarization direction 44.

FIG. 4 illustrates an electronic driving scheme of the first preferred embodiment that achieves fast, power-fed switching between two polarized states. The line graph (a) of fig. 4 shows the drive signal applied to the first TN device 26, and the line graph (b) of fig. 4 shows the drive signal applied to the second TN device 28.

At the beginning of a first sub-frame，t=t₀An initial start at-V_HHigh voltage level + V_HApplied to the first TN device 26 and having a voltage level + V initially at zero_HTo a second TN device 28. Voltage + V_Hand-V_HAre of equal strength and the nematic liquid crystal material reacts equally to it because it does not react to polarity. Which achieves a net DC balance using equal but opposite sign drive voltages to maintain long term stability of the liquid crystal material. Wherein V_HIs typically 25 volts, but it may be higher or lower depending on the desired switching speed and the threshold voltage of the liquid crystal material. The first TN apparatus 26 is already at the high voltage intensity level V_HAnd the second TN arrangement 28 from 0 to + V_HIs a powered transition, compensation is quickly achieved and the resulting polarization direction remains at the vertical direction 0 ° for this period, as shown in diagram (e) of fig. 4. Graphs (c) and (d) of fig. 4 show that the interlayer tilt angle of the molecular axis direction in the middle of the first and second TN devices 26 and 28 is nearly 90 ° at this voltage (see also fig. 3B). At t = t₁When, V_L(wherein in this case V_L=0) is applied to the TN devices 26 and 28 simultaneously, and it selects t early enough in the first subframe period₁So that the liquid crystal material ends at t = t in the first sub-frame₂Is released substantially to its equilibrium state. This relaxation is shown in plots (C) and (d) of fig. 4 as the decay of the interlayer tilt angle over this period (see also fig. 3C). The TN devices 26 and 28 compensate each other during the entire first subframe, initially statically, and later dynamically when the TN devices 26 and 28 are sequentially released. While the release is ongoing, at t = t₁From + V among the two TN devices 26 and 28_HThe unpowered, slow-transitioning optical effect to zero remains "secret" (i.e., optically invisible to the viewer), and the output polarization remains vertically polarized at 0 ° during the entire first subframe, as shown in diagram (e) of fig. 4. At the end of the first sub-frame, TN devices 26 and 28 are both in the low voltage state shown in FIG. 3A.

At the beginning of the second sub-frame, t = t₂The TN apparatus 26 is set at a highVoltage level + V_HIs turned on again, and the TN apparatus 28 is maintained at the low voltage level V_LAs shown in fig. 4, line graphs (a) and (b) (see also fig. 3D), and these driving voltages remain unchanged until t = t₃Until the end of the second sub-frame. At t = t₂To switch the first TN arrangement 26 from zero to + V_HIs a powered transition and is therefore extremely fast. During the second subframe, the TN devices 26 and 28 no longer compensate each other, and the combination now acts like a 90 ° polarization rotator, as shown in diagram (e) of FIG. 4, in which the first TN device 26 does not contribute to the input polarization, while the second TN device 28 performs polarization direction rotation.

Starting at t = t₃The next sub-frame is an inverted first sub-frame in which the applied drive signal voltages have the same magnitude but opposite signs to maintain DC balance. In the same way, the subsequent sub-frame is an inverted second sub-frame. The drive signal waveform continues to repeat after the last sub-frame shown in fig. 4. The curve portions in plots (c), (d), and (e) of FIG. 4 are the same in one of the voltage-reversed sub-frames as in the first and second sub-frames, respectively, because nematic liquid crystals do not react to polarity. This polarization switching process may continue indefinitely, with the liquid crystal device combining light polarized in the vertical direction at 0 ° during odd subframes and light polarized in the horizontal direction at 90 ° during even subframes.

Plots (f) and (g) of fig. 4 show the output transmission that would be seen by an observer wearing passive eyewear or a passive decoder comprising a first viewing device, e.g., a vertical orientation analyzing polarizer located in the left eyepiece lens, and a second viewing device, e.g., a horizontal orientation analyzing polarizer located in the right eyepiece lens. The output polarizer 60 shown in FIGS. 3A, 3B, 3C, and 3D represents one of the two analyzing polarizers of the passive decoder. In this configuration, the left eyepiece lens will be on during odd subframes and off during even subframes, while the right eyepiece lens will be on during even subframes and off during odd subframes. This embodiment would be suitable for viewers some distance away from the polarization switch, which may be attached to the image source 22, and polarization encoded left and right eye images transmitted through the air, as is the case in movie theaters. Stereoscopic 3D viewing is performed when the image source 22 displays a left eye image during odd sub-frames and a right eye image during even sub-frames. The optical transition shown in plots (f) and (g) of fig. 4 is extremely rapid because it is a powered transition. The slower, unpowered transition to reset the liquid crystal device remains a stealth state and is never optically apparent.

The system described in the first preferred embodiment switches the linearly polarized light by 90 between the vertical polarization and the horizontal polarization. Rotating the input polarizer 24 and TN units 26 and 28 by 45 DEG will cause the polarization modulator 20 to switch linearly polarized light between +45 DEG and-45 DEG, which is also applicable to a passive spectacle system as long as the polarizer in the lens of each eyepiece is also rotated by 45 deg.

The polarization rotator of the first preferred embodiment may also be implemented to switch between right-handed and left-handed circularly polarized light by placing a quarter-wave plate at the output of the combined TN devices 26 and 28, with a principal axis oriented at 45 to the direction of linear polarization of light propagating from the exit surface 56 of the second TN device 28. In this case, the lens of the passive glasses will also be provided with a quarter-wave retarder film laminated in front of the polarizing film. The quarter wave film may be in the form of a multilayer film that is colorless or in the form of a simpler monolayer film that is colored.

Those skilled in the art will recognize that there is considerable freedom in the order of polarity inversion of the voltages applied to the first and second TN devices 26 and 28 of the first preferred embodiment to maintain DC balance. For example, the amplitude + V within an individual sub-frame is cut off_Hand-V_HThe unipolar drive signal pulses may also have an amplitude of + V, as shown in the graphs (a) and (b) of FIG. 4_HAnd-V_HThis will thus automatically achieve DC balance on a sub-frame basis one by one, as shown in fig. 5. In addition, the drive signal waveform applied to either or both of the first and second TN devices 26 and 28 may have a polarity reversed from that shown in the diagrams (a) and (b) of FIG. 4, without departing from the principle of operation described. The drive signal waveforms applied to the first and second TN apparatuses 26 and 28 may also be interchanged with one another.

The driving scheme of FIG. 4 goes from 0 to V_HCan be controlled by applying an intensity V_HApplying an intensity V before the pulse wave_ODIs made faster by brief over-voltage drive (over-drive) pulses of which | V_OD|>|V_HL. Which amplitude and width of the overvoltage drive pulse are selected such that when the molecular axial field in the liquid crystal material reaches a value corresponding to a steady state V_HAt the state of voltage, the V_ODThe pulse is turned off and V is applied_HA pulse wave. This is illustrated in graphs (a) and (b) of fig. 6 to be compared with graphs (a) and (b) of fig. 4. With this over-voltage driving mechanism, it is possible to lower V_HWhile still maintaining a fast response time. The use of such over-voltage drive pulses can significantly reduce power consumption, which is an important factor in battery operated devices such as some active 3D glasses.

FIG. 7 shows a second preferred embodiment of the driving signal conditions of the TN polarization modulator 20, in which an analyzing or output polarizer 60 is combined with the polarization modulator 20 to enable it to be used as a light shutter in active glasses for stereoscopic 3D viewing for use with the image source 22 displaying sub-frames of left and right eye images. The left and right eyepiece lens assemblies in the active glasses have the same structure, each including a first TN device 26 and a second TN device 28, as shown in FIG. 3A, disposed between an input polarizer 24 and an output polarizer 60. The axes of polarization of light transmission of the input polarizer 24 and the output polarizer 60 are arranged at right angles to each other. The drive signal waveforms of the first and second TN devices 26 and 28 of the right eyepiece lens are shown in the graphs (a) and (b) of FIG. 7, and those concerning the left eyepiece lens are shown in the graphs (c) and (d) of FIG. 7. The driving signal waveform for the left eye is the same as that for the right eye, except that the phases are shifted from each other by the time length of one sub-frame. The optical transmission of the right eye lens is shown in plot (e) of FIG. 7, where the right eye lens is shown turned off during the left eye image sub-frame and turned on during the right eye image sub-frame. Similarly, the optical transmission of the left eye lens is shown in plot (f) of FIG. 7, where the left eye lens is shown on during the left eye image sub-frame and off during the right eye image sub-frame. This second embodiment is particularly suitable for use when used with an ultra-high speed imager, such as Texas Instruments (Texas Instruments) DLP imaging devices, which employ digitally controlled micro-mirrors (micromorrs). Since it is a digital device, the DLP represents the encoding of gray levels through a series of digital pulses throughout the sub-frame period. When used with a DLP imaging device, a very fast optical shutter not only maintains a high overall transmission when the shutter is open, but also avoids attenuation of the basic gray scale information at the beginning or end of each sub-frame, which would otherwise degrade the quality of the image reproduction when a slow-acting shutter is used.

FIG. 8 shows a third preferred embodiment of the driving conditions of active glasses for stereoscopic 3D viewing. The driving conditions of this third embodiment are similar to those shown in the second embodiment of fig. 7, except for the waveform of the driving signal of the blank period generated immediately before the start of each sub-frame. For some imaging devices, it takes a certain length of time to update the image. For example, while the right eye image is being written to the top of the screen, the lower portion of the screen will still display the previous left eye image. Therefore, for the period of time when the right-eye image is being updated, the shutter lenses of both eyes are closed to avoid the unpleasant interference or ghost phenomenon. The situation is similar when the left-eye image is being updated.

Graphs (a) and (b) in fig. 8 show drive signal waveforms of the first and second TN devices 26 and 28, respectively, in the right eyepiece lens. For subgraph from the left eyeBeginning t of frame₀To t₁During the time period, the first and second TN devices 26 and 28 of the right eye receive the high intensity voltage level V_HTherefore, TN devices 26 and 28 compensate each other, resulting in a light blocking state as shown by the optical response curve of diagram (e) in FIG. 8. A low intensity voltage level V during the remaining portion of the left-eye image sub-frame_LIn this case zero, is applied to the first and second TN devices 26 and 28 of the right eyepiece lens so that they attenuate but maintain dynamic compensation so that the right eyepiece lens remains closed. Time t₁May occur within the period Lu while the left eye image is being updated, or it may occur within or after the update period Lu. At the beginning of the right image sub-frame where the right image is being updated during the period Ru, the first and second TN devices 26 and 28 of the right lens receive V_LSo that TN devices 26 and 28 compensate each other, resulting in a light blocking state, as shown by the optical response curve of line (e) in FIG. 8. At the beginning of period R, when the right eye image has been updated, the first TN device 26 of the right eye lens is turned on to a high voltage intensity level V_HWhile the second TN apparatus 28 is maintained at V_LSo that the right eyepiece lens is turned on during this period, allowing the viewer to see the updated right eye image.

Graphs (c) and (d) in fig. 8 show drive signal waveforms of the first and second TN devices 26 and 28 in the left eyepiece lens, respectively. It should be noted that the left eye lens driving signal waveform is simply a phase shifted version of the right eye lens driving signal waveform, which is shifted by the time length of one sub-frame. The optical response shown in line (f) of fig. 8 is therefore only a phase shift distortion of the right eye response shown in line (e) of fig. 8. Refer to diagrams (e) and (f) of FIG. 8, which achieve the desired optical response for both eyes. In the update periods Lu and Ru, both the right and left eyepiece lenses are turned off. In the period of the part L of the left eye sub-frame after the left eye image is completely updated, only the left eye lens is opened; during the period of the completely updated part R of the right-eye image in the right-eye sub-frame, only the right eyepiece lens is opened.

In addition to the TN mode, other liquid crystal electro-optic modes may be used to perform polarization state compensation. A fourth preferred embodiment uses two electric field controlled birefringence (ECB) liquid crystal devices. ECB liquid crystal devices include two forms, using liquid crystal materials having positive dielectric anisotropy and using liquid crystal materials having negative dielectric anisotropy. The latter form is also known as Vertically Aligned (VA) or Vertically Aligned (VAN) mode. When used in accordance with the present disclosure, both positive and negative types are suitable for use in a polarization modulator.

FIGS. 9A, 9B, 9C, and 9D show an example of a polarization modulator 80 using a two-positive ECB mode liquid crystal device. FIG. 9A shows an input polarizer 82 on the left, followed by a first ECB liquid crystal device 84 and a second ECB liquid crystal device 86, combined in optical series. The first ECB device 84 is constructed with liquid crystal material contained between glass substrates 88, the glass substrates 88 having a light-transmissive electrode layer 90 formed on an upper inner surface thereof. The liquid crystal material comprises an electrode surface contacting molecular axis 92_CAnd electrode surface not contacting molecular axis 92_n. The second ECB device 86 is constructed with liquid crystal material contained between glass substrates 94, the glass substrates 94 having light transmissive electrode layers 96 formed on their upper inner surfaces. The liquid crystal material comprises electrode surface contact molecular axis 98_cAnd electrode surface not contacting molecular axis 98_n. The two ECB liquid crystal devices 84 and 86 satisfy the conditions mentioned earlier for compensation. Light propagating from image source 22 exits polarizer 82 with an input polarization direction 100, which is shown as a tilted cylinder, indicating that the direction of polarization makes an angle of +45 with the plane of the drawing.

FIG. 9A shows a low voltage level V of a driving signal_LFrom the display driver circuit 102 to both ECB devices 84 and 86. Drive signal level V_LBelow the ECB threshold voltage or even equal to zero. At this voltage, the molecular axis 92 in the first ECB device 84 is_CAnd 92_nIn the plane of the drawing andparallel to the substrate 88, and the molecular axis 98 in the second ECB device 86_CAnd 98_nLying in a plane perpendicular to the plane of the drawing and parallel to the substrate 94. By means of a cylinder 92 representing the molecular axis of the area viewed from the side in the first ECB device 84_CAnd 92_nAnd a bottom-view cylinder 98 in the second ECB apparatus 86_CAnd 98_nThis condition is displayed. Surface contact molecular axes 92 relative to the inner surfaces of substrates 88 and 94, respectively_CAnd 98_CThe pretilt angle (pretilt angle) of (a) is not shown in the figure. Within each ECB device 84 and 86, the domains are axially parallel to each other. At the applied drive signal level V_LHere, both ECB devices 84 and 86 are provided with an in-plane retardation (in-plane retardation)₀The characteristics are the same. In FIG. 9A, the two ECB devices 84 and 86 compensate each other so that the polarization state of the incident light remains unchanged after passing through their combination.

FIG. 9B shows the high voltage level V of the same driving signal_HTo both the first and second ECB devices 84 and 86 such that the molecules are oriented in the axial direction 92_nAnd 98_nAre respectively arranged approximately perpendicular to the boundaries of the liquid crystal device defined by electrode layers 90 and 96, but with the molecular axis direction 92_CAnd 98_CThe thin surface layer is not. Due to the molecular axial direction 92_CAnd 98_CWith a thin surface layer, each ECB device 84 and 86 has a slight residual in-plane retardation_R(ii) a But because of the ECB devices 84 and 86_RAre orthogonally arranged so they still compensate each other.

FIG. 9C shows the signal level V at the driving signal level_HIs removed from the ECB devices 84 and 86 and replaced with the drive signal level V_LA snapshot at a later point in time of the axial orientation of the molecule in a short time is taken by the switches 104 in the display driver circuit 102₁And 104₂As shown by the switch position. Shown in the first ECB device 84 with the surface not in contact with the molecular axis 92_nThe small arrow 110 in the central molecular axis indicates that the central molecular axis is rotating back to that shown in FIG. 9AThe parallel state process. The same rotation occurs in the second ECB device 86, e.g., respectivelyAnd the arrows 112 pointing into and out of the plane of the drawing as indicated by the |. Surface-untouched molecular axial direction 92_nIs released in the first ECB device 84 by rotation in the plane of the drawing, with the surface not contacting the molecular axis 98_nBy being perpendicular to the molecular axis 92_nAnd an axis lying in the plane of the drawing is rotated as a center to be released in the second ECB device 86. In this case it is dynamically compensated.

FIG. 9D shows the first ECB device 84 being driven with a high voltage level V of the driving signal_HIs turned on and ECB device 86 is maintained at V_LIn this case. The combination of the ECB devices 84 and 86 is no longer compensated because the drive signals applied to the ECB devices 84 and 86 are different. The first ECB device 84 introduces a residual in-plane retardance_RAnd the second ECB device 86 introduces an in-plane retardation₀Thereby causing a total delay₀-_RSince the slow axes of the residual and in-plane retardance are at 90 deg. to each other. By using₀-_R= λ/2 obtains a 90 ° polarization rotation of the polarization modulator 80, where λ is the design wavelength of the light, as shown by the output polarization direction 110.

A fourth embodiment using two ECB devices 84 and 86 constructed with a nematic liquid crystal mixture having positive dielectric anisotropy has been experimentally realized. Each ECB device is made using Indium Tin Oxide (ITO) coated glass substrates and liquid crystal molecules are axially aligned with rubbed polyimide (rubbbedpolyimide) such that the rubbing directions of the top and bottom substrates are anti-parallel to each other when the two substrates are assembled. The pretilt angle in the axial direction of the surface contact molecules is about 4 DEG, and a cell gap d of 2.5 μm is provided by spacers in the sealing material. The ECB liquid crystal device was filled with a nematic liquid crystal mixture MLC-7030 available from Merck KGaA of Darmstadt, Germany. The MLC-7030 mixture has a birefringence of 0.1102.

Fig. 10A and 10B show waveforms of driving signals applied to the first and second ECB devices 84 and 86. In this example, the sub-frame time duration is 5.0 ms, corresponding to a frequency of 200 Hz. Which in this example selects the bipolar drive signal pulses to provide DC balance in each subframe, as described earlier. A 0.25 millisecond wide +20 volt pulse followed by a 0.25 millisecond wide-20 volt pulse is applied to both ECB devices 84 and 86 at the beginning of the first subframe. After these pulses, for the remainder of the 5 millisecond sub-frame, both ECB devices 84 and 86 receive 0 volts. At the beginning of the second subframe, the first ECB device 84 receives a 2.5 millisecond wide +20 volt pulse followed by a 2.5 millisecond wide-20 volt pulse while the second ECB device 86 is held at 0 volts. FIG. 10C shows the measured optical response when the polarization modulator 80 is positioned between orthogonally arranged polarizers, wherein the alignment direction of the first ECB device 84 is at a 45 angle to the input polarization direction 100. It was measured at 25 ℃. Both the off and on times are on the order of milliseconds or less and no optical behavior appears with dynamic compensation occurring in the period between 0.5 and 5 milliseconds, meaning that the attenuation of the molecular axial fields in the ECB devices 84 and 86 follow each other very precisely. FIG. 10D is an enlarged version of FIG. 10C near the transition showing the optical shutter having an on time of about 60 microseconds and an off time of about 80 microseconds. These short reaction times are sufficient to allow operation at switching frequencies up to 480 Hz.

A fifth preferred embodiment is a polarization state modulator using a two pi cell instead of two ECB liquid crystal devices. Like ECB devices, pi cells are a liquid crystal device having an in-plane retardation controlled by a voltage. The pi cell has a structure similar to that of a positive ECB liquid crystal device except that the polyimide rubbing direction of the assembled substrates is a parallel direction rather than an opposite parallel direction. However, the molecular axial field in a π cell is quite different from that of a positive ECB cell, in that the surface in the middle of the liquid crystal layer that is not in contact with the molecular axial is perpendicular to the cell boundary in both high and low voltage driving signal states, and in that most of the switching occurs near the cell boundary.

FIGS. 11A, 11B, 11C, and 11D show an example of a polarization modulator 120 using a two π liquid crystal cell. FIG. 11A shows an input polarizer 82 on the left, followed by a first π cell 122 and a second π cell 124, combined in optical series. The ECB devices 84 and 86 in fig. 9A, 9B, 9C, and 9D show a surface-contact molecular axis-parallel alignment, while the pi cells 122 and 124 show an anti-parallel alignment of the surface-contact molecular axes; otherwise, the liquid crystal devices are similar to each other and their corresponding components are denoted by the same reference numerals. Which arrange pi cells 122 and 124 such that projections of their optical axes onto the light propagation surfaces (i.e., entrance surface 54 and exit surface 56) of pi cells 122 and 124 are orthogonally related to each other. The two pi cells 122 and 124 satisfy the conditions set forth earlier for compensation. Light propagating from image source 22 exits polarizer 82 with an input polarization direction 100, which is shown as a tilted cylinder, indicating that the direction of polarization is at an angle of +45 ° to the plane of the drawing.

FIG. 11A shows a low voltage level V of a driving signal_LFrom display driver circuit 126 to pi cells 122 and 124. Drive signal level V_LCommonly referred to as a bias voltage (bias voltage), to prevent the transition of the molecular axial domain structure within the pi cell to an undesired splay state structure. For this reason, the drive signal level V_LAnd in general is not zero. At the applied drive signal level V_LHere, the surface in the first π liquid crystal cell 122 is not in contact with the molecular axis 130_nIs located in the plane of the drawing, and the surface of the second pi cell 124 is not in contact with the molecular axis 132_nIs in a plane perpendicular to the plane of the drawing and the substrate 94. At the applied drive signal level V_LHere, both π liquid crystal cells 122 and 124 have an in-plane retardation₀The characteristics are the same. In FIG. 11A, the two π liquid crystal cells 122 and 124 compensate each other such that the polarization state of the incident light remains unchanged after passing through their combination.

FIG. 11B shows the high voltage level V of the same driving signal_HIs applied to both the first pi cell 122 and the second pi cell 124 such that the molecular axis 130 near the boundaries of the liquid crystal device_nAnd 132_nAre arranged more perpendicularly to the substrates 88 and 94, respectively. Due to the molecular axial direction 130_cAnd 132_cWith a thin surface layer, each pi cell 122 and 124 has a small residual in-plane retardation_R(ii) a But because of the pi cells 122 and 124_RThe slow axes of (a) are orthogonally arranged so they still compensate each other.

FIG. 11C shows the signal level V at the driving signal level_HIs removed from pi liquid crystal cells 122 and 124 and is replaced with drive signal level V_LA snapshot at a later point in time of the axial orientation of the molecule in a brief period of time is taken by the switches 134 in the drive circuit 126, respectively₁And 134₂As shown by the switch position. Shown in the first pi cell 122 is the surface-untouched molecular axis 130_nThe small arrow 140 above indicates that it is rotating back to the drive signal level V shown in FIG. 11A_LThe state is in progress. The same rotation occurs in the second pi cell 124, e.g. respectivelyAnd the arrows 142 which move into and out of the plane of the drawing as indicated by the |. Surface uncontacted molecular axial direction 130_nIs released in the first pi cell 122 by rotation in the plane of the drawing, without the surface contacting the molecular axis 132_nBy being perpendicular to the molecular axis 132_nAnd rotates about an axis lying in the plane of the drawing to relax in the second pi cell 124. In this case it is dynamically compensated.

FIG. 11D shows first π liquid crystal cell 122 being driven with a high voltage intensity level V of a driving signal_HIs turned on and the second pi cell 124 is maintained at V_LIn this case. Because the drive signals applied to pi cells 122 and 124 are already different, the combination of pi cells 122 and 124 is no longer compensated. First pi liquid crystal cell 122To produce a residual in-plane retardation_RAnd the second pi cell 124 leads out an in-plane retardation₀Thereby causing a total delay₀-_RSince the slow axes of the two in-plane retardations are at 90 ° to each other. By using₀-_R= λ/2 obtains a 90 ° polarization rotation of the polarization modulator 120, where λ is the designed wavelength of the light, as shown by the output polarization direction 110.

The voltage level V of the pi cell is due to the occurrence of the splay state_LCannot be set to zero, and this slows down V_HTo V_LIf the pi cell can be switched to a state with an intensity less than V_LIdeally even zero, the level transition can be faster. However, by switching to a value less than V_LIf the voltage is maintained for only a brief period of time, it is possible to accelerate the transition. This is known as under-voltage drive (under-drive) technology. The undervoltage driving voltage is V_UDIn which V is_UD<V_L. The under-voltage driving technique can also be combined with the over-voltage driving technique shown in FIG. 6 to obtain faster rise and fall times. FIG. 12 shows over-voltage driving and V_UDCombination of undervoltage driving of = 0. Line drawing (a) of fig. 12 shows the drive signal waveform applied to the first pi liquid crystal cell 122, and line drawing (b) of fig. 12 shows the drive signal waveform applied to the second pi liquid crystal cell 124.

Fig. 13A and 13B show an example in which a polarization modulator 80' having a first ECB device 84 and a second ECB device 86 as shown in fig. 9A, 9B, 9C, and 9D is used in a stereoscopic 3D viewing system 150 using passive glasses 152. In this example, an input polarizer 82 'is a linear polarizer having a perpendicular input polarization direction 100', and the first and second ECB devices 84 and 86 in the inactive state are essentially half-wave optical retarders. The slow shaft 154 of the first ECB device 84 is oriented at +45 relative to the vertical axis, while the slow shaft 156 of the second ECB device 86 is oriented at-45 relative to the vertical axis. Output polarizers 60R and 60L are located in the right and left eyepiece lenses of the passive glasses 152 worn by the viewer, respectively. In this example, the polarizing plate 60R located in front of the right eye (R eye) is a linear polarizing plate having a horizontal polarization direction (90 °), and the polarizing plate 60L located in front of the left eye (L eye) is a linear polarizing plate having a vertical polarization direction (0 °). The system 150 may be used to view stereoscopic images in a direct viewing system in which the image source 22 is a television screen having the polarization modulator 80' disposed thereon. The system 150 may also be used to view images in a stereoscopic projection system in which the polarization modulator 80' is placed in or in front of a projector-type image source 22, the projector-type image source 22 projecting polarization modulated images onto a screen for viewing by a viewer wearing passive glasses 152.

The basic operation of the example of FIGS. 13A and 13B is also described below with reference to FIGS. 10A, 10B, and 10C. Referring to fig. 13A, during the first sub-frame when the ECB devices 84 and 86 receive the same voltage, the output polarization direction 110 'is polarized to the same 0 ° (vertical) direction as the input polarization direction 100'. The image projected onto the right eyepiece lens is blocked because the orientation of the transmission axis of its associated polarizing plate 60R is arranged at 90 DEG, while the image projected onto the left eyepiece lens is transmitted because the orientation of the transmission axis of its associated polarizing plate 60L is arranged at 0 deg. Thus, during the first sub-frame of the viewing scene for the left eye being displayed by image source 22, it is transmitted (light rectangle 158) to the left eye and blocked (dark rectangle 160) from the right eye.

Referring to FIG. 13B, during the second sub-frame, V_HApplied to a first ECB device 84 and V_LApplied to the second ECB device 86 resulting in a net half wave retardation at the design wavelength. At the design wavelength, this combination of applied voltages has the effect of rotating the 0 DEG input polarization 100 'by 90 DEG, so that the output polarization direction 110' becomes 90 DEG (horizontal). At this time, the image is transmitted (bright rectangle 160) to the right eye through its associated 90 ° polarizer 60R, and blocked (dark rectangle 158) to the left eye through its associated polarizer 60L oriented at 0 °. Thus, during the time that image source 22 is displaying the second sub-frame of the right-eye viewing scene, it is transmitted to the right eyeBut blocked from the left eye.

However, during the second subframe, the combination of the first and second ECB devices 84 and 86 only exhibits the property of a half-wave retarder at the design wavelength, which is typically 550 nm where the eye is most sensitive. At wavelengths outside the design wavelength, the output polarization state 110 'is no longer the linear input polarization state 100' rotated by 90, but is an elliptical polarization state. This non-ideal behavior causes a clear, on-state transmission of light through the system 150 to be attenuated at off-design wavelengths and, more importantly, causes light leakage through the system 150 in a light blocking state at off-design wavelengths. This light leakage results in unpleasant ghost images visible to viewers of the 3D image. This non-ideality of clear and light blocking states does not occur during the first subframe because the ECB devices 84 and 86 compensate for all wavelengths, resulting in linear output polarization in a vertical direction for all wavelengths. For this case, light leakage in the light blocking state may be considerably low because it substantially depends only on the quality of the polarizing plate used.

FIG. 14 shows simulated optical transmission spectra of clear and light blocking states generated during the first and second sub-frames of the system 150 shown in FIGS. 13A and 13B. For simplicity, the simulation used an ideal polarizer, defined as having 50% transmission with unpolarized light. The liquid crystal material used was MLC-7030, available from Merck GmbH, Darmstadt, Germany. During the first subframe, a right eye light transmission curve 162 indicates 0% transmission and a left eye light transmission curve 164 indicates 50% transmission throughout the visible spectrum. However, during the second subframe, at the design wavelength, in this example 550 nm, a right eye light transmission curve 166 indicates 50% transmission and a left eye light transmission curve 168 indicates only 0% transmission. Light transmission curves 166 and 168 show that at other wavelengths, light transmission in the clear, on state is reduced and light transmission in the light blocking state is increased.

One disadvantage of the system 150 is the amount of light leakage in the left eye light blocking state at wavelengths other than the design wavelength. This means that the right eye image leaks out and is seen by the left eye as an unpleasant ghost. The analog contrast for the left eye is only 38.1. Another disadvantage of the system 150 is that when the viewer's head is tilted sideways, additional ghosting effects are created because the output polarization direction 110 of the modulator 80' is no longer orthogonally aligned with the polarization axis of one of the polarizers in the passive glasses 152, thereby allowing an unwanted polarization component to leak out in the light blocking state of each eye.

By adding an outer quarter wave film with its slow axis oriented at +45 to the vertical axis and the output polarization direction, as described in the earlier paragraph, the foregoing example can be made insensitive to the tilt angle of the viewer's head, at least at the design wavelength. The quarter-wave film enables the polarization modulator to switch between right-handed and left-handed circularly polarized light, rather than orthogonal linear polarization states. Quarter wave films may also be placed on the light input side of the passive glasses to interpret the right and left hand circular polarization. For the right eye, the quarter wave film is oriented so that its slow axis is at-45 ° to the vertical axis; for the left eye, the quarter-wave film is oriented such that its slow axis is at +45 ° to the vertical axis. The quarter-wave film is followed by a linear polarizer whose polarization directions are oriented at 90 DEG for both eyes. An example of an implementation of this mechanism in a stereoscopic 3D viewing system 150 'is shown in fig. 15A and 15B, where the viewing system 150' is a modified variant of the system 150.

The basic operation of the example of FIGS. 15A and 15B is described below. Referring to fig. 15A, during the first subframe, the ECB devices 84 and 86 receive the same voltage and thus compensate each other such that the output polarization direction 110 'is polarized to the same 0 ° (vertical) direction as the input polarization direction 100'. The orientation of its slow axis 182 is arranged to be +45 deg. of the outer quarter wave film 180 with respect to the vertical axis, and the orientation of its slow axis 186 is arranged to be-45 deg. of the quarter wave film 184 with respect to the vertical axis in the right eyepiece lens of the passive glasses 152' such that the light maintains linear polarization at 0 deg.. This polarization direction is at right angles to the 90 DEG transmission axis of the right eyepiece polarizer 60R so that the image projected onto the right eyepiece of the passive glasses 152' is blocked to the right eye. For the left eye lens of the passive glasses 152', the slow axis 190 of a quarter-wave film 188 is oriented at +45 °, which is in the same direction as the slow axis 182 of the outer quarter-wave film 180 at the output surface of the ECB device 86, so that the quarter-wave films 180 and 188 add up a half-wave optical retarder. This half-wave retardation combination rotates the linear input polarization direction 100 'by 90 deg. so that it is parallel to the transmission direction of the left eyepiece lens polarizer 60L', resulting in a clear, on state.

Referring to FIG. 15B, during the second sub-frame, V_HApplied to an ECB device 84, and V_LTo the ECB device 86. The combination of ECB devices 84 and 86 thus acts as a half-wave plate (HWP) with its slow axis oriented at-45 DEG with respect to the vertical axis. For the right eye lens of the passive glasses 152 ', the quarter-wave films 180 and 184 compensate each other, causing a half-wave overall retardation in the overall optical path that rotates the linear input polarization direction 100' by 90 ° to be parallel to the transmission direction of the right eye lens polarizer 60R, resulting in a clear, on state. For the left eye lens of the passive glasses 152', the combination of the quarter wave film 188 and the outer quarter wave film 180 is equivalent to a half wave retarder having its slow axis oriented at +45 ° to the vertical axis. This combination compensates for the half-wave retardation of the combination of ECB devices 84 and 86, resulting in the light exiting the quarter-wave film 188 being linearly polarized at 0 deg. to be at right angles to the transmission direction of the left eye lens polarizer 60L', resulting in a blocked state for the left eye.

Further detailed studies of fig. 15A and 15B show that light leakage may occur in the light blocking state for the left eye. During the first sub-frame, the ECB devices 84 and 86 use the same liquid crystal mixture and have the same cell gap, so they compensate each other at all wavelengths of image-carrying light. Similarly, if the outer quarter wave film 180 and quarter wave films 184 and 188 in passive glasses 152' are made of the same material, these films will also compensate each other in the right eyepiece lens for all wavelengths. As a result, the image carrying light projected to the right eye lens is completely blocked from the right eye for all wavelengths.

However, during the second subframe, the ECB devices 84 and 86 sum to a HWP of-45 relative to the vertical axis, while the outer quarter wave film 180 and the quarter wave film 188 in the left eyepiece lens sum to a HWP of +45 relative to the vertical axis. The two HWP combinations compensate each other to achieve a light-leak-free light blocking state for the left eye under two additional conditions: (1) the combination of ECB devices 84 and 86 is such that the design wavelength of a half-wave retarder is the same as the design wavelength of quarter-wave films 180, 184, and 188 in system 150'; and (2) the wavelength dispersion in birefringence of the liquid crystal materials used in the ECB devices 84 and 86 is substantially the same as the wavelength dispersion in birefringence of the quarter-wave films 180, 184, and 188 in the system 150'.

The nominal retardation (nominal retardance) of commercially available quarter-wave films is 140 nm, corresponding to a design wavelength of 560 nm. Since other retardation values cannot be obtained, the condition (1) can be satisfied by selecting the liquid crystal mixture, selecting the cell gap of the ECB devices 84 and 86, and fine-tuning the voltage V applied to the ECB devices 84 and 86_HAnd V_LTo meet the half-wave optical retardation condition at 560 nm.

Because of the limited number of commercially available quarter-wave film types, the most practical way to satisfy condition (2) is to select a liquid crystal mixture whose wavelength dispersion matches that of one of the available quarter-wave films.

One measure of wavelength dispersion can be taken as the ratio of the birefringence of a material at 450 nm relative to its birefringence at 650 nmD, i.e. D = Δ n₄₅₀/Δn₆₅₀. The company Nitto-Denko of tokyo, japan supplies a wide band colorless form of quarter wave film and a colored form of quarter wave film. The broadband colorless film of Nitto-Denko has D =1.00 and is a laminated structure of two or more birefringent films. Nitto-Denko supplies two different types of colored quarter wave films, including NAF type with D =1.02 made from polyvinyl alcohol (pviyl alcohol) and NRF type with D =1.14 made from polycarbonate (polycarbonate). Since it is known that there is no liquid crystal mixture with D =1.00 or even 1.02, the satisfaction of condition (2) requires the selection of a liquid crystal mixture with the desired material properties, such as birefringence and viscosity, for an ECB device, and with a dispersion value that matches that of polycarbonate-type quarter-wave films. The aforementioned Merck liquid crystal MLC-7030 meets ECB device requirements and has a dispersion D =1.15, which is approximately equal to the dispersion of polycarbonate having a dispersion D = 1.14.

It performs simulations to quantify the amount of leakage that would be obtained from the system 150 'for the case where MLC-7030 is used in ECB devices 84 and 86 and where the three available quarter-wave film types are each employed in the remainder of the system 150'. FIG. 16 shows simulated transmission spectra for the light blocking state for the left eye, where curve 200 represents a multi-layer colorless quarter wave film, curve 202 represents a colored polyvinyl alcohol quarter wave film, and curve 204 represents a colored polycarbonate quarter wave film. It is evident from the curves 200, 202, and 204 that the use of a colorless quarter wave film results in the greatest amount of light leakage, followed by the polyvinyl alcohol quarter wave film immediately thereafter. The significant reduction in the amount of light leakage of the polycarbonate quarter-wave film is due to the close matching of the wavelength dispersion between the polycarbonate and the liquid crystal, thereby achieving a high contrast stereoscopic image that is substantially free of the ghost effect.

FIG. 17 shows simulation results of optical transmission of a stereoscopic system 150' in which MLC-7030 is used in ECB devices 84 and 86 and quarter-wave films 180, 184, and 188 are best constructed of polycarbonate. The spectra of the right and left eye light pass states (curves 206 and 208) are almost identical, as are the light block states of the right and left eyes (curves 210 and 212). Each eye's symmetrical behavior and indestructible light blocking state is a desirable property for a stereoscopic viewing mechanism to avoid the ghost effect and color shift. The comparative difference between the linear polarization system 150 of fig. 14 and the optimized circular polarization system 150' of fig. 17 is extremely significant. In the linear polarization system 150, light leakage in the blocking state at one eye introduces substantial aliasing effects, and spectral differences in the clear, on state of the right and left eyes introduce a binocular color shift into the image that is difficult to correct.

FIG. 18 shows the actual measurement of optical transmission for a stereo system 150' using commercially available polarizers for the best case, where the dispersion of the liquid crystal mixture is approximately matched to that of the quarter wave film, which in this case is MLC-7030 liquid crystal and polycarbonate quarter wave film, respectively. The light leakage for the right eye (curve 206) and the left eye (curve 208) is almost zero for most of the visible spectrum; while the clear, on-state of the right eye (curve 210) and left eye (curve 212) are substantially identical, which verifies the simulation of fig. 17. The significant oscillations in the curves 206 and 208 are due to interference effects in the ECB devices 84 and 86 that are not taken into account in generating the simulation shown in fig. 17.

Fig. 19 shows a stereoscopic 3D viewing system 150 "using passive glasses 152" comprising protective cellulose Triacetate (TAC) carrier layers 220, 222, and 224. TAC layers 220, 222, and 224 are also included in the stereoscopic viewing systems 150 and 150', but are not shown in fig. 13A, 13B, 15A, and 15B for clarity, since the illustration does not involve viewing angle considerations. TAC layers 220 and 222 are respectively located inside the polarizing plates 60R and 60L' in the eyepiece lens; and the TAC layer 224 is located inside the polarizer 82 'in the polarization modulator 80'. The TAC layers located outside the polarizers 60R, 60L ', and 82' have no influence on the viewing angle characteristics, and thus are not shown in FIG. 19. The TAC layers may be modeled using negative uniaxial birefringent films having an out-of-plane retardation (out-of-plane retardation) of about-40 nm and an optical axis perpendicular to the films. Because of the birefringence, the ECB devices 84 and 86, the quarter wave films 180, 184, and 188, and the TAC layers 220, 222, and 224 all affect the contrast of the off-axis field of view. FIG. 19 shows that the system 150 "also includes an optional positive C-type film compensator 226 between the polarization modulator 80" and the passive glasses 152 ". A positive C-type film is a positive uniaxial birefringent film whose slow axis is perpendicular to the film.

Computer simulations show that it is possible to greatly improve the off-axis contrast for the right and left eyes by placing the positive C-type thin film compensator 226 in the position shown in fig. 19 and optimizing its out-of-plane retardation to 280 nm. Depending on the application, the positive C-shaped film compensator 226 may be laminated to the outside of the outer quarter wave film 180 of the modulator 80 "or partially outside the quarter wave films 184 and 188 of the passive glasses 152".

FIGS. 20A, 20B, 20C, and 20D show simulated iso-contrast (iso-contrast) viewing angle plots for the system 150 ", where FIGS. 20A and 20B show the system characteristic viewing angle performance for the left and right eyes, respectively, without the selective positive C-type film compensator 226, and FIGS. 20C and 20D show the system characteristic viewing angle performance for the left and right eyes, respectively, with the selective positive C-type film compensator. These diagrams are contours of contrast observed at polar angles (polarangles) from 0 ° to 60 ° and azimuths from 0 ° to 360 °. These iso-contrast maps provide a useful measure of the amount of ghosting that exists in off-axis views. Contrast contours of 20 and 100 are shown. Fig. 20A and 20B show that without the positive C-type thin film compensator 226, the system characteristic range of high contrast viewing angle for the right eye is significantly narrower than for the left eye. FIGS. 20C and 20D show that the addition of the +280 nm positive C-type film compensator 226 can significantly expand the system characteristic range of viewing angles for which a high contrast is observed by the right eye, thereby making the range of high contrast viewing directions for the right and left eyes substantially the same.

It is also possible to insert the positive C-type film compensator 226 in a position in the optical path other than the position shown in fig. 19, such as between the ECB device 86 and the outer quarter wave film 180, between the input polarizer TAC layer 224 and the first ECB device 84, or between the quarter wave films 184 and 188 and the polarizer TAC layers 220 and 222 attached thereto. Simulations show that placing the positive C-type film compensator 226 in these alternative positions expands the viewing angle to a level that is not achievable with placing the positive C-type film compensator 226 in the position shown in fig. 19.

In the passive eyewear embodiment of FIG. 19, a positive C-shaped film compensator 226 is placed over the polarization modulator 80 ". A positive C compensator 226 may also be used for active glasses, as shown by an eyepiece 230 in FIG. 21. In the case of active glasses, the right and left eye lens have the same structure, except that the active glasses are driven in a phase-different manner, as illustrated in FIG. 7 and the description of the TN apparatus in the previous related paragraphs.

The simulated iso-contrast plots of fig. 22A and 22B show that the range of high contrast viewing angles with minimal ghosting effects can be significantly extended by incorporating a 400 nm ortho-C film 226 into the optical path as illustrated in fig. 21. For this example, as shown in FIG. 22B, the contrast ratio is at least 100:1 up to 25 polar angle, and at least 20:1 up to 35 polar angle. Simulations show that positive C-type films with out-of-plane retardation in the range 350 nm to 400 nm provide good results and are optimal for this example 400 nm.

The foregoing embodiments illustrated in FIGS. 19 and 21 employ ECB devices having positive dielectric anisotropy. The following describes embodiments in which the liquid crystal device employs a liquid crystal material having a negative dielectric anisotropy, which is used in a vertical nematic (VAN) device.

FIGS. 23A, 23B, 23C, and 23D show an example of a polarization modulator 240 using a two VAN mode liquid crystal device. FIG. 23A shows an input polarizer 82 on the left, followed by a first VAN liquid crystal device 244 and a second VAN liquid crystal device 246, combined in optical sequence. The first VAN device 244 is liquidThe crystalline material is built up between glass substrates 248, the glass substrates 248 having a light-transmissive electrode layer 250 formed on the upper inner surface thereof. The liquid crystal material comprises electrode surface contact molecular axial direction 252_cAnd electrode surface not contacting molecular axis 252_n. The second VAN device 246 is constructed with liquid crystal material contained between glass substrates 254, the glass substrates 254 having light transmissive electrode layers 256 formed on their upper inner surfaces. The liquid crystal material comprises electrode surface contact molecular axis 258_cAnd the electrode surface is not in contact with the molecular axis 258_n. The two VAN liquid crystal devices 244 and 246 satisfy the conditions mentioned earlier for compensation. Light propagating from image source 22 exits polarizer 82 with an input polarization direction 100, which is shown as a tilted cylinder, indicating that the direction of polarization makes an angle of +45 with the plane of the drawing.

FIG. 23A shows a low voltage level V of a driving signal_LFrom display driver circuit 262 to both VAN devices 244 and 246. Drive signal level V_LBelow the VAN threshold voltage or even equal to zero. At this voltage, the molecules in the first VAN device 244 are axially 252_CAnd 252_nIs perpendicular to the substrate 248 and the molecular axis 258 in the second VAN device 246_CAnd 258_nIs perpendicular to the substrate 254. By means of a cylinder 252 representing the molecular axis of the region viewed from the side in the first VAN device 244_cAnd 252_nAnd a cylinder 258 viewed from the side in the second VAN device 246_cAnd 258_nThis condition is displayed. A surface contact molecular axis 252 with respect to the normal direction of the substrates 248 and 254, respectively_cAnd 258_CThe slight pretilt angle of (c) is not shown in the figure. Within each VAN device 244 and 246, the domain molecular axes are parallel to each other. At the applied drive signal level V_LHere, both VAN devices 244 and 246 are provided with a residual in-plane delay_RThe characteristics are the same; but due to VAN devices 244 and 246_RThe slow axes of the light beams are arranged orthogonally so that they still compensate each other and the polarization state of the incident light remains unchanged after passing through the combination.

FIG. 23B shows the same driveHigh voltage strength level V of moving signal_HTo both the first VAN device 244 and the second VAN device 246 such that the molecules are oriented axially 252_nAnd 258_nAre aligned approximately parallel to the boundaries of the liquid crystal device defined by electrode layers 250 and 256, respectively.

FIG. 23C shows the signal level V at the driving signal level_HIs removed from VAN devices 244 and 246 and converted to drive signal level V_LA snapshot at a later point in time of the axial orientation of the molecule over a brief period of time is taken by the switches 264 in the display driver circuit 262, respectively₁And 264₂As shown by the switch position. Shown in the first VAN device 244 with the surface not in contact with the molecular axis 252_nThe small arrow 270 in the central molecular axis direction indicates that the central molecular axis is rotating back to the vertically aligned state shown in FIG. 23A. The same rotation occurs in the second VAN device 246, e.g., respectivelyAnd the arrows 272 which move into and out of the plane of the drawing as indicated by the |. Surface-untouched molecular axial 252_nIs released in the first VAN device 244 by rotation in the plane of the drawing, with the surface not contacting the molecular axis 258_nReleased in the second VAN device 246 by rotation in a plane perpendicular to the plane of the drawing and perpendicular to the substrate. In this case it is dynamically compensated.

Fig. 23D shows the first VAN device 244 remaining at V_LAnd the second VAN device 246 is driven with a high voltage level V of the driving signal_HConducting the circuit. The combination of the VAN devices 244 and 246 is not compensated for because the drive signals applied to the VAN devices 244 and 246 are already different. The first VAN device 244 introduces a residual in-plane delay_RAnd the second VAN device 246 introduces an in-plane delay₀Thereby causing a total delay₀-_RSince the slow axes of the residual and in-plane retardance are at 90 deg. to each other. By using₀-_R= λ/2 obtaining a 90 DEG polarization rotation of the polarization modulator 240, where λ is the design wavelength of the light, e.g., the output polarizationShown in the orientation 110.

The compensation of viewing angle for active glasses using VAN devices is described below with reference to FIGS. 24A and 24B, in which a modulator element stack 230 'shown in FIG. 24A does not incorporate compensation, while a modulator element stack 230' shown in FIG. 24B includes two commercially available Nitto-Denko 55-275 biaxial retarder films 274 and 276. The slow axis of the upper retarder 274 is parallel to the transmission axis of the adjacent polarizer 60, and the slow axis of the lower retarder 276 is parallel to the transmission axis of the adjacent polarizer 82. The transmission axes of the polarizing plates 60 and 82 are arranged orthogonally to each other. The liquid crystal material in the VAN device is a negative dielectric heterogeneous material MLC-7026-100, available from Merck Gmb of Darmstadt, Germany. Of course, other embodiments employing different retarders and liquid crystal mixtures would also be suitable for implementing the disclosed compensation.

The simulated iso-contrast plots of FIGS. 25A and 25B show that the range of high contrast viewing angles can be significantly enhanced by the addition of two Nitto55-275 compensator films 274 and 276 as shown in FIG. 24B. FIG. 25A shows that, without compensation, the VAN contrast is at least 100:1 up to 19 polar angle and at least 20:1 up to 30 polar angle. This is comparable to the uncompensated ECB case of fig. 22. However, FIG. 25B shows that after compensation, the VAN contrast is at least 100:1 and up to 30, and at least 20:1 and up to 45. This is much more exaggerated than the uncompensated ECB case of fig. 22, where the contrast is at least 100:1 up to 25 °, and 20:1 up to 35 °.

The VAN device may also be used for stereoscopic viewing systems using passive glasses, as shown in fig. 26. Simulated contrast images of the left and right eyes are shown in fig. 27A and 27B. A comparison of fig. 27A and 27B with fig. 20A and 20B shows that the angular range of the high contrast field of view of the polarization modulator using the VAN device is wider than that of the ECB device without film compensation.

Fig. 20A, 20B, 20C, and 20D show ECB devices, while fig. 27A and 27B show VAN devices, when passive glasses are used with the disclosed polarization modulator, the viewing angle characteristics of the right and left eyes are completely different. In order to achieve the best high contrast viewing angle range for each eye, the eyepieces are compensated separately.

For example, fig. 28 shows a stereoscopic viewing system employing the VAN device shown in fig. 23A, 23B, 23C, and 23D and passive glasses 152' ″ in which different compensation films are used in the eyepieces of the right and left eyes. In the right eye mirror, a biaxial compensator 280 having an in-plane retardation of 62 nm and an out-of-plane retardation of 309 nm is disposed between the TAC layer 220 and the quarter-wave film 184. The slow axis of the biaxial compensator 280 is oriented at 90 DEG, which is parallel to the transmission axis of the adjacent polarizer 60R. In the left eyepiece, the viewing angle compensation is achieved by the combination of a 150 nm positive C-type film 282 placed in front of the quarter wave film 188 in the left eyepiece and a 100 nm positive A-type film 284 placed between the TAC layer 222 and the quarter wave film 188, wherein the slow axis of the positive A-type film 284 is oriented parallel to the transmission axis of the adjacent polarizer 60L'. A positive A-type film is a positively uniaxially birefringent film with its slow axis in the plane of the film.

FIGS. 29A and 29B show simulated isometric views of the left and right eyes of the compensated eyepiece configuration illustrated in FIG. 28, respectively. Compared to the uncompensated stereoscopic viewing system shown in fig. 27A and 27B, a much wider high contrast viewing angle is achieved.

For example, fig. 30 shows a stereoscopic viewing system employing the ECB apparatus shown in fig. 9A, 9B, 9C, and 9D and passive glasses 152 "", in which different compensation films are used in the eyepieces of the right and left eyes. In the right eye mirror, a 300 nm positive C-type film 286 is placed in front of the quarter wave film 184 and a 350 nm positive A-type film 288 is placed between the TAC layer 220 of polarizers and the quarter wave film 184, wherein the orientation of the slow axis of the positive A-type film 288 is arranged parallel to the transmission axis of the adjacent polarizer 60R. In the left eyepiece, the viewing angle compensation is achieved by the combination of a 150 nm positive C-type film 282 placed in front of the quarter wave film 188 in the left eyepiece and a 100 nm positive A-type film 284 placed between the TAC layer 222 of the polarizers and the quarter wave film 188, wherein the slow axis of the positive A-type film 284 is oriented parallel to the transmission axis of the adjacent polarizer 60L'.

FIGS. 31A and 31B show simulated isometric views of the left and right eyes of the compensated eyepiece configuration illustrated in FIG. 30, respectively. It achieves a much wider range of high contrast viewing angles than the uncompensated stereo system shown in fig. 20A and 20B, or even the compensated system shown in fig. 20C and 20D, where both eyes receive the same angle compensator.

In many applications of the disclosed polarization state modulator, the temperature may not remain constant and may vary over a wide range, such as in the case of a modulator included in a projector during warm-up time. Since the material properties of the liquid crystal are known to be temperature dependent, in particular its birefringence, any change in temperature will shift the wavelength of the half-wave optical retardation state of the combined first and second liquid crystal devices away from the design wavelength. This deviation will cause a brightness loss and color shift in the bright state of stereoscopic viewing systems employing active or passive glasses. In addition, in these systems employing passive glasses, any shift in the wavelength set for the half-wave state away from the quarter-wave film design wavelength (which is relatively insensitive to temperature) will result in increased ghosting in the image transmitted to the viewer's left eye.

For a nematic liquid crystal, the temperature dependence Δ n of birefringence can be roughly estimated by the following equation:

wherein T is the degree of temperature in absolute temperature unit, T_clpIs a nematic isotropic transition temperature in absolute temperature units, Δ n₀Represents a virtual birefringence of a perfectly ordered nematic liquid crystal having a unit order parameter. By normalizing the birefringence to a fixed temperature, e.g., 20 ℃, Δ n in the formula₀The equation is solved.

FIG. 32 shows simulated temperature dependence of birefringence for ECB mixture MLC-7030 and VAN mixture MLC-7026-100 normalized to that at 20 ℃, where ECB mixture MLC-7030 has a birefringence of 0.1126 at 20 ℃ and T at 75 ℃_clpWhile the VAN mixture MLC-7026-100 has a birefringence of 0.1091 at 20 ℃ and a T of 80.5 ℃_clp. The shape of these curves may deviate slightly from the actual measurement, but the trend should still be similar.

FIG. 33 is a graph showing simulated voltage dependence of phase shift at 20 ℃ for an ECB device filled with ECB mixture MLC-7030 and having a cell gap of 3.0 microns and a pretilt angle of 3 °, where the phase shift is in degrees. The phase shift monotonically decreases from about 220 ° to 0 ° when the voltage is applied. This curve applies to either the first or second ECB device. Since the slow axes of the two ECB devices are arranged orthogonally to each other, the phase shift of one of the ECB devices is subtracted from the phase shift of the other ECB device. During the second sub-frame, a high voltage level V_HApplied to the first ECB device to cause a phase delay_RWhile a low voltage strength V which may be zero_LApplied to a second ECB device, causing a delay₀. The polarized light is rotated 90 degrees to achieve the best performance,₀-_R= λ/2, which is a 180 ° phase shift at the design wavelength λ. Voltages may be applied to either or both of the ECB devices to achieve the 180 DEG phase shift condition. In the example shown in FIG. 33, V is set_HApplication of =20.8V to the first ECB device results in a phase shift of 9.1 DEG, and V is applied_LApplication of =2.2V to a second ECB deviceA 189.1 DEG phase shift to achieve a predetermined 180 DEG phase shift difference for the combination.

Inspection view 33 finds that there are many possible voltage pairs (V) that will provide a 180 overall phase shift_H、V_L). FIG. 34 shows these voltage pairs in the form of a family of curves with fixed 180 phase shifts, each curve corresponding to a temperature of 20 ℃, 30 ℃, 40 ℃, or 45 ℃ respectively, where simulations take into account the reduction in birefringence as MLC-7030 temperature increases, as shown in FIG. 32. FIG. 34 clearly shows that it is possible to adjust V individually or in combination depending on the temperature_HAnd V_LSo as to maintain a predetermined 180 DEG phase shift. Which can be particularly advantageous for simultaneous adjustment of V_HAnd V_LBoth to maintain a fixed 180 DEG phase shift. This is seen by the control lines in FIG. 34, where V_HAnd V_LBoth increase with decreasing temperature. Increasing V with decreasing temperature_HA fast on-time is ensured at lower temperatures, where the viscosity of the liquid crystal is significantly higher.

FIG. 35 shows a simplified functional block diagram of a circuit 300 that may be used to adjust V_HAnd V_LTo maintain a fixed 180 DEG phase shift over a wide temperature range. In circuit 300, a temperature sensor 302 measures the operating temperature of liquid crystal devices 304 and 306 and adjusts V via processing circuit 308 in response to the stored phase shifts of liquid crystal devices 304 and 306_HAnd V_LThe level of the voltage. V_HAnd V_LThe level of which is then sent to timing and drive circuit 310, which applies the drive waveform to liquid crystal devices 304 and 306. Performed according to the temperature pairs V measured by the sensor 302_HAnd V_LThe control procedure for level adjustment may be determined from a curve like that of FIG. 34, where the actual measured curve would be stored and used, rather than an approximation of the simulation.

FIG. 36 is a graph showing simulated voltage dependence of phase shift at 20 ℃ for a VAN device filled with the VAN mixture MLC-7026-100 and having a 3.0 micron cell gap and a 87 DEG pretilt angleThe offset is in degrees. The phase shift increases monotonically from near 0 to about 205 while the voltage is applied. This curve applies to either the first or second VAN device. Since the slow axes of the two VAN devices are arranged orthogonally to each other, the phase offset of one VAN device is subtracted from the phase offset of the other VAN device. During the second sub-frame, a high voltage level V_HApplied to a second VAN device to cause a phase delay₀While a low voltage strength V which may be zero_LApplied to the first VAN device to cause a phase delay_R. The polarized light is rotated 90 DEG to achieve the best performance₀-_R= λ/2, which is a 180 ° phase shift at the design wavelength λ. Voltages are applied to either or both of the VAN devices to achieve the 180 DEG phase shifted state. In the example shown in FIG. 36, V is set to_HApplication of =24.9V to the second VAN device results in a phase shift of 204.7 DEG, and V is applied_LApplication of =2.2V to the first VAN device results in a phase shift of 24.7 °, achieving the combined predetermined 180 ° phase shift difference.

Inspection view 36 finds that there are many possible voltage pairs (V) that will provide an overall phase shift of 180 DEG_H、V_L). FIG. 37 shows these voltage pairs in the form of a family of fixed 180 phase shifts curves, each corresponding to a temperature of 20 ℃, 30 ℃, 40 ℃, or 45 ℃ respectively, wherein the simulation takes into account the reduction in birefringence as MLC-7026 and 100 temperatures are increased, as shown in FIG. 32. FIG. 36 clearly shows that it is possible to adjust V individually or in combination depending on the temperature_HAnd V_LSo as to maintain a predetermined 180 DEG phase shift. Which can be particularly advantageous for simultaneous adjustment of V_HAnd V_LBoth to maintain a fixed 180 DEG phase shift. This is seen by the control lines in FIG. 37, where V_HAnd V_LBoth increase with decreasing temperature. Increasing V with decreasing temperature_HA fast on-time is ensured at lower temperatures, where the viscosity of the liquid crystal is significantly higher.

It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the invention should, therefore, be determined only by the following claims.

Claims (31)

1. An optical polarization state modulator for time division multiplexed stereoscopic three-dimensional image viewing by a viewer, the modulator receiving light in alternating sequence in an input polarization state and carrying first and second perspective images of a scene in different first and second subframes including updated image portions, comprising:

first and second liquid crystal devices combined in an optical train such that polarized light propagating therethrough undergoes a change in polarization state in accordance with voltages applied to the first and second liquid crystal devices;

the first and second liquid crystal devices having first and second sets of molecular axes, respectively, and being constructed and oriented such that, upon removal of an applied equal voltage, the molecular axes in the first and second sets release in coordination with each other and thereby dynamically offset polarization state changes such that a plurality of wavelengths of incident light passing through and out of the combination of the first and second liquid crystal devices are in the input polarization state;

a driving circuit for sending first and second driving signals to the first and second liquid crystal devices, respectively, the first and second driving signals including lower intensity levels establishing lower intensity molecular axial field states for the first and second liquid crystal devices, and the first and second driving signals including pulses having lower to higher intensity level powered transitions establishing higher intensity molecular axial field states for the first and second liquid crystal devices;

the first and second drive signals cooperate with one another during a subframe of the first and second subframes to cause, in the first and second liquid crystal devices, formation of the higher intensity molecular axial field state from which molecules are axially relaxed during an update image portion of such a subframe, such that molecules located in the first and second groups are axially offset from the change in polarization state and thereby impart a first output polarization state equal to the input polarization state to the combined image carrying polarized light passing through the first and second liquid crystal devices; and is

The first and second drive signals cooperate with each other during the other of the first and second sub-frames to cause, in different first and second liquid crystal devices, formation of the lower and higher intensity molecular axial field states during an update image portion of the other sub-frame such that molecular axes in the first and second groups are not offset by a change in polarization state and thereby carry a second output polarization state in which polarized light imparted to the image passing through the combination of the first and second liquid crystal devices is different from the first output polarization state.

2. The optical polarization state modulator of claim 1, wherein one of the first and second sets of molecular axes is arranged as a 90 ° rotated mirror image of the other of the first and second sets of molecular axes.

3. The optical polarization state modulator of claim 2, wherein the first and the second liquid crystal devices are of the twisted nematic type.

4. An optical polarization state modulator according to claim 3, wherein the first and second liquid crystal devices comprise equal optically active dopants but opposite optical activities.

5. The optical polarization state modulator of claim 2, wherein the first and second liquid crystal devices comprise light propagation surfaces and are of a pi cell type with optical axes arranged such that projections of the optical axes on the light propagation surfaces are orthogonally related to each other.

6. The optical polarization state modulator of claim 2, wherein the first and second liquid crystal devices are of the electric-field-controlled birefringence type with the alignment layer surface-contact molecular axis direction arranged such that the surface-contact molecular axis direction of one of the first and second liquid crystal devices is orthogonal with respect to the surface-contact molecular axis direction of the other of the first and second liquid crystal devices.

7. The optical polarization state modulator of claim 1, having a light entrance surface and a light exit surface, and further comprising:

an image source and an input polarizer optically coupled to each other, the image source producing the first and second perspective images in alternating order, and light in the input polarization state carrying the first and second perspective images exiting the input polarizer to be projected onto the light entrance surface; and

a passive decoder comprising first and second viewing devices separated from the light exit surface by a transmission medium and configured to receive image-carrying polarized light in the first and second output polarization states during different ones of the first and second sub-frames, the first viewing device comprising a first polarizer having a first transmission polarization axis directed to transmit light in the first output polarization state and block light in the second output polarization state, and the second viewing device comprising a second polarizer having a second transmission polarization axis directed to transmit light in the second output polarization state and block light in the first output polarization state, thereby presenting the first and second perspective images to the viewer during different ones of the first and second sub-frames.

8. The optical polarization state modulator of claim 1, having a light entrance surface and a light exit surface, and further comprising:

an image source for emitting light carrying the first and second perspective images, traveling through a transmission medium, and traveling through an input polarizer to produce the light in the input polarization state carrying the first and second perspective images for projection onto the light entry surface; and

an analyzing polarizer optically connected to the light exit surface, wherein the image in one of the first and second output polarization states carries polarized light therethrough to present a corresponding one of the first and second perspective images to the viewer.

9. The optical polarization state modulator of claim 8, wherein the input polarizer and the analyzing polarizer have an input filter transmission polarization axis and an analyzing filter transmission polarization axis, respectively, in transverse relation to each other.

10. The optical polarization state modulator of claim 1, wherein light entry and exit surfaces connect different ones of the first and second liquid crystal devices, wherein the first and second liquid crystal devices are constructed from liquid crystal material having birefringence characteristics, and wherein off-axis birefringence effects of the liquid crystal material contribute to a system characteristic viewing angle range, and further comprising:

an input polarizer in the input polarization state and carrying the light of the first and second perspective images to exit through the input polarizer to be projected on the light entrance surface;

an output polarizer receiving image carrying polarized light in the first and second output polarization states during the different first and second subframes; and

a birefringent compensator positioned between the input and output polarizers to at least partially offset the off-axis birefringence effect to produce a system viewing angle range that is wider than the system's characteristic viewing angle range.

11. The optical polarization state modulator of claim 10, wherein the birefringent compensator comprises a C-type compensator.

12. The optical polarization state modulator of claim 11, wherein the type of the first and the second liquid crystal devices is an electric field controlled birefringence type.

13. The optical polarization state modulator of claim 11, wherein the input polarizer and the output polarizer have an input filter transmission polarization axis and an output filter transmission polarization axis, respectively, in transverse relation to each other, and wherein the C-type compensator is located between the light exit surface and the output polarizer.

14. The optical polarization state modulator of claim 13, wherein the first and the second liquid crystal devices are of the electrically controlled birefringence type.

15. The optical polarization state modulator of claim 10, wherein the birefringent compensator comprises first and second biaxial birefringent layers having first and second slow axes, respectively, aligned transversely.

16. The optical polarization state modulator of claim 15, wherein the first and second liquid crystal devices are of the vertical nematic type.

17. The optical polarization state modulator of claim 15, wherein the input polarizer and the output polarizer have an input filter transmission polarization axis and an output filter transmission polarization axis, respectively, in transverse relation to each other, and wherein the first and second liquid crystal devices are located between the first and second birefringent biaxial birefringent layers.

18. The optical polarization state modulator of claim 17, wherein the first and second liquid crystal devices are of the vertical nematic type.

19. The optical polarization state modulator of claim 1, wherein light entry and exit surfaces are connected to different ones of the first and second liquid crystal devices, and wherein the first and second liquid crystal devices are constructed from liquid crystal material characterized by birefringence exhibiting wavelength dispersion that contributes to a system characteristic contrast, and further comprising:

an input polarizer in the input polarization state and carrying the light of the first and second perspective images to exit through the input polarizer to be projected on the light entrance surface;

a quarter-wave optical retarder located adjacent the light exit surface to impart circular polarization to the image carrying light in the first and second polarization states; and

a passive viewing decoder comprising first and second viewing devices separated from the light exit surface and the quarter-wave optical retarder by a transmission medium and configured to receive image-carrying circularly polarized light in the first and second output polarization states during different ones of the first and second subframes and to remove circularly polarized light from the image-carrying circularly polarized light,

the first viewing device includes a first quarter-wave optical retarder having a slow axis and cooperates with a first polarizer having a first transmission axis oriented with respect to the slow axis of the first quarter-wave optical retarder to transmit light of the first output polarization state and block light of the second output polarization state,

the second viewing device includes a second quarter-wave optical retarder having a slow axis and cooperating with a second polarizer having a second transmission axis oriented with respect to the slow axis of the second quarter-wave optical retarder to transmit light of the second output polarization state and to block light of the first output polarization state,

the quarter-wave optical retarder is located adjacent to the light exit surface, the first quarter-wave optical retarder, and the second quarter-wave optical retarder, each constructed of a birefringent material having a wavelength dispersion characteristic that substantially matches a wavelength dispersion in a birefringence of the liquid crystal material to present the first and second perspective images to the viewer during different ones of the first and second sub-frames that exhibit a contrast that is higher than a contrast of the system characteristic.

20. The optical polarization state modulator of claim 19, further comprising a C-type compensator between the light exit surface and the first and second polarizers of the passive viewing decoder.

21. The optical polarization state modulator of claim 20, wherein the C-type compensator is attached to the quarter-wave optical retarder adjacent to the light exit surface.

22. The optical polarization state modulator of claim 20, wherein the C-type compensator comprises first and second portions attached to the first and second quarter-wave optical retarders, respectively.

23. The optical polarization state modulator of claim 19, further comprising first and second birefringent compensators respectively included in the first and second viewing devices and characterized by different birefringent properties.

24. The optical polarization state modulator of claim 23, wherein at least one of the first and second birefringent compensators comprises a stack of a-type and C-type compensators.

25. The optical polarization state modulator of claim 24, wherein the first and second birefringent compensators each comprise a stack of a-type and C-type compensators having different optical retardation values.

26. The optical polarization state modulator of claim 25, wherein the first and the second liquid crystal devices are of the electrically controlled birefringence type.

27. The optical polarization state modulator of claim 24, wherein the other birefringent compensator of the first and second birefringent compensators comprises a biaxial birefringent layer.

28. The optical polarization state modulator of claim 27, wherein the first and second liquid crystal devices are of the vertical nematic type.

29. The optical polarization state modulator of claim 1, further comprising:

a temperature sensor operatively connected to the first and second liquid crystal devices for measuring device operating temperature information;

a memory store containing temperature dependent phase shift response data corresponding to molecular axial field states of the first and second liquid crystal devices; and

processing circuitry operatively coupled to the drive circuitry to generate first and second drive signals establishing a half-wavelength polarization state change, the processing circuitry accessing stored phase shift response data corresponding to operating temperature information of the measurement device and causing the drive circuitry to generate the first and second drive signals at higher and lower intensity levels that maintain a substantially fixed phase shift corresponding to the half-wavelength polarization state change over a wide temperature range.

30. The optical polarization state modulator of claim 29, wherein the stored temperature-dependent phase shift response data is generated by actual measurement.

31. The optical polarization state modulator of claim 29, wherein the stored temperature-dependent phase shift response data is generated by simulation.