WO2025078173A1 - Optical element, switchable light filter, lighting device, and screen - Google Patents

Abstract

The invention relates to an optical element (1) comprising a plurality of light-absorbing transition dipole moments. The plurality of transition dipole moments are aligned parallel to a first preferred direction, which can be selected for the first optical element (1), with a tolerance τ of τ ≤20° or vary about same permanently or at least in a first state, wherein the transition dipole moments aligned parallel to the first preferred direction with a tolerance τ have a density N₁, and the remaining transition dipole moments have a density N₂ such that light which is incident on the first optical element (1) is transmitted or partly absorbed on the basis of the direction of incidence thereof relative to the first optical element (1) and the polarization state of the light. The inequality (I) is satisfied for the ratio of the respective densities N₁ and N₂ on the optical element (1) permanently or at least in the aforementioned first state such that higher tolerance values τ are at least partly compensated for on the basis of the correspondingly increased relation of the densities N₁ and N₂ of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the optical element (1) at angles greater than 45° is amplified. The invention additionally relates to first and second switchable light filters (5, 5a), lighting devices, and screens.

Description

Title

[0001] Optical element, switchable light filter, lighting device and screen

Technical field of the invention

[0002] In recent years, great strides have been made in widening the viewing angle of LCDs. However, there are often situations in which this very large viewing area of a screen can be a disadvantage. Information, such as banking details or other personal and sensitive data, is also becoming increasingly available on mobile devices such as notebooks and tablet PCs. Accordingly, people need control over who can see this sensitive data; they must be able to choose between a wide viewing angle – a public mode – in order to share information on their display with others, e.g., when viewing holiday photos or for advertising purposes. On the other hand, they need a narrow viewing angle – in a private mode – if they want to keep the image information confidential.

[0003] A similar problem arises in vehicle construction: There, the driver must not be distracted by image content, such as digital entertainment programs, when the engine is running, while the passenger also wants to consume these while driving. Therefore, a screen is required that can switch between the corresponding display modes.

[0004] Additional films based on micro-louvres have already been used for mobile displays to achieve visual data protection. However, these films were not switchable or reversible; they always had to be applied and removed manually. They also had to be transported separately from the display when not in use. A significant disadvantage of using such louvre films is the associated light loss. [0005] US Pat. No. 6,765,550 B2 describes such a privacy screen using micro-louvres. The major disadvantage is the mechanical removal or installation of the filter, as well as the loss of light in protected mode.

[0006] US Pat. No. 5,993,940 A describes the use of a film with small, strip-shaped prisms evenly arranged on its surface to achieve a private mode, i.e., a restricted viewing mode with a narrow viewing angle range. Development and production are technically quite complex.

[0007] In WO 2012/033583 A1, switching between unobstructed and restricted view is achieved by controlling liquid crystals between so-called "chromonic" layers. This results in a loss of light and requires considerable technical effort.

[0008] US 2012/0235891 A1 describes a very complex backlight in a screen. According to Figs. 1 and 15, not only are several light guides used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform the light from the rear illumination on its way to the front illumination. This is expensive and technically complex to implement and also involves light loss. According to the variant shown in Fig. 17 in US 2012/0235891 A1, both light sources 4R and 18 produce light with a narrow illumination angle, with the light from the rear light source 18 first being converted, at great expense, into light with a wide illumination angle. This complex conversion, as already mentioned above, significantly reduces brightness.

[0009] US 2013/0308185 A1 describes a special light guide formed with steps, which radiates light over a large area in different directions, depending on the direction from which it is illuminated from a narrow side. In conjunction with a transmissive image display device, e.g. an LC display, a screen that can be switched between free and restricted viewing modes can be created. A disadvantage here is, among other things, that the restricted viewing effect can only be created for left/right or for up/down, but not for left/right/up/down simultaneously, as is the case for certain payment transactions are necessary. In addition, even in restricted view mode, some residual light is still visible from blocked viewing angles.

[0010] WO 2015/121398 A1 by the applicant describes a screen with two operating modes, in which scattering particles are present in the volume of the corresponding light guide for switching between the operating modes. However, the polymer scattering particles selected therein generally have the disadvantage that light is coupled out of both large surfaces, whereby approximately half of the useful light is emitted in the wrong direction, namely toward the background lighting, where it cannot be adequately recycled due to the structure. Furthermore, the polymer scattering particles distributed in the volume of the light guide can, under certain circumstances, particularly at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

[001 1] The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the basic screen and/or require a complex and expensive optical element for mode switching and/or reduce the resolution in the freely viewable, public mode and/or have visual artifacts on very high-resolution displays.

Description of the invention

[0012] The object of the invention is therefore to describe an optical element in which light incident on the optical element is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties - not primarily depending on its position. Production-related alignment tolerances are to be counteracted by the optical effect. Switchable light filters that use the optical element are intended to influence the transmission of light as a function of the angle - optionally with respect to a seated or standing observer - with switching between at least two operating states. In particular, the transmission behavior is to be switchable for specific directions. Furthermore, lighting devices and screens that use the switchable light filters are to be disclosed. [0013] This object is achieved according to the invention by a first optical element comprising

• a multitude of light-absorbing transition dipole moments, where the corresponding absorption cross sections are designated o _a bs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans,

• wherein the majority of the transition dipole moments are permanently or at least in a first state with a tolerance T with T<19°, preferably with T<15°, aligned parallel to a first preferred direction selectable for the first optical element or vary around this (the tolerance T applies in both directions, i.e. for the value of a respective angle plus ±T), wherein the first preferred direction is arranged at an angle α to the perpendicular bisector of the first optical element, wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the first preferred direction have a density Ni and the remaining transition dipole moments (i.e. those lying outside the described tolerance range) have a density of N2,

ist, so dass höhere Toleranzwerte T aufgrund der dann entsprechend erhöhten Relation der Dichten Ni und N2 der Übergangsdipolmomente mindestens teilweise kompensiert werden, wodurch die Absorption von p-polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der ersten Ebene, auf das optische Element einfällt, verstärkt wird. • so that light incident on the first optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element and its polarization state, wherein on the optical element permanently or at least in said first state for the ratio of the respective densities Ni and N2 of the transition dipole moments the inequality is satisfied

so that higher tolerance values T are at least partially compensated due to the correspondingly increased relation of the densities Ni and N2 of the transition dipole moments, whereby the absorption of p-polarized light incident on the optical element at angles greater than 45°, measured in the first plane, is increased.

[0014] The multitude of light-absorbing transition dipole moments are arranged in a layer at least 0.2 micrometers thick. Several, e.g. by OCA (Optically Clear Adhesive) and/or substrates. The layer thicknesses of the transition dipole moments can be, for example, from 0.2 pm to 50 pm, preferably 0.2 pm to 20 pm, particularly preferably 1 pm to 10 pm. In a preferred embodiment, two layers of transition dipole moments are present in the first optical element 1, each of which is between 1 pm and 10 pm thick, wherein both each have a substrate (e.g. consisting of TAC or another non- or slightly birefringent material with a thickness preferably less than 100 pm), wherein furthermore the two substrates with the respective layers of transition dipole moments are glued to one another by means of an OCA (Optically Clear Adhesive).

der dann entsprechend erhöhten Relation der Dichten Ni und N2 der Übergangsdipolmomente mindestens teilweise kompensiert werden. [0015] The invention therefore shows a way of counteracting production-related limitations in the alignment of the transition dipole moments on the optical element by using only optical elements in which the inequality / N ₂ > 10 - is fulfilled permanently or at least in the said first state for the ratio of the respective densities Ni and N2 of the transition dipole moments, so that production-related higher tolerance values T due to

the correspondingly increased relation of the densities Ni and N2 of the transition dipole moments can be at least partially compensated.

[0016] Preferably, the inequality N / N ₂ > is satisfied permanently or at least in said first state for the ratio of the respective densities Ni and N2 of the transition dipole moments.

[0017] The greater the relative proportion of the remaining transition dipole moments (i.e. those lying outside the described tolerance range) with the density of N2, the worse the general transmission of the optical element is in all directions.

[0018] The relationship between the densities Ni and N2 can be measured indirectly, for example, using DLS (Dynamic Light Scattering) methods, which are known in the art and therefore will not be further explained here. By adhering to the inequality according to the invention, appropriate first optical elements can then be evaluated and selected after their production. [0019] A further object of the invention is achieved according to the invention by a first switchable light filter comprising a first optical element as described above,

Means for selectively generating a first electric field EF1 or a second electric field EF2, e.g. ITO layers with a connected signal generator, a liquid crystal layer arranged behind or in front of the first optical element in the viewing direction, on which layer the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarization state of light passing through it when the liquid crystal layer is arranged in front of the first optical element in the viewing direction, a first linear polarization filter arranged in front of the liquid crystal layer in the viewing direction, so that the transmission properties of the first switchable light filter differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the second electric field EF2 is applied.

[0020] In the case that the transition dipole moments are variable, e.g. via so-called guest-host liquid crystal cells, such guest-host liquid crystal cells may, but do not have to, correspond directly to the aforementioned liquid crystal layer.

[0021] In a preferred embodiment, light penetrating the liquid crystal layer is transmitted essentially unchanged when the second electric field EF2 is applied, while when the first electric field EF1 is applied, the incident light is circularly or elliptically polarized, or the polarization of the light is rotated by 90°. Essentially, this means that at the interfaces, the orientation of the liquid crystal molecules is determined by electric fields and surface-induced forces. Thus, the liquid crystal molecules are not ideally aligned, leading to an undesirable, slight change in polarization.

[0022] For designs with TN liquid crystal cells, the following applies: The alignment of the liquid crystal molecules typically differs by 90° at the large surfaces that define the liquid crystal layer. Such alignment is supported by PMI or PVA and additional mechanical or optical surface treatment. For TN liquid crystal layers, it is also generally true that when switching between the electric fields EF1 and EF2, the majority of the liquid crystals in the liquid crystal layer are rotated out of plane by 75 to 90 degrees. In the case of IPS and FFS liquid crystal layers, the rotations of the LC molecules are less than 45°, typically about 25° to 40° in plane.

[0023] The first electric field EF1 can, for example, have a field strength other than 0 V/pm, approximately on the order of 1 V/pm, e.g., as a square wave at 1 kHz, while the second electric field EF2, for example, has a field strength of 0 V/pm. However, the configuration can also be exactly the opposite, or such that both fields EF1 and EF2 are not field-free.

[0024] If the liquid crystal layer is arranged behind the first optical element in the viewing direction, the liquid crystal layer is preferably subjected to linearly polarized light or elliptically polarized light, in which the ratio of the major to the minor semi-axis is at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by linear polarization filters in the light path, or by λ/4 layers when using circularly polarized light.

[0025] Advantageously, the first optical element (and any further such optical element, if present) and/or the liquid crystal layer and/or the means for generating a first or second electric field EF1, EF2 are divided into several separately switchable segments, so that local switchability between the respectively possible operating states is enabled.

[0026] In a further embodiment, the switchable light filter comprises at least two first optical elements, wherein a birefringent layer is optionally arranged between at least two such first optical elements. Furthermore, the at least two first optical elements could, if appropriate, have different thicknesses of the layers containing the plurality of light-absorbing transition dipole moments, but this is not required.

[0027] A further object of the invention is further achieved by a second switchable light filter comprising a second optical element, in turn comprising • a multitude of light-absorbing transition dipole moments, where the corresponding absorption cross sections are designated o _a bs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans,

• wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a guest-host arrangement in a second liquid crystal layer,

• wherein furthermore, the majority of the transition dipole moments are aligned, at least in a first state, with a tolerance T with T<19°, preferably with T<15°, parallel to a second preferred direction selectable for the second optical element or vary around this (here too, the tolerance T applies in both directions, i.e. ±T), wherein the second preferred direction is arranged at an angle α to the perpendicular bisector of the second optical element, wherein the angle α is measured in a selectable second plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the second preferred direction have a density Ni and the remaining transition dipole moments have a density of N2,

• so that light which is incident on the second optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element, its polarization state and the state of the second liquid crystal layer at the location of the light incidence,

Means for selectively generating at least one first electric field EF1 or one second electric field EF2, wherein the respective electric field acts on the second liquid crystal layer, so that the transmission properties of the second switchable light filter differ between a first operating mode B1, in which the first electric field EF1 is applied and the second liquid crystal layer is in said first state, and a second operating mode B2, in which the second electric field EF2 is applied and the second liquid crystal layer is in a second state, wherein on the second optical element at least in said first state for the ratio of the respective densities Ni and N2 of the

"

erfüllt ist, so dass höhere Toleranzwerte T aufgrund der dann entsprechend erhöhten Relation der Dichten Ni und N2 der Übergangsdipolmomente mindestens teilweise kompensiert werden, wodurch die Absorption von p-polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der zweiten Ebene, auf das zweite optische Element einfällt, verstärkt wird. Transition dipole moments the inequality

fulfilled so that higher tolerance values T are at least partially compensated due to the correspondingly increased relation of the densities Ni and N2 of the transition dipole moments, whereby the absorption of p-polarized light incident on the second optical element at angles greater than 45°, measured in the second plane, is increased.

[0028] It should be noted at this point that, in the case of the second switchable light filter, it can generally be the case that the at least one first state of the majority of the transition dipole moments can practically also correspond to a second state for a second operating mode B2, which will be described in more detail below. This means, in particular, that the boundary conditions for the first state—concerning, among other things, the tolerance and the alignment with the second preferred direction—also apply to the second state.

[0029] The means for selectively generating at least one first electric field EF1 or one second electric field EF2 can also advantageously be designed such that different electric fields EF1, EF2 (or possibly different electric fields EF3, EF4, etc.) are present at different positions on the liquid layer at the same time. This allows for a position-dependent partial switching of the operating modes of the second switchable light filter.

welches unter Winkeln von größer 45°, gemessen in der zweiten Ebene, auf das zweite optische Element einfällt, abgeschwächt wird. Auf diese Weise wird entlang der zweiten Ebene in dem besagten zweiten Zustand eine höhere Transmission erreicht. Die Betriebsart B2 erlaubt dann bei Betrachtung entlang der zweiten Ebene mindestens teilweise einen größeren Betrachtungswinkel, also einen freien Sichtmodus. [0030] For the second switchable light filter, it can optionally apply that on its second optical element, at least in the said second state (for the second operating mode B2), for the ratio of the respective densities Ni and N2 of the transition dipole moments, deviating from the first state, the inequality V, / /V ₂ 10 - 3 ■ iog ₁₀ is satisfied, whereby the absorption of p-polarized light,

which is incident on the second optical element at angles greater than 45°, measured in the second plane, is attenuated. In this way, a higher transmission is achieved along the second plane in the second state. Operating mode B2 then allows, at least in part, a larger viewing angle when viewed along the second plane, i.e., a free view mode.

[0031] Advantageously, both in the first switchable light filter and in the second switchable light filter, the potential differences between the electrodes for generating the first electric field EF1 and the second electric field EF2 by at least 2 volts. Further operating modes B3, B4, etc. can be explicitly provided with electric fields EF3, EF4, etc. that differ from the electric fields EF1 and EF2. Furthermore, the operating modes B1, B2, etc. can also differ locally on the switchable light filter, as already indicated above.

[0032] The invention also comprises a first illumination device in a first embodiment for a screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising

- a surface-like backlight that radiates light and is optionally directly luminous (e.g. with an LED matrix), as well as

- a first or second switchable light filter according to the invention arranged in front of the background lighting in the viewing direction as described above.

[0033] In addition to this, the invention also comprises a first screen in a first embodiment, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, comprising

- a first lighting device as described above,

- and, if no first linear polarization filter is arranged in the first or second switchable light filter of the first illumination device, a second linear polarization filter arranged in front of the background illumination in the viewing direction, whereby light emanating from the background illumination and penetrating the second linear polarization filter is restricted in its propagation directions, and

- a transmissive image display device which is arranged in front of the first or second switchable light filter in the viewing direction,

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0034] It is preferred that the first or second linear polarization filter is arranged in the transmissive image display device or is a part of it. [0035] Furthermore, the invention comprises a second screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising

- an image display device, whereby in principle any type of image display device is suitable, for example LC panel, OLED, microLED and others,

- in the viewing direction in front of the image display device, a first or second switchable light filter according to the invention as described above,

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0036] Optionally, the first or second switchable light filter may be subsequently attached to the image display device by a user and/or reversibly. In this case, a light filter may be sold as a so-called "after-market product."

[0037] For some of the first and second screens described above, it may be advantageous when used in cars if a third optical element is arranged in front of the transmissive image display device in the viewing direction, which third optical element comprises:

• a large number of light-absorbing transition dipole moments; the dye mass density is greater than 1% or even greater than 10%.

• wherein the majority of the transition dipole moments are aligned, at least in a first state, with a tolerance of a maximum of 20° (alternatively 10°) parallel to a third preferred direction selectable for the third optical element or fluctuate around this, wherein the third preferred direction is arranged at an angle oci to the perpendicular bisector of the third optical element (for example, ai = 0°, ai = ±2° or amount(oci) > 2° can apply), wherein the angle oci is measured in a selectable third plane containing said perpendicular bisector; the third plane is preferably perpendicular to the first plane of the first optical element, • so that light which is incident on the third optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the third optical element and its polarization state.

[0038] This latter embodiment advantageously provides for reduced transmission in the vertical direction and can thus reduce or completely avoid reflections of image content displayed on the said screen on the windscreen in the vehicle.

[0039] Furthermore, the invention comprises a second illumination device for a screen in a second embodiment, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising

- a surface-like extended backlight that radiates light in a limited angular range and which is optionally designed to be directly illuminated, and

- a plate-shaped light guide located in front of the background lighting in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume,

- illuminants arranged laterally on at least one narrow side of the light guide, and

- optionally a linear polarization filter,

- a first or second switchable light filter according to the invention arranged in the viewing direction in front of the background lighting (this also includes a position in front of a screen with which the lighting device is used), as described above,

- wherein in operating mode B2 the backlight is switched on and the lamps are switched off, and wherein in operating mode B1 at least the lamps are switched on, and

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0040] By “limited angular range” is meant within the scope of the invention, and in particular with regard to the background lighting, that the corresponding Luminance is concentrated to at least 80% or 90% in a defined angular range, while there may still be residual light outside the defined, restricted angular range, which is usually due to technical reasons. Ideally, such residual light is minimal and decreases at larger angles. In order to achieve particularly strong minimization, a corresponding light filter is used in addition to the backlight that emits light in a restricted angular range. This also applies to the variant described below with a light guide that emits or decouples light predominantly in a restricted angular range. In contrast to this embodiment of the invention, luminance curves of backlights are typically bell-shaped, particularly over horizontal (and possibly also vertical) angular ranges, although this does not necessarily mean that there is a true concentration of luminance around a smaller angular range.

[0041] Furthermore, the invention comprises a third illumination device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode,

- a surface-like backlight that radiates light in an unrestricted angular range and that is optionally designed to be directly luminous (e.g. by means of a locally dimmable LED matrix lighting unit), as well as

- a plate-shaped light guide located in front of the background lighting in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume, wherein said output coupling elements output the majority of the light coupled laterally into at least one narrow side of the light guide (i.e., more than half, preferably more than 80% or 90%) into a limited angular range,

- illuminants arranged laterally on at least one narrow side of the light guide, and

- optionally a linear polarization filter,

- a first or second switchable light filter according to the invention arranged in the viewing direction in front of the background illumination, preferably in front of the light guide, as described above, - where in operating mode B2 the backlight is off and the lamps are switched on, and where in operating mode B1 at least the backlight is switched on (optionally the lamps can also be switched on), and

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0042] Furthermore, the invention comprises a fourth illumination device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising

- a surface-like backlight which emits light in a limited angular range and - to limit the angular range of said light - contains a first optical element as described above, and which is optionally designed to be directly luminous, and

- a plate-shaped light guide located in front of the background lighting in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume,

- illuminants arranged laterally on at least one narrow side of the light guide, and

- where in operating mode B2 the backlight is switched on and the lamps are switched off, and where in operating mode B1 at least the lamps are switched on.

[0043] The first, second, third and/or fourth illumination devices are advantageously combined with a transmissive screen, such as an LC panel, to produce a screen that can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode.

[0044] Advantageously, the transition dipole moments of a first optical element (or optionally, if present, also of a further first or second or third optical element) are formed as at least one, at least two, at least three or more dichroic dye(s) which are arranged in a guest-host Arrangement mixed with the liquid crystals. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0045] In contrast, the transition dipole moments can also be embedded in a liquid crystal layer in a non-fixed manner as a guest-host arrangement, so that the transition dipole moments can be varied in their orientation and/or their magnitude between the first and at least one second state depending on the influence of this liquid crystal layer.

[0046] The dichroic dye molecules within a guest-host arrangement generally align parallel to the liquid crystal molecules.

[0047] Alternatively, the first optical element may be formed as a laminate of layers of polymer film polarizers.

[0048] The aforementioned embodiments also ensure that the first optical element is non-periodic in its structure. This is advantageous because, in interaction with pixel structures of screens, there is no risk of artifacts such as moiré effects.

[0049] The first (or second) preferred direction can, for example, enclose an angle between 0° and 45° to a surface normal of the first (or second) optical element. Furthermore, it is possible for the first or second preferred direction to vary over the surface of the first (or second) optical element. For the purposes of the invention, the average, weighted preferred direction then applies. The first and second preferred directions can also be identical or differ in their orientation by only a few degrees (maximum 10°) and both can, in particular, be perpendicular to the respective optical element. This is a preferred case. However, depending on the application, it is also possible for the first and second preferred directions to differ from one another by more than 10°.

[0050] Furthermore, it is possible that at least two such preferred directions differ by more than 10° in a selectable plane and/or that the respective preferred direction of a transition dipole moment is selectable depending on its position in the first or second optical element. [0051] Furthermore, it may be advantageous if the first or second optical element is divided into different regions (A1, A2, ...) along a selectable reference line, wherein a separate region-preferred direction can be selected for each region (A1, A2, ...), which applies to all transition dipole moments within a region (A1, A2, ...), wherein all region-preferred directions are different in pairs and point in the direction of a viewer up to a maximum tolerance of ±10 degrees. This arrangement has the advantage that the viewer perceives a screen with a first or second switchable light filter as homogeneously illuminated in the restricted viewing mode.

[0052] The transition dipole moment – also referred to as the transition matrix element – is a quantum mechanical vector quantity associated with a specific transition between an initial state – usually the ground state – and a final state – usually an excited state – of a system, i.e., an atom, molecule, or solid, and corresponds to the electric dipole moment associated with this transition. The direction of the vector defines the polarization of the transition, which in turn determines how the system interacts with an electromagnetic wave with a given polarization. For example, during the transition from the ground state to the excited state, light of the corresponding polarization is absorbed. The magnitude of the vector corresponds to the strength of the interaction or the transition probability.

[0053] The first (second) preferred direction corresponds to the orientation of the transition dipole moments of the first (second) optical element for a given propagation direction of light, in which the absorption is the same for any polarization of the light.

[0054] The first or second light filter may further comprise a polarization filter, which is arranged upstream or downstream of the first or second optical element in the direction of incidence. Alternatively or additionally, a λ/4 layer is also conceivable, for example, in the case of incident circularly polarized light, which is converted into (essentially) linearly polarized light due to this layer.

[0055] A first exemplary manufacturing variant for a first optical element using the guest-host principle is based on mixtures of dichroic dyes or dichroic dye mixtures with liquid crystal mixtures or -compounds, and comprises the following steps for production (with reference to US 9,481,658 B2 or WO2021/177308A1 paragraph 37ff)

A low- or non-birefringent substrate is coated with a film that determines the orientation of the molecules relative to the surface, usually parallel or perpendicular to the surface. Polymers, preferably polyvinyl alcohol or polyimides, are used for this purpose.

- Optional: Optical or mechanical treatment of the surfaces to improve the subsequent quality of molecular alignment.

- Application of the mixture of dichroic dye and thermotropic liquid crystalline compounds or polymers.

When exposed to light, the side chains are locally condensed, causing birefringence along the surface.

[0056] An alternative, second manufacturing variant uses thermotropic, liquid crystalline dichroic dyes (reference to JP201 1 -237513A), and comprises the following steps:

Preparation of the corresponding dyes and addition of a polar group.

- Application of the dye mixture as well as photo-alignment and curing of the dye mixture using polarized light.

[0057] The following materials are suitable for various manufacturing variants, although this list is not exhaustive:

- As polymer substrate with low or no birefringence: preferably TAC,

- As dichroic substances or mixtures: Dichroic dyes (preferably azo dyes) or dichroic metal nanoparticles (preferably gold, silver, copper, and aluminum); these are usually single dyes or mixtures of typically up to three different dyes to enable absorption across the entire spectrum,

- For surface treatment by alignment of dyes or liquid crystalline substances: polymers, preferably polyvinyl alcohol or polyimides,

For thermotropic liquid crystalline compounds or polymers, reference is made to JP 201 1 -237513A as an example. - As chemical groups for crosslinking, which are bound to the thermotropic liquid crystalline compounds or polymers (crosslinking): metaacrylic groups, epoxy groups, oxetanyl groups, and styrene groups, preferably methacrylic groups. Alternatively, these can be polymerizable liquid crystal compounds, such as those described in JP 6268730B2.

Polymerizable liquid crystalline dichroic dyes, for example azo dyes

[0058] The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e., each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01%, preferably from 1% to 15%, of the material of the respective layers of the optical element in question. In special cases, the concentration can even reach 95% in the case of liquid-crystalline dichroic dyes. The thickness of the layers is preferably in the range from 0.2 pm to 50 pm, more preferably in the range from 0.5 pm to 20 pm, all boundary values included. The dyes or dye mixtures for different layers within an optical element can be different, but do not have to be.

[0059] In all embodiments of the invention, the tolerance T may be 5° < T < 19° or 10° < T < 19°, for example, T = 5° or T = 10° or T = 15° or T = 19°. For very small proportions of dipole moments N2, for which, for example, N1/N1 > 50 applies, the tolerance T may also be greater than 19° without jeopardizing the inventive success.

[0060] Furthermore, it is advantageous if, in the described image display devices (the screens) and/or lighting devices, such a luminance curve is implemented, which, for example, at angles (in particular horizontal from the viewer's perspective or measured in the first plane) from 25°, emits or transmits less than 80%, preferably less than 60% of the maximum radiated or transmitted luminance in (any) direction. In this way, the privacy protection is enhanced at such angular ranges, because the said luminance curve corresponds to the respective transmission curve of a screen and/or lighting device used with a described switchable light filter. In this context, it is also advantageous if, for example, such a luminance curve has approximately 50% of the maximum luminance at an angle of 45°.

[0061] For certain applications in which viewers wearing (usually vertically) linearly polarized sunglasses look at a previously described screen with a first or second switchable light filter, it may be advantageous to further arrange an λ/4 layer in the light path in the viewing direction in front of the screen to convert linearly polarized light into circularly polarized light. Depending on the state of the first liquid crystal layer in the first switchable light filter or depending on the state of the second liquid crystal layer in the second switchable light filter, left- or right-rotating, circularly polarized light would then be generated, which - with reduced brightness - is visible through said sunglasses. In the case of crossed linear polarizations of sunglasses and the light emanating from the screen, if no such λ/4 layer is present, it could otherwise happen, depending on the state, that a viewer wearing such sunglasses essentially sees no image.

[0062] A switchable light filter, a lighting device, or such a screen as described above is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access system. Switching between the aforementioned operating modes is possible to protect sensitive data, i.e., to display it perceptibly for only one viewer, or alternatively, to display image content simultaneously for multiple viewers.

[0063] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0064] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations indicated, but also in other combinations or in isolation, without departing from the scope of the present invention. Short description of the drawings

[0065] The invention will be explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are merely illustrative and are not to be interpreted as limiting. For example, a description of an exemplary embodiment with a plurality of elements or components should not be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments. To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. They show: Fig. 1 shows a schematic diagram of the structure of a first optical element,

Fig. 2 to Fig. 5 show exemplary normalized transmission curves for the incidence of light of two different polarizations (s, p) into the first optical element, plotted against a viewing angle on the first optical element for the respective values of N1/N2,

Fig. 6a shows a comparison of exemplary normalized transmission curves for the incidence of light of two different polarizations into a first optical element, plotted on a linear scale over a viewing angle on the first optical element for two different values of T and for the same ratio N1/N2,

Fig. 6b the comparison from Fig. 6a, here in logarithmic scale on the ordinate, Fig. 7a a comparison of two further exemplary normalized transmission curves each for the incidence of light of two different polarizations into a first optical element, plotted on a linear scale over a viewing angle on the first optical element for two different values of the ratio N1/N2 and with the same value for T,

Fig. 7b the comparison from Fig.7a, here in logarithmic scale on the ordinate, Fig. 8 and Fig. 9 further exemplary normalized transmission curves each for the incidence of light of two different polarizations into the first optical element, plotted against a viewing angle on the first optical element, wherein the inequality according to the invention is not observed in each case,

Fig. 10 shows the schematic diagram of an exemplary structure of a first switchable light filter,

Fig. 11 shows the schematic diagram of an exemplary structure of a second switchable light filter,

Fig. 12 shows the schematic diagram of an exemplary structure of a lighting device in a first embodiment with a first or second switchable light filter, Fig. 13 shows the schematic diagram of an exemplary structure of a screen in a first embodiment, which can be operated in at least two operating modes B1 and B2, with a first or second switchable light filter,

Fig. 14 shows the schematic diagram of an exemplary structure of a screen in a second embodiment, which can be operated in at least two operating modes B1 and B2, with a first or second switchable light filter,

Fig. 15 shows the schematic diagram of an exemplary structure of a lighting device in a second embodiment, with a first or second switchable light filter,

Fig. 16 shows the schematic diagram of an exemplary structure of a lighting device in a third embodiment, with a first or second switchable light filter, and

Fig. 17 shows the schematic diagram of an exemplary structure of a lighting device in a fourth embodiment, with a first optical element.

Detailed description of the drawings

[0066] The drawings are not to scale and merely represent schematic diagrams. In some cases, selected rays are shown, whereas in reality, a multitude of light rays are present. The same applies to the representation of selected dipole moments in Fig. 1. Where necessary, viewing directions should always be assumed to be from above in the plane of the page.

[0067] Fig. 1 shows a schematic diagram of the construction of a first optical element 1 in a permanent (or possibly first) state. The first optical element 1 comprises • a multitude of light-absorbing transition dipole moments (represented by the arrows), whereby the corresponding absorption cross sections are designated Oabs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans,

• wherein the majority of the transition dipole moments are permanently or at least in a first state with a (selectable) tolerance T with T < 19° parallel to a first preferred direction selectable for the first optical element 1 (see dashed line) or vary around this (the tolerance T applies in both directions, i.e. for the value of a respective angle plus ±T), wherein the first preferred direction is arranged at an angle α to the perpendicular bisector (see dashed line) of the first optical element 1, wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the first preferred direction have a density Ni and the remaining transition dipole moments (i.e. those lying outside the described tolerance range) have a density of N2; in the drawing Fig. 1, for a few transition dipole moments selected as examples, it is shown to which density Ni or N2 these respectively belong,

ist, so dass höhere Toleranzwerte T aufgrund der dann entsprechend erhöhten Relation der Dichten Ni und N2 der Übergangsdipolmomente mindestens teilweise kompensiert werden, wodurch (mindestens) die Absorption von p-polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der ersten Ebene, auf das optische Element 1 einfällt, verstärkt wird. • so that light incident on the first optical element 1 is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element 1 and its polarization state, wherein on the optical element 1 permanently or at least in said first state for the ratio of the respective densities Ni and N2 of the transition dipole moments the inequality is satisfied

so that higher tolerance values T are at least partially compensated due to the correspondingly increased relation of the densities Ni and N2 of the transition dipole moments, whereby (at least) the absorption of p-polarized light which is incident on the optical element 1 at angles greater than 45°, measured in the first plane, is increased.

[0068] The plurality of light-absorbing transition dipole moments are arranged in a layer at least 0.2 micrometers thick. Several, e.g. Layers of transition dipole moments separated by OCA (optically clear adhesive) and/or substrates may be present. The layer thicknesses of the transition dipole moments can, for example, be in the sum of all layers present, from 0.2 pm to 50 pm, preferably 0.2 pm to 20 pm, particularly preferably 1 pm to 10 pm. In particular embodiments, at least two layers of transition dipole moments separated by at least one substrate are present.

erfüllt ist, so dass herstellungsbedingte höhere Toleranzwerte T aufgrund der dann entsprechend erhöhten Relation der Dichten Ni und N₂ der Übergangsdipolmomente mindestens teilweise kompensiert werden. Dadurch wird nicht allein die Absorption von p-polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der ersten Ebene, auf das optische Element 1 einfällt, verstärkt, sondern es wird auch die normierte Transmission in der Vorzugsrichtung bzw. in einem Winkelbereich von einigen Grad darüber hinaus erhöht. [0069] The invention shows a way of counteracting manufacturing-related limitations in the alignment of the transition dipole moments on the optical element 1 by using only optical elements ₁ in which the inequality N / N ₂ >

is met, so that manufacturing-related higher tolerance values T are at least partially compensated due to the correspondingly increased ratio of the densities Ni and N ₂ of the transition dipole moments. This not only increases the absorption of p-polarized light incident on the optical element 1 at angles greater than 45°, measured in the first plane, but also increases the normalized transmission in the preferred direction or in an angular range of a few degrees beyond that.

[0070] The greater the proportion of the remaining transition dipole moments (i.e. those lying outside the described tolerance range) with the density of N ₂ , the worse the general transmission of the optical element 1 in general, ie in all directions.

[0071] Approximate, quantitative simulations of the normalized transmission of the first optical element 1 are shown for various parameters in Figs. 2 to 9. The transmissions were calculated independently for s- and p-polarized light incident on an optical element 1. In each of these drawings, the normalized transmission is shown with a dashed line for incident s-polarized light and with a solid line for p-polarized light. The starting point for calculating the transmission through an absorbing layer is the Labert-Beer law. In addition to the thickness of the layer, it also includes the product of the extinction coefficient and the concentration of the absorbing species. In the present case, the extinction is approximately equated with the absorption, since the absorption is caused by very small molecules that only weakly absorb the light. scatter. These absorption coefficients are denoted by a _s and a _p . These coefficients describe the absorption for s- and p-polarized light, respectively. For the ideal alignment of the molecules parallel to the first preferred direction, a _s reaches the numerical value 0. This describes the propagation of s-polarized light that propagates in the plane that includes the surface normal and the first preferred direction. In order to also describe the propagation of p-polarized light, a _p must be modified using Malus' law. The following relationships are assumed in the simulation:

[0072] Firstly, Fig. 2 shows a first exemplary normalised transmission curve for the incidence of light of two different polarisations (s, p) into a first optical element 1, plotted against a viewing angle on a first optical element 1 for a first value of N1/N2 = 6.66. This ratio of N1/N2 satisfies the aforementioned inequality N _± / _N2 > 10 - 3 ■ iog ₁₀ [ ⁽² ° _1O ~ ^T ' ⁾ ] for example for T = 1 ⁰ (here, strictly speaking, N1/ _N2 > 6.16 would be sufficient). It can be seen that even a relatively poor ratio of the densities of the transition dipole moments for p-polarised light, which is therefore relatively easy to achieve for the production of a first optical element 1, produces an absorption of more than 90% at 45 °.

[0073] Furthermore, Fig. 3 shows a second exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element 1 for a second value of NI/N2 = 7. This corresponds to the limit of the said inequality for T = 10°. It can also be seen here that even a relatively poor, but therefore relatively easy to achieve for the production of a first optical element 1, ratio of the densities of the transition dipole moments for p-polarized light produces an absorption of more than 90% at approximately 45°.

[0074] Furthermore, Fig. 4 shows a third exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element for a third value of NI/N2 = 10. This corresponds to the limit of the said inequality for T = 19°. The significantly larger tolerance T = 19° compared to the conditions according to Figs. 2 and 3 is also (over-)compensated here by the relative increased density N1 of the transition dipole moments. The for p-polarized light desired absorption at about 45° is improved compared to the case where the above inequality is not fulfilled.

[0075] Furthermore, Fig. 5 shows a fourth exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element for a fourth value of N1/N2=16>10. This corresponds to a tightening of the limit of the said inequality for T=19°, where - as described above - at least N1/N2>10 should apply. The comments given above for Fig. 4 apply accordingly.

[0076] Fig. 6a shows a comparison of exemplary normalized transmission curves for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted on a linear scale over a viewing angle onto the first optical element 1 for two different values of T (T = 0° with thin lines and T = 10° with thick lines) and for the same exemplary ratio N1/N2 = 16.7. Qualitatively, the curves for s- and p-polarized light are comparable. It is obvious that the transmission for T = 10° is visibly reduced compared to T = 0°, which is due to the larger tolerance T = 10° of the alignment of the transition dipole moments compared to the first preferred direction: poorly aligned transition dipole moments, i.e. those with a larger tolerance, result in a larger Os.

[0077] Figure 6b shows the comparison from Figure 6a, but on a logarithmic scale on the ordinate. It can be seen that, for absolute values of the viewing angle of more than 25°, both sets of parameters exhibit approximately the same transmission or absorption for p-polarized light, but the total transmission for T=10° is reduced by a factor of approximately 1.2 in the first preferred direction for p-polarized light and in many directions for s-polarized light, which is undesirable.

[0078] In contrast, Fig. 7a shows a comparison of exemplary normalized transmission curves for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted on a linear scale over a viewing angle on the first optical element 1 for two different values of the ratio N1/N2 (16.7 with thick lines and 29.2 with thin lines) and with the same value for T = 10°. It is obvious that the transmission is noticeably reduced for the lower ratio N1/N2 = 16.7 compared to N1/N2 = 29.2.

[0079] Fig. 7b shows the comparison from Fig. 7a, but on a logarithmic scale on the ordinate. It can be seen that, for absolute values of the viewing angle of more than approximately 30°, both sets of parameters have approximately the same transmission or absorption for p-polarized light, but the total transmission for NI/N ₂ = 16.7 in the first preferred direction for p-polarized light and in many directions for s-polarized light is reduced by a factor of approximately 1.2, which is undesirable. The comparison of the drawings Figs. 6b and 7b therefore allows the conclusion that the absorption for p-polarized light, even at angles with absolute values of less than approximately 30°, can be achieved by a lower tolerance T rather than by a higher ratio of the densities NI/N ₂ .

[0080] Finally, Fig. 8 and Fig. 9 show further exemplary normalized transmission curves, each for the incidence of light of two different polarizations (p, s) into a first optical element 1, plotted against a viewing angle on the first optical element 1, wherein the inventive inequality n/cf is observed in each case: The absorption for p-polarized light is in no way capable of generating a transmission of less than 10% at angles of, for example, ±45°. According to Fig. 8, the ratio NI/N ₂ =3, and according to Fig. 9, NI/N ₂ =1.5.

[0081] Furthermore, Fig. 10 shows the schematic diagram of an exemplary structure of a first switchable light filter 5. This comprises a first optical element 1 as described above,

(not shown in the drawing) means for selectively generating a first electric field EF1 or a second electric field EF2, e.g. ITO layers with a connected signal generator, a liquid crystal layer 3 arranged in the viewing direction (here) in front of the first optical element 1, on which the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarization state of light passing through it, since the liquid crystal layer 3 is arranged in the viewing direction in front of the first optical element 1, a first linear polarization filter X arranged in the viewing direction in front of the liquid crystal layer 3, so that the transmission properties of the first switchable light filter 5 differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the second electric field EF2 is applied.

[0082] In the case that the transition dipole moments are variable, e.g. via so-called guest-host liquid crystal cells, such guest-host liquid crystal cells may, but do not have to, correspond directly to the aforementioned liquid crystal layer.

[0083] In a preferred embodiment, light penetrating the liquid crystal layer is transmitted substantially unchanged when the first electric field EF1 is applied, while when the second electric field EF2 is applied, the incident light is circularly or elliptically polarized or the polarization of the light is rotated by (substantially) 90°.

[0084] If the liquid crystal layer 3 is arranged behind the first optical element 1 in the viewing direction, the liquid crystal layer 3 is preferably subjected to linearly polarized light or elliptically polarized light, in which the ratio of the major to the minor semi-axis is at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by linear polarization filters in the light path, or by λ/4 layers when using circularly polarized light.

[0085] Furthermore, Fig. 11 shows the schematic diagram of an exemplary structure of a second switchable light filter 5a, comprising a second optical element 2, in turn comprising

• a multitude of light-absorbing transition dipole moments, where the corresponding absorption cross sections are designated o _a bs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans,

• wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a guest-host arrangement in a second liquid crystal layer 3a,

• further wherein the majority of the transition dipole moments are arranged at least in a first state with a tolerance T with T < 19° parallel to a value for the second optical element 2 is aligned in a selectable first preferred direction or varies around it (here too, the tolerance T applies in both directions, i.e. ±T), wherein the second preferred direction is arranged at an angle α to the perpendicular bisector of the second optical element 2, wherein the angle α is measured in a selectable second plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the second preferred direction have a density Ni and the remaining transition dipole moments have a density of N2 (for an explanation of the geometric relationships, Fig. 1 can be used in a figurative sense; in this case, the optical element 1 would correspond to the second liquid crystal layer 3a),

• so that light which is incident on the second optical element 2 is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element 2, its polarization state and the state of the second liquid crystal layer 3a at the location of the light incidence,

ist, so dass höhere Toleranzwerte T aufgrund der dann entsprechend erhöhten Relation der Dichten Ni und N2 der Übergangsdipolmomente mindestens teilweise kompensiert werden, wodurch die Absorption von p-polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der zweiten Ebene, auf das zweite optische Element 2 einfällt, verstärkt wird. [0086] Die Mittel zur wahlweisen Erzeugung mindestens eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2 können auch vorteilhaft so ausgelegt sein, dass an unterschiedlichen Positionen auf der Flüssigkeitsschicht 3a zur gleichen Zeit unterschiedliche elektrische Felder EF1 , EF2 (oder ggf. davon verschiedene elektrische Felder EF3, EF4, etc.) anliegen. Damit kann eine positionsabhängige Teilumschaltung der Betriebsarten des zweiten schaltbaren Lichtfilters 5a erreicht werden. (not shown in the drawing) means for selectively generating at least a first electric field EF1 or a second electric field EF2, wherein the respective electric field acts on the second liquid crystal layer 3a, so that the transmission properties of the second switchable light filter 5a differ between a first operating mode B1, in which the first electric field EF1 is applied and the second liquid crystal layer 3a is in said first state, and a second operating mode B2, in which the second electric field EF2 is applied and the second liquid crystal layer 3a is in a second state, wherein on the second optical element 2, at least in said first state, the inequality is satisfied for the ratio of the respective densities Ni and N2 of the transition dipole moments

so that higher tolerance values T are at least partially compensated due to the correspondingly increased relation of the densities Ni and N2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the second optical element 2 at angles greater than 45°, measured in the second plane, is increased. [0086] The means for selectively generating at least one first electric field EF1 or one second electric field EF2 can also advantageously be designed such that different electric fields EF1, EF2 (or possibly different electric fields EF3, EF4, etc.) are present at different positions on the liquid layer 3a at the same time. This allows for a position-dependent partial switching of the operating modes of the second switchable light filter 5a.

polarisiertem Licht, welches unter Winkeln von größer 45°, gemessen in der zweiten Ebene, auf das zweite optische Element 2 einfällt, abgeschwächt wird. Auf diese Weise wird entlang der zweiten Ebene in dem besagten zweiten Zustand eine höhere Transmission erreicht. [0087] For the second switchable light filter 5a, it can optionally apply that on its second optical element 2 at least in the said second state for the ratio of the respective densities Ni and IXL of the transition dipole moments, the is satisfied, whereby the absorption of p-

polarized light incident on the second optical element 2 at angles greater than 45°, measured in the second plane, is attenuated. In this way, a higher transmission is achieved along the second plane in said second state.

[0088] Advantageously, the potential differences between the electrodes for generating the first electric field EF1 and the second electric field EF2 differ by at least 2 volts in both the first switchable light filter 5 and the second switchable light filter 5a. Further operating modes B3, B4, etc. with electric fields EF3, EF4, etc. that differ from the electric fields EF1 and EF2 can be explicitly provided. Furthermore, the operating modes B1, B2, etc. can also differ locally on the first or second switchable light filter 5, 5a, as already indicated above.

[0089] In Fig. 12, the schematic diagram of an exemplary structure of a lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, is shown in a first embodiment with a first or second switchable light filter 5, 5a, comprising

- a surface-like backlight 8, which radiates light and which is optionally directly luminous (e.g. with a locally dimmable LED matrix), as well as - a first or second switchable light filter 5, 5a arranged in front of the background lighting 8 in the viewing direction as described above.

[0090] Furthermore, Fig. 13 shows the schematic diagram of an exemplary structure of a screen in a first embodiment, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, with a first or second switchable light filter 5, 5a, which comprises

- a first lighting device as described above,

- and, if no first linear polarization filter is arranged in the first or second switchable light filter 5, 5a of the first illumination device, a second linear polarization filter P arranged in front of the background illumination 8 in the viewing direction, whereby light emanating from the background illumination 8 and penetrating the second linear polarization filter P is restricted in its propagation directions, and

- a transmissive image display device 11, which is arranged in front of the first or second switchable light filter 5, 5a in the viewing direction,

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0091] It is preferred that the first or second linear polarization filter P is arranged in the transmissive image display device 11 or is a part of it, e.g. as a rear (light input side) polarization filter of an LCD panel as image display device 11.

[0092] Furthermore, Fig. 14 shows the schematic diagram of an exemplary structure of a screen in a second embodiment, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, comprising

- an image display device 12, whereby in principle any type of image display device 12 is possible, for example LC panel, OLED, microLED or others, - in the viewing direction in front of the image display device 12, a first or second switchable light filter 5, 5a according to the invention as described above,

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0093] Furthermore, Fig. 15 shows the schematic diagram of an exemplary structure of a lighting device for a screen in a second embodiment, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, in a second embodiment, with a first or second switchable light filter 5, 5a, comprising

- a surface-like extended background illumination 8b, which radiates light in a limited angular range (indicated by the 2 almost parallel exemplary beams), and which is optionally designed to be directly luminous, and

- a plate-shaped light guide 9 located in front of the background lighting 8b in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume,

- illuminants 10 arranged laterally on at least one narrow side of the light guide, and

- optionally a linear polarization filter (not shown in the drawing),

- a first or second switchable light filter 5, 5a arranged in the viewing direction in front of the background lighting 8b (this also includes a position in front of a screen with which the lighting device is used), as described above,

- wherein in operating mode B2 the background lighting 8b is switched on and the illuminants 10 are switched off, and wherein in operating mode B1 at least the illuminants 10 are switched on, so that the light guide 9 emits light in an unrestricted angular range, which is indicated in Fig. 15 with the exemplary three beams which are not approximately parallel, and

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied. [0094] Within the scope of the invention, and particularly with regard to the backlight or the screen, "limited angular range" means that the corresponding luminance is concentrated to at least 80% or 90% in a defined angular range, while outside the defined limited angular range there may still be residual light, which is usually due to technical reasons. Ideally, such residual light is minimal and decreases at larger angles. To achieve particularly strong minimization, a corresponding light filter is used in addition to the backlight emitting light in a limited angular range. This also applies to the variant described below with a light guide that emits or decouples light predominantly in a limited angular range. In contrast to this embodiment of the invention, luminance curves of backlights are typically bell-shaped, particularly across horizontal (and possibly also vertical) angular ranges, although there need not be a true concentration of luminance around a smaller angular range.

[0095] Fig. 16 shows the schematic diagram of an exemplary structure of a lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, in a third embodiment, with a first or second switchable light filter 5, 5a, comprising

- a surface-like backlight 8c, which radiates light in an unrestricted angular range (which is indicated in Fig. 16 with the exemplary three beams that are not approximately parallel), and which is optionally designed to be directly luminous (e.g. by means of a locally dimmable LED matrix lighting unit), and

- a plate-shaped light guide 9c located in front of the background lighting 8c in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume, wherein said output coupling elements output the majority of the light coupled laterally into at least one narrow side of the light guide 9c (i.e., more than half, preferably more than 80% or 90%) into a limited angular range (see two almost parallel beams), - illuminants 10 arranged laterally on at least one narrow side of the light guide 9c, and

- a first or second switchable light filter 5, 5a arranged in the viewing direction in front of the background illumination 8c, preferably in front of the light guide 9c, as described above,

- wherein in operating mode B2 the backlight 9c is switched off and the lamps 10 are switched on, and wherein in operating mode B1 at least the backlight 8c is switched on (optionally the lamps 10 can also be switched on), and

- where in operating mode B2 the second electric field EF2 is applied and where in operating mode B1 the first electric field EF1 is applied.

[0096] Finally, Fig. 17 shows the schematic diagram of an exemplary structure of a lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, in a fourth embodiment, with a first optical element 1, comprising

- a surface-like extended background illumination 8b, which emits light in a limited angular range (see two almost parallel beams) and which contains a first optical element 1, as described above, to limit the angular range, and which is optionally constructed to be directly luminous, and

- a plate-shaped light guide 9 located in front of the background lighting 8b in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume,

- illuminants 10 arranged laterally on at least one narrow side of the light guide 9, and

- wherein in operating mode B2 the background lighting 8b is switched on and the illuminants 10 are switched off, and wherein in operating mode B1 at least the illuminants 10 are switched on, so that the light guide 9 emits light in an unrestricted angular range, which is indicated in Fig. 17 with the exemplary three beams which are not approximately parallel. [0097] The first, second, third and/or fourth illumination devices are advantageously combined with a transmissive screen, such as an LC panel, to produce a screen that can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode.

[0098] Advantageously, the transition dipole moments of a first optical element 1 (or optionally, if present, also of a further first or second optical element 1, 2) are formed as one or more dichroic dye(s) mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0099] In contrast, the transition dipole moments can also be embedded in a non-fixed manner as a guest-host arrangement in a liquid crystal layer, so that the transition dipole moments can be varied in their orientation and/or their magnitude between the first and at least one second state depending on the influence of this liquid crystal layer.

[0100] The dichroic dye molecules generally align parallel to the liquid crystal molecules.

[0101] The aforementioned embodiments also ensure that the first optical element is non-periodic in its structure. This is advantageous because, in interaction with pixel structures of screens, there is no risk of artifacts such as moiré effects.

[0102] The first (or second) preferred direction can, for example, enclose an angle between 0° and 45° to a surface normal of the first (or second) optical element 1, 2. Furthermore, it is possible for the first preferred direction to vary across the surface of the first (or second) optical element 1, 2. For the purposes of the invention, the average, weighted preferred direction then applies.

[0103] The first or second switchable light filter 5, 5a may further comprise a polarization filter which is assigned to the first or second optical element 1, 2 is arranged upstream or downstream in the direction of incidence. Alternatively or additionally, a 4X layer is also conceivable, for example, in the case of incident circularly polarized light, which is converted into (essentially) linearly polarized light due to this layer.

[0104] The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e., each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01%, preferably from 1% to 15%, of the material of the respective layers of the optical element in question. In special cases, the concentration can even reach 95% in the case of liquid-crystalline dichroic dyes. The thickness of the layers is preferably in the range from 0.2 pm to 50 pm, more preferably in the range from 0.5 pm to 20 pm, all boundary values included. The dyes or dye mixtures for different layers within an optical element 1, 2 can be different, but do not have to be.

[0105] The invention solves the stated problem: An optical element has been described in which light incident on the optical element is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties - not primarily but depending on its position. Production-related alignment tolerances are counteracted by the optical effect. By switchable light filters that use the optical element according to the invention, the transmission of light is influenced depending on the angle - optionally with respect to a seated or standing observer - whereby switching between at least two operating states is possible. In particular, the transmission behavior is switchable for specific directions. Furthermore, lighting devices and screens using the switchable light filters are disclosed.

erstes optisches Element 2 zweites optisches Element[0106] The invention described above, in conjunction with an image display device, can be advantageously used wherever confidential data is displayed and/or entered, such as when entering a PIN or for data display at ATMs or payment terminals, or when entering a password or reading emails on mobile devices. As described above, the invention can also be used in cars to selectively withhold disturbing image content from the driver or passenger. List of reference symbols

first optical element 2 second optical element

3 first liquid crystal layer

3a second liquid crystal layer

5 first switchable light filter

5a second switchable light filter 8 backlight

8b Backlight

8c backlight

9 light guides

10 Light source 11 , 12 Image display device

P, X polarizing filters

Claims

Patent claims 1 . First optical element (1 ), comprising • a multitude of light-absorbing transition dipole moments, where the corresponding absorption cross sections are designated o _a bs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans, • wherein the majority of the transition dipole moments are permanently or at least in a first state with a tolerance T with T<19° aligned parallel to a first preferred direction selectable for the first optical element (1) or vary around this, wherein the first preferred direction is arranged at an angle α to the perpendicular bisector of the first optical element (1), wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the first preferred direction have a density of Ni and the remaining transition dipole moments have a density of N2, • so that light which is incident on the first optical element (1 ) is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element (1 ) and its polarization state, characterized in that on the optical element (1 ) permanently or at least in the said first state for the ratio of the respective densities Ni and N2 of the transition dipole moments the inequality / N ₂ > 10 - is fulfilled, so that higher tolerance values T due to the then

correspondingly increased relation of the densities Ni and N2 of the transition dipole moments are at least partially compensated, whereby the absorption of p-polarized light which is incident on the optical element (1) at angles greater than 45°, measured in the first plane, is increased. 2. First switchable light filter (5), comprising a first optical element (1) according to claim 1, Means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, on which layer the first electric field (EF1) or the second electric field (EF2) acts and which, depending thereon, influences the polarization state of light passing through it when the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, a first linear polarization filter (X) arranged in front of the liquid crystal layer (3) in the viewing direction, so that the transmission properties of the first switchable light filter (5) differ between a first operating mode B1, in which the first electric field (EF1) is applied, and a second operating mode B2, in which the second electric field (EF2) is applied. 3. Second switchable light filter (5a), comprising a second optical element (2), in turn comprising • a multitude of light-absorbing transition dipole moments, where the corresponding absorption cross sections are designated o _a bs and apply either to one wavelength, to several selected wavelengths or on average to several wavelengths in the light spectrum visible to humans, • wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a guest-host arrangement in a second liquid crystal layer (3a), • further wherein the majority of the transition dipole moments are aligned, at least in a first state, with a tolerance T with T<19° parallel to a second preferred direction selectable for the second optical element (2) or vary around this, wherein the second preferred direction is arranged at an angle α to the perpendicular bisector of the second optical element (2), wherein the angle α is measured in a selectable second plane containing said perpendicular bisector, and wherein the transition dipole moments aligned with a tolerance T parallel to the second preferred direction have a density Ni and the remaining transition dipole moments have a density of N2, • so that light which is incident on the second optical element (2) is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element (2), its polarization state and the state of the second liquid crystal layer (3a), Means for selectively generating at least a first electric field (EF1) or a second electric field (EF2), wherein the respective electric field acts on the second liquid crystal layer (3a), so that the transmission properties of the second switchable light filter (5a) differ between a first operating mode B1, in which the first electric field (EF1) is applied and the second liquid crystal layer (3a) is in said first state, and a second operating mode B2, in which the second electric field (EF2) is applied and the second liquid crystal layer (3a) is in a second state, characterized in that on the second optical element ( ₂ ), at least in said first state, the inequality ]

is fulfilled, so that higher tolerance values T are at least partially compensated due to the then correspondingly increased relation of the densities Ni and N ₂ of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the second optical element (2) at angles greater than 45°, measured in the second plane, is increased. 4. Second switchable light filter (5a) according to claim 3, characterized in that on the second optical element (2) at least in said second state for the ratio of the respective densities Ni and N ₂ of the transition dipole moments, the inequality A^ / Af ₂ < 10 - 3 ■ iog ₁₀ is satisfied

whereby the absorption of p-polarized light incident on the second optical element (2) at angles greater than 45°, measured in the second plane, is attenuated. 5. First switchable light filter (5) according to claim 2 or second switchable light filter (5a) according to claim 3 or 4, characterized in that the potential differences between the electrodes for generating the first electric field (EF1) and the second electric field (EF2) differ by at least 2 volts. 6. First switchable light filter (5) according to claim 2, characterized in that the first optical element (1) and/or the liquid crystal layer (3) and/or the means for generating a first or second electric field EF1, EF2 is divided into a plurality of separately switchable segments, so that local switchability between the respectively possible operating states is enabled. 7. Lighting device for a screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a surface-like extended background lighting (8) which radiates light and which is optionally constructed to be directly luminous, and - a first switchable light filter (5) according to claim 2 arranged in front of the background illumination (8) in the viewing direction, or a second switchable light filter (5a) according to one of claims 3 or 4. 8. A screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a lighting device according to claim 7, - if no first linear polarization filter (X) is arranged in the first or second switchable light filter (5, 5a) of the illumination device, a second linear polarization filter (P) arranged in front of the background illumination (8) in the viewing direction, whereby light emanating from the background illumination and penetrating the second linear polarization filter (P) is restricted in its propagation directions, and - a transmissive image display device (11) which is arranged in front of the first or second switchable light filter (5, 5a) in the viewing direction, - wherein in operating mode B2 the second electric field (EF2) is applied and in operating mode B1 the first electric field (EF1) is applied. 9. Screen according to claim 8, characterized in that the first or second linear polarization filter (P, X) is arranged in the transmissive image display device (11) or is a part of it. 10. A screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - an image display device (12), - in the viewing direction in front of the image display device (12) a switchable first light filter (5) according to claim 2 or a second light filter (5a) according to claim 3 or 4, - wherein in operating mode B2 the second electric field (EF2) is applied and in operating mode B1 the first electric field (EF1) is applied. 11. Lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising - a surface-like extended backlight (8b) which radiates light in a limited angular range and which is optionally designed to be directly luminous, and - a plate-shaped light guide (9) located in front of the background lighting (8b) in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume, - illuminants (10) arranged laterally on at least one narrow side of the light guide (9), and - optionally a linear polarization filter (P), - a first switchable light filter (5) according to claim 2 arranged in front of the background illumination (8b) in the viewing direction, or a second switchable light filter (5a) according to claim 3 or 4, - wherein in operating mode B2 the background lighting (8b) is switched on and the lamps (10) are switched off, and wherein in operating mode B1 at least the lamps (10) are switched on, and - wherein in operating mode B2 the second electric field (EF2) is applied and in operating mode B1 the first electric field (EF1) is applied. 12. Lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising - a surface-like extended backlight (8c), which radiates light in an unrestricted angular range and which is optionally designed to be directly luminous, and - a plate-shaped light guide (9c) located in front of the background lighting (8c) in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume, wherein said output coupling elements output light coupled laterally into at least one narrow side of the light guide (9c) predominantly into a limited angular range, - illuminating means (10) arranged laterally on at least one narrow side of the light guide (9c), and - optionally a linear polarization filter (P), - a first switchable light filter (5) according to claim 2 arranged in front of the background illumination (8c) in the viewing direction, or a second switchable light filter (5a) according to claim 3 or 4, - wherein in operating mode B2 the backlight (8c) is switched off and the lamps (10) are switched on, and wherein in operating mode B1 at least the backlight (8c) is switched on, and - wherein in operating mode B2 the second electric field (EF2) is applied and in operating mode B1 the first electric field (EF1) is applied. 13. Lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising - a surface-like backlight (8b) which emits light in a limited angular range and contains a first optical element (1) according to claim 1, and which is optionally constructed to be directly luminous, and - a plate-shaped light guide (9) located in front of the background lighting (8b) in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume, - illuminants (10) arranged laterally on at least one narrow side of the light guide (9), and - wherein in operating mode B2 the background lighting (8b) is switched on and the lamps (10) are switched off, and wherein in operating mode B1 at least the lamps (10) are switched on.