JP2025151995A - Backlight system and display device - Google Patents

Abstract

A backlight system with excellent light utilization efficiency and a display device using the backlight system are provided.
[Solution] A backlight system comprising a backlight, a Pancharatnam Berry phase diffraction grating, and a first λ/4 plate in this order, and emitting light that is emitted from the backlight and passes through the Pancharatnam Berry phase diffraction grating and the first λ/4 plate.
[Selected Figure] Figure 3

Description

The following disclosure relates to a backlight system and a display device.

The mainstream backlight structure used in recent LCD displays is the edge type, in which light emitted from LED light sources placed on the side is guided by a light guide plate and then concentrated toward the front using a diffusion sheet and prism sheet.

Patent Document 1 discloses an optical sheet for a backlight, in which a diffusion sheet and a prism sheet are arranged in this order from the light source side. In this optical sheet, the exit surface of the diffusion sheet and the entrance surface of the prism sheet are bonded together via a reflective layer with openings, preventing light with a large angle of incidence from entering the prism sheet.

JP 2009-210798 A

In conventional backlight systems, when a prism sheet is used, some of the light that enters the prism sheet is emitted as unnecessary light called side lobe light, reducing light utilization efficiency.

Furthermore, the optical sheet for backlights described in Patent Document 1 is difficult to form a reflective layer on, and if light does not enter the openings between the reflective layers, the number of reflections increases, resulting in reduced light utilization efficiency.

The present invention was made in consideration of the above-mentioned current situation, and aims to provide a backlight system with excellent light utilization efficiency, and a display device using such a backlight system.

(1) One embodiment of the present invention is a backlight system that includes, in this order, a backlight, a Pancharatnam Berry phase diffraction grating, and a first λ/4 plate, and that emits light that is emitted from the backlight and has passed through the Pancharatnam Berry phase diffraction grating and the first λ/4 plate.

(2) In addition to the configuration of (1), an embodiment of the present invention provides a backlight system in which the Pancharatnam-Berry phase diffraction grating includes a retardation layer containing a cured polymerizable liquid crystal, a slow axis of the polymerizable liquid crystal periodically rotates in a first direction from one end of the retardation layer to the other end within the plane of the retardation layer, and a molecular orientation pattern Φ(x) [°], which is an orientation direction of the polymerizable liquid crystal arranged at a position a distance x [μm] in the first direction from a position where the slow axis of the polymerizable liquid crystal is parallel to the first direction, satisfies the following formula (1):
In the formula, Λ represents the pitch [μm] at which the slow axis of the polymerizable liquid crystal rotates by 180° in the plane of the retardation layer, and m, n, and A are arbitrary constants.

(3) Furthermore, in addition to the configuration of (1) or (2) above, one embodiment of the present invention is a backlight system further comprising a diffusion sheet between the backlight and the Pancharatnam Berry phase diffraction grating.

(4) Furthermore, in one embodiment of the present invention, in addition to the configuration of (1), (2), or (3) above, the backlight system further comprises a refractive optical element on the opposite side of the first λ/4 plate from the Pancharatnam Berry phase diffraction grating.

(5) Furthermore, in one embodiment of the present invention, in addition to the configuration of (4) above, the refractive optical element is a lenticular lens or a prism sheet.

(6) Furthermore, in addition to the configuration of (1), (2), (3), (4), or (5) above, one embodiment of the present invention is a backlight system further comprising a second λ/4 plate between the backlight and the Pancharatnam-Berry phase diffraction grating, and the light emitted from the backlight is linearly polarized.

(7) Furthermore, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), or (6) above, the Pancharatnam-Berry phase diffraction grating has, in a planar view, a plurality of regions having different orientation pitches, the plurality of regions including a first region facing a first backlight region in the backlight and a second region facing a second backlight region in the backlight that is farther from the light source of the backlight than the first backlight region, and the orientation pitch in the second region is smaller than the orientation pitch in the first region.

(8) Another embodiment of the present invention is a display device comprising the backlight system of (1), (2), (3), (4), (5), (6), or (7) above, and a liquid crystal panel that displays an image using light emitted from the backlight system.

(9) Furthermore, in one embodiment of the present invention, in addition to the configuration of (7) above, the display device further comprises a reflective polarizing plate between the backlight system and the liquid crystal panel.

The present invention provides a backlight system with excellent light utilization efficiency, and a display device using the backlight system.

FIG. 1 is a schematic diagram showing the configuration of a conventional backlight system. FIG. 10 is a cross-sectional view of a prism sheet for explaining side lobe light. 1 is a schematic diagram illustrating a configuration of a backlight system and a display device according to a first embodiment. FIG. 1 is a cross-sectional view schematically showing the configuration of a PBP diffraction grating. FIG. 1 is a plan view schematically showing the configuration of a PBP diffraction grating. 1 is a photograph of a PBP diffraction grating taken with a polarizing microscope. 1A and 1B are schematic diagrams illustrating the function of a PBP diffraction grating when circularly polarized light is incident thereon. 1 is a schematic diagram illustrating a diffraction angle θ of a PBP diffraction grating and a light intensity distribution U(θ) on a screen placed on the exit light side of the PBP diffraction grating. 10 is a graph showing the molecular orientation pattern Φ(x) of the PBP diffraction grating of the reference example. 10 is a graph showing the light intensity distribution U(θ) of the PBP diffraction grating of the reference example. 10 is a graph showing the molecular orientation pattern Φ(x) of the PBP diffraction grating of the second embodiment. 10 is a graph showing the light intensity distribution U(θ) of the PBP diffraction grating of the second embodiment. 10A and 10B are schematic diagrams illustrating the state of light transmitted through the PBP diffraction gratings of the second embodiment and the reference example. FIG. 10 is a schematic diagram showing the configuration of a backlight system and a display device according to a third embodiment. FIG. 10 is a schematic diagram illustrating the configuration of a backlight system and a display device according to a fourth embodiment. FIG. 10 is a cross-sectional view schematically showing the structure of a prism sheet included in a backlight system according to a fourth embodiment. FIG. 2 is a schematic diagram for explaining the characteristics of a diffraction element. FIG. 2 is a schematic diagram for explaining the characteristics of a refractive element. FIG. 10 is a schematic diagram illustrating the configuration of a backlight system and a display device according to a fifth embodiment. 1 is a schematic diagram showing a head-mounted display including a display panel that emits display light uniformly in the normal direction within a plane. 1 is a schematic diagram showing a head-mounted display having a display panel that emits display light obliquely at both ends. FIG. 10 is a schematic diagram showing the configuration of a backlight system and a display device according to a sixth embodiment. 5A and 5B are diagrams illustrating a method for measuring the angle of emitted light in the first embodiment.

(Summary of the Disclosure)
Fig. 1 is a schematic diagram showing the configuration of a conventional backlight system. Fig. 2 is a cross-sectional view of a prism sheet for explaining side lobe light. In this disclosure, arrows in the figure indicate the traveling direction of light rays. In this disclosure, angles represent values when the normal direction to the display surface is defined as 0°, and correspond to the magnitude of the inclination from the normal direction to the display surface.

As shown in Figure 1, in conventional backlight systems, prism sheets 12a and 12b, whose ridgelines intersect perpendicularly with the diffusion sheet 11, are stacked on top of the backlight 100. These sheets adjust the angle of the light beam emitted from the backlight system, which indicates its maximum intensity (also known as the "peak angle"), to 0° (the normal direction of the display surface). However, as shown in Figure 2, the direction of the light beam changes depending on the angle and position at which the light strikes the prism sheet 12, so not all light can be used uniformly as effective light. Light emitted as a side lobe (side lobe light) is emitted without being reused. In the configuration of Figure 1, the two prism sheets 12a and 12b each generate side lobe light, so conventional backlight systems have issues with light utilization efficiency.

The backlight system of the present disclosure uses a Pancharatnam-Berry Phase (PBP) diffraction grating instead of the prism sheet. A PBP diffraction grating is an optical film made by UV-curing polymerizable liquid crystals, and is an optical element whose diffraction angle can be controlled by changing the rotation period of the liquid crystal orientation. A PBP diffraction grating can increase the amount of front-emitting light from a backlight system by bending the direction of incident light toward the front. Furthermore, because light can be bent uniformly within the plane, unnecessary light such as side lobe light is not generated. Due to these factors, the backlight system of the present disclosure can achieve high light utilization efficiency without the need for complex stacking of optical films.

Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the same reference numerals will be used in different drawings to designate the same parts or parts having similar functions, and repeated explanations will be omitted where appropriate.

(Embodiment 1)
The configuration of the backlight system of embodiment 1 and a display device including the backlight system will be described with reference to Figures 3 to 7. Figure 3 is a schematic diagram showing the configuration of the backlight system and display device according to embodiment 1. Figure 4 is a cross-sectional view showing a schematic configuration of a PBP diffraction grating. Figure 5 is a plan view showing a schematic configuration of a PBP diffraction grating. Figure 6 is a photograph of a PBP diffraction grating taken with a polarizing microscope. Figure 7 is a schematic diagram explaining the function of a PBP diffraction grating when circularly polarized light is incident thereon.

As shown in FIG. 3, the backlight system of embodiment 1 includes a backlight 100, a Pancharatnam-Berry Phase (PBP) diffraction grating 150, and a first λ/4 plate 180, arranged in this order. The display device of embodiment 1 includes a reflective polarizing plate 200 and a liquid crystal panel (LCD) 300, arranged in this order, in front of the backlight system. The liquid crystal panel 300 displays images using light emitted from the backlight system.

The backlight 100 is not particularly limited, and can be, for example, a conventional backlight used in technical fields such as liquid crystal display devices. An edge-type backlight including a light source such as an LED and a light guide plate is preferably used as the backlight 100.

As shown in Figure 4, the PBP diffraction grating 150 is an optical film having a retardation layer 158 formed by applying an alignment film 154 provided on a substrate 152 made of a glass substrate, PET film, or the like, to which an alignment treatment has been applied, followed by application of a polymerizable liquid crystal 156, and then photo-curing the polymerizable liquid crystal 156 with ultraviolet light.

The PBP diffraction grating 150 can be fabricated, for example, by methods described in International Publication No. 2019/189818 and Published Japanese Translation of PCT International Publication No. 2008-532085.

As shown in FIG. 5 , in a planar view, the orientation direction of the cured polymerizable liquid crystal 156 periodically rotates within the plane of the retardation layer 158 in a first direction (the x-axis direction in FIG. 5 ) from one end of the retardation layer 158 to the other end, but does not periodically rotate in the y-axis direction, which is perpendicular to the x-axis direction. Here, the long axis of the cured polymerizable liquid crystal 156 is the slow axis. That is, in the retardation layer 158 provided in the PBP diffraction grating 150, the direction of the slow axis derived from the cured polymerizable liquid crystal 156 changes while continuously rotating along the x-axis direction in the plane. As shown in FIG. 6 , the direction of the slow axis can be confirmed using a polarizing microscope or an Axoscan (AxoMetrics). As shown in FIG. 7 , the PBP diffraction grating 150 diffracts incident right-handed circularly polarized light (RCP) in the +θ direction and left-handed circularly polarized light (LCP) in the -θ direction. In other words, it is a polarization-dependent diffraction element in which the diffraction direction reverses depending on the type of polarization.

The PBP diffraction grating 150 can control the diffraction angle of incident light by changing the rotation period of the liquid crystal orientation (hereinafter also referred to as the "orientation pitch"). The orientation pitch may be varied within the plane of the PBP diffraction grating 150. This makes it possible to form multiple regions within the plane of the PBP diffraction grating 150 that have different diffraction angles according to the incident angle distribution of the incident light.

The alignment film 154 has a patterned in-plane alignment control force. Specifically, the alignment film 154 has an alignment control force that aligns the polymerizable liquid crystal 156 so that the slow axis of the retardation layer 158 rotates periodically in-plane.

The alignment film 154 can be made of materials commonly used in the field of liquid crystal panels, such as polymers with polyimide in the main chain, polymers with polyamic acid in the main chain, and polymers with polysiloxane in the main chain. The alignment film 154 can be formed by applying an alignment film material to the substrate 152. The application method is not particularly limited, and can be, for example, flexographic printing, inkjet application, etc.

The type of alignment film 154 is not particularly limited, and may be a rubbed alignment film that has been subjected to rubbing as an alignment treatment, or a photo-alignment film that has photo-functional groups and has been subjected to photo-alignment as an alignment treatment. However, from the perspective of patterning the in-plane alignment control force into a complex pattern, a photo-alignment film is preferable.

The retardation layer 158 is obtained by polymerizing polymerizable liquid crystal 156. The type of polymerizable liquid crystal 156 is not particularly limited, and conventionally known polymerizable liquid crystal compounds can be used. Those that polymerize and harden upon exposure to ultraviolet (UV) light are preferably used. Examples of polymerizable liquid crystal 156 include polymers with side chains that combine mesogenic groups such as biphenyl, terphenyl, naphthalene, phenylbenzoate, azobenzene, and derivatives thereof with photoreactive groups such as cinnamoyl, chalcone, cinnamylidene, β-(2-phenyl)acryloyl, cinnamic acid, and derivatives thereof, and with main chains that include structures such as acrylate, methacrylate, maleimide, N-phenylmaleimide, and siloxane.

The polymerizable liquid crystal 156 may be a homopolymer consisting of a single repeating unit, or a copolymer consisting of two or more repeating units with different side chain structures. The copolymer may be of an alternating type, a random type, a graft type, or the like.

The diffraction efficiency η of the PBP diffraction grating 150 is expressed as η = sin ² (Δndπ/λ) using the thickness d and birefringence Δn of the retardation layer 158, and is 100% efficient when the retardation Δnd = λ/2. Therefore, the retardation layer 158 is usually designed so that Δnd is λ/2. Because the PBP diffraction grating 150 functions as a λ/2 plate, incident circularly polarized light is converted into counter-rotating circularly polarized light and then emitted.

A λ/4 plate is a component that imparts a phase difference equivalent to 1/4 of the wavelength of transmitted light. For example, it is a component that imparts a phase difference of 117.5 nm or more and 157.5 nm or less to light with a wavelength of 550 nm. The first λ/4 plate 180 can be, for example, a conventionally known plate used in technical fields such as liquid crystal display devices.

The reflective polarizing plate 200 is not particularly limited, and can be, for example, a conventional polarizing plate used in the technical field of liquid crystal display devices, etc.

The liquid crystal panel (LCD) 300 is not particularly limited, and can be, for example, a conventionally known one used in the technical field of liquid crystal display devices, etc.

The principle by which light utilization efficiency is improved in embodiment 1 will be explained with reference to FIG. 3. The left-handed circularly polarized light (LCP) component contained in the light (incident angle θ) emitted from the backlight 100 and incident on the PBP diffraction grating 150 is diffracted by −θ by the PBP diffraction grating 150 and becomes right-handed circularly polarized light (RCP). The light that passes through the PBP diffraction grating 150 and becomes right-handed circularly polarized light (RCP) is converted into linearly polarized light (LP1) by the first λ/4 plate 180. This linearly polarized light (LP1) is parallel to the transmission axis of the reflective polarizer 200, and therefore passes through the reflective polarizer 200. On the other hand, the right-handed circularly polarized light (RCP) component contained in the light (incident angle θ) incident on the PBP diffraction grating 150 is diffracted by +θ by the PBP diffraction grating 150 and becomes left-handed circularly polarized light (LCP). The light that passes through the PBP diffraction grating 150 and becomes left-handed circularly polarized light (LCP) is converted into linearly polarized light (LP2) by the first λ/4 plate 180. This linearly polarized light LP2 is perpendicular to the transmission axis of the reflective polarizer 200, and is therefore reflected by the reflective polarizer 200. This reflected light is returned to the backlight 100 and recycled. As a result, ideally, all light is ultimately emitted in the front direction, improving light utilization efficiency.

(Embodiment 2)
In the second embodiment, a configuration for further improving light utilization efficiency will be described in comparison with the reference example. FIG. 8 is a schematic diagram illustrating the diffraction angle θ of the PBP diffraction grating and the light intensity distribution U(θ) on a screen arranged on the output light side of the PBP diffraction grating. FIG. 9 is a graph showing the molecular orientation pattern Φ(x) of the PBP diffraction grating of the reference example. FIG. 10 is a graph showing the light intensity distribution U(θ) of the PBP diffraction grating of the reference example. Note that in this disclosure, "molecular orientation pattern Φ(x)" may also be simply referred to as "molecular orientation Φ(x)." FIG. 11 is a graph showing the molecular orientation pattern Φ(x) of the PBP diffraction grating of the second embodiment. FIG. 12 is a graph showing the light intensity distribution U(θ) of the PBP diffraction grating of the second embodiment. FIG. 13 is a schematic diagram illustrating the state of light transmitted through the PBP diffraction gratings of the second embodiment and the reference example.

The diffraction angle of the PBP diffraction grating 150 depends on the wavelength. The diffraction angle can be calculated using, for example, Fraunhofer diffraction. In a planar view, if the molecular orientation of the polymerizable liquid crystal 156 positioned at a distance x [μm] in the first direction from the position where the slow axis of the polymerizable liquid crystal 156 is parallel to the first direction (the x-axis direction in the figure) is Φ(x) [°], when light is incident on the screen 50 from the PBP diffraction grating 150 with a diffraction angle θ [°] as shown in Figure 8, the light intensity distribution U(θ) on the screen 50 is expressed by the following equation (2):

In this formula, θ is the diffraction angle (°) of the PBP diffraction grating 150, λ is the wavelength of light (nm), and k is a proportionality constant. In this embodiment and reference example, the value of k is set so that the value obtained by integrating U(θ) over the range of θ = -π to π is 100%. U(θ) in this case is also called the diffraction efficiency.

Here, for the sake of simplicity of calculation, the case where the orientation pitch is relatively large (the diffraction angle is 7.9°) will be described.
As shown in Fig. 9, for the PBP diffraction grating of the reference example having a molecular orientation pattern of "Φ(x) = x × 180°/4 μm," the light intensity distribution U(θ) was calculated when the wavelength of incident light was 450 nm, 550 nm, and 650 nm. This corresponds to the calculation for an orientation pitch of 4 μm.

The results are shown in Figure 10. For example, when the wavelength of the incident light is 550 nm, U(θ) is 100% at θ = 7.9°. This indicates that all incident light is bent in the direction of θ = 7.9°, which can be easily verified experimentally. The problem with this reference example was that the diffraction angles of red light R, green light G, and blue light B were different from one another. This can cause color breakup and other degradation in display quality.

Therefore, in this embodiment, U(θ) was calculated using the molecular orientation "Φ(x) = kx + m × sin(nx + A)." Specifically, the following formula (1) was used, where k = 180°/Λ in "Φ(x) = kx + m × sin(nx + A)."

More specifically, as shown in FIG. 11 , U(θ) was calculated assuming Λ=4 μm (i.e., k=180°/4 μm), n=2π/800 μm, and A=0. This is a calculation used in the concept of FM modulation, and it is known that multiple peaks appear. Looking at the calculation results in FIG. 12 , the peaks for each wavelength were split and overlapped. As a result, as shown in FIG. 13 , it was found that the PBP diffraction grating 150 of the second embodiment, which satisfies the above formula (1), is more capable of solving the color breakup problem than the PBP diffraction grating 150 of the reference example. Note that, because the second term on the right side of the above formula (1) is small, FIGS. 9 and 11 appear similar, but are actually different graphs.
Similarly, when the diffraction angle is 78°, the problem of color breakup can be solved by appropriately setting the optimum values of A, k, n, and m.

(Embodiment 3)
Fig. 14 is a schematic diagram showing the configuration of a backlight system and a display device according to embodiment 3. As shown in Fig. 14, in embodiment 3, a diffusion sheet 120 is provided between a backlight 100 and a PBP diffraction grating 150. The diffusion sheet 120 is not limited as long as it has the function of diffusing and transmitting incident light, and for example, a conventionally known diffusion sheet used in technical fields such as liquid crystal display devices can be used.
To increase the diffraction angle of the PBP diffraction grating 150, a finer orientation pitch is required. However, a finer orientation pitch results in less uniform alignment of the liquid crystals, making haze more likely to occur. This places limitations on the selection of materials that allow for uniform alignment of the liquid crystals. The use of a diffusion sheet 120 makes it possible to reduce the diffraction angle of the PBP diffraction grating 150. For example, as shown in FIG. 14 , when the peak angle of the light emitted from the backlight 100 is 78°, the diffraction angle required for the PBP diffraction grating 150 can be reduced from 78° to 45°. A diffraction angle of 45° results in the orientation pitch of the PBP diffraction grating 150 being 0.8 μm, which increases the orientation pitch and improves the productivity of the PBP diffraction grating 150. Furthermore, the use of a diffusion sheet 120 also prevents color breakup, a phenomenon in which the optical paths of light rays separate into wavelengths (e.g., white light splitting into multiple monochromatic lights) when passing through the PBP diffraction grating 150.

(Embodiment 4)
Fig. 15A is a schematic diagram showing the configuration of a backlight system and a display device according to embodiment 4. Fig. 15B is a cross-sectional view showing the structure of a lenticular lens included in the backlight system according to embodiment 4. As shown in Fig. 15A, in embodiment 4, a lenticular lens 190, which is a refractive optical element (refractive element), is provided between a first λ/4 plate 180 and a reflective polarizing plate 200. The type of the refractive element is not particularly limited, and a prism sheet may be used instead of the lenticular lens 190.
FIG. 16A is a schematic diagram illustrating the characteristics of a diffractive element. FIG. 16B is a schematic diagram illustrating the characteristics of a refractive element. As shown in FIGS. 16A and 16B, the diffractive element 15 bends light with longer wavelengths more, while the refractive element 19 bends light with shorter wavelengths more. Therefore, white light W incident on the diffractive element 15 or the refractive element 19 is color-separated into red light R, green light G, and blue light B. Therefore, by combining the PBP diffraction grating 150 with the refractive element 19, color breakup caused by the wavelength dependence of the diffraction angle of the PBP diffraction grating 150 can be reduced. When a refractive element (lenticular lens 190) is arranged as in this embodiment, as shown in FIG. 15A, the light color-separated by the PBP diffraction grating 150 overlaps again at the lenticular lens 190, improving color separation. Furthermore, the lenticular lens 190 can focus light more in the front direction, thereby improving front brightness.

(Embodiment 5)
17 is a schematic diagram showing the configuration of a backlight system and a display device according to embodiment 5. As shown in FIG. 17 , in embodiment 5, a second λ/4 plate 130 is provided between the backlight 100 and the PBP diffraction grating 150. When the light emitted from the backlight 100 is linearly polarized light LP, by inserting the second λ/4 plate 130 before the PBP diffraction grating 150, only circularly polarized light (here, left-handed circularly polarized light LCP) is incident on the PBP diffraction grating 150. Therefore, the PBP diffraction grating 150 diffracts all of the incident left-handed circularly polarized light LCP in the forward direction, thereby improving the light utilization efficiency.

(Embodiment 6)
Fig. 18 is a schematic diagram showing a head-mounted display including a display panel that emits display light uniformly in the normal direction within a plane. Fig. 19 is a schematic diagram showing a head-mounted display including a display panel that emits display light in oblique directions at both ends. Fig. 20 is a schematic diagram showing the configuration of a backlight system and a display device according to a sixth embodiment.

For example, in a head-mounted display (HMD), a lens 400 is used between the liquid crystal panel 300 (which serves as a display) and the viewer's eye E, enlarging the image displayed on the liquid crystal panel 300. As shown in FIG. 18, if the liquid crystal panel 300 is designed so that the brightness in the normal direction is highest uniformly across the surface, the amount of light reaching the eye E will be slightly reduced at the edges of the liquid crystal panel 300. Therefore, by changing the light path as shown in FIG. 19, the light utilization efficiency at the edges of the liquid crystal panel 300 can be improved. In embodiment 6, the light propagation direction shown in FIG. 19 is achieved by varying the orientation pitch of the PBP diffraction element across the surface. Specifically, the orientation pitch is increased on the side closer to the LED light source 110, and the orientation pitch is decreased as the distance from the LED light source 110 increases. This results in a peak angle for the emitted light that achieves high light utilization efficiency.

To achieve the light propagation direction shown in Figure 19, for example, the orientation pitch at the end of the light guide plate on the LED light source side (an example of a first region facing the first backlight region in the backlight) is adjusted to 1.2 μm, the orientation pitch at the center of the light guide plate (an example of a second region facing the second backlight region that is farther from the backlight light source than the first backlight region in the backlight) is adjusted to 0.5 μm, and the orientation pitch at the end of the light guide plate opposite the LED light source (an example of a second region facing the second backlight region that is farther from the backlight light source than the first backlight region in the backlight) is adjusted to 0.4 μm.

(Verification experiment)
21 is a diagram illustrating a method for measuring the angle of output light in Example 1. In this verification experiment, laser light (wavelength 532 nm) was used as incident light, and an optical system in which the "reflective polarizing plate" was replaced with a general polarizing plate 210 was used to measure whether diffraction of the PBP diffraction grating 150 actually occurred according to the principle.

A measurement sample of the PBP diffraction grating 150 was prepared in the following procedure.
(1) A photoisomerizable photo-alignment film was applied onto a glass substrate.
(2) The photo-alignment film was subjected to an alignment treatment by irradiating it with polarized ultraviolet (UV) light having a wavelength of 365 nm at 100 mJ/cm ² , and then the photo-alignment film was baked in an oven at 160° C. for 20 minutes.
(3) A polymerizable liquid crystal compound was applied onto the photo-alignment film using a spin coater rotating at 1000 rpm.
(4) The applied polymerizable liquid crystal compound was irradiated with 3 J/cm ² of unpolarized UV light having a wavelength of 365 nm to harden the polymerizable liquid crystal compound, thereby completing a measurement sample of the PBP diffraction grating 150.

The completed measurement sample of the PBP diffraction grating 150 had a structure in which a photo-alignment film and a polymerizable liquid crystal layer were laminated on a glass substrate. This measurement sample was subjected to an alignment process so that the distance by which the alignment direction of the polymerizable liquid crystal rotates by 180° (alignment pitch Δ) was 0.5 μm. The diffraction angle θ of the PBP diffraction grating 150 is calculated using the following formula. In this verification experiment, θ = 78°, as shown in Figure 21.
θ=arcsin(λ/Δ)

A retardation film made of COP (cycloolefin polymer) was used as the first λ/4 plate 180.

In the optical system shown in Figure 21, the angle of the exiting light was measured when the incident angle of the incident light on the PBP diffraction grating 150 was set to 78°. According to the above calculation, the exiting light would be at 0°, but the experimental result was 0°. This experimental result confirmed that the light utilization efficiency can be improved by the method disclosed in this disclosure.

Reference numeral 11: Diffusion sheet 12, 12a, 12b: Prism sheet 15: Diffraction element 19: Refractive element 50: Screen 100: Backlight 110: LED light source 120: Diffusion sheet 130: Second λ/4 plate 150: Pancharatnam-Berry phase (PBP) diffraction grating 152: Base material 154: Alignment film 156: Polymerizable liquid crystal 158: Retardation layer 180: First λ/4 plate 190: Lenticular lens 200: Reflective polarizing plate 210: Polarizing plate 300: Liquid crystal panel (LCD)
400: Lens LCP: Left circularly polarized LP, LP1, LP2: Linearly polarized RCP: Right circularly polarized

Claims (9)

Backlight and
Pancharatnam Berry phase grating;
a first λ/4 plate in this order,
A backlight system that outputs light that is emitted from the backlight and passes through the Pancharatnam Berry phase diffraction grating and the first λ/4 plate. the Pancharatnam-Berry phase diffraction grating comprises a retardation layer containing a cured product of a polymerizable liquid crystal;
a slow axis of the polymerizable liquid crystal periodically rotates in a first direction from one end of the retardation layer to the other end of the retardation layer within the plane of the retardation layer,
The backlight system of claim 1, wherein the molecular orientation pattern Φ(x) [°], which is the orientation direction of the polymerizable liquid crystal arranged at a position a distance x [μm] in the first direction from a position where the slow axis of the polymerizable liquid crystal is parallel to the first direction, satisfies the following formula (1):
In the formula, Λ represents the pitch [μm] at which the slow axis of the polymerizable liquid crystal rotates by 180° in the plane of the retardation layer, and m, n, and A are arbitrary constants. The backlight system of claim 1, further comprising a diffusion sheet between the backlight and the Pancharatnam Berry phase diffraction grating. The backlight system of claim 1 further comprising a refractive optical element on the opposite side of the first λ/4 plate from the Pancharatnam Berry phase diffraction grating. The backlight system of claim 4, wherein the refractive optical element is a lenticular lens or a prism sheet. further comprising a second λ/4 plate between the backlight and the Pancharatnam Berry phase grating;
The backlight system according to claim 1 , wherein the light emitted from the backlight is linearly polarized light. The Pancharatnam Berry phase diffraction grating has a plurality of regions having different orientation pitches in a plan view,
the plurality of regions include a first region facing a first backlight region of the backlight and a second region facing a second backlight region of the backlight that is farther from a light source of the backlight than the first backlight region of the backlight,
The backlight system according to claim 1 , wherein the alignment pitch in the second region is smaller than the alignment pitch in the first region. A backlight system according to any one of claims 1 to 7;
a display device comprising a liquid crystal panel that displays an image using light emitted by the backlight system; The display device described in claim 8, further comprising a reflective polarizing plate between the backlight system and the liquid crystal panel.