Light guide plate for image display

Abstract

To provide an image display light guide plate capable of suppressing deterioration of a hologram layer.SOLUTION: An image display light guide plate 8 is provided, comprising a base material 1, a glass layer 3, and a hologram layer 4 arranged in the described order in a thickness direction.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a light guide plate for image display.

Background Art

[0002] In a display device, a light guide plate for image display may be used. For example, in a display device using VR (Virtual Reality) technology or AR (Augmented Reality) technology, a light guide plate for image display in which a hologram layer is supported by a transparent substrate is used. In the hologram layer, holograms having various optical functions, such as waveguide, reflection, and diffraction functions, are formed. In such a light guide plate, in order to achieve weight reduction and high strength, the adoption of a resin substrate has been considered. However, the hologram material for forming the hologram layer may erode the resin substrate due to temperature changes or the like. Further, when a resin substrate is used, the hologram material may deteriorate due to moisture absorption.

[0003] Patent Document 1 describes forming a photosensitive material layer for forming a hologram on an optically transparent resin-made substrate and coating the photosensitive material layer with an aqueous polymer protective barrier. Patent Document 1 suggests that the aqueous polymer protective barrier is provided for the purpose of withstanding attack by moisture.

[0004] Patent Document 2 describes a laminate having a hologram sandwiched between resin substrates via an optical adhesive, and a hologram laminate in which the entire outer peripheral portion is covered with a protective coating layer. Patent Document 2 describes that the protective coating layer may be a coating that enhances airtightness and gas barrier properties.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] However, the techniques described in Patent Document 1 and Patent Document 2 have the following problems. In the technique described in Patent Document 1, a resin substrate is adhered to the surface of the photosensitive material layer on the side opposite to the aqueous polymer protective barrier. Therefore, there is a risk that the photosensitive material may erode the resin substrate in a high-temperature environment. Furthermore, since the resin substrate contains moisture inside, the moisture diffuses into the photosensitive material layer through the adhesion surface with the photosensitive material layer. In addition, since the substrate is exposed to the outside, moisture continues to penetrate into the substrate from the outside. As a result, moisture penetrates into the photosensitive material layer via the resin substrate, and even if the moisture is shielded by the aqueous polymer protective barrier, it is impossible to suppress the deterioration of the photosensitive material layer over time due to the moisture from the substrate side. In the technique described in Patent Document 2, the entire outer periphery of the laminate including the hologram is sealed by a protective coating layer. Therefore, it is possible to suppress the penetration of moisture from the outside of the hologram laminate into the inside and the deterioration of the hologram. However, the moisture contained in the resin substrate during the formation of the protective coating layer is confined inside the protective coating layer. As a result, there is a risk that the moisture contained in the resin substrate may penetrate into the hologram and the deterioration of the hologram over time may progress. In particular, in a high-temperature environment, the influence of moisture release from the resin substrate becomes significant.

[0007] An object of the present invention is to provide a light guide plate for image display that can suppress the deterioration of the hologram layer. MEANS FOR SOLVING THE PROBLEMS

[0008] The present invention has the following aspects. [1] A light guide plate for image display, comprising a base material, a glass layer, and a hologram layer, wherein the base material, the glass layer, and the hologram layer are arranged in this order in the thickness direction. [2] The light guide plate for image display according to [1], wherein the base material is a transparent material. [3] The light guide plate for image display according to [1] or [2], wherein the base material is a resin base material. [4] The light guide plate for image display according to any one of [1] to [3], wherein the thickness of the glass layer is 500 μm or less. [5] The light guide plate for image display according to any one of [1] to [4], wherein the refractive index of the glass layer is 1.48 or more. [6] The light guide plate for image display according to any one of [1] to [5], wherein the base material contains at least one resin selected from the group consisting of an acrylic resin, a cyclic polyolefin resin, and a polycarbonate resin. [7] The light guide plate for image display according to any one of [1] to [6], wherein the glass layer is disposed on the hologram layer. [8] The light guide plate for image display according to any one of [1] to [7], wherein an adhesive layer is disposed between the base material and the glass layer. [9] A light guide plate for image display having a distal glass layer, a base material, a proximal glass layer, and a hologram layer, wherein the distal glass layer, the base material, the proximal glass layer, and the hologram layer are disposed in this order in the thickness direction. However, the distal glass layer is a glass layer located on the side opposite to the side of the hologram layer of the base material, and the proximal glass layer is a glass layer located between the hologram layer and the base material.

[10] The light guide plate for image display according to [9], wherein the distal glass layer is at least one outermost layer of the light guide plate for image display.

[11] The light guide plate for image display according to [9] or

[10] , wherein the proximal glass layer is disposed on the hologram layer.

[12] The light guide plate for image display according to any one of [9] to

[11] , wherein an adhesive layer is disposed between the distal glass layer and the base material.

[13] The light guide plate for image display according to any one of [9] to

[12] , wherein an adhesive layer is disposed between the base material and the proximal glass layer. [Effect of the Invention]

[0009] According to the present invention, it is possible to provide a light guide plate for image display that can suppress deterioration of the hologram layer.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In all the drawings, even if the embodiments are different, the same or corresponding members are denoted by the same reference numerals, and common descriptions are omitted. “(Meth)acrylic” is a general term for “acrylic” and “methacrylic”.

[0012] <First Embodiment> A light guide plate for image display according to a first embodiment of the present invention will be described. The light guide plate for image display of the present embodiment has a base material, a glass layer, and a hologram layer. In the light guide plate for image display of the present embodiment, the base material, the glass layer, and the hologram layer are arranged in this order in the thickness direction. An adhesive layer may be disposed between the base material and the glass layer as necessary.

[0013] The glass layer may be disposed on only one surface of the hologram layer, but it is preferably disposed on both surfaces of the hologram layer. When the glass layer is disposed on both surfaces of the hologram layer, deterioration of the hologram layer can be more reliably suppressed.

[0014] One or more transparent layers may be disposed on at least one surface of the base material. One or more transparent layers may be disposed between the glass layer and the hologram layer. Examples of the type of the transparent layer include a hard coat layer or an anchor coat layer.

[0015] In the light guide plate for image display according to the present embodiment, two base materials may be provided with the glass layer and the hologram layer interposed therebetween.

[0016] The material of the base material is not particularly limited as long as it has a function as a light guide plate for image display, but a transparent material is preferable from the viewpoint of preventing deterioration of image information. As the base material, an inorganic base material such as glass can be used because of its excellent optical properties, but a resin base material is preferable because it can achieve weight reduction and high strength of the light guide plate for image display.

[0017] When a resin base material is used as the base material, the surface of the resin base material may be subjected to a surface modification treatment in order to improve the adhesion to an adhesive layer described later. Examples of the surface modification treatment include treatments such as corona discharge treatment, flame treatment, oxidation treatment, plasma treatment, and lamination of a primer layer.

[0018] The hologram layer is sandwiched between two base materials. The light guide plate for image display according to the present embodiment has an incident portion for incident image light and a display portion for displaying an image by the image light. The hologram layer is disposed between the incident portion and the display portion. In the hologram layer, at least a diffraction grating pattern for guiding the image light incident from the incident portion to the display portion and emitting it from the display portion is formed. The diffraction grating pattern in the display portion transmits at least a part of the external light incident from the outside of the light guide plate for image display.

[0019] The image light incident on the incident portion is guided in the hologram layer and emitted from the display portion to the outside. On the other hand, as a result of the external light also passing through the base material and the display portion, an observer of the display portion can observe both the image light and the external light within the visual field. The light guide plate for image display of the present embodiment is suitably used for a display device using VR technology or AR technology, etc. For example, the light guide plate for image display of the present embodiment may be used for devices such as a combiner of a head-up display (HUD) for automotive use or a holographic optical element (HOE) typified by a reflector for a reflective liquid crystal display device, in addition to display applications.

[0020] As the material of the hologram layer, a known resin material for hologram formation can be used. Examples of the resin material for hologram formation include a hologram recording material composed of a thermosetting resin having at least one ethylene oxide ring soluble in a solvent and polymerizable cationically and an ethylenic monomer polymerizable radically (Japanese Patent Laid-Open Nos. 8-1676, 8-1677, 8-1678, 8-1679).

[0021] Hereinafter, based on FIG. 1, a detailed configuration of an example of the light guide plate for image display of the present embodiment will be described. FIG. 1 is a schematic cross-sectional view showing an example of a light guide plate for image display according to the first embodiment of the present invention.

[0022] In the light guide plate 8 for image display shown in FIG. 1, in the thickness direction, a first base material 1, a first adhesive layer 2, a first glass layer 3, a hologram layer 4, a second glass layer 5, a second adhesive layer 6, and a second base material 7 are arranged in this order. The plan view shape of the light guide plate 8 for image display is not particularly limited. For example, the light guide plate 8 for image display may be shaped to be attachable to the display device to be used. For example, the light guide plate 8 for image display may be a rectangular plate larger than the shape for attachment to the display device. In this case, before being assembled to the display device, the light guide plate 8 for image display is formed by being cut into a shape attachable to the display device. The light guide plate 8 for image display may be flat plate-shaped or, if necessary, may be curved plate-shaped. Hereinafter, an example in the case where the light guide plate 8 for image display is composed of a flat plate having a rectangular shape in plan view will be described.

[0023] The first base material 1 is disposed at the outermost part in the thickness direction of the light guide plate 8 for image display. The first base material 1 is disposed on the surface on the display image emission side in the light guide plate 8 for image display. The first base material 1 has the same shape as the outer shape of the light guide plate 8 for image display. The image light emitted from the hologram layer 4 and the external light transmitted through the second base material 7 and the hologram layer 4 described later are transmitted through the first base material 1. The thickness of the first base material 1 is not particularly limited. For example, the thickness of the first base material 1 may be 0.1 to 10 mm.

[0024] As described above, the material for forming the first base material 1 is preferably a resin material. The resin material is not particularly limited. Examples of the resin material include polyolefin resins such as homopolymers or copolymers of olefins such as ethylene, propylene, and butene, considering optical properties such as transparency and refractive index of light, as well as physical properties such as impact resistance, heat resistance, and durability; amorphous polyolefin resins such as cyclic polyolefin; polyester resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN); cellulose resins such as cellulose, acetyl cellulose, triacetyl cellulose, diacetyl cellulose, and cellophane; polyamide resins such as nylon 6, nylon 66, nylon 12, and copolymer nylon; ethylene-vinyl acetate copolymer partial hydrolyzate (EVOH); polyimide resins; polyetherimide resins; polysulfone resins; polyethersulfone resins; polyetheretherketone resins; polycarbonate resins; polyvinyl butyral resins; polyarylate resins; fluororesins; poly(meth)acrylic resins; styrene resins such as polystyrene; polyvinyl alcohol; ethylene-vinyl alcohol copolymer; polyvinyl chloride; polyvinylidene chloride; polyphenylene sulfide; polyurethane; phenol resins; epoxy resins; polyarylate resins; polynorbornene; styrene-isobutylene-styrene block copolymer (SIBS); allyl diglycol carbonate; and organic materials such as biodegradable resins. The first substrate 1 may be formed of two or more materials, or may have a laminated structure in which two or more materials are laminated. From the viewpoint of transparency, it is preferable to use either one or both of polycarbonate and poly(meth)acrylic resin as the resin material. Also, from the viewpoint of processability resistance, it is preferable to use at least one selected from the group consisting of poly(meth)acrylic resin, epoxy resin, and cyclic polyolefin as the resin material.

[0025] In the light guide plate 8 for image display, if the adhesion between the first base material 1 and the first glass layer 3 is good, the first adhesive layer 2 is not essential. However, especially when the resin material described above is used as the first base material 1, it is preferable to employ the first adhesive layer 2 in order to maintain excellent adhesion to the first glass layer 3. The material for forming the first adhesive layer 2 is not particularly limited as long as it has good adhesiveness between the first base material 1 and the first glass layer 3. Examples of the material for the first adhesive layer 2 include curable resin compositions containing ultraviolet curable resins or thermosetting resins. Examples of the ultraviolet curable resin or the thermosetting resin include acrylic resins, urethane resins, epoxy resins, and silicone resins. In the curable resin composition constituting the first adhesive layer 2, at least one additive selected from the group consisting of silane coupling agents, sensitizers, crosslinking agents, ultraviolet absorbers, polymerization inhibitors, surfactants, fillers, release agents, and thermoplastic resins may be added within a range that does not impair the object of the present invention. These additives may be used in combination of two or more. Examples of the thermoplastic resin include fluorine-based resins, polyethersulfone-based resins, polycarbonate-based resins, acrylic-based resins, silicone-based resins, cycloolefin-based resins, thermoplastic polyimide-based resins, polyamide-based resins, polyamideimide-based resins, polyarylate-based resins, polysulfone-based resins, polyetherimide-based resins, polyetheretherketone-based resins, polyethersilicon-based resins, polyester-based resins, and polyphenylene sulfide-based resins. Since the total light transmittance of the first adhesive layer 2 can impart an excellent luminance value to the light guide plate 8 for image display, it is preferably 80% or more, more preferably 90% or more. Since the refractive index of the first adhesive layer 2 can impart an excellent luminance value to the light guide plate 8 for image display and tends to prevent color unevenness, it is preferably n±0.20, more preferably n±0.15, and even more preferably n±0.10 with respect to the average refractive index n of the first base material 1 and the first glass layer 3. Since the thickness (lower limit value) of the first adhesive layer 2 tends to be able to maintain excellent adhesiveness between the first base material 1 and the first glass layer 3, 1 μm or more is preferable, and 5 μm or more is more preferable. Further, since the thickness (upper limit value) of the first adhesive layer 2 tends to be able to suppress color unevenness of the light guide plate, 700 μm or less is preferable, and 100 μm or less is more preferable. The method for forming the first adhesive layer 2 is not particularly limited. For example, the first adhesive layer 2 can be formed by laminating a glass plate that will become the first glass layer 3 on an adhesive applied on the first base material 1 and treating this laminate by a method such as pressing, nip roll, or thermal lamination. When this adhesive is a curable resin composition, a curing step of the adhesive may be included in the formation of the first adhesive layer 2.

[0026] The first glass layer 3 is disposed between the first base material 1 and a hologram layer 4 described later. In the configuration shown in FIG. 1, the first glass layer 3 is in close contact with the surfaces of the first adhesive layer 2 and the hologram layer 4, respectively. The first glass layer 3 prevents gas outside the light guide plate 8 for image display and gas permeating from the first base material 1 from permeating into the hologram layer 4. The lower the oxygen permeability and the water vapor permeability of the first glass layer 3, the more preferable. The oxygen permeability of the first glass layer 3 is preferably 3 / m 2 ·day or less. The water vapor permeability of the first glass layer 3 is preferably 1 g / m 2 ·day or less, and more preferably 0.5 g / m 2 ·day or less.

[0027] The refractive index of the first glass layer 3 may be higher than that of the first substrate 1. For example, when the resin material described above is used as the material of the first substrate 1, the refractive index of the first glass layer 3 is preferably 1.48 or more, and more preferably 1.48 to 3.0. When the refractive index of the first glass layer 3 is higher than that of the first substrate 1, the light transmitted through the first substrate 1 via the first glass layer 3 enters from the optically dense first glass layer 3 into the optically rough first substrate 1. Therefore, the emission angle of the light from the first glass layer 3 toward the first substrate 1 increases according to the refractive index difference between the first glass layer 3 and the first substrate 1. Thereby, the FOV (field of view) in the light guide plate 8 for image display can be made wider.

[0028] Since the thickness (lower limit value) of the first glass layer 3 tends to be excellent in gas barrier properties together with mechanical strength, it is preferably 10 μm or more, and more preferably 30 μm or more. Further, the thickness (upper limit value) of the first glass layer 3 is preferably 200 μm or less, more preferably 100 μm or less, still more preferably 75 μm or less, and even more preferably 50 μm or less because it tends to be excellent in optical properties as a light guide plate such as light transmittance. Examples of the material of the first glass layer 3 include borosilicate glass, alkali-free glass, low-alkali glass, soda-lime glass, and sol-gel glass, and glass obtained by subjecting these glasses to heat treatment or surface treatment. As the material of the first glass layer 3, alkali-free glass is preferable from the viewpoint of avoiding coloring due to impurities in the glass material. The method for forming the first glass layer 3 is not particularly limited. For example, the first glass layer 3 can be formed by a slot down draw method, a fusion method, or a float method. Further, the first glass layer 3 may be formed using a commercially available glass plate. When forming the first glass layer 3 using a commercially available glass plate, the commercially available glass plate can be used as it is, or the commercially available glass plate can be polished to a desired thickness and then used. Examples of the commercially available glass plate include EAGLE2000 (alkali-free glass, manufactured by Corning), AN100 (aluminosilicate glass, manufactured by AGC), OA-10G (alkali-free glass, manufactured by Nippon Electric Glass), and D263 (borosilicate glass, manufactured by Schott).

[0029] The hologram layer 4 is laminated on the surface of the first glass layer 3. The configuration of the hologram layer 4 is not particularly limited. An appropriate diffraction grating corresponding to the functions required for the light guide plate 8 for image display is formed in the hologram layer 4.

[0030] The second glass layer 5 is disposed on the surface of the hologram layer 4 opposite to the surface on which the first glass layer 3 is formed. The configuration of the second glass layer 5 is the same as that exemplified in the description of the first glass layer 3. However, the material, thickness, etc. of the second glass layer 5 may be different from those of the first glass layer 3.

[0031] The second substrate 7 is laminated on the surface of the second adhesive layer 6 on the side opposite to the first adhesive layer 2 via the hologram layer 4. The same configuration as that exemplified in the description of the first substrate 1 is used for the second substrate 7. However, the thickness, material, etc. of the second substrate 7 may be different from those of the first substrate 1. In particular, since the second substrate 7 is disposed on the surface of the light guide plate 8 for image display on the external light incident side opposite to the display image emission side, a material having a higher surface hardness than the first substrate 1 may be used. As the second adhesive layer 6, the same configuration as that exemplified in the description of the first adhesive layer 2 is used. However, the thickness, material, etc. of the second adhesive layer 6 may be different from those of the first adhesive layer 2.

[0032] The light guide plate 8 for image display can be manufactured, for example, as follows. The first base material 1 and the second base material 7 are prepared, and the first adhesive layer 2 and the first glass layer 3 are respectively disposed on the surface of the first base material 1, and the second adhesive layer 6 and the second glass layer 5 are respectively disposed on the surface of the second base material 7. For example, a photosensitive material for hologram formation is applied to the surface of the first glass layer 3 in the first base material 1 to which the first glass layer 3 is adhered. At this time, a transparent seal layer having the same thickness as the hologram layer 4 may be provided on the outer peripheral portion of the first glass layer 3. In this case, the photosensitive material is applied to a recess formed and surrounded by the seal layer. The seal layer seals the outer peripheral portion of the hologram layer 4 after the formation of the hologram layer 4. Thereafter, the second base material 7 to which the second glass layer 5 is adhered is placed on the photosensitive material with the second glass layer 5 facing the photosensitive material. However, the above manufacturing sequence is an example. For example, the photosensitive material may be applied to the second base material 7 to which the second glass layer 5 is adhered, and then the first base material 1 to which the first glass layer 3 is adhered may be placed on the photosensitive material. Thereafter, a laminate in which the first base material 1, the first adhesive layer 2, the first glass layer 3, the photosensitive material, the second glass layer 5, the second adhesive layer 6, and the second base material 7 are bonded together is manufactured by vacuum pressing. Thereafter, an interference fringe corresponding to the diffraction pattern is formed in the photosensitive material of the laminate to obtain a hologram layer 4 in which a diffraction grating is recorded. In this way, the light guide plate 8 for image display is manufactured.

[0033] According to the light guide plate 8 for image display, the first adhesive layer 2 and the first glass layer 3, and the second adhesive layer 6 and the second glass layer 5 are respectively disposed between the first base material 1 and the hologram layer 4, and between the second base material 7 and the hologram layer 4. The gas barrier properties of the first base material 1 and the second base material 7 vary depending on the type of the material of the base material. For example, when the material of the base material is a resin material, that is, when the base material is a resin base material, the gas barrier property of the base material is significantly lower than when the material of the base material is glass, that is, when it is a glass base material. Therefore, the resin base material has higher hygroscopicity and water vapor permeability than the glass base material. Therefore, when a resin base material is used as the first base material 1 and the second base material 7, the gas outside the light guide plate 8 for image display permeates through the first base material 1 and the second base material 7 to some extent or accumulates inside. In this case, moisture particularly easily accumulates in the first base material 1 and the second base material 7. However, the gas and moisture that penetrate into the first base material 1 and the second base material 7 from the outside can be blocked by the first glass layer 3 and the second glass layer 5 even if they diffuse inside the light guide plate 8 for image display. Thereby, in the light guide plate 8 for image display, the penetration of gas and water vapor into the hologram layer 4 is suppressed. In this way, by suppressing the penetration of moisture into the hologram layer 4, the deterioration of the hologram layer 4 is prevented even when a resin base material is used. Furthermore, in the light guide plate 8 for image display, since it has the above-described layer structure, the hologram layer 4 is not in contact with the first base material 1 and the second base material 7. Thereby, even when the light guide plate 8 for image display is disposed in a high-temperature environment, it is prevented that the hologram layer 4 erodes this resin base material.

[0034] In particular, when the refractive index of the first glass layer 3 is higher than the refractive index of the first base material 1, as described above, the emission angle of light from the first glass layer 3 toward the first base material 1 becomes larger. Similarly, when the refractive index of the second glass layer 5 is higher than the refractive index of the second base material 7, the emission angle of the light incident from the outside on the second base material 7 side and transmitting through the light guide plate 8 for image display into the second glass layer 5 is narrowed. For this reason, compared with the case where the second glass layer 5 is not provided, light in a wider angle range is incident on the light guide plate 8 for image display from the outside on the second base material 7 side, and compared with the case where the first glass layer 3 is not provided, the light is emitted in a wider angle range. As a result, the range of the field of view of external light is further widened, and the FOV on the display side is also widened. Regarding the image light from the hologram layer 4, as described above, as a result of the increase in the emission angle of the light from the first glass layer 3 toward the first base material 1, the FOV of the display screen is widened compared to the case where the first glass layer 3 is not provided. In particular, in the present embodiment, by arranging the first glass layer 3 and the second glass layer 5 on the hologram layer 4, the diffusion position of the external light and the image light approaches the diffraction position of the hologram layer 4 constituting the display screen. For this reason, a clearer image can be observed from a wider range of angles compared to the case where the first glass layer 3 and the second glass layer 5 are provided at positions distant from the hologram layer 4.

[0035] An example of a method for measuring the luminance value and FOV in the light guide plate 8 for image display will be briefly described. FIG. 2 is a schematic front view for explaining a method for measuring the luminance value and FOV.

[0036] As shown in FIG. 2, to measure the luminance value and FOV of the light guide plate 8 for image display, a display device 10 is manufactured using the light guide plate 8 for image display. The display device 10 includes an image light projection unit 18 and an incident optical system 19 in addition to the light guide plate 8 for image display. The image light projection unit 18 projects image light to be displayed on the light guide plate 8 for image display according to an image signal sent from a controller (not shown). The incident optical system 19 includes, for example, a prism. The incident optical system 19 makes the image light emitted from the image light projection unit 18 enter an incident portion 8a provided on the surface of the light guide plate 8 for image display. For example, the incident portion 8a is provided on the surface on the side of the first base material 1. The image light incident on the incident portion 8a reaches the display diffraction grating portion 4c of the hologram layer 4 via the waveguide diffraction grating portion 4b formed in the hologram layer 4. In the display diffraction grating portion 4c, the image light is diffracted at positions corresponding to the respective display pixels. The diffracted light is emitted to the outside from a display portion 8d on the surface of the light guide plate 8 for image display. In the example shown in FIG. 2, the display portion 8d is formed at a position spaced apart from the incident portion 8a on the surface on the side of the first base material 1.

[0037] The luminance value and FOV of the light guide plate 8 for image display are measured by placing the display device 10 on the measuring device 15. The measuring device 15 includes a holding table (not shown), a luminance meter 14, and a goniostage (not shown). The holding table holds the display device 10. The luminance meter 14 measures the luminance value of the received light. The goniostage supports the luminance meter 14 so that it can swing on a circumference centered on its rotation center. The distance d between the luminance meter 14 and the display surface 4a is a distance corresponding to the position of the user's eyes when the display device 10 is mounted. For example, when the display device 10 is a head-mounted display, the distance d is set to 15 mm.

[0038] The luminance value of the light guide plate 8 for image display is measured by placing the luminance meter 14 at a position where the swing angle θ = 0° (see the luminance meter 14 on the solid line O in Fig. 2). The display device 10 is placed by the holding table at a position where the center of the display surface 4a faces the luminance meter 14 on the measurement optical axis of the luminance meter 14. The luminance value is the luminance measured by the luminance meter 14 when the display device 10 displays a white image with the maximum luminance. In the measurement of the FOV of the light guide plate 8 for image display, with a white image with the maximum luminance displayed on the display device 10, the luminance is measured while changing the swing angle θ. Using the luminance corresponding to the disappearance of the white image as the threshold value, the angular range in which the luminance equal to or higher than the threshold value is obtained is determined. When the luminance equal to or higher than the threshold value is obtained in the range from -θ1 to +θ2, the FOV is θ1 + θ2. The FOV of the light guide plate 8 for image display is preferably 30 to 160°. Also, the larger the FOV, the wider the viewing angle of the image, the larger the amount of image information, and the wider the range of applicable uses. When the FOV is smaller than the upper limit value of the above range, the amount of light extracted for a certain area tends to increase, and the image becomes brighter, which is preferable.

[0039] For example, the closer the distance between the first glass layer 3 and the second glass layer 5 and the hologram layer 4, the more the FOV can be improved by about 10° or more. The distance is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 100 nm or less.

[0040] As described above, according to the present embodiment, it is possible to provide a light guide plate for image display that can suppress deterioration of the hologram layer.

[0041] In the above embodiment, an example in which the glass layers are disposed on the front and back surfaces of the hologram layer has been described. However, for the purpose of shielding moisture or the like that penetrates through the base material and for the purpose of preventing contact between the hologram layer and the base material, the glass layer may be disposed between the base material and the hologram layer via an intermediate layer. However, in this case, if the hygroscopicity of the intermediate layer sandwiched between the glass layer and the hologram layer is high, there is a risk that moisture will penetrate through the side surface of the intermediate layer. Therefore, it is more preferable that the intermediate layer between the glass layer and the hologram layer is formed of a material with low hygroscopicity. When the intermediate layer between the glass layer and the hologram layer has hygroscopicity, it is more preferable to reduce the thickness of the intermediate layer. In this case, since the exposed area of the side surface of the intermediate layer that can be a penetration port for moisture is reduced, the amount of moisture absorbed can be reduced.

[0042] <Second Embodiment> A light guide plate for image display according to a second embodiment of the present invention will be described. The light guide plate for image display of the present embodiment has a distal glass layer, a base material, a proximal glass layer, and a hologram layer. However, the distal glass layer is a glass layer located on the side opposite to the hologram layer side of the base material, and the proximal glass layer is a glass layer located between the hologram layer and the base material. In the light guide plate for image display of the present embodiment, the distal glass layer, the base material, the proximal glass layer, and the hologram layer are arranged in this order in the thickness direction. An adhesive layer may be disposed between the distal glass layer and the base material, and between the base material and the proximal glass layer, respectively, as necessary. From the viewpoint of suppressing damage to the surface of the light guide plate for image display, it is preferable that the distal glass layer is the outermost layer on at least one side of the light guide plate for image display, and more preferably the outermost layer on both sides of the light guide plate for image display. The proximal glass layer may be disposed on only one surface of the hologram layer, but it is preferably disposed on both surfaces of the hologram layer. When the proximal glass layer is disposed on both surfaces of the hologram layer, deterioration of the hologram layer can be more reliably suppressed.

[0043] One or more transparent layers may be disposed on at least one surface of the distal glass layer. One or more transparent layers may be disposed between the distal glass layer and the base material, and between the proximal glass layer and the hologram layer, respectively. Examples of the type of the transparent layer include a hard coat layer or an anchor coat layer.

[0044] Hereinafter, based on FIG. 3, a detailed configuration of an example of the light guide plate for image display according to the present embodiment will be described. FIG. 3 is a schematic cross-sectional view showing an example of the light guide plate for image display according to the second embodiment of the present invention. In the light guide plate 17 for image display shown in FIG. 3, in the thickness direction, a third glass layer 9, a third adhesive layer 11, a first base material 1, a first adhesive layer 2, a first glass layer 3, a hologram layer 4, a second glass layer 5, a second adhesive layer 6, a second base material 7, a fourth adhesive layer 12, and a fourth glass layer 13 are arranged in this order.

[0045] The light guide plate 17 for image display according to the present embodiment is the same as the first embodiment except that the third glass layer 9 and the fourth glass layer 13, and the third adhesive layer 11 and the fourth adhesive layer 12 are provided outside the first base material 1. Therefore, the configurations of the first base material 1, the first adhesive layer 2, the first glass layer 3, the hologram layer 4, the second glass layer 5, the second adhesive layer 6, and the second base material 7 in the present embodiment are the same as those exemplified in the descriptions of the first base material 1, the first adhesive layer 2, the first glass layer 3, the hologram layer 4, the second glass layer 5, the second adhesive layer 6, and the second base material 7 in the first embodiment, respectively. Hereinafter, the third glass layer 9 and the fourth glass layer 13, and the third adhesive layer 11 and the fourth adhesive layer 12 will be described.

[0046] The third glass layer 9 and the fourth glass layer 13 are arranged at the outermost part in the thickness direction of the light guide plate 17 for image display, and can prevent the surface of the light guide plate 17 for image display from being damaged and protect the first base material 1 from the gas outside the light guide plate 17 for image display. For the configurations of the third glass layer 9 and the fourth glass layer 13, the same configurations as those exemplified in the description of the first glass layer 3 in the first embodiment of the present invention are used. However, the materials, thicknesses, etc. of the third glass layer 9 and the fourth glass layer 13 may all be different from those of the first glass layer 3.

[0047] In the light guide plate 17 for image display of the present embodiment, if the adhesion between the third glass layer 9 and the first base material 1 and the adhesion between the second base material 7 and the fourth glass layer 13 are good, the third adhesive layer 11 and the fourth adhesive layer 12 are not essential respectively. However, especially when the resin materials described above are used as the first base material 1 and the second base material 7, in order to maintain excellent adhesion with the third glass layer 9 and the fourth glass layer 13, it is preferable to adopt the third adhesive layer 11 and the fourth adhesive layer 12. For the configurations of the third adhesive layer 11 and the fourth adhesive layer 12, the same configurations as those exemplified in the description of the first adhesive layer 2 in the first embodiment of the present invention are used. However, the materials, thicknesses, etc. of the third adhesive layer 11 and the fourth adhesive layer 12 may all be different from those of the first adhesive layer 2.

Examples

[0048] Hereinafter, Example 1 of the embodiment will be described together with Example 2 and Comparative Example 1. The present invention is not limited to the examples described below, and various modifications are possible without departing from the gist of the present invention.

[0049] Table 1 shows the materials and plate thicknesses of the base materials (resin base materials) used in the examples and comparative examples, the arrangement of the glass layers (thin glass plates), and the evaluation results (evaluations).

[0050]

Table 1

[0051] [Example 1] Example 1 is an example corresponding to the light guide plate 8 for image display of the first embodiment (see FIG. 1). As shown in Table 1, acrylic resin (PMMA) is used as the material of the first substrate 1 and the second substrate 7 (referred to as "resin substrate" in Table 1). The shapes of the first substrate 1 and the second substrate 7 are both rectangular plates of 60 mm × 60 mm × 1 mm. The hologram layer 4 is common to the examples and comparative examples. As the photosensitive material for forming the hologram layer 4, 100 parts by weight of bisphenol-based epoxy resin jER (registered trademark) 1007 (degree of polymerization n = 10.8, epoxy equivalent: 1750 - 2200, manufactured by Mitsubishi Chemical Corporation), 50 parts by weight of triethylene glycol diacrylate, 5 parts by weight of 4,4'-bis(tert-butylphenyl)iodonium hexafluorophosphate, and 0.5 parts by weight of 3,3'-carbonylbis(7-diethylamino)coumarin are used. A resin composition obtained by mixing and dissolving them in 100 parts by weight of 2-butanone is used. The thickness of the hologram layer 4 is 5 μm. The size of the hologram layer 4 in plan view is 50 mm × 50 mm.

[0052] Next, the manufacturing process of the light guide plate 8 for image display in Example 1 will be described. The light guide plate 8 for image display is manufactured by performing the following substrate preparation process, thin glass bonding process, and light guide plate manufacturing process in this order.

[0053] <Substrate Preparation Process> In the substrate preparation process, the first substrate 1 and the second substrate 7 are washed and dried. Hereinafter, when there is no need to distinguish between the first substrate 1 and the second substrate 7, they are simply referred to as substrates. The substrate is ultrasonically cleaned for 5 minutes while being immersed in a 5% surfactant aqueous solution of semiclean (registered trademark) M-LO (manufactured by Yokohama Oil & Fat Co., Ltd.), which is a neutral cleaning agent. Thereafter, the substrate is ultrasonically cleaned for 5 minutes while being immersed in ultrapure water. Further, the substrate is rinsed with ultrapure water, and after air drying, it is dried in an oven at 100°C under a nitrogen atmosphere. Thereafter, the air-dried evaluation sample B is UV-ozone cleaned in a UV-ozone cleaner for 1 minute. Thus, the substrate preparation process is completed.

[0054] <Glass layer adhesion process> In the glass layer adhesion process, the first glass layer 3 is adhered to the surface of the first substrate 1 via the first adhesive layer 2. On the first substrate 1, a thermally and ultraviolet curable epoxy resin (trade name "KRX-690-5", manufactured by ADEKA Corporation) is applied as an uncured product of the first adhesive layer 2 so that the cured thickness is 3 μm with a bar coater. Further, as the first glass layer 3, a thin plate glass (trade name "OA-10G", thickness: 50 μm, manufactured by Nippon Electric Glass Co., Ltd.) is laminated on the first substrate 1 with a hand roll (hardness: 90°) facing the coating surface of the first adhesive layer 2. A high-pressure mercury lamp (integrated light quantity: 370 mJ / cm 2 ) is irradiated, and then, by heat treatment at 150°C for 30 minutes in a hot air circulation dryer, the thermally and ultraviolet curable resin composition is cured to obtain the first adhesive layer 2. The first glass layer 3, the first adhesive layer 2, and the first substrate 1 are arranged so that the outer shape ranges (edge portions) match. Thus, the glass layer adhesion process is completed. Hereinafter, the substrate with the glass layer disposed thereon is referred to as an intermediate laminate.

[0055] <Light guide plate manufacturing process> In the light guide plate manufacturing process, the light guide plate 8 for image display is manufactured using two intermediate laminates. A seal layer having a width of 5 mm and a thickness of 5 μm is applied to the peripheral edge portion of the glass layer of one of the intermediate laminates. As the seal layer, Photo Adhesive Hard Lock (registered trademark) OP-1045K (manufactured by Denki Kagaku Kogyo Co., Ltd.) is used. Thereby, a seal layer step intermediate laminate having an opening surrounded by the seal layer with a size of 50 mm × 50 mm is prepared. Thereafter, on this intermediate laminate, the photosensitive material described above as the hologram photo-polymer material is applied by spin coating. The photosensitive material is applied so that its thickness after drying becomes 5 μm. Thereafter, the other intermediate laminate is laminated on the seal layer and the photosensitive material such that its glass layer faces the glass layer of the intermediate laminate with the seal layer, and press-bonded under reduced pressure. The conditions for press-bonding are an absolute pressure of 5 kPa, a temperature of 70 °C, and a press pressure of 0.04 MPa. Thereafter, a diffraction grating is recorded on the photosensitive material of the press-bonded laminate. In this step, the temperature of the laminate is maintained at 20 °C. The diffraction grating forms interference fringes on the laminate by irradiating the laminate with two laser beams and adjusting the respective irradiation angles and intensities so that a necessary diffraction pattern is formed. Thereby, the diffraction grating is recorded on the photosensitive material. This diffraction grating diffracts each of the lights in the wavelength regions of red, green, and blue that are incident as image light incident on the incident portion, and is a color display diffraction grating that is emitted from the display portion at positions corresponding to the pixels of the image light. Thereafter, while maintaining the laminate at 20 °C, ultraviolet light (wavelength 365 nm, irradiance 80 W / cm 2 ) is irradiated onto the entire surface from one side direction of the laminate for 30 seconds. As the light source of the ultraviolet light, a high-pressure mercury lamp is used. Thereby, the seal layer is cured, and the light guide plate 8 for image display of Example 1 is formed.

[0056] [Example 2] Example 2 is an example corresponding to the light guide plate 17 for image display of the second embodiment (see FIG. 3). As shown in Table 1, Example 2 is the same as Example 1 except that a glass layer is disposed on the outermost surface. Hereinafter, the description will focus on the differences from Example 1.

[0057] The light guide plate 17 for image display of Example 2 has a third glass layer 9, a third adhesive layer 11, a first base material 1, a first adhesive layer 2, a first glass layer 3, a hologram layer 4, a second glass layer 5, a second adhesive layer 6, a second base material 7, a fourth adhesive layer 12, and a fourth glass layer 13 laminated in this order (see Fig. 3). In Example 2, in the glass layer bonding step described above, the third glass layer 9 and the first glass layer 3 are bonded to the surfaces of the first base material 1 via the third adhesive layer 11 and the first adhesive layer 2 respectively to obtain an intermediate laminate. Note that the materials, thicknesses, coating methods, and heat treatment conditions of the third adhesive layer 11 and the first adhesive layer 2 are the same as those of the first adhesive layer 2 in Example 1. Using two such obtained intermediate laminates, a light guide plate 17 for image display is manufactured in the same manner as in Example 1. A seal layer is provided on one base material, a photosensitive material is applied, and then the other base material is laminated so that the base materials face each other. Thereafter, a light guide plate 17 for image display is manufactured by performing the same reduced-pressure pressing and diffraction grating formation as in Example 1.

[0058] [Comparative Example 1] Comparative Example 1 is a comparative example corresponding to the light guide plate 16 for image display (see Fig. 4). As shown in Table 1, Comparative Example 1 is the same as Example 1 except that no glass layer is arranged. Hereinafter, the description will focus on the differences from Example 1.

[0059] The light guide plate 16 for image display of Comparative Example 1 is a light guide plate for image display in which a first base material 1, a hologram layer 4, and a second base material 7 are laminated in this order (see Fig. 4). A light guide plate 16 for image display is manufactured in the same manner as in Example 1. A seal layer is provided on one base material, a photosensitive material is applied, and then the other base material is laminated so that the base materials face each other. Thereafter, a light guide plate 16 for image display is manufactured by performing the same reduced-pressure pressing and diffraction grating formation as in Example 1.

[0060] [Evaluation Method] Next, the evaluation methods for each example and each comparative example will be described. As evaluations, luminance value measurement, FOV evaluation, and sharpness evaluation of the displayed image are performed.

[0061] <Luminance value measurement> Samples for measuring the luminance value are prepared in three types: a sample for performing a humidification test (referred to as "after humidification" in Table 1), a sample for performing a heating test (referred to as "after heating" in Table 1), and a sample for which neither a humidification test nor a heating test is performed (referred to as "initial" in Table 1). For the humidification test and the heating test, a small environmental tester SH-241 (product name; manufactured by Espec Corporation) is used. The test conditions for the humidification test are 60°C, 90% RH, and 500 hours. The test conditions for the heating test are 85°C and 500 hours. The luminance value of the measurement sample is performed based on the measurement method described above in the embodiment. Each measurement sample is assembled into the above-described display device, respectively. As the luminance meter 14, a luminance meter BM-8 (product name; manufactured by Topcon Corporation) is used. The measurement angle is 1°. The distance d from the display surface 3a is 15 mm. When the luminance value is 3000 nit or more, it is determined to be good (good, described as "○" in Table 1). When the luminance value is 1000 nit or more and less than 3000 nit, it is determined to be fair (fair, described as "△" in Table 1). When the luminance value is less than 1000 nit, it is determined to be no good (no good, described as "×" in Table 1).

[0062] <FOV measurement> The FOV measurement is performed using a display device assembled with a sample for which neither a humidification test nor a heating test is performed among the samples for measuring the luminance value. The measurement of the FOV is performed based on the measurement method described above in the embodiment. As the luminance meter 14, a luminance meter BM-8 (product name; manufactured by Topcon Corporation) is used. The measurement angle is 1°. The distance d from the display surface 3a is 15 mm. When the FOV is 45° or more, it is determined to be very good (very good, described as "◎" in Table 1). When the FOV is 35° or more and less than 45°, it is determined to be good (good, described as "○" in Table 1). When the FOV is 24° or more and less than 35°, it is determined to be fair (fair, described as "△" in Table 1). When the FOV is less than 24°, it is determined to be no good (no good, described as "×" in Table 1).

[0063] <Evaluation of the sharpness of the displayed image> The sharpness of the displayed image is evaluated using the display device used for FOV measurement. As the input images used for evaluation, a white image and a character display image are used. The evaluation is performed by visually determining the appearance of the white image and the character display image. As the character image, "ABCDE" within 10 mm × 100 mm is displayed. When no rainbow color is visible in the white image and the characters are clearly visible in the character display image, it is determined to be good (good, described as "○" in Table 1). When a slight rainbow color is visible in the white image and the characters are clearly visible in the character display image, it is determined to be fair (fair, described as "△" in Table 1). When rainbow color is visible in at least a part of the white image and the outlines of the characters are blurred in the character display image, it is determined to be no good (no good, described as "×" in Table 1).

[0064] [Evaluation results] As shown in Table 1, the luminance values of Examples 1 and 2 are all evaluated as "good" regardless of the presence or absence of the humidification test and the heating test. In contrast, the luminance value of Comparative Example 1 is evaluated as "fair" when neither the humidification test nor the heating test is performed, but all are evaluated as "no good" in the measurement samples after either the humidification test or the heating test. The reason for this is that in Examples 1 and 2, the thin glass plate prevents the penetration of moisture into the hologram layer, and as a result, the hologram layer is suppressed from eroding the resin substrate. Therefore, the diffraction performance of the hologram layer is good, and there is no disturbance in the optical path in the resin substrate. On the other hand, in Comparative Example 1, all the luminance values are inferior to those in Examples 1 and 2. In the case of Comparative Example 1, the penetration of moisture from the resin substrate cannot be prevented. Therefore, it is considered that the decrease in the luminance value after the humidification test in Comparative Example 1 is caused by the penetration of moisture from the outside into the hologram layer, resulting in the deterioration of the hologram layer 4 and the impairment of the diffraction performance. In particular, the decrease in the luminance value after the heating test in Comparative Example 1 is mainly considered to be due to the fact that in the heating test, the measurement sample is exposed to a higher temperature than in the humidification test. That is, when heated, the material of the hologram layer in contact with the resin substrate erodes the resin substrate, so that the interface between the hologram layer and the resin substrate is deformed or the optical properties of the resin substrate deteriorate. As a result, it is considered that the luminance value deteriorates due to an increase in the scattering and absorption of the transmitted light of the resin substrate or a change in the optical path. In the case of Examples 1 and 2, since the hologram layer is not in contact with the resin substrate, even when heated, the material of the hologram layer does not erode the resin substrate.

[0065] As shown in Table 1, the FOV of Example 1 is evaluated as "very good" and "good", respectively. On the other hand, the FOV of Comparative Example 1 is evaluated as acceptable. The reason for this is considered to be the same as the reason for the difference in the luminance value evaluation described above. That is, in Examples 1 and 2, the glass layer prevents the penetration of moisture into the hologram layer and the erosion of the resin substrate, and as a result, no decrease in the FOV is observed. On the other hand, in Comparative Example 1, the penetration of moisture into the hologram layer and the erosion of the resin substrate occur, resulting in a decrease in the FOV.

[0066] The sharpness of the display images in Examples 1 and 2 (referred to as "sharpness" in Table 1) is determined to be "good". In contrast, the sharpness of the display images in Comparative Example 1 is all determined to be "unacceptable (×)". This reason is considered to be the same as the reason for the difference in the luminance value evaluation described above. That is, in Examples 1 and 2, as a result of the glass layer preventing the penetration of moisture into the hologram layer and the erosion of the resin base material, clear images are observed. In contrast, in Comparative Example 1, as a result of the penetration of moisture in the hologram layer and the erosion of the resin base material, clear images are not observed.

[0067] As described above, the preferred embodiments and examples of the present invention have been described, but the present invention is not limited to these embodiments and examples. Additions, omissions, substitutions, and other changes to the configuration are possible without departing from the spirit of the present invention. Also, the present invention is not limited by the foregoing description and is limited only by the appended claims.

Industrial Applicability

[0068] The light guide plate for image display of the present invention can suppress the deterioration of the hologram layer and is useful, for example, for display device applications of VR and AR applications. For example, the light guide plate for image display of the present invention is useful for display device applications such as head-up displays, wearable displays, and head-mounted displays.

Explanation of Reference Numerals

[0069] 1... First base material (base material), 2... First adhesive layer, 3... First glass layer (glass layer), 4... Hologram layer, 4a... Display surface, 4b... Waveguide diffraction grating portion, 4c... Diffraction grating portion for display, 5... Second glass layer, 6... Second adhesive layer, 7... Second base material, 8, 16, 17... Light guide plate for image display, 8a... Incident portion, 8d... Display portion, 9... Third glass layer, 10... Display device, 11... Third adhesive layer, 12... Fourth adhesive layer, 13... Fourth glass layer, 14... Luminance meter, 15... Measuring device, 18... Image light projection portion, 19... Incident optical system, d... Distance, θ... Swing angle, O... Solid line

Claims

1. It has a distal glass layer, a substrate, a proximal glass layer, and a hologram layer, and the distal glass layer, the substrate, the proximal glass layer, and the hologram layer are arranged in this order in the thickness direction. A light guide plate for image display, wherein the material of the proximal glass layer is selected from the group consisting of borosilicate glass, alkali-free glass, low-alkali glass, soda-lime glass, and sol-gel glass. However, the distal glass layer is a glass layer located on the side opposite to the hologram layer side of the substrate, and the proximal glass layer is a glass layer located between the hologram layer and the substrate.

2. The light guide plate for image display according to claim 1, wherein the distal glass layer is at least one outermost layer of the light guide plate for image display.

3. The light guide plate for image display according to claim 1 or 2, wherein the proximal glass layer is disposed on the hologram layer.

4. The light guide plate for image display according to any one of claims 1 to 3, wherein an adhesive layer is disposed between the distal glass layer and the substrate.

5. The light guide plate for image display according to any one of claims 1 to 4, wherein an adhesive layer is disposed between the substrate and the proximal glass layer.

6. The light guide plate for image display according to any one of claims 1 to 5, wherein the material of the proximal glass layer is alkali-free glass.

7. The light guide plate for image display according to any one of claims 1 to 6, wherein the thickness of the proximal glass layer is 10 μm or more and 100 μm or less.

Images