JP7582541B2 - Volume hologram, Exit pupil expansion element, Head-mounted display device - Google Patents

Description

The present invention relates to a volume hologram, an exit pupil expansion element, and a head-mounted display device.

In a close-proximity display such as a head-mounted display, one of the mechanisms for displaying an image is to expand the exit pupil (see, for example, Patent Document 1). The exit pupil expansion element used for this expansion of the exit pupil is configured with a waveguide and two or three types of diffractive optical elements, and the diffractive optical element at the entrance diffracts light such as an image, causing the first-order diffracted light to enter the waveguide. Within the waveguide, the light propagates by total reflection, and at the diffractive optical element at the exit, the light undergoes first-order diffraction and exits, and also undergoes zero-order diffraction reflection and propagation, and the exit pupil is expanded by repeating the first-order diffraction emission and zero-order diffraction reflection and propagation at the diffractive optical element at the exit.

In such an exit pupil expansion element, a relief-type diffractive optical element is used as described in Patent Document 1, but if a volume hologram can be used, particularly for the light entrance portion, it is possible to improve the light utilization efficiency.
However, volume holograms tend to be highly wavelength selective, and if the usable light band is not appropriately selected, when using a broadband light source such as an LED, some light will not be diffracted and will not be used, which can reduce the light utilization efficiency.
Furthermore, even when a laser light source or the like is used, if the wavelength selectivity of the volume hologram is too high, the desired diffraction efficiency may not be obtained due to temperature changes, and only dark images may be observed, or the color balance of the observed image may be shifted.

Special Publication No. 2006-510059

The objective of the present invention is to provide a volume hologram, an exit pupil expansion element, and a head-mounted display device that can increase the efficiency of light utilization.

The present invention solves the above problems by the following means. Note that, for ease of understanding, the following description will use symbols corresponding to the embodiments of the present invention, but the present invention is not limited to these.

The first invention is a volume hologram (21, 21B) placed on the entrance section (21, 21B) of an exit pupil expansion element that includes a flat waveguide section (10a) that guides light, an entrance section (21, 21B) that introduces light into the waveguide section (10a), and an expansion section (22) that is placed on the waveguide section (10a) or is integrally formed with the waveguide section (10a) and expands the exit pupil by dividing it into a plurality of sections, the volume hologram (21, 21B) having a half-width of 5 nm or more in a spectral distribution curve.

The second invention relates to the volume hologram (21, 21B) according to the first invention, wherein when the film thickness of the volume hologram (21, 21B) is t (μm) and the refractive index modulation amount is Δn,
The volume hologram (21, 21B) is characterized in that the relationship Δn≧5.87×10 ⁻³ ×ln(t)−1.29×10 ⁻² is satisfied.

A third invention is a volume hologram (21, 21B) according to the first or second invention, characterized in that, when the film thickness of the volume hologram (21, 21B) is t (μm) and the refractive index modulation amount is Δn, the volume hologram (21, 21B) satisfies the relationship 0.0200t ^-1.00 ≦Δn≦0.395t ^-1.25 .

A fourth invention is a volume hologram (21, 21B) according to any one of the first to third inventions, characterized in that, when the diffraction angle of light incident on the volume hologram (21, 21B) at an incident angle θ ₁ is θ ₂ , the volume hologram (21, 21B) satisfies both of the relationships θ ₂ ≧-0.326θ ₁ +100 and θ ₂ ≦0.159θ ₁ +76.8 within the ranges of -45°≦θ ₁ ≦45° and ₄₅ °≦θ 2 ≦135°.

The fifth invention is an exit pupil expansion element including a volume hologram (21, 21B) according to any one of the first to fourth inventions in the entrance portion (21, 21B).

The sixth invention is a head-mounted display device that includes a volume hologram (21, 21B) according to any one of the first to fourth inventions in the entrance portion (21, 21B) and allows the display to be superimposed on a see-through field of view.

The present invention provides a volume hologram, an exit pupil expansion element, and a head-mounted display device that can increase the efficiency of light utilization.

1 is a diagram showing an overview of an embodiment of a head-mounted display device 1 using a volume hologram according to the present invention. 2 is a plan view of an exit pupil expansion element 40 according to the present invention. 3 is a cross-sectional view of the exit pupil expansion element 40 taken along the line indicated by the arrows A-A in FIG. 2. 13 is a diagram showing an exit pupil expansion element 40B configured to expand the exit pupil in two-dimensional directions. FIG. 4 is a diagram similar to FIG. 3 showing an exit pupil expansion element 40C in the case where a reflection type volume hologram 21B (entrance portion 21B) is arranged. 1 is a diagram showing an example of the spectral distribution curve of a light source and the distribution of the diffraction efficiency of a volume hologram when the half-value width of the efficiency of the volume hologram is 4 nm. 1 is a diagram showing an example of the spectral distribution curve of a light source and the distribution of the diffraction efficiency of a volume hologram when the half-width of the efficiency of the volume hologram is 10 nm. FIG. 13 is a diagram showing an example of a volume hologram peak shift due to temperature change when the half-width of the efficiency of the volume hologram is 4 nm. FIG. 13 is a diagram showing an example of a shift in the peak of a volume hologram due to a change in temperature when the half-width of the efficiency of the volume hologram is 10 nm. 13 is a diagram showing the distribution of diffraction efficiency versus wavelength when the wavelength selectivity of the volume hologram 21 is very high. FIG. 11 is a diagram showing an example of the distribution of diffraction efficiency at 20° C. of the volume hologram 21 of this embodiment. 13 is a diagram showing the distribution of diffraction efficiency at 40° C. of the volume hologram 21 of this embodiment. FIG. 13 is a diagram showing the distribution of diffraction efficiency at 0° C. of the volume hologram 21 of the present embodiment. FIG. 13 is a diagram showing distribution curves of diffraction efficiency when the half-width is 5 nm, 10 nm, and 20 nm. FIG. 13 is a diagram showing simulation conditions. FIG. 11 is a diagram showing a summary of the results of a simulation in which the half-width is determined by changing the film thickness t and Δn. 1 is a graph showing an approximation formula showing a preferable combination range of film thickness t and Δn obtained from the half-width. 13 is a diagram summarizing the results of a simulation in which the film thickness t and Δn are changed to obtain the maximum value of the diffraction efficiency. 1 is a graph showing an approximation formula showing a preferable combination range of film thickness t and Δn obtained from the maximum value of diffraction efficiency. FIG. 13 is a diagram showing simulation conditions focusing on the incident angle and the exit angle. 11 is a diagram showing a summary of the results of a simulation in which the half-width is obtained by changing the incident angle θ ₁ and the exit angle θ ₂ . 1 is a graph showing an approximation formula indicating a preferable combination range of an incident angle θ ₁ and an exit angle θ ₂ , which is determined from a half-width. 11 is a diagram showing a summary of the results of a simulation in which the maximum value of diffraction efficiency is obtained by changing the incident angle θ ₁ and the exit angle θ ₂ . 1 is a graph showing an approximation formula showing a preferable combination range of an incident angle θ ₁ and an exit angle θ ₂ obtained from a maximum value of diffraction efficiency.

The best mode for carrying out the present invention will be described below with reference to the drawings.

(Embodiment)
FIG. 1 is a diagram showing an outline of an embodiment of a head-mounted display device 1 using a volume hologram according to the present invention.
Note that the figures shown below, including FIG. 1, are schematic views, and the size and shape of each part are appropriately exaggerated or omitted to facilitate understanding.
In the following description, specific numerical values, shapes, materials, etc. are given, but these can be changed as appropriate.
In this specification, the terms plate, sheet, film, etc. are used, and in general usage, they are used in the order of thickness, plate, sheet, film, etc., and this specification follows that order. However, since such distinction in usage has no technical meaning, these terms can be used interchangeably as appropriate.
In this specification, the sheet surface refers to the surface of each sheet that is in the planar direction of the sheet when viewed as a whole.
It should be noted that the specific numerical values specified in this specification and claims should be treated as including a general error range. In other words, a difference of about ±10% means that there is no substantial difference, and values set within a range slightly exceeding the numerical range of this invention should be interpreted as being substantially within the scope of this invention.

The head-mounted display device 1 includes an image source 50 and an exit pupil expansion element 40, which expands the image light from the image source 50 by dividing the exit pupil into multiple parts and outputs it to the observer's eye E. The head-mounted display device 1 can be attached to the head, for example, in the form of glasses or a head mount, but details of these will be omitted.

The image source 50 irradiates image light to the entrance portion 21 of the exit pupil expansion element 40. For example, a microdisplay can be used as the image source 50. As the light source of the image source 50, a narrow-band light source such as a laser light source or a wide-band light source such as an LED can be used.

FIG. 2 is a plan view of an exit pupil expanding element 40 according to the present invention.
FIG. 3 is a cross-sectional view of the exit pupil expansion element 40 taken along the line indicated by the arrows AA in FIG.
The exit pupil expansion element 40 of the present embodiment is a sheet-like element having an optical function of expanding the exit pupil by dividing it into a plurality of parts, and is configured by laminating a base material layer 10 and a pattern-imparting layer 20. The exit pupil expansion element 40 may be configured in a film or plate shape.
The base layer 10 is used as a base when manufacturing the exit pupil expansion element 40, and is a layer that constitutes a major part of the waveguide section through which light is guided.
The shape-imparting layer 20 is laminated on the surface side of the base material layer 10. The shape-imparting layer 20 has an incident portion 21 and an exit portion 22 on its surface.

The base layer 10 and the shape-imparting layer 20 constitute a waveguide (core) 10a for guiding light such as image light incident from the incident portion 21 toward the exit portion 22. Therefore, the base layer 10 and the shape-imparting layer 20 are made of materials having substantially the same or very similar refractive indexes for the guided light. In addition, the region between the incident portion 21 and the exit portion 22 is a region that guides (propagates) light, and is therefore referred to as the waveguide 10a. This waveguide 10a guides light by repeatedly reflecting it inside.
Examples of materials constituting the base layer 10 include PET, polycarbonate, acrylic resin, glass, etc. Examples of materials constituting the shape-imparting layer 20 include ultraviolet-curable resin materials such as acrylic ultraviolet-curable resins, etc. Both of the materials constituting the base layer 10 and the materials constituting the shape-imparting layer 20 are transparent to the guided light.

In this embodiment, the base layer 10 and the mold-imparting layer 20 form the core (waveguide section 10a), and the air (atmosphere) surrounding them forms the clad, so that light can be guided using the base layer 10 and the mold-imparting layer 20 as the waveguide section 10a. Therefore, the material used for the base layer 10 and the mold-imparting layer 20 has a higher refractive index than the air (atmosphere) that forms the clad. This allows light to be efficiently guided while being reflected at the interface between the core formed by the base layer 10 and the mold-imparting layer 20 and the clad formed by the air (atmosphere).

The entrance portion 21 is configured by attaching a volume hologram onto the base layer 10 or onto the pattern-imparting layer 20. In the following description, the entrance portion 21 will also be referred to as the volume hologram 21.
Light that travels perpendicular to the sheet surface of the exit pupil expansion element 40 and enters the entrance portion 21 is diffracted by the volume hologram 21 of the entrance portion 21, and the diffracted light travels into the base material layer 10 (waveguide portion 10a). The volume hologram 21 is configured so that the diffraction angle of this diffracted light satisfies the total reflection condition of the waveguide portion 10a. The volume hologram 21 of the entrance portion 21 will be described in detail later.

The light that has been guided through the waveguide 10a reaches the exit portion 22. As shown in Fig. 3, the exit portion 22 has a relief-type diffraction grating. The diffraction grating of the exit portion 22 causes the light that has been guided through the waveguide 10a to exit from the exit portion 22.
The diffraction grating of the emission section 22 is composed of, for example, a three-level diffraction grating. The diffraction grating of the emission section 22 is repeatedly arranged at a constant pitch P and has high refractive index portions 221 extending in one direction perpendicular to the direction of the arrangement.

Since the diffraction grating of the exit section 22 does not have a diffraction efficiency of 100%, only a portion of the light that first reaches the diffraction grating of the exit section 22 is emitted, and the remaining light continues to be totally reflected and travels further downstream in the waveguiding direction (the direction away from the entrance section 21, the right side in FIG. 3) and reaches the diffraction grating of the exit section 22 again. Only a portion of the light that reaches the diffraction grating of the exit section 22 again is also emitted, and the remaining light continues to be totally reflected. By repeating this, multiple exit pupils are formed, and the exit pupil can be expanded. In this way, in this embodiment, the exit section 22 also functions as an expansion section that expands the exit pupil. By expanding (splitting) the exit pupil, any of the exit pupils can be observed even if the observation position (eye position) moves, and usability can be improved without being limited to a specific eye position. Note that in FIG. 3, the exit pupil is simplified to be divided into three exit pupils, but in reality, it is expanded (divided) into more exit pupils.

FIG. 4 is a diagram showing an exit pupil expansion element 40B configured to expand the exit pupil in two-dimensional directions.
The exit pupil extension element 40 described above is an element that extends the exit pupil only in one direction (one-dimensional direction: the longitudinal direction of the exit pupil extension element 40 in FIG. 2). Alternatively, an exit pupil extension element 40B that extends the exit pupil in two orthogonal directions (two-dimensional directions) may be used.
Like the previously described exit pupil extension element 40, the exit pupil extension element 40B includes a base layer 10 and a patterning layer 20. The patterning layer 20 of the exit pupil extension element 40B includes an entrance portion 21, a second extension portion 23, and an exit portion 22B on its surface.

The entrance portion 21 is composed of a volume hologram 21, similar to the exit pupil expansion element 40 described above.
The second extension section 23 is composed of a two-level diffraction grating, and the high refractive index section 231 is arranged to extend in a direction inclined by 45° with respect to the direction in which the light guided from the incident section 21 travels. The second extension section 23 deflects the direction in which the light incident from the incident section 21 travels by 90° and advances it to the exit section 22B, and expands the exit pupil in a direction intersecting with the direction in which the exit section 22 (first extension section) expands the light by 90°. Note that, although FIG. 4 shows the exit pupil expanded to three, in reality, the exit pupil is divided and expanded into a number of lines according to the number of repeated reflections in the second extension section 23.

Emission section 22B differs from emission section 22 of exit pupil expansion element 40 described above in that it is located at a different position and that the arrangement direction of high refractive index section 221B is different. Light deflected and expanded by second expansion section 23 reaches emission section 22B, and similar to emission section 22 of exit pupil expansion element 40 described above, emission section 22B divides and expands the exit pupil while sequentially emitting light. The direction in which emission section 22B expands the exit pupil is perpendicular to the direction in which second expansion section 23 expands the exit pupil.

In the above example, a transmissive volume hologram is used as the incident portion 21 (volume hologram 21), but a reflective volume hologram can also be used as the incident portion that introduces light into the waveguide portion.
FIG. 5 is a diagram similar to FIG. 3, showing an exit pupil expansion element 40C in the case where a reflection type volume hologram 21B (entrance portion 21B) is arranged.

In the head-mounted display device 1 of this embodiment, whether the above-mentioned exit pupil extension element 40, exit pupil extension element 40B, or exit pupil extension element 40C is used, the volume holograms 21 and 21B are used for the entrance portions 21 and 21B. This is because the volume holograms 21 and 21B can achieve higher diffraction efficiency than a relief-type diffraction grating, and therefore can take in more image light into the wave guide portion 10a. Below, the volume holograms 21 and 21B are described in more detail. In the following description, the transmission-type volume hologram 21 and the reflection-type volume hologram 21B are not differentiated from each other, and are simply referred to as volume hologram 21.

The photosensitive material for forming the volume hologram 21 may be, for example, a known photosensitive material for volume holograms 21, such as a silver halide material, a dichromated gelatin emulsion, a photopolymerizable resin, or a photocrosslinkable resin.

[Photosensitive material for forming hologram]
The photosensitive material for forming a hologram according to the present invention contains a photopolymerizable monomer, a photopolymerization initiator, a sensitizing dye that sensitizes the photopolymerization initiator, and a binder resin, and the photopolymerizable monomer includes at least a photoradical polymerizable monomer and a photocationic polymerizable monomer.

<Photopolymerizable Monomer>
The photopolymerizable monomer in the present invention is a compound that undergoes a polymerization or dimerization reaction upon irradiation with light and is capable of diffusing and migrating within the hologram recording layer.
Examples of the photopolymerizable monomer in the present invention include photoradical polymerizable monomers, photocationic polymerizable monomers, and photodimerizable compounds, and include at least photoradical polymerizable monomers and photocationic polymerizable monomers.
The photoradical polymerizable monomer and the photocationic polymerizable monomer will be described below.

(Photo-radical polymerizable monomer)
The photoradical polymerizable monomer used in the present invention is not particularly limited as long as it is a compound that is polymerized by the action of an active radical generated from a photoradical polymerization initiator described later, for example, by laser irradiation, when a hologram recording layer is formed using the photosensitive material for hologram formation of the present invention, but it is preferable to use a compound having at least one ethylenically unsaturated double bond in the molecule. For example, unsaturated carboxylic acid, unsaturated carboxylate, ester of unsaturated carboxylic acid and aliphatic polyhydric alcohol compound, amide bond of unsaturated carboxylic acid and aliphatic polyhydric amine compound, etc. can be mentioned.

Examples of the above photoradical polymerizable monomers include methyl methacrylate, hydroxyethyl methacrylate, lauryl acrylate, N-acryloylmorpholine, 2-ethylhexyl carbitol acrylate, isobornyl acrylate, methoxypropylene glycol acrylate, 1,6-hexanediol diacrylate, tetraethylene glycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, acrylamide, methacrylamide, styrene, 2-bromostyrene, phenyl acrylate, 2-phenoxyethyl acrylate, 2,3-naphthalenedicarboxylic acid (acryloxyethyl) monoester, methylphenoxyethyl acrylate, nonylphenoxyethyl acrylate, β- Acryloxyethyl hydrogen phthalate, phenoxy polyethylene glycol acrylate, 2,4,6-tribromophenyl acrylate, diphenic acid (2-methacryloxyethyl) monoester, benzyl acrylate, 2,3-dibromopropyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate, 2-naphthyl acrylate, N-vinylcarbazole, 2-(9-carbazolyl)ethyl acrylate, triphenylmethyl thioacrylate, 2-(tricyclo[5,2,102.6]dibromodecylthio)ethyl acrylate, S-(1-naphthylmethyl)thioacrylate, dicyclopentanyl acrylate, methylenebisacrylamide, polyethylene glycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate acrylate, diphenic acid (2-acryloxyethyl) (3-acryloxypropyl-2-hydroxy) diester, 2,3-naphthalene dicarboxylic acid (2-acryloxyethyl) (3-acryloxypropyl-2-hydroxy) diester, 4,5-phenanthrene dicarboxylic acid (2-acryloxyethyl) (3-acryloxypropyl-2-hydroxy) diester, dibromneopentyl glycol diacrylate, dipentaerythritol hexaacrylate, 1,3-bis[2-acryloxy-3-(2,4,6-tribromophenoxy)propoxy]benzene, diethylenedithioglycol diacrylate, 2,2-bis(4-acryloxyethoxyphenyl)propane, bis(4-acryloxydiethoxyphenyl)methane, bis(4-acryloxyethoxy-3,5-dibromophenyl) Examples of such compounds include methane, 2,2-bis(4-acryloxyethoxyphenyl)propane, 2,2-bis(4-acryloxydiethoxyphenyl)propane, 2,2-bis(4-acryloxyethoxy-3,5-dibromophenyl)propane, bis(4-acryloxyethoxyphenyl)sulfone, bis(4-acryloxydiethoxyphenyl)sulfone, bis(4-acryloxypropoxyphenyl)sulfone, bis(4-acryloxyethoxy-3,5-dibromophenyl)sulfone, and compounds in which the acrylate in the above is replaced with a methacrylate, as well as ethylenically unsaturated double bond-containing compounds containing at least two or more S atoms in the molecule as described in JP-A-2-247205 and JP-A-2-261808. These compounds may be used alone or in combination of two or more.

The average refractive index of the photoradical polymerizable monomer is preferably larger than the average refractive index of the photocationic polymerizable monomer described below, and is preferably 0.02 or more larger. This is because if the difference in the average refractive index between the photoradical polymerizable monomer and the photocationic polymerizable monomer is smaller than the above value, the desired refractive index modulation amount (Δn) may not be obtained.

(Photocationic Polymerizable Monomer)
The photocationically polymerizable monomer used in the present invention is a compound that undergoes cationic polymerization by the Bronsted acid or Lewis acid generated by decomposition of a photocationic polymerization initiator described below upon exposure to energy irradiation. Examples of the monomer include cyclic ethers, thioethers, and vinyl ethers having functional groups such as epoxy groups and oxetane groups.
In addition, when a photoradical polymerizable monomer and a photocationic polymerizable monomer are used in combination, it is preferable that the polymerization of the photoradical polymerizable monomer is carried out in a composition having a relatively low viscosity, and therefore it is preferable that the photocationic polymerizable monomer in the present invention is liquid at room temperature.

Examples of the photocationically polymerizable monomer include diglycerol diether, pentaerythritol polydiglycidyl ether, 1,4-bis(2,3-epoxypropoxyperfluoroisopropyl)cyclohexane, sorbitol polyglycidyl ether, 1,6-hexanediol glycidyl ether, polyethylene glycol diglycidyl ether, and phenyl glycidyl ether.

In the present invention, only one of the above-mentioned photocationically polymerizable monomers may be used, or two or more of them may be used in combination.

The total content of the photopolymerizable monomers in the photosensitive material for forming a hologram of the present invention is preferably 8.5 to 85 parts by mass, and more preferably 8.5 to 70 parts by mass, based on 100 parts by mass of the total solid content of the photosensitive material for forming a hologram. Here, the solid content refers to components other than the solvent, and includes monomers that are liquid at room temperature.
If the total content of the photopolymerizable monomers is less than the above range, a large refractive index modulation amount (Δn) cannot be obtained, and there is a possibility that a volume hologram recording body with high brightness cannot be obtained, whereas if the total content of the photopolymerizable monomers is greater than the above range, there is a relative decrease in the binder resin content, and there is a possibility that the hologram recording layer cannot be maintained.

In addition, in the photopolymerizable monomer in the photosensitive material for forming a hologram of the present invention, the content ratio of the photoradical polymerizable monomer to the photocationic polymerizable monomer is preferably within a range of 30 to 90 parts by mass of the photocationic polymerizable monomer per 100 parts by mass of the photoradical polymerizable monomer, and more preferably within a range of 50 to 80 parts by mass.

<Photopolymerization initiator>
As the photopolymerization initiator constituting the photosensitive material for forming a hologram of the present invention, a photoradical polymerization initiator and a photocationic polymerization initiator can be used.

(Photoradical polymerization initiator)
Examples of the photoradical polymerization initiator used in the present invention include imidazole derivatives, bisimidazole derivatives, N-arylglycine derivatives, organic azide compounds, titanocenes, aluminate complexes, organic peroxides, N-alkoxypyridinium salts, and thioxanthone derivatives. More specifically, 1,3-di(t-butyldioxycarbonyl)benzophenone, 3,3',4,4'-tetrakis(t-butyldioxycarbonyl)benzophenone, 3-phenyl-5-isoxazolone, 2-mercaptobenzimidazole, bis(2,4,5-triphenyl)imidazole, 2,2-di Examples of the methoxy-1,2-diphenylethan-1-one (trade name Irgacure 651, manufactured by BASF Corporation), 1-hydroxy-cyclohexyl-phenyl-ketone (trade name Irgacure 184, manufactured by BASF Corporation), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (trade name Irgacure 369, manufactured by BASF Corporation), bis(η5-2,4-cyclopentadiene-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium (trade name Irgacure 784, manufactured by BASF Corporation), and the like are not limited thereto.

Examples of photocationic polymerization initiators include sulfonic acid esters, imide sulfonates, dialkyl-4-hydroxysulfonium salts, arylsulfonic acid-p-nitrobenzyl esters, silanol-aluminum complexes, (η6-benzene)(η5-cyclopentadienyl)iron(II), and more specifically, benzoin tosylate, 2,5-dinitrobenzyl tosylate, N-tosiphthalic acid imide, but are not limited to these.

Examples of initiators that can be used as both photoradical polymerization initiators and photocationic polymerization initiators include aromatic iodonium salts, aromatic sulfonium salts, aromatic diazonium salts, aromatic phosphonium salts, triazine compounds, and iron arene complexes. More specifically, iodonium salts such as diphenyliodonium, ditolyliodonium, bis(p-tert-butylphenyl)iodonium, bis(p-chlorophenyl)iodonium chlorides, bromides, borofluorides, hexafluorophosphate salts, and hexafluoroantimonate salts, triphenylsulfonium , sulfonium chlorides, bromides, boron fluorides, hexafluorophosphate salts, hexafluoroantimonate salts, etc., such as 4-tert-butyltriphenylsulfonium and tris(4-methylphenyl)sulfonium, and 2,4,6-substituted-1,3,5-triazine compounds, such as 2,4,6-tris(trichloromethyl)-1,3,5-triazine, 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine, and 2-methyl-4,6-bis(trichloromethyl)-1,3,5-triazine, are included, but are not limited to these.

From the viewpoint of stabilizing the recorded hologram, it is preferable that the photopolymerization initiator is one that is decomposed after the hologram is recorded.

The content of the photopolymerization initiator in the photosensitive material for hologram formation of the present invention is preferably 0.04 to 6.5 parts by mass, more preferably 1.8 to 5.0 parts by mass, based on 100 parts by mass of the total solid content of the photosensitive material for hologram formation.
If the content of the photopolymerization initiator is less than the above range, the photopolymerizable monomer may not be sufficiently polymerized, and the desired refractive index modulation amount (Δn) may not be obtained, whereas if the content of the photopolymerization initiator is more than the above range, the unreacted photopolymerization initiator may deteriorate the hologram characteristics.

<Sensitizing dye>
The sensitizing dye in the present invention is generally a component that absorbs light and has the function of increasing the sensitivity of the photopolymerization initiator to the recording light. By using the sensitizing dye, the photosensitive material for forming a hologram becomes active to visible light, and it becomes possible to record a hologram using visible laser light.

Examples of sensitizing dyes that can be used in the present invention include thiopyrylium salt dyes, merocyanine dyes, quinoline dyes, styrylquinoline dyes, coumarin dyes, ketocoumarin dyes, thioxanthene dyes, xanthene dyes, oxonol dyes, cyanine dyes, rhodamine dyes, pyrylium ion dyes, cyclopentanone dyes, cyclohexanone dyes, and diphenyliodonium ion dyes. Specific examples of cyanine dyes and merocyanine dyes include 3,3'-dicarboxyethyl-2,2'-thiocyanine bromide, 1-carboxymethyl-1'-carboxyethyl-2,2'-quinocyanine bromide, 1,3'-diethyl-2,2'-quinothiacyanine iodide, 3-ethyl-5-[(3-ethyl-2(3H)-benzothiazolylidene)ethylidene]-2-thioxo-4-oxazolidine, 3,9-diethyl-3'-carboxymethyl-2, Examples of coumarin dyes and ketocoumarin dyes include, but are not limited to, 3-(2'-benzimidazole)-7-diethylaminocoumarin, 3,3'-carbonylbis(7-diethylaminocoumarin), 3,3'-carbonylbiscoumarin, 3,3'-carbonylbis(5,7-dimethoxycoumarin), and 3,3'-carbonylbis(7-acetoxycoumarin).

When high transparency is required for the volume hologram recording body described below, it is preferable to use a material that is easily decolorized by decomposition during the heating step or light irradiation step after the interference exposure step, and for example, a dye that is generally easily decomposed by light, such as a cyanine dye, is preferable. This is because by leaving it under indoor light or sunlight for several hours to several days, the dye in the volume hologram recording body is decomposed and no longer has an absorption wavelength range in the visible light range, and a volume hologram recording body with high transparency can be obtained.

The content of the sensitizing dye is preferably 0.001 to 2.0 parts by mass, and more preferably 0.001 to 1.2 parts by mass, based on 100 parts by mass of the total solid content of the photosensitive material for forming a hologram.
If the content of the sensitizing dye is more than the above range, when high transparency is required, the decomposition of the dye by light irradiation may not be sufficient, resulting in a colored hologram recording layer. On the other hand, if the content is less than the above range, the sensitivity of the photopolymerization initiator may not be sufficiently sensitized, and the photosensitive material for forming a hologram may become inactive to visible light.

<Binder resin>
The binder resin improves the film-forming properties and film thickness uniformity of the hologram recording layer, and has the function of stabilizing the hologram formed by polymerization due to light irradiation. It also contributes to increasing the refractive index modulation amount (Δn) of the hologram recording layer and improving the heat resistance and mechanical properties, etc.
The binder resin used in the present invention is preferably one or more selected from thermoplastic resins and thermosetting resins. Among them, it is preferable to use at least a thermoplastic resin, and further, it is preferable to use a thermoplastic resin and a thermosetting resin in combination from the viewpoints of increasing the refractive index modulation amount (Δn) and improving heat resistance and mechanical properties.

(Thermoplastic resin)
Examples of thermoplastic resins include polyvinyl acetate, polyvinyl butyrate, polyvinyl formal, polyvinyl carbazole, polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polymethyl methacrylate, polyethyl acrylate, polybutyl acrylate, polymethacrylonitrile, polyethyl methacrylate, polybutyl methacrylate, polyacrylonitrile, poly-1,2-dichloroethylene, ethylene-vinyl acetate copolymer, syndiotactic polymethyl methacrylate, poly-α-vinyl naphthalate, polycarbonate, cellulose acetate, cellulose triacetate, cellulose acetate butyrate, polystyrene, poly-α-methylstyrene, poly-o-methylstyrene, poly-p -methylstyrene, poly-p-phenylstyrene, poly-2,5-dichlorostyrene, poly-p-chlorostyrene, poly-2,5-dichlorostyrene, polyarylate, polysulfone, polyethersulfone, styrene-acrylonitrile copolymer, styrene-divinylbenzene copolymer, styrene-butadiene copolymer, styrene-maleic anhydride copolymer, ABS resin, polyethylene, polyvinyl chloride, polypropylene, polyethylene terephthalate, polyvinylpyrrolidone, polyvinylidene chloride, hydrogenated styrene-butadiene-styrene copolymer, transparent polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, copolymer of a cyclic aliphatic (meth)acrylic ester and methyl (meth)acrylate, and the like.
The thermoplastic resins may be used alone or in combination of two or more.

The thermoplastic resin used in the binder resin of the present invention preferably contains a polyacrylic acid ester in terms of increasing the refractive index modulation amount (Δn).

Examples of the polyacrylic acid ester used in the present invention include polymethyl acrylate, polyethyl acrylate, poly n-propyl acrylate, poly n-butyl acrylate, polybenzyl acrylate, poly n-hexyl acrylate, polyisopropyl acrylate, polyisobutyl acrylate, poly-t-butyl acrylate, polycyclohexyl acrylate, polyphenyl acrylate, poly 1-phenylethyl acrylate, poly 2-phenylethyl acrylate, polyfurfuryl acrylate, polydiphenylmethyl acrylate, polypentachlorophenyl acrylate, and polynaphthyl acrylate. The poly(meth)acrylic acid ester may further contain a hydrolyzate of the poly(meth)acrylic acid ester. Among them, the thermoplastic resin used in the binder resin in the present invention is preferably polymethyl methacrylate and a copolymer of polymethyl methacrylate and poly(meth)acrylic acid ester from the viewpoint of increasing the refractive index modulation amount (Δn) and storage stability.

The weight average molecular weight of the thermoplastic resin used for the binder resin in the present invention is preferably within the range of 20,000 to 150,000, more preferably within the range of 80,000 to 150,000, and even more preferably within the range of 120,000 to 140,000, from the viewpoints of the diffusive movement ability of the photopolymerizable monomer during hologram recording and the high-temperature storage stability.
In the present invention, the weight average molecular weight refers to a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

The glass transition temperature (Tg) of the thermoplastic resin used as the binder resin in the present invention is preferably within the range of 60°C to 150°C, and more preferably within the range of 70°C to 120°C.
If the glass transition temperature (Tg) is higher than the above range, the diffusion and migration ability of the photopolymerizable monomer is poor, the desired refractive index modulation amount (Δn) cannot be obtained, and a volume hologram recording medium with high brightness may not be obtained. On the other hand, if the glass transition temperature (Tg) is lower than the above range, the binder resin softens during high-temperature storage, causing interference fringes to become distorted, and the desired refractive index modulation amount (Δn) may not be obtained.
The glass transition temperature (Tg) can be measured using a differential scanning calorimeter (DSC) or the like.

(Thermosetting resin)
When a thermosetting resin is used as the binder resin in the present invention, the strength of the hologram recording layer is increased by being cured in a heating step, the volume hologram recording characteristics are improved, and a stable layer structure can be formed. In addition, a part of the functional group of the thermosetting resin interacts with a part of the functional group of the photopolymerizable monomer by light irradiation, and a chemical bond can be formed. In this case, the photopolymerizable monomer is fixed by the light irradiation step after the volume hologram recording, and the strength of the volume hologram recording body can be increased.

The thermosetting resin used as the binder resin in the present invention is not particularly limited, and monomers, oligomers, and polymers having a thermosetting group can be suitably used.
Examples of the thermosetting resin include compounds containing a hydroxyl group, a mercapto group, a carboxyl group, an amino group, an epoxy group, an oxetane group, an isocyanate group, a carbodiimide group, an oxazine group, and a metal alkoxide. In the present invention, it is more preferable to use a compound containing an epoxy group and an oxetane group, and it is even more preferable to use an epoxy group-containing compound. The epoxy group-containing compound used in the present invention is not particularly limited as long as it is a resin containing one or more epoxy groups in one molecule.

Among the compounds having an epoxy group, examples of monofunctional epoxy compounds having one epoxy group include phenyl glycidyl ether, p-tert-butylphenyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, allyl glycidyl ether, 1,2-butylene oxide, 1,3-butadiene monoxide, 1,2-epoxydodecane, epichlorohydrin, 1,2-epoxydecane, styrene oxide, and cyclohexene oxide, and among these, those containing a polymerizable unsaturated bond include 3-methacryloyloxymethylcyclohexene oxide, 3-acryloyloxymethylcyclohexene oxide, 3-vinylcyclohexene oxide, and glycidyl (meth)acrylate.

Examples of polyfunctional epoxy compounds having two or more epoxy groups include bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol S diglycidyl ether, brominated bisphenol A diglycidyl ether, brominated bisphenol F diglycidyl ether, brominated bisphenol S diglycidyl ether, epoxy novolac resin, hydrogenated bisphenol A diglycidyl ether, hydrogenated bisphenol F diglycidyl ether, hydrogenated bisphenol S diglycidyl ether, 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-meta-dioxane, bis(3,4-epoxycyclohexylmethyl)adipate, vinylcyclohexene oxide, 4-vinylepoxycyclohexane, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, ester, 3,4-epoxy-6-methylcyclohexyl-3',4'-epoxy-6'-methylcyclohexanecarboxylate, methylene bis(3,4-epoxycyclohexane), dicyclopentadiene diepoxide, di(3,4-epoxycyclohexylmethyl)ether of ethylene glycol, ethylene bis(3,4-epoxycyclohexanecarboxylate), dioctyl epoxyhexahydrophthalate, di-2-ethylhexyl epoxyhexahydrophthalate, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ethers, 1,1,3-tetradecadiene dioxide, limonene dioxide, 1,2,7,8-diepoxyoctane, 1,2,5,6-diepoxycyclooctane, and the like.
In addition, epoxy group-containing polymers are also preferably used. Examples of epoxy group-containing polymers include copolymers using monomers having epoxy groups or glycidyl groups as copolymerization components. Examples of the monomers having epoxy groups or glycidyl groups include glycidyl esters of α,β-unsaturated carboxylic acids such as glycidyl (meth)acrylate and glycidyl maleate.
These epoxy compounds may be used alone or in combination of two or more.

The weight average molecular weight of the thermosetting resin used as the binder resin in the present invention is preferably within the range of 5,000 to 100,000, and more preferably within the range of 10,000 to 50,000 in terms of improving the refractive index modulation amount (Δn) and the film strength of the hologram recording layer.

When a thermoplastic resin and a thermosetting resin are used in combination as a binder resin, the blending ratio of the thermoplastic resin to the thermosetting resin is preferably within the range of thermoplastic resin/thermosetting resin = 50/50 to 90/10 by mass, more preferably within the range of 60/40 to 90/10, and even more preferably within the range of 70/30 to 80/20, in terms of improving the refractive index modulation amount (Δn) and the strength of the volume hologram recording medium.

The content of the binder resin in the photosensitive material for forming a hologram of the present invention is preferably 1 to 40 parts by mass, and more preferably 25 to 35 parts by mass, per 100 parts by mass of the total solid content of the photosensitive material for forming a hologram, from the viewpoints of improving the refractive index modulation amount (Δn) and the strength of the volume hologram recording medium.

<Other ingredients>
The photosensitive material for forming a hologram of the present invention may contain, if necessary, fine particles, a thermal polymerization inhibitor, a silane coupling agent, a colorant, etc., insofar as the effect of the present invention is not impaired.
For example, when it is desired to impart good foil cutting properties, it is preferable to use fine particles.
As the fine particles, for example, organic fine particles containing low-density polyethylene, high-density polyethylene, polypropylene, poly(meth)acrylic, etc. as a resin skeleton, inorganic particles such as silica, mica, talc, titania, etc., can be used, and these fine particles may be used alone or in combination of two or more. Among the above, fluorine-based fine particles, which are fine particles of a fluorine-containing resin in which part or all of the hydrogen atoms in the skeleton or side chain of the resin of the organic fine particles are substituted with fluorine atoms, or titania fine particles are preferred.

The photosensitive material for forming a hologram of the present invention may be coated using a solvent as necessary. When the photosensitive material for forming a hologram contains a component that is liquid at room temperature, a coating solvent may not be required at all.
Examples of the solvent include ketone-based solvents such as methyl ethyl ketone, acetone, and cyclohexanone; ester-based solvents such as ethyl acetate, butyl acetate, and ethylene glycol diacetate; aromatic solvents such as toluene and xylene; cellosolve-based solvents such as methyl cellosolve, ethyl cellosolve, and butyl cellosolve; alcohol-based solvents such as methanol, ethanol, and propanol; ether-based solvents such as tetrahydrofuran and dioxane; halogen-based solvents such as dichloromethane and chloroform; and mixed solvents thereof.

It should be noted that the above-mentioned materials are merely examples of materials that can be used to form a volume hologram, and the materials are not limited to these.
Moreover, the volume hologram 21 can be formed using such a photosensitive material for forming a hologram in the same manner as a conventionally known method for forming a hologram.

The image source 50 uses, as a light source, for example, an LED light source or a laser light source.
When an LED light source is used as the image source 50, the band of light that reaches the volume hologram 21 is wide. Here, when the band of the volume hologram 21 is narrower than the band of the light source, light of wavelengths that are not diffracted cannot be effectively utilized, resulting in low efficiency.
FIG. 6 is a diagram showing an example of the spectral distribution curve of a light source and the distribution of the diffraction efficiency of a volume hologram when the half-value width of the efficiency of the volume hologram is 4 nm.
FIG. 7 is a diagram showing an example of the spectral distribution curve of a light source and the distribution of the diffraction efficiency of a volume hologram when the half-value width of the efficiency of the volume hologram is 10 nm.
6 and 7, it is assumed that the peak of the light quantity of the light source coincides with the peak of the diffraction efficiency. When the half-width of the efficiency of the volume hologram is 4 nm, the amount of available light is significantly reduced, and it is not desirable for the half-width of the volume hologram to become smaller than this. Therefore, it is desirable for the half-width of the volume hologram to be at least 5 nm or more.

FIG. 8 is a diagram showing an example of the peak shift of a volume hologram due to a temperature change when the half-width of the efficiency of the volume hologram is 4 nm.
FIG. 9 is a diagram showing an example of a volume hologram peak shift caused by temperature change when the half-width of the efficiency of the volume hologram is 10 nm.
The efficiency peak of a volume hologram may shift due to temperature changes. For example, when the temperature changes by 20° C., the efficiency peak of the volume hologram may shift by about 2 nm. In this case, the wavelength of available light shifts, which may cause a chromaticity shift.
As shown in FIGS. 8 and 9, it is understood that a wider half-value width of the efficiency of a volume hologram is also advantageous in terms of preventing chromaticity shift.

In addition, when a laser light source is used as the image source 50, the band of light that reaches the volume hologram 21 is very narrow. When using laser light, changes in diffraction efficiency due to temperature changes become a bigger problem. More specifically, if the wavelength selectivity of the volume hologram 21 is too high, the desired diffraction efficiency cannot be obtained due to temperature changes, and only dark images can be observed, or the color balance of the observed image may be shifted.

Fig. 10 is a diagram showing the distribution of diffraction efficiency versus wavelength when the wavelength selectivity of the volume hologram 21 is very high. The horizontal axis of Fig. 10 indicates wavelength, and the vertical axis indicates the diffraction efficiency of the volume hologram 21. In Fig. 10, the distribution of diffraction efficiency at room temperature is shown by a solid line, at high temperatures by a dashed line, and at low temperatures by a dashed line.
As shown by the solid line in Fig. 10, the volume hologram 21 has very high wavelength selectivity, and at first glance appears ideal because it hardly diffracts light outside the band of the laser light. However, when the temperature of the volume hologram 21 changes, the selectively diffracted wavelength changes. As shown in Fig. 10, at high temperatures, the diffracted wavelength shifts to the longer wavelength side, and at low temperatures, the diffracted wavelength shifts to the shorter wavelength side. This is thought to be due to the expansion or contraction of the volume hologram 21 due to temperature changes. As shown in Fig. 10, at both high and low temperatures, the wavelength band diffracted by the volume hologram 21 is significantly shifted from the band of the laser light, and a sufficient amount of diffracted light cannot be obtained.

Therefore, in this embodiment, a volume hologram 21 that is less affected by temperature changes is created.
FIG. 11 is a diagram showing an example of the distribution of the diffraction efficiency at 20° C. of the volume hologram 21 of this embodiment.
The volume hologram 21 of this embodiment shown in Fig. 11 has a half-width of 5 nm in the spectral distribution curve. In this specification, the half-width in the spectral distribution curve is the width of the curve at half the peak value in the spectral distribution curve (wavelength range, also called full width at half maximum). The vertical axis of the spectral distribution curve may be the light amount itself or the diffraction efficiency. In this embodiment, the distribution curve of the diffraction efficiency is taken as the spectral distribution curve.

The spectral distribution curve may be obtained by measuring the laser light that is actually diffracted and reflected, or by measuring the laser light that is not diffracted and reflected by the volume hologram 21 and is transmitted directly through the volume hologram 21. The measurement may also be performed using a spectrophotometer (Shimadzu Corporation's "UV-3100PC", compliant with JIS K 0115) or the like.

In addition, in FIG. 11, the band of the laser light emitted by the image source 50 is also shown, as in FIG. 10. The band of the laser light emitted by the image source 50 in this embodiment has a wavelength width of 1 nm centered at 530 nm. As shown in FIG. 11, at 20°C, the peak position of the diffraction efficiency and the band of the laser light emitted by the image source 50 approximately coincide, and the laser light can be diffracted and reflected efficiently.

FIG. 12 is a diagram showing the distribution of the diffraction efficiency at 40° C. of the volume hologram 21 of this embodiment.
At 40°C, the distribution curve of the diffraction efficiency (spectral distribution curve) shifts by approximately 2 nm to the long wavelength side, but since the half-width of the spectral distribution curve of the volume hologram 21 of this embodiment is 5 nm, the decrease in diffraction efficiency is suppressed to approximately half, and images can be displayed satisfactorily.

FIG. 13 is a diagram showing the distribution of the diffraction efficiency at 0° C. of the volume hologram 21 of this embodiment.
Even at 0°C, the distribution curve of the diffraction efficiency (spectral distribution curve) shifts by approximately 2 nm to the short wavelength side, but since the half-width of the spectral distribution curve of the volume hologram 21 of this embodiment is 5 nm, the decrease in diffraction efficiency is suppressed to approximately half, and images can be displayed satisfactorily.

Therefore, it is desirable for the volume hologram 21 to have a half-width in the spectral distribution curve of 5 nm or more.
More preferably, the volume hologram 21 has a half-width of 10 nm or more in the spectral distribution curve.
More preferably, the volume hologram 21 has a half-width of 20 nm or more in the spectral distribution curve.
FIG. 14 is a diagram showing distribution curves of diffraction efficiency when the half-width is 5 nm, 10 nm, and 20 nm.
As shown in FIG. 14, the wider the half-width, the less susceptible the effect of temperature changes can be.
Furthermore, the wavelength width at the position of 90% of the maximum efficiency is preferably 5 nm or more, and more preferably 10 nm or more, because the efficiency hardly changes even if the central wavelength is shifted, and therefore the effect of temperature change can be further reduced.

Next, a more specific configuration of the volume hologram 21 having a wide half-width in the spectral distribution curve as described above will be described.
Since the half-width of the diffracted light of the volume hologram 21 is considered to be significantly affected by the film thickness and the refractive index modulation amount Δn, the optimal ranges for these are first defined.
15A and 15B are diagrams showing the simulation conditions, in which Fig. 15A shows a list of the simulation conditions, and Fig. 15B shows the conditions of the incidence angle and the emission angle.
Here, a simulation of the volume hologram 21 was performed under the conditions shown in Fig. 15 to investigate the influence of the film thickness and the refractive index modulation amount Δn on the half-width. The refractive index modulation amount Δn varies greatly depending on the selection and composition of materials before exposure, but it can also be changed by the exposure conditions, so it is quite possible to obtain a desired value. The simulation results shown below use the average value of P-polarized light and S-polarized light.

The simulation of the reflection hologram was performed based on Kogelnik's Coupled Wave theory (The Bell System Technical Journal, Vol. 48, No. 9, pp. 2909-2947 (Nov. 1969)).
The diffraction efficiency η of a reflection hologram is given by the following equation (A) when absorption in the medium can be ignored.

η=1/[1+(1-ξ ² /ν ² )/sinh ² {√(ν ² -ξ ² )}]...(A)
Here, ν and ξ are given by the following equations:

ν=πtΔn/{λ√(cosθ _R・cosθ _S )}
ξ=t/2×(k _Rz +K _z -k _Sz )
however,
t: thickness of photosensitive material λ: wavelength in vacuum θ _R : angle between incident light and normal vector of hologram surface θ _S : angle between diffracted light and normal vector of hologram surface k _Rz : hologram of wave vector of incident light Component in the normal direction of the surface k _Sz : Component of the wave vector of the diffracted light in the normal direction of the hologram surface K _z : Component of the diffraction grating vector in the normal direction of the hologram surface Δn: Amplitude of the refractive index modulation of the hologram medium n: Hologram medium is the average refractive index of

Here, the wave vector k of the light is given by |k| = 2πn/λ, and the diffraction grating vector K is a vector perpendicular to the interference fringe plane of the volume hologram, and is given by |K| = 2π/Λ, where Λ is the period of the interference fringes (grating spacing).

(FWHM simulation with film thickness t and Δn as variables)
16 is a diagram summarizing the results of a simulation in which the half-width was obtained by changing the film thickness t and Δn. In FIG. 16, 15 types of film thickness t and 20 types of Δn were combined to perform a total of 300 types of simulation.
16, it is clear that a thinner film thickness t is preferable and a higher value of Δn is preferable, but it is difficult to quantitatively grasp this. Therefore, from the results shown in FIG. 16, a formula was created to determine a preferable range for the combination of film thickness t and Δn.
16 can be divided into a plurality of regions within its numerical range, a data set consisting of combinations of film thickness t and Δn that provide the boundary half-width was collected, and an approximation formula expressing the data set was obtained. The boundaries were set to half-widths of 5 nm, 10 nm, 15 nm, 20 nm, and 30 nm.

FIG. 17 is a graph showing an approximation formula showing a preferable combination range of film thickness t and Δn obtained from the half-width.
FIG. 17 also shows the results of polynomial approximation performed for each of the half-widths of 5 nm, 10 nm, 15 nm, 20 nm, and 30 nm, which are the boundary values described above.

From the approximation result of a half-width of 5 nm,
Δn≧5.87×10 ⁻³ ×ln(t)−1.29×10 ⁻²
By satisfying this relationship, the half width can be set to 5 nm or more.

More preferably, from the approximation result of a half-width of 10 nm,
Δn≧1.10×10 ⁻² ×ln(t)−1.42×10 ⁻²
By satisfying this relationship, the half width can be set to 10 nm or more.

More preferably, from the approximation result of the half-width of 15 nm,
Δn≧4.60×10 ^-3 ×ln(t)+2.26×10 ^-2
By satisfying this relationship, the half width can be set to 15 nm or more.

More preferably, from the approximation result of the half-width of 20 nm,
Δn≧8.40×10 ^-3 ×ln(t)+2.42×10 ^-2
By satisfying this relationship, the half width can be set to 20 nm or more.

More preferably, from the approximation result of the half-width of 30 nm,
Δn≧6.31×10 ^-4 ×ln(t)+7.47×10 ^-2
By satisfying this relationship, the half width can be set to 30 nm or more.

(Simulation of Diffraction Efficiency with Film Thickness t and Δn as Variables)
When the thickness t and Δn are changed, not only the half width described above but also the diffraction efficiency is affected. Therefore, taking the diffraction efficiency into consideration, the appropriate ranges of the thickness t and Δn are further limited.
Fig. 18 is a diagram summarizing the results of a simulation in which the maximum diffraction efficiency was obtained by changing the film thickness t and Δn. As in the case of Fig. 16, Fig. 18 also summarizes the results of a total of 300 simulations performed by combining 15 different film thicknesses t and 20 different Δn.
18, it is clear that a thicker film thickness t is desirable and a higher value of Δn is desirable, but it is difficult to quantitatively grasp this. Therefore, similar to the case of FIG. 16, a preferable range of the combination of film thickness t and Δn was expressed as a formula from the results shown in FIG.
Specifically, the boundaries were set at which the maximum value of the diffraction efficiency was 0.02, 0.05, 0.1, 0.4, 0.6, and 0.8.

FIG. 19 is a graph showing an approximation formula showing a preferable combination range of film thickness t and Δn obtained from the maximum value of the diffraction efficiency.
FIG. 19 also shows the results of polynomial approximation for each of the maximum diffraction efficiency values of 0.02, 0.05, 0.1, 0.4, 0.6, and 0.8, which are the boundary values described above.

From the results of FIG.
0.0200t ^-1.00 ≦Δn≦0.395t ^-1.25
By satisfying this relationship, the maximum value of the diffraction efficiency can be kept within the range of 0.02 to 0.8.

Also, more preferably,
0.0374t ^-0.886 ≦Δn≦0.188t ^-1.13
By satisfying this relationship, the maximum value of the diffraction efficiency can be kept within the range of 0.05 to 0.6.

Further, more preferably,
0.0517t ^-0.843 ≦Δn≦0.133t ^-1.10
By satisfying this relationship, the maximum value of the diffraction efficiency can be kept within the range of 0.1 to 0.4.

(FWHM simulation with incident angle _θ1 and exit angle _θ2 as variables)
In the above-mentioned simulation, the incident angle and the exit angle were fixed at 0° and 125°, respectively. The incident angle of 0° and the exit angle of 120° are one of the most suitable combinations when a head-mounted display device 1 is assumed. However, the combination of the incident angle and the exit angle is not limited to this, and can be appropriately changed in design. Therefore, a simulation was performed assuming a case where the incident angle and the exit angle are changed.

Fig. 20 shows simulation conditions focusing on the angle of incidence and the angle of emergence, Fig. 20(a) shows a list of simulation conditions, Fig. 20(b) shows a state in which the angle of incidence θ ₁ and the angle of emergence θ ₂ are changed in the case of a transmission-type volume hologram 21, and Fig. 20(c) shows a state in which the angle of incidence θ ₁ and the angle of emergence θ ₂ are changed in the case of a reflection-type volume hologram 21B.
Here, a simulation of the volume hologram 21 (21B) was performed under the conditions shown in FIG. 20, and the influence of the incident angle θ ₁ and the exit angle θ ₂ on the half-width was examined.

Fig. 21 is a diagram summarizing the results of a simulation in which the half-width was obtained by changing the incident angle θ ₁ and the exit angle θ _2. Fig. 21 summarizes the results of a simulation performed by combining 19 different incident angles θ ₁ from -45° to 45° and 16 different exit angles θ ₂ from 45° to 135°. Note that the exit angles of 45° to 80° are transmission type volume holograms, and the exit angles of 100° to 135° are reflection type volume holograms.
21, it is clear that the larger the incident angle _θ1 , the wider the half-width, and the farther the exit angle _θ2 is from 90°, the wider the half-width, but it is difficult to quantitatively grasp this. Therefore, from the results shown in FIG. 21, the preferable range for the combination of the incident angle _θ1 and the exit angle _θ2 was expressed in a formula in the same manner as in the case of the film thickness t and Δn described above. The boundaries were set at 10 nm and 15 nm for the half-width for the transmission type and the reflection type, respectively.

FIG. 22 is a graph showing an approximation formula indicating a preferable combination range of the incident angle θ ₁ and the exit angle θ ₂ , which is determined from the half-width.
FIG. 22 also shows the results of linear approximation for each of the boundary values of 10 nm for the transmission type, 10 nm for the reflection type, 15 nm for the transmission type, and 15 nm for the reflection type.

From the results of the transmission type 10 nm and the reflection type 10 nm, within the range of −45°≦θ ₁ ≦45° and 45°≦θ ₂ ≦135°,
θ ₂ ≧-0.326 _{θ 1} +100
,and,
θ ₂ ≦0.159 _{θ 1} +76.8
By satisfying these relationships, the half width can be set to 10 nm or more.

From the results of the transmission type 15 nm and the reflection type 15 nm, within the range of −45°≦θ ₁ ≦45° and 45°≦θ ₂ ≦135°,
θ ₂ ≧−θ ₁ +147
,and,
θ ₂ ≦0.235 _{θ 1} +70.3
By satisfying these relationships, the half width can be set to 15 nm or more.

FIG. 23 is a diagram for explaining a state in which multiple diffractions occur in the volume hologram 21. In FIG.
The diffraction efficiency of the volume hologram 21 is preferably within an appropriate range. For example, as shown in FIG. 23, the light diffracted by the volume hologram 21 may be totally reflected by the main surface of the base layer 10 and then enter the volume hologram 21 again. The light that enters the volume hologram 21 again may be diffracted by the volume hologram 21 again. The light diffracted by the volume hologram 21 again does not go into the base layer 10, but exits from the volume hologram 21. For this reason, if the diffraction efficiency of the volume hologram 21 is too high, the light that is finally guided from the entrance part 21 to the exit part 22 will be reduced if the light is diffracted by the volume hologram 21 multiple times. The inventors of the present invention confirmed that the light that enters the entrance part 21 and is diffracted by the volume hologram 21 enters the volume hologram 21 again about 1 to 5 times, particularly 4 times, after being totally reflected by the main surface of the base layer 10.

Figure 23 is a table showing the ratio of light finally guided from the entrance part 21 to the exit part 22 in relation to the diffraction efficiency of the volume hologram 21 and the number of times the light is diffracted by the volume hologram 21 and then re-enters the volume hologram 21. That is, it shows the ratio of light guided from the entrance part 21 to the exit part 22 when the light entering the entrance part 21 is 100%. As can be seen from Figure 23, when the number of times the light is diffracted by the volume hologram 21 and then re-enters the volume hologram 21 is one or more, the diffraction efficiency of the volume hologram 21 is preferably 90% or less. Also, when the number of times the light is diffracted by the volume hologram 21 and then re-enters the volume hologram 21 is three or more, the diffraction efficiency of the diffraction grating is preferably 1% or more and 80% or less (underlined in Figure 23), and more preferably 5% or more and 60% or less. Furthermore, if the light is diffracted once by the volume hologram 21 and then enters the volume hologram 21 again four or more times, the diffraction efficiency of the diffraction grating is preferably 10% or more and 40% or less (underlined in Figure 23).

The volume hologram 21 used in this embodiment can be an optimum volume hologram 21 for the usage conditions, which is selected by appropriately combining the preferred ranges of the film thickness t and Δn, and the preferred ranges of the incident angle _θ1 and the exit angle _θ2 described above.
For the measurement of the film thickness of an actually produced volume hologram, it is advisable to measure it by observing the cross section with a SEM. Also, as a method for measuring Δn, it is advisable to carry out a simulation calculation of the diffraction efficiency based on the measurement results of the film thickness, diffraction angle, and wavelength, and calculate Δn by fitting it to the diffraction efficiency measurement results.

As described above, the head mounted display device 1 of this embodiment uses the volume hologram 21, and since the half width of the diffracted light is 5 nm or more, it is possible to efficiently use the diffracted light. Furthermore, since the volume hologram 21 of this embodiment is within the above-mentioned preferred ranges of film thickness t and Δn, and the preferred ranges of incident angle _θ1 and exit angle _θ2 , it is possible to realize a volume hologram 21 with a wide half width and good diffraction efficiency.

(Modifications)
The present invention is not limited to the above-described embodiment, and various modifications and variations are possible, and these are also within the scope of the present invention.

(1) In each embodiment, a single wavelength has been described, but the present invention can also be applied to cases where there are multiple wavelengths, such as when laser light corresponding to the three colors RGB is used. In that case, a hologram layer may be formed separately for each RGB wavelength and then laminated, or interference fringes for each RGB wavelength may be formed in the same layer.

(2) In each embodiment, each numerical range defined as a desirable range does not have to be satisfied over the entire area of the volume hologram. For example, the angle of incidence or the angle of emergence for the volume hologram does not have to satisfy the desirable numerical range over the entire area of the volume hologram. For example, it is sufficient that the center of the volume hologram satisfies the condition of the angle range defined here.

(3) In each embodiment, an example has been described in which the volume hologram 30 is applied to a head-mounted display device. However, the present invention is not limited to this example, and the volume hologram of the present invention may be used in a display device that is not mounted on the head.

(4) In each embodiment, an example has been described in which the emission section 22, the second extension section 23, and the emission section 22B are provided with a diffraction grating formed of a fine uneven shape. This is not limited to the above, and for example, one or all of these may be formed from a volume hologram. Note that if all are formed from a volume hologram, the shape-imparting layer can be omitted.

The embodiments and variations can be used in any suitable combination, but detailed explanations will be omitted. Furthermore, the present invention is not limited to the embodiments described above.

1 Head-mounted display device 10 Base material layer 10a Waveguide section 20 Shape-imparting layer 21 Volume hologram (incident section)
21B Volume hologram (entrance part)
22 Exit section 22B Exit section 23 Second extension section 30 Volume hologram 40 Exit pupil extension element 40B Exit pupil extension element 40C Exit pupil extension element 50 Image source 221 High refractive index section 221B High refractive index section 231 High refractive index section

Claims (5)

A waveguide portion configured in a flat plate shape for guiding light;
an input section for inputting light into the waveguide section;
an extension section that is disposed on the waveguide section or is integral with the waveguide section and divides an exit pupil into a plurality of sections and expands the exit pupil;
The half-width in the spectral distribution curve is 5 nm or more,
When the film thickness of the volume hologram is t (μm) and the refractive index modulation amount is Δn,
Δn≧6.31×10 ^-4 ×ln(t)+7.47×10 ^-2
A volume hologram that satisfies the relationship. 2. The volume hologram according to claim 1,
When the film thickness of the volume hologram is t (μm) and the refractive index modulation amount is Δn,
0.0200t ^-1.00 ≦Δn≦0.395t ^-1.25
To satisfy the relationship
A volume hologram characterized by: 3. The volume hologram according to claim 1,
When the diffraction angle of light incident on the volume hologram at an incident angle θ ₁ is θ ₂ ,
Within the range of −45°≦θ ₁ ≦45° and 45°≦θ ₂ ≦135°,
θ ₂ ≧-0.326 _{θ 1} +100
,and,
θ ₂ ≦0.159 _{θ 1} +76.8
To satisfy both of the above relationships,
A volume hologram characterized by: An exit pupil expansion element including a volume hologram according to any one of claims 1 to 3 at its entrance portion. A head-mounted display device that includes a volume hologram according to any one of claims 1 to 3 at its entrance and allows the display to be superimposed on a see-through field of view.

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