Sony Patent | Optical device, image display device, and display apparatus

Patent: Optical device, image display device, and display apparatus

Drawings: Click to check drawins

Publication Number: 20210026140

Publication Date: 20210128

Applicant: Sony Corporation

Assignee: Sony

Abstract

An optical device includes a first A deflecting member, a first B deflecting member, a first C deflecting member, a second A deflecting member, a second B deflecting member, and a second C deflecting member. Light emitted from an image forming device that enters the first A deflecting member and is emitted toward a pupil of an observer via the first B deflecting member and the first C deflecting member. Light emitted from the image forming device that enters the second A deflecting member is emitted toward the pupil of the observer via the second B deflecting member and the second C deflecting member. A direction of light deflected by the first A deflecting member that is orthogonally projected on a light guide plate is opposite a direction of light deflected by the second A deflecting member that is orthogonally projected on the light guide plate.

Claims

1. An optical device, on which light emitted from an image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device comprising: a light guide plate; a first deflecting unit; and a second deflecting unit, wherein the first deflecting unit includes a first A deflecting member, a first B deflecting member, and a first C deflecting member, the second deflecting unit includes a second A deflecting member, a second B deflecting member, and a second C deflecting member, part of the light emitted from the image forming device enters the first A deflecting member, the light incident on the first A deflecting member is deflected by the first A deflecting member, enters the first B deflecting member by total reflection inside of the light guide plate, is deflected by the first B deflecting member, enters the first C deflecting member by total reflection inside of the light guide plate, is deflected by the first C deflecting member, and is emitted toward a pupil of an observer, at least remaining part of the light emitted from the image forming device enters the second A deflecting member, the light incident on the second A deflecting member is deflected by the second A deflecting member, enters the second B deflecting member by total reflection inside of the light guide plate, is deflected by the second B deflecting member, enters the second C deflecting member by total reflection inside of the light guide plate, is deflected by the second C deflecting member, and is emitted toward the pupil of the observer, and assuming that a direction obtained when a propagation direction of the light deflected by the first A deflecting member is orthogonally projected on the light guide plate is a first direction, and that a direction obtained when a propagation direction of the light deflected by the second A deflecting member is orthogonally projected on the light guide plate is a second direction, the first direction is opposite to the second direction.

2. The optical device according to claim 1, wherein assuming that a point on the light guide plate, at which light emitted from the center point of an image forming region of the image forming device collides with the light guide plate, is an origin, an axis line of the light guide plate that passes through the origin and is directed toward the first direction is an X-axis, an axis in a thickness direction of the light guide plate that passes through the origin is an Z-axis, and an axis that is orthogonal to the X-axis and the Z-axis is a Y-axis, the first deflecting unit and the second deflecting unit are disposed symmetrically to a YZ plane.

3. The optical device according to claim 1, wherein the first A deflecting member and the second A deflecting member each include a volume hologram diffraction grating, and satisfy k.sup.X.sub.1-A+k.sup.X.sub.2-A=0, k.sup.Y.sub.1-A=k.sup.Y.sub.2-A, and k.sup.Z.sub.1-A=k.sup.Z.sub.2-A, where a wave vector of the first A deflecting member is k.sup.v.sub.1-A, an X component, a Y component, and a Z component of k.sup.v.sub.1-A are k.sup.X.sub.1-A, k.sup.Y.sub.1-A, and k.sup.Z.sub.1-A, respectively, a wave vector of the second A deflecting member is k.sup.v.sub.2-A, and an X component, a Y component, and a Z component of k.sup.v.sub.2-A are k.sup.X.sub.2-A, k.sup.Y.sub.2-A, and k.sup.Z.sub.2-A, respectively.

4. The optical device according to claim 3, wherein the first C deflecting member and the second C deflecting member each include a volume hologram diffraction grating, and satisfy k.sup.X.sub.1-C+k.sup.X.sub.2-C=0, k.sup.Y.sub.1-C=k.sup.Y.sub.2-C, and k.sup.Z.sub.1-C=k.sup.Z.sub.2-C, where a wave vector of the first C deflecting member is k.sup.v.sub.1-C, an X component, a Y component, and a Z component of k.sup.v.sub.1-C are k.sup.X.sub.1-C, k.sup.Y.sub.1-C, and k.sup.Z.sub.1-C, respectively, a wave vector of the second C deflecting member is k.sup.v.sub.2-C, and an X component, a Y component, and a Z component of k.sup.v.sub.2-C are k.sup.X.sub.2-C, k.sup.Y.sub.2-C, and k.sup.Z.sub.2-C, respectively.

5. The optical device according to claim 4, wherein the first B deflecting member and the second B deflecting member each include a volume hologram diffraction grating, and satisfy k.sup.X.sub.1-B+k.sup.X.sub.2-B=0, k.sup.Y.sub.1-B=k.sup.Y.sub.2-B, and k.sup.Z.sub.1-B=k.sup.Z.sub.2-C, where a wave vector of the first B deflecting member is k.sup.v.sub.1-B, an X component, a Y component, and a Z component of k.sup.v.sub.1-B are k.sup.X.sub.1-B, k.sup.Y.sub.1-B, and k.sup.Z.sub.1-B, respectively, a wave vector of the second B deflecting member is k.sup.v.sub.2-B, and an X component, a Y component, and a Z component of k.sup.v.sub.2-B are k.sup.X.sub.2-B, k.sup.Y.sub.2-B, and k.sup.Z.sub.2-B, respectively.

6. The optical device according to claim 5, wherein k.sup.v.sub.1-A+k.sup.v.sub.1-B+k.sup.v.sub.1-C=0, and k.sup.v.sub.2-A+k.sup.v.sub.2-B+k.sup.v.sub.2-C=0 are satisfied.

7. The optical device according to claim 1, wherein the part of the light that is emitted from the image forming device and enters the first A deflecting member enters the second A deflecting member.

8. The optical device according to claim 7, wherein the first A deflecting member and the second A deflecting member partially overlap.

9. The optical device according to claim 1, wherein the first A deflecting member and the first B deflecting member are laminated, and the second A deflecting member and the second B deflecting member are laminated.

10. The optical device according to claim 1, wherein the first A deflecting member, the first B deflecting member, and the first C deflecting member each include a volume hologram diffraction grating, and satisfy .eta..sub.1-B/.eta..sub.1-A<1, and .eta..sub.1-C/.eta..sub.1-A<1 where with respect to the light emitted from the image forming device, an average diffraction efficiency of the first A deflecting member is .eta..sub.1-A, an average diffraction efficiency of the first B deflecting member is .eta..sub.1-B, and an average diffraction efficiency of the first C deflecting member is .eta..sub.1-C, and the second A deflecting member, the second B deflecting member, and the second C deflecting member each include a volume hologram diffraction grating, and satisfy .eta..sub.2-B/.eta..sub.2-A<1, and .eta..sub.2-C/.eta..sub.2-A<1 where with respect to the light emitted from the image forming device, an average diffraction efficiency of the second A deflecting member is .eta..sub.2-A, an average diffraction efficiency of the second B deflecting member is .eta..sub.2-B, and an average diffraction efficiency of the second C deflecting member is .eta..sub.2-C.

11. An image display device, comprising: an image forming device; and an optical device, on which light emitted from the image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device including a light guide plate, a first deflecting unit, and a second deflecting unit, wherein the first deflecting unit includes a first A deflecting member, a first B deflecting member, and a first C deflecting member, the second deflecting unit includes a second A deflecting member, a second B deflecting member, and a second C deflecting member, part of the light emitted from the image forming device enters the first A deflecting member, the light incident on the first A deflecting member is deflected by the first A deflecting member, enters the first B deflecting member by total reflection inside of the light guide plate, is deflected by the first B deflecting member, enters the first C deflecting member by total reflection inside of the light guide plate, is deflected by the first C deflecting member, and is emitted toward a pupil of an observer, at least remaining part of the light emitted from the image forming device enters the second A deflecting member, the light incident on the second A deflecting member is deflected by the second A deflecting member, enters the second B deflecting member by total reflection inside of the light guide plate, is deflected by the second B deflecting member, enters the second C deflecting member by total reflection inside of the light guide plate, is deflected by the second C deflecting member, and is emitted toward the pupil of the observer, and assuming that a direction obtained when a propagation direction of the light deflected by the first A deflecting member is orthogonally projected on the light guide plate is a first direction, and that a direction obtained when a propagation direction of the light deflected by the second A deflecting member is orthogonally projected on the light guide plate is a second direction, the first direction is opposite to the second direction.

12. A display apparatus, comprising: a frame to be mounted to a head of an observer; and an image display device attached to the frame, the image display device including an image forming device, and an optical device, on which light emitted from the image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device including a light guide plate, a first deflecting unit, and a second deflecting unit, wherein the first deflecting unit includes a first A deflecting member, a first B deflecting member, and a first C deflecting member, the second deflecting unit includes a second A deflecting member, a second B deflecting member, and a second C deflecting member, part of the light emitted from the image forming device enters the first A deflecting member, the light incident on the first A deflecting member is deflected by the first A deflecting member, enters the first B deflecting member by total reflection inside of the light guide plate, is deflected by the first B deflecting member, enters the first C deflecting member by total reflection inside of the light guide plate, is deflected by the first C deflecting member, and is emitted toward a pupil of an observer, at least remaining part of the light emitted from the image forming device enters the second A deflecting member, the light incident on the second A deflecting member is deflected by the second A deflecting member, enters the second B deflecting member by total reflection inside of the light guide plate, is deflected by the second B deflecting member, enters the second C deflecting member by total reflection inside of the light guide plate, is deflected by the second C deflecting member, and is emitted toward the pupil of the observer, and assuming that a direction obtained when a propagation direction of the light deflected by the first A deflecting member is orthogonally projected on the light guide plate is a first direction, and that a direction obtained when a propagation direction of the light deflected by the second A deflecting member is orthogonally projected on the light guide plate is a second direction, the first direction is opposite to the second direction.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an optical device, an image display device including such an optical device, and a display apparatus including such an image display device, and more specifically, to a display apparatus used in a head-mounted display (HMD).

BACKGROUND ART

[0002] In recent years, the development of a head-mounted display (HMD), which displays an image from an image forming device on an optical device disposed in front of observer’s eyes, has been intensively promoted. Various types of head-mounted displays have been studied, but for the head-mounted displays, there is a strong need for a wider angle of view of a displayed image so as to provide a more realistic image. To address such a demand, a head-mounted display including three deflecting means disposed on a light guide plate constituting an optical device is well known, for example, from U.S. Patent Application Laid-Open No. 2006/0132914A1 or U.S. Patent Application Laid-Open No. 2014/0330966A1.

[0003] Further, Japanese Patent Application Laid Open No. 2009-133998 discloses an image display device including

[0004] (A) an image forming device including a plurality of pixels arranged in a two-dimensional matrix,

[0005] (B) a collimating optical system for collimating light emitted from the pixels of the image forming device to be parallel light, and

[0006] (C) an optical device, in which the light collimated to be a plurality of parallel light beams having different traveling orientations in the collimating optical system is incident, guided, and emitted, the optical device including [0007] (a) a light guide plate, in which incident light propagates by total reflection, and from which the incident light is emitted, [0008] (b) a first diffraction grating member disposed on the light guide plate and including a reflective volume hologram diffraction grating that diffracts and reflects the light incident on the light guide plate such that the light incident on the light guide plate is totally reflected inside the light guide plate, and [0009] (c) a second diffraction grating member disposed on the light guide plate and including a reflective volume hologram diffraction grating that diffracts and reflects the light propagated by total reflection inside of the light guide plate and emits the light from the light guide plate, in which [0010] assuming that the center of the first diffraction grating member is an origin, a normal of the first diffraction grating member that passes through the origin or a normal with a direction toward the collimating optical system side as a positive direction is an X.sub.i axis, an axis line of the light guide plate that passes through the origin, is orthogonal to the X.sub.1 axis, and has a direction toward the second diffraction grating member side as a positive direction is a Y.sub.i axis, [0011] central light emitted from the pixel at the center of the image forming device and passing through the center of the collimating optical system is optically parallel to the X.sub.iY.sub.i plane and intersects with the X.sub.iZ.sub.i plane at a sharp angle.

CITATION LIST

Patent Literature

[0012] Patent Literature 1: U.S. Patent Application Laid-Open No. 2006/0132914A1

[0013] Patent Literature 2: U.S. Patent Application Laid-Open No. 2014/0330966A1

[0014] Patent Literature 3: Japanese Patent Application Laid Open No. 2009-133998

DISCLOSURE OF INVENTION

Technical Problem

[0015] However, in the head-mounted displays disclosed in the above two U.S. Patent Publications, light emitted from the center point of the image forming region of the image forming device is perpendicularly incident on the deflecting means, but the head-mounted displays fail to cope with a request for further widening the angle of view of a display image. Further, the image display device disclosed in Japanese Patent Application Laid Open No. 2009-133998 includes only two diffraction grating members of the first diffraction grating member and the second diffraction grating member, and thus a display image region in the light guide plate can be enlarged only in one direction propagating from the first diffraction grating to the second diffraction grating. However, providing three diffraction grating members can enlarge the display image region in the light guide plate in two directions.

[0016] Therefore, it is an object of the present disclosure to provide an optical device having a configuration or a structure that can further widen the angle of view of a display image, an image display device including such an optical device, and a display apparatus including such an image display device.

Solution to Problem

[0017] An optical device according to the present disclosure to achieve the above-mentioned object is an optical device, on which light emitted from an image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device including: a light guide plate; a first deflecting unit; and a second deflecting unit. The first deflecting unit includes a first A deflecting member, a first B deflecting member, and a first C deflecting member. The second deflecting unit includes a second A deflecting member, a second B deflecting member, and a second C deflecting member. Part of the light emitted from the image forming device enters the first A deflecting member. The light incident on the first A deflecting member is deflected by the first A deflecting member, enters the first B deflecting member by total reflection inside of the light guide plate, is deflected by the first B deflecting member, enters the first C deflecting member by total reflection inside of the light guide plate, is deflected by the first C deflecting member, and is emitted toward a pupil of an observer. At least remaining part of the light emitted from the image forming device enters the second A deflecting member. The light incident on the second A deflecting member is deflected by the second A deflecting member, enters the second B deflecting member by total reflection inside of the light guide plate, is deflected by the second B deflecting member, enters the second C deflecting member by total reflection inside of the light guide plate, is deflected by the second C deflecting member, and is emitted toward the pupil of the observer. Assuming that a direction obtained when a propagation direction of the light deflected by the first A deflecting member is orthogonally projected on the light guide plate is a first direction, and that a direction obtained when a propagation direction of the light deflected by the second A deflecting member is orthogonally projected on the light guide plate is a second direction, the first direction is opposite to the second direction. Note that the term “total reflection” means total internal reflection or total reflection inside of the light guide plate.

[0018] An image display device according to the present disclosure to achieve the above-mentioned object is an image display device including: an image forming device; and an optical device, on which light emitted from the image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device including the optical device according to the present disclosure.

[0019] A display apparatus according to the present disclosure to achieve the above-mentioned object is a display apparatus including: a frame to be mounted to a head of an observer; and an image display device attached to the frame, the image display device including an image forming device, and an optical device, on which light emitted from the image forming device is incident, in which the light is guided, and from which the light is emitted, the optical device including the optical device according to the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIGS. 1A and 1B are respectively schematic perspective views of an optical device of a first embodiment and schematic views of the optical device of the first embodiment seen from above.

[0021] FIG. 2 is a schematic perspective view of a first deflecting unit constituting the optical device of the first embodiment.

[0022] FIG. 3 is a schematic perspective view of a second deflecting unit constituting the optical device of the first embodiment.

[0023] FIGS. 4A and 4B are respectively schematic cross-sectional views of the optical device of the first embodiment and schematic views of the optical device of the first embodiment seen from the side.

[0024] FIGS. 5A and 5B are respectively schematic views of optical devices of a second embodiment and a third embodiment seen from above.

[0025] FIG. 6 is a schematic view of a display apparatus of the first embodiment seen from above.

[0026] FIG. 7 is a schematic view of the display apparatus of the first embodiment seen from the front.

[0027] FIGS. 8A, 8B, and 8C are conceptual diagrams of an image forming device in the display apparatus of the first embodiment.

[0028] FIGS. 9A and 9B are conceptual diagrams of arrangements of the image forming device, a light guide plate, a first A deflecting member, a first B deflecting member, and the like in the optical device of the first embodiment.

[0029] FIGS. 10A and 10B are conceptual diagrams of wave vectors or the like of the first A deflecting member, the first B deflecting member, and a first C deflecting member.

[0030] FIGS. 11A and 11B are conceptual diagrams of other wave vectors or the like of the first A deflecting member, the first B deflecting member, and the first C deflecting member.

[0031] FIGS. 12A, 12B, and 12C are conceptual diagrams of modified examples of the optical device of the first embodiment.

[0032] FIGS. 13A, 13B, 13C, 13D, 13E, 13F, 13G, and 13H are conceptual diagrams of other modified examples of the optical device of the first embodiment.

[0033] FIG. 14 is a graph showing a relationship between an incident angle of light to the first A deflecting member or a second A deflecting member and a diffraction angle of the first A deflecting member or the second A deflecting member, with a pitch d being as a parameter.

MODE(S)* FOR CARRYING OUT THE INVENTION*

[0034] Hereinafter, the present disclosure will be described on the basis of embodiments with reference to the drawings. The present disclosure is not limited to the embodiments, and various numerical values and materials in the embodiments are illustrative. Note that the description will be given in the following order.

1. General Description of Optical Device, Image Display Device, and Display Apparatus of Present Disclosure

2. First Embodiment (Optical Device, Image Display Device, and Display Apparatus of Present Disclosure)

3. Second Embodiment (Modification of First Embodiment)

4. Third Embodiment (Modification of First and Second Embodiments)

5. Others

[0035] <General Description of Optical Device, Image Display Device, and Display Apparatus of Present Disclosure>

[0036] In an optical device or the like of the present disclosure, assuming that a point on a light guide plate, at which light emitted from the center point of an image forming region of an image forming device collides with the light guide plate, is an origin, an axis line of the light guide plate that passes through the origin and is directed toward a first direction is an X-axis, an axis in a thickness direction of the light guide plate that passes through the origin is an Z-axis, and an axis that is orthogonal to the X-axis and the Z-axis is a Y-axis, a first deflecting unit and a second deflecting unit may be disposed symmetrically to a YZ plane.

[0037] In the optical device or the like of the present disclosure including the favorable form described above, the first A deflecting member and the second A deflecting member may each include a volume hologram diffraction grating, and satisfy k.sup.X.sub.1-A+k.sup.X.sub.2-A=0,

k.sup.Y.sub.1-A=k.sup.Y.sub.2-A, and

k.sup.Z.sub.1-A=k.sup.Z.sub.2-A.

[0038] where a wave vector of the first A deflecting member is k.sup.v.sub.1-A, an X component, a Y component, and a Z component of k.sup.v.sub.1-A are k.sup.X.sub.1-A, k.sup.Y.sub.1-A, and k.sup.Z.sub.1-A, respectively, a wave vector of the second A deflecting member is k.sup.v.sub.2-A, and an X component, a Y component, and a Z component of k.sup.v.sub.2-A are k.sup.X.sub.2-A, k.sup.Y.sub.2-A, and k.sup.Z.sub.2-A, respectively. In this case, the first C deflecting member and the second C deflecting member may each include a volume hologram diffraction grating, and satisfy

k.sup.X.sub.1-C+k.sup.X.sub.2-C=0,

k.sup.Y.sub.1-C=k.sup.Y.sub.2-C, and

k.sup.Z.sub.1-C=k.sup.Z.sub.2-C,

[0039] where a wave vector of the first C deflecting member is k.sup.v.sub.1-C, an X component, a Y component, and a Z component of k.sup.v.sub.1-C are k.sup.X.sub.1-C, k.sup.Y.sub.1-C, and k.sup.Z.sub.1-C, respectively, a wave vector of the second C deflecting member is k.sup.v.sub.2-C, and an X component, a Y component, and a Z component of k.sup.v.sub.2-C are k.sup.X.sub.2-C, k.sup.Y.sub.2-C, and k.sup.Z.sub.2-C, respectively. Furthermore, the first B deflecting member and the second B deflecting member may each include a volume hologram diffraction grating, and satisfy

k.sup.X.sub.1-B+k.sup.X.sub.2-B=0,

k.sup.Y.sub.1-B=k.sup.Y.sub.2-B, and

k.sup.Z.sub.1-B=k.sup.Z.sub.2-C,

[0040] where a wave vector of the first B deflecting member is k.sup.v.sub.1-B, an X component, a Y component, and a Z component of k.sup.v.sub.1-B are k.sup.X.sub.1-B, k.sup.Y.sub.1-B, and k.sup.Z.sub.1-B, respectively, a wave vector of the second B deflecting member is k.sup.v.sub.2-B, and an X component, a Y component, and a Z component of k.sup.v.sub.2-B are k.sup.X.sub.2-B, k.sup.Y.sub.2-3, and k.sup.Z.sub.2-B, respectively. Furthermore,

k.sup.X.sub.1-A+k.sup.X.sub.2-B=0,

k.sup.Y.sub.1-B=k.sup.Y.sub.2-C, and

k.sup.Z.sub.2-A=k.sup.Z.sub.2-B=0, and

k.sup.Y.sub.2-B+k.sup.Y.sub.2-C=0,

may be satisfied. Furthermore,

k.sup.v.sub.1-A+k.sup.v.sub.1-B+k.sup.v.sub.2-C=0, and

k.sup.v.sub.2-A+k.sup.v.sub.2-B+k.sup.v.sub.2-C=0

may be satisfied. Thus, the light incident on the first A deflecting member and the second A deflecting member, and the light emitted from the first C deflecting member and the second C deflecting member, have a conjugate relation. Note that, as described above, the vector is expressed by attaching a superscription letter “v”, and the X, Y, and Z components of the vector are expressed by attaching superscription letters “X”, “Y”, and “Z” as described above.

[0041] Depending on the arrangement of the first A deflecting member, the first B deflecting member, and the first C deflecting member, the second A deflecting member, the second B deflecting member, and the second C deflecting member with respect to the light guide plate, the volume hologram diffraction grating may be transmissive or reflective. The volume hologram diffraction grating means a hologram diffraction grating that diffracts only +1 order diffracted light.

[0042] Furthermore, in the optical device or the like of the present disclosure including the favorable form described above, the part of the light that is emitted from the image forming device and enters the first A deflecting member may enter the second A deflecting member. That is, the first A deflecting member and the second A deflecting member may partially overlap. Specifically, the first A deflecting member has an end portion in the X direction, and the second A deflecting member has an end portion in the -X direction. The end portion of the first A deflecting member in the X direction and the end portion of the second A deflecting member in the -X direction may overlap each other.

[0043] Furthermore, in the optical device or the like of the present disclosure including the favorable form described above, the first A deflecting member and the first B deflecting member may be laminated, and the second A deflecting member and the second B deflecting member may be laminated. Alternatively, the first B deflecting member and the first C deflecting member may be laminated, and the second B deflecting member and the second C deflecting member may be laminated. The first A deflecting member, the first B deflecting member, and the first C deflecting member may be laminated, and the second A deflecting member, the second B deflecting member, and the second C deflecting member may be laminated.

[0044] Furthermore, in the optical device or the like of the present disclosure including the favorable form described above, the first A deflecting member, the first B deflecting member, and the first C deflecting member may each include a volume hologram diffraction grating, and satisfy

.eta..sub.1-B/.eta..sub.1-A<1, and

.eta..sub.1-C/.eta..sub.1-A<1

[0045] where with respect to the light emitted from the image forming device, an average diffraction efficiency of the first A deflecting member is .eta..sub.1-A, an average diffraction efficiency of the first B deflecting member is .eta..sub.1-B, and an average diffraction efficiency of the first C deflecting member is .eta..sub.1-C, and

[0046] the second A deflecting member, the second B deflecting member, and the second C deflecting member may each include a volume hologram diffraction grating, and satisfy

.eta..sub.2-B/.eta..sub.2-A<1, and

.eta..sub.2-C/.eta..sub.2-A<1

[0047] where with respect to the light emitted from the image forming device, an average diffraction efficiency of the second A deflecting member is .eta..sub.2-A, an average diffraction efficiency of the second B deflecting member is .eta..sub.2-B, and an average diffraction efficiency of the second C deflecting member is .eta..sub.2-C.

[0048] It is favorable to satisfy .eta..sub.1-B.ltoreq.0.2, .eta..sub.2-B.ltoreq.0.2, .eta..sub.1-C.ltoreq.0.2, and .eta..sub.2-C.ltoreq.0.2. Here, a diffraction efficiency .eta. is expressed by I.sub.1/I.sub.0, where the light intensity of the light incident on the volume hologram diffraction grating is I.sub.0, and the light intensity of the +1 order diffracted light diffracted by the volume hologram diffraction grating is I.sub.1. The diffraction efficiency can be controlled, for example, by the thickness of the volume hologram diffraction grating. That is, if the thickness of the volume hologram diffraction grating is reduced, the value of the diffraction efficiency n is lowered. Further, as a refractive index modulation degree .DELTA.n in the volume hologram diffraction grating becomes larger, the value of the diffraction efficiency .eta. becomes lower. For example, assuming that, with the diffraction efficiency .eta.=0.2, when the light (light amount=1.0) incident on the volume hologram diffraction grating is emitted from the volume hologram diffraction grating, LI.sub.1 represents the amount of light emitted from a region of the volume hologram diffraction grating closest to a light incident portion of the volume hologram diffraction grating, LI.sub.2 represents the amount of light emitted from the next closest region of the volume hologram diffraction grating, LI.sub.3 represents the amount of light emitted from the third closest region of the volume hologram diffraction grating, and LI.sub.4 represents the amount of light emitted from the fourth closest region of the volume hologram diffraction grating,

LI.sub.1=1.0.times.0.2=0.2,

LI.sub.2=(1.0-0.2).times.0.2=0.16,

LI.sub.3=(1.0-0.2-0.16).times.0.2=0.128, and

LI.sub.4=(1.0-0.2-0.16-0.128).times.0.2=0.102.

[0049] In the following description, for simplicity of explanation, the first A deflecting member and the second A deflecting member are collectively referred to as the first A deflecting member or the like, the first B deflecting member and the second B deflecting member are collectively referred to as the first B deflecting member or the like, and the first C deflecting member and the second C deflecting member are collectively referred to as the first C deflecting member or the like in some cases.

[0050] In the optical device or the like of the present disclosure including the favorable forms described above, the light emitted from the center point of the image forming region of the image forming device may also be configured to be incident perpendicularly to the first A deflecting member and the second A deflecting member, or may also be configured to be incident at a certain angle that is not perpendicular.

[0051] Further, all of the light deflected by the first A deflecting member may be incident on the first B deflecting member, all of the light deflected by the first B deflecting member may be incident on the first C deflecting member, all of the light deflected by the second A deflecting member may be incident on the second B deflecting member, and all of the light deflected by the second B deflecting member may be incident on the second C deflecting member. However, actually, part of the light deflected by the first A deflecting member and the second A deflecting member, and part of the light deflected by the first B deflecting member and the second B deflecting member may be lost in the light guide plate.

[0052] Furthermore, in the optical device or the like of the present disclosure including the favorable forms described above, the refractive index of the material constituting the light guide plate may be 1.5 or more, favorably 1.6 or more. The refractive index of the material constituting the volume hologram diffraction grating may be 1.5 or more, favorably 1.6 or more.

[0053] In the optical device or the like of the present disclosure including the favorable forms described above, the optical device is of a semi-transmissive type (see-through type). Specifically, at least the part of the optical device that faces an eyeball (pupil) of an observer is set to be semi-transmissive (see-through), and the outside may be viewed through this part of the optical device (specifically, at least the first C deflecting member and the second C deflecting member). Here, the term “semi-transmissive” does not mean transmitting or reflecting 1/2 (50%) of the incident light, but it is used in the sense of transmitting part of the incident light and reflecting the remaining part.

[0054] The image display device or the display apparatus of the present disclosure allows image display of a single color (e.g., green). On the other hand, if the image display of color is performed, the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like can be configured by laminating P layers of diffraction grating layers each made of a volume hologram diffraction grating so as to correspond to the diffraction of P types of light having different P types (e.g., P=3, three types of red, green, and blue) of wavelength bands (or wavelengths). Interference fringes corresponding to one type of wavelength band (or wavelength) are formed in each grating layer. Alternatively, in order to correspond to the diffraction of the P types of light having different P types of wavelength bands (or wavelengths), P types of interference fringes may be formed in the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a single diffraction grating layer. Alternatively, for example, the following structure may also be adopted: the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts and reflects light having a red wavelength band (or wavelength) are disposed on the first light guide plate; the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a green wavelength band (or a wavelength) are disposed on the second light guide plate; the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a blue wavelength band (or wavelength) are disposed on the third light guide plate; and the first light guide plate, the second light guide plate, and the third light guide plate are laminated with gaps between those light guide plates. Alternatively, for example, the following structure may also be adopted: the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts and reflects light having a red wavelength band (or wavelength) are disposed on one surface of the first light guide plate; the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a green wavelength band (or a wavelength) are disposed on the other surface of the first light guide plate; the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a blue wavelength band (or wavelength) are disposed on the second light guide plate; and the first light guide plate and the second light guide plate are laminated with gaps between those light guide plates. Alternatively, for example, the following structure may be adopted: the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts and reflects light having a red wavelength band (or wavelength), and the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a green wavelength band (or a wavelength) are laminated on one surface of a light guide plate; the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like each including a diffraction grating layer made of a volume hologram diffraction grating that diffracts light having a blue wavelength band (or wavelength) are disposed on the other surface of the light guide plate. Alternatively, it is also possible to form P types of volume hologram diffraction gratings in the single diffraction grating layer. If these configurations are employed, it is possible to achieve an increase in diffraction efficiency, an increase in diffraction acceptance angle, and an optimization of the diffraction angle when light having the respective wavelength bands (or wavelengths) is diffracted in the first A deflecting member or the like, the first B deflecting member or the like, and the like and the first C deflecting member or the like. It is favorable to dispose a protective member such that the volume hologram diffraction grating does not come into direct contact with the atmosphere.

[0055] Examples of the material constituting the volume hologram diffraction grating include a photopolymer material. Constituent materials and basic structures of the volume hologram diffraction grating in the optical device or the like of the present disclosure may be the same as the constituent materials and structures of the conventional volume hologram diffraction gratings. In the volume hologram diffraction grating, interference fringes are formed over the surface from its inside, and the method of forming such interference fringes themselves may be the same as the conventional forming methods. Specifically, for example, the material constituting the volume hologram diffraction grating (e.g., a photopolymer material) is irradiated with object light from a first predetermined direction on one side, and at the same time, the material constituting the volume hologram diffraction grating is irradiated with reference light from a second predetermined direction on the other side, and the interference fringes formed by the object light and the reference light may be recorded inside the material constituting the volume hologram diffraction grating. The first predetermined direction, the second predetermined direction, and the wavelengths of the object light and the reference light are appropriately selected, and thus a desired pitch of the interference fringes at the surface of the volume hologram diffraction grating, and a desired inclination angle (slant angle) of the interference fringes can be obtained. The inclination angle of the interference fringe means the angle formed by the interference fringe and the surface of the volume hologram diffraction grating. In a case of a laminated structure of the P layers of the diffraction grating layers each made of the volume hologram diffraction grating, such lamination of the diffraction grating layers may be performed by, after the P layers of the diffraction grating layers are separately manufactured, laminating (bonding) the P layers of the diffraction grating layers by using an ultraviolet-curable adhesive, for example. Further, the P layers of the diffraction grating layers may be manufactured by, after a single diffraction grating layer is manufactured using a photopolymer material having viscosity, sequentially bonding thereon a photopolymer material having viscosity, to manufacture the diffraction grating layers.

[0056] The inclination angle (slant angle) of the interference fringes may be constant in the volume hologram diffraction grating or may be changed depending on the value of the angle of view of an image incident on the volume hologram diffraction grating. If the inclination angle of the interference fringes is changed depending on the value of the angle of view of the incident image, the inclination angle may be changed continuously or stepwisely.

[0057] Any photopolymer material can be used as long as the material constituting the volume hologram diffraction grating (photopolymer material constituting photosensitive material precursor layer before being irradiated with object light and reference light) includes at least a photopolymerizable compound, a binder resin, and a photopolymerization initiator. As the photopolymerizable compound, for example, it is possible to use known photopolymerizable compounds such as an acrylic monomer, a methacrylic monomer, a styrene-based monomer, a butadiene-based monomer, a vinyl-based monomer, and an epoxy-based monomer. These compounds may be copolymers or may be monofunctional or polyfunctional. Further, these monomers may be used alone or in combination. Any known binder resin may be used as the above-mentioned binder resin. Specifically, a cellulose acetate-based resin, an acrylic-based resin, an acrylic acid ester-based resin, a methacrylic acid resin, an epoxy-based resin, a urethane-based resin, a polypropylene resin, a polyvinyl ether resin, a polycarbonate resin, a polyamide resin, a polyvinyl acetate, a vinyl chloride-based resin, a urea-based resin, a styrene-based resin, a butadiene-based resin, a natural rubber-based resin, a polyvinyl carbazole, a polyethylene glycol, a phenol-based resin, a copolymer including the above-mentioned resins, gelatin, and the like may be used. The binder resins may also be used alone or in combination. Any known photopolymerization initiator may be used as the above-mentioned photopolymerization initiator. The photopolymerization initiators may be used alone or in combination. The photopolymerization initiator may be used in combination with at least one photosensitizer pigment. A plasticizer, a chain transfer agent, and other additives may be added to the photosensitive material precursor layer, as appropriate. The protective layer for protecting the volume hologram diffraction grating may include any transparent material. The protective layer may be formed through coating or laminating a material made into a film in advance onto the photosensitive material precursor layer. The material constituting the protective layer may be, for example, a polyvinyl alcohol (PVA) resin, an acrylic resin, a polyurethane resin, a polyethylene terephthalate (PET) resin, a cellulose triacetate (TAC) resin, a polymethyl methacrylate (PMMA) resin, a polypropylene resin, a polycarbonate resin, and a polyvinyl chloride resin.

[0058] In the image display device or the image display device in the display apparatus of the present disclosure including the various favorable forms described above, the image forming device may have a plurality of pixels arranged in a two-dimensional matrix. Note that an image forming device having such a configuration is referred to as “image forming device of a first configuration”, for the purpose of convenience.

[0059] The image forming device of the first configuration may include: an image forming device including a reflective spatial light modulator and a light source; an image forming device including a transmissive spatial light modulator and a light source; and an image forming device including a light emitting element such as an organic electro luminescence (EL) element, an inorganic EL element, a light emitting diode (LED), or a semiconductor laser element. Of those, the image forming device including a reflective spatial light modulator and a light source or the image forming device including an organic EL element is favorably used. The spatial light modulator may include a transmissive or reflective liquid crystal display device including a light valve such as LCOS (Liquid Crystal On Silicon), and a digital micromirror device (DMD). The light source may include a light emitting element. Furthermore, the reflective spatial light modulator may include a liquid crystal display device, and a polarizing beam splitter that reflects part of light from the light source, guides the part of the light to the liquid crystal display device, passes therethrough part of the light reflected by the liquid crystal display device, and guides the part of the light to the optical system. The light emitting element constituting the light source may include a red light emitting element, a green light emitting element, a blue light emitting element, and a white light emitting element. Alternatively, red light, green light, and blue light emitted from the red light emitting element, the green light emitting element, and the blue light emitting element may be mixed and made uniform in luminance by using a light pipe, to thereby obtain white light. As the light emitting element, for example, a semiconductor laser device, a solid-state laser, and an LED can be exemplified. The number of pixels may be determined on the basis of the specifications required for the image display device. As a specific value of the number of pixels, 320.times.240, 432.times.240, 640.times.480, 1024.times.768, 1920.times.1080, and the like can be exemplified.

[0060] Alternatively, in the image display device or the image display device in the display apparatus of the present disclosure including the favorable forms described above, the image forming device may be configured to include a light source and scanning means for scanning parallel light emitted from the light source. Note that an image forming device having such a configuration is referred to as “image forming device of a second configuration”, for the purpose of convenience.

[0061] The light source in the image forming device of the second configuration may include a light emitting element. Specifically, a red light emitting element, a green light emitting element, a blue light emitting element, and a white light emitting element can be used. Alternatively, red light, green light, and blue light emitted from the red light emitting element, the green light emitting element, and the blue light emitting element may be mixed and made uniform in luminance by using a light pipe, to thereby obtain white light. As the light emitting element, for example, a semiconductor laser device, a solid-state laser, and an LED can be exemplified. The number of pixels (virtual pixels) in the image forming device of the second configuration may also be determined on the basis of the specifications required for the image display device. As a specific value of the number of pixels (virtual pixels), 320.times.240, 432.times.240, 640.times.480, 1024.times.768, 1920.times.1080, and the like can be exemplified. Further, in the case of performing image display of color and in the case where the light source includes a red light emitting element, a green light emitting element, and a blue light emitting element, for example, it is favorable to perform color synthesis using a cross rhythm. The scanning means may include, for example, a galvano mirror or a micro electro mechanical system (MEMS) including micromirrors rotatable in two-dimensional directions, which horizontally and vertically scans the light emitted from the light source.

[0062] In the image forming device of the first configuration or the image forming device of the second configuration, the light made to be a plurality of parallel light beams in the optical system (which is an optical system that collimates the light emitted from the image forming device to obtain parallel light and may be referred to as a “parallel light emitting optical system”, specifically, e.g., a collimating optical system or a relay optical system) is caused to enter the light guide plate. Such a request for being parallel light is based on the fact that the light wavefront information obtained when the light beams enter the light guide plate needs to be stored even after the light beams are emitted from the light guide plate through the first A deflecting member or the like, the first B deflecting member or the like, and the first C deflecting member or the like. Note that, in order to generate a plurality of parallel light beams, specifically, for example, a light emitting unit of the image forming device may be positioned at a position of the focal length in the parallel light emitting optical system, for example. The parallel light emitting optical system has a function of converting position information of a pixel into angle information in the optical system. Examples of the parallel light emitting optical system may include an optical system having a positive optical power as a whole, in which a convex lens, a concave lens, a free curved surface prism, and a hologram lens are used alone or in combination.

[0063] In order to cause the light emitted from the parallel light emitting optical system to enter the first A deflecting member and the second A deflecting member, appropriate light guide means only needs to be disposed between the parallel light emitting optical system, and the first A deflecting member and the second A deflecting member. A reflecting mirror can be used as the light guiding means. Further, the light emitted from the parallel light emitting optical system may be directly condensed on the first A deflecting member and the second A deflecting member.

[0064] The light guide plate has two parallel surfaces (first surface and second surface). Assuming that the surface of the light guide plate on which the light is incident is a light guide plate incident surface and that the surface of the light guide plate from which the light is emitted is a light guide plate emission surface, the light guide plate incident surface and the light guide plate emission surface may be configured by the first surface. Alternatively, the light guide plate incident surface may be configured by the first surface, and the light guide plate emission surface may be configured by the second surface.

[0065] Examples of the material constituting the light guide plate may include glass containing optical glass such as quartz glass or BK7, and a plastic material (e.g., a PMMA, a polycarbonate resin, an acrylic resin, an amorphous polypropylene-based resin, or a styrene-based resin containing an AS resin). The shape of the light guide plate is not limited to be flat, and it may have a curved shape. As a material having a refractive index of 1.5 or more, BK7, a polycarbonate resin, an amorphous polypropylene-based resin, and a styrene-based resin including an AS resin can be exemplified. As a material having a refractive index of 1.6 or more, an acrylic resin can be exemplified.

[0066] The image display device may include a dimmer. That is, the optical device may overlap at least part of the dimmer. More specifically, at least the first C deflecting member or the like of the optical device favorably overlap with the dimmer.

[0067] Specifically, the dimmer can be configured to include

[0068] a first substrate,

[0069] a second substrate facing the first substrate,

[0070] a first transparent electrode provided on the opposing surface of the first substrate facing the second substrate,

[0071] a second transparent electrode provided on the opposing surface of the second substrate facing the first substrate, and

[0072] a dimming layer sandwiched between the first transparent electrode and the second transparent electrode.

Note that, during operation of the dimmer, during operation of the dimmer, for example, a higher voltage is applied to the first transparent electrode than a voltage for the second transparent electrode.

[0073] The dimming layer may be an optical shutter that applies a color change of a substance generated by an oxidation-reduction reaction of an inorganic or organic electrochromic material. Specifically, the dimming layer may include an inorganic or organic electrochromic material. Further, the dimming layer may have a laminated structure of inorganic electrochromic material layers of a WO.sub.3 layer/Ta.sub.2O.sub.5 layer/Ir.sub.XSn.sub.1-XO layer, or a laminated structure of inorganic electrochromic material layers of a WO.sub.3 layer/Ta.sub.2O.sub.5 layer/IrO.sub.x layer, from the first transparent electrode side. Instead of the WO.sub.3 layer, a MoO.sub.3 layer or a V.sub.2O.sub.5 layer can be used. Alternatively, a ZrO.sub.2 layer or a zirconium phosphate layer may be used instead of the IrO.sub.x layer. Alternatively, a Prussian blue complex/nickel-substituted Prussian blue complex or the like may be used. As the organic electrochromic material, for example, an electrochromic material disclosed in Japanese Patent Application Laid Open No. 2014-111710 or Japanese Patent Application Laid Open No. 2014-159385 may also be used.

[0074] Alternatively, the dimming layer may include an electrophoretic dispersion or may include an optical shutter by an electrodeposition method (electrodeposition and electric field deposition) applying an electrodeposition and dissociation phenomenon generated by a reversible oxidation-reduction reaction of metal (e.g., silver particles), that is, an electrolyte containing metal ions.

[0075] Here, the electrophoretic dispersion contains a large number of charged electrophoretic particles and a dispersion medium having a color different from that of the electrophoretic particles. For example, in the case where the first transparent electrode is patterned, and the second transparent electrode is not patterned (so-called solid electrode configuration) and in the case where the electrophoretic particles are negatively charged, when a relatively negative voltage is applied to the first transparent electrode, and a relatively positive voltage is applied to the second transparent electrode, the negatively charged electrophoretic particles migrate so as to cover the second transparent electrode. Therefore, a light shielding rate in the dimmer has a high value. On the other hand, to the contrary, when a relatively positive voltage is applied to the first transparent electrode, and a relatively negative voltage is applied to the second transparent electrode, the electrophoretic particles migrate so as to cover the first transparent electrode. Therefore, the light shielding rate in the dimmer has a low value. Appropriately performing the voltage application to such transparent electrodes allows the light shielding rate in the dimmer to be controlled. The voltage may be direct current or alternating current. Any shape of the patterned first transparent electrode may be used if the electrophoretic particles migrate so as to cover the first transparent electrode and the value of the light shielding rate in the dimmer can be optimized when the light shielding rate in the dimmer has a low value. The shape of the patterned first transparent electrode may be determined by performing various tests. If desired, an insulating layer may be formed over the transparent electrode. As the material constituting the insulating layer, for example, a colorless transparent insulating resin may be used. Specifically, for example, an acrylic resin, an epoxy-based resin, a fluorine-based resin, a silicone-based resin, a polyimide-based resin, a polystyrene-based resin, or the like may be used.

[0076] As the material constituting the transparent first substrate and second substrate constituting the dimmer, specifically, a transparent glass substrate of soda-lime glass, white plate glass, or the like, a plastic substrate, a plastic sheet, and a plastic film may be exemplified. Here, the plastic may include polyethylene terephthalate, polyethylene naphthalate, polycarbonate, cellulose esters such as cellulose acetate, fluoropolymers such as copolymers of polyvinylidene fluoride or polytetrafluoroethylene and hexafluoropropylene, polyethers such as polyoxymethylene, polyacetal, polystyrene, polyethylene, polypropylene, polyolefin such as methylpentene polymer, polyimides such as polyamideimide or polyetherimide, polyamide, polyethersulfone, polyphenylene sulfide, polyvinylidene fluoride, tetraacetylcellulose, brominated phenoxy, polyarylate, polysulfone, and the like. The plastic sheet and the plastic film may have stiffness for which they are not bent easily, or may be flexible. If the first substrate and the second substrate are made of a transparent plastic substrate, a barrier layer made of an inorganic material or an organic material may be formed in the inner surface of the substrate.

[0077] The first substrate and the second substrate are sealed by a sealing member at the outer edge and are bonded to each other. Various resins of a thermosetting type, a photocurable type, a moisture curable type, an anaerobic curable type, and the like, such as an epoxy-based resin, an urethane-based resin, an acrylic resin, a vinyl acetate-based resin, an ene-thiol-based resin, a silicone-based resin, and a modified polymer resin, may be used as the sealing member, which is also referred to as a sealing agent.

[0078] If one of the substrates constituting the dimmer also serves as a constituent member of the optical device (specifically, a protective member disposed such that the volume hologram diffraction grating does not come into direct contact with the atmosphere), it is possible to reduce the weight of the entire display device, and there is no possibility of providing a discomfort feeling to a user of the display apparatus. Note that the other substrate may have a thinner configuration than the one of the substrates.

[0079] The first transparent electrode may be patterned or not. The second transparent electrode may be patterned or not. Specific examples of the materials constituting the first transparent electrode and the second transparent electrode may include, but are not limited to, an indium-tin composite oxide (including ITO, Indium Tin Oxide, Sn-doped In.sub.2O.sub.3, crystalline ITO, and amorphous ITO), fluorine-doped SnO.sub.2 (FTO), IFO (F-doped In.sub.2O.sub.3), antimony-doped SnO.sub.2 (ATO), SnO.sub.2, ZnO (including Al-doped ZnO or B-doped ZnO), an indium-zinc composite oxide (IZO, Indium Zinc Oxide), a spinel-type oxide, an oxide having a YbFe.sub.2O.sub.4 structure, and conductive polymers such as polyaniline, polypyrrole, and polythiophene. Further, it is also possible to use a combination of two or more types of them. The first transparent electrode and the second transparent electrode may be formed on the basis of physical vapor deposition methods (PVD methods) such as a vacuum vapor deposition method and a sputtering method, various chemical vapor deposition methods (CVD methods), various coating methods, and the like, and patterning can be performed in any method such as an etching method, a lift-off method, and a method using various masks.

[0080] The dimmer may be disposed on a front portion. In this case, the front portion may have a rim, and the dimmer may be fitted into the rim. Further, in the display apparatus of the present disclosure including various favorable forms described above, the optical device and the dimmer may be disposed in this order from the observer side, or the dimmer and the optical device may be disposed in this order.

[0081] An illuminance sensor (environmental illuminance measurement sensor) for measuring the illuminance of the environment, in which the display apparatus is placed, is further provided, and the light shielding rate of the dimmer can be controlled on the basis of a measurement result of the illuminance sensor (environmental illuminance measurement sensor). Alternatively, an illuminance sensor (environmental illuminance measurement sensor) for measuring the illuminance of the environment, in which the display apparatus is placed, is further provided, and the luminance of an image formed by the image forming device can be controlled on the basis of a measurement result of the illuminance sensor (environmental illuminance measurement sensor). These forms may be used in combination.

[0082] Alternatively, a second illuminance sensor for measuring the illuminance based on the light passing through the dimmer from the external environment (for the purpose of convenience, referred to as transmitted light illuminance measurement sensor in some cases) is further provided, and the light shielding rate of the dimmer can be controlled on the basis of a measurement result of the second illuminance sensor (transmitted light illuminance measurement sensor). Alternatively, a second illuminance sensor (transmitted light illuminance measurement sensor) for measuring the illuminance based on the light passing through the dimmer from the external environment is further provided, and the luminance of an image formed by the image forming device can be controlled on the basis of a measurement result of the second illuminance sensor (transmitted light illuminance measurement sensor). Note that the second illuminance sensor (transmitted light illuminance measuring sensor) is favorably disposed on the observer side relative to the optical device. At least two second illuminance sensors (transmitted light illuminance measurement sensors) may be disposed to measure the illuminance based on the light passing through a portion of a high light shielding rate and to measure the illuminance based on the light passing through a portion of a low light shielding rate. These forms may be used in combination. Furthermore, these forms may be combined with a form for performing control on the basis of the measurement result of the illuminance sensor (environmental illuminance measurement sensor) described above.

[0083] The illuminance sensor (environmental illuminance measurement sensor, transmitted light illuminance measurement sensor) may be formed of a well-known illuminance sensor, and the illuminance sensor may be controlled on the basis of a well-known control circuit.

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