Optical system and image display device

Abstract

Provided are an optical system that can improve the usage efficiency of image light from a display element, and an image display device. A light guide member (5) of the optical system comprises an emission extension region (53) having a diffractive structure that splits, in a first propagation direction intersecting a thickness direction of a main body part (50), image light (L1B) that enters inside the main body part (50) from an incident region (51) and propagates in the first propagation direction into a plurality of image light (L1C) propagating in a second propagation direction intersecting the first propagation direction, and causes the plurality of image light to be emitted from the light guide member (5). In a case assuming that, in at least a portion of the emission extension region (53), the refractive indices of the medium are equal between the incident side and emission side of the image light with respect to the emission extension region (53), an emission angle θ_out of image light (LIC) emitted at the highest diffraction efficiency from the emission extension region (53) in a prescribed plane (P1) including the normal (N) of the emission extension region (53) is 15-45°, and the incident angle θ_in of image light (LIB) incident on the emission extension region (53) in the prescribed plane (P1) is at least 14° greater than the emission angle θ_out.

Description

Optical system and image display device

[0001] The present disclosure relates to an optical system and an image display device.

[0002] A vehicle information projection system that uses a head-up display device to display augmented reality (AR) information has been disclosed. The head-up display device projects light representing a virtual image (image light from a display element) onto the windshield of the vehicle, allowing the driver to view the virtual image together with the actual view outside the vehicle.

[0003] Patent document 1 describes a display system that displays a virtual image, and describes using a diffraction element to diffract a light beam emitted from inside a waveguide (light guide) and change the direction of travel of the light beam.

[0004] Special table 2018-506068 publication

[0005] In the display system described in Patent Document 1, it is desirable to improve the utilization efficiency of image light from the display element.

[0006] The present disclosure provides an optical system and an image display device that can improve the utilization efficiency of image light from a display element.

[0007] According to one aspect of the present disclosure, there is provided an optical system including a light-guiding member that guides image light forming an image output from a display element to a user's field of view as an optical image. The light-guiding member includes a plate-shaped main body, an entrance region formed in the main body for allowing the image light to enter the main body so that the image light propagates within the main body, and an exit extension region formed in the main body and having a diffractive structure that splits the image light propagating in a first propagation direction intersecting a thickness direction of the main body into a plurality of image light beams propagating in a second propagation direction intersecting the first propagation direction, in the first propagation direction, and outputs the split image light from the light-guiding member. When the refractive index of the medium on the incident side of the image light relative to the output extension region and the refractive index of the medium on the exit side of the image light relative to the output extension region are equal, the output angle θ_out of the image light emitted from the output extension region with the highest diffraction efficiency in a specified plane including the normal to the output extension region is 15° or more and 45° or less, and the incident angle θ_in of the image light incident on the output extension region in the specified plane is 14° or more greater than the output angle θ_out.

[0008] An image display device according to one aspect of the present disclosure includes the above optical system and a display element.

[0009] The aspects of the present disclosure can improve the utilization efficiency of image light from a display element.

[0010] 3 is a schematic diagram of an example of the configuration of a moving body equipped with an image display device according to an embodiment; FIG. 4 is an explanatory diagram of an optical path of image light emitted from a display element of the image display device of FIG. 1; FIG. 5 is a perspective view of an example of the configuration of a light guide member of the image display device of FIG. 1; FIG. 6 is a schematic plan view of the light guide member of FIG. 3; FIG. 7 is a schematic diagram of a volume hologram element; FIG. 8 is a graph showing an example of the relationship between the angle of incidence and the angle of emission; FIG. 9 is an explanatory diagram of the diffraction effect of image light by the extended emission region of the light guide member of FIG. 3;

[0011] Hereinafter, embodiments will be described in detail with reference to the drawings as appropriate. However, more detailed description than necessary may be omitted. For example, detailed description of already well-known matters or redundant description of substantially identical configurations may be omitted. This is to avoid unnecessary redundancy in the following description and to facilitate understanding by those skilled in the art. Note that the inventor(s) provide the accompanying drawings and the following description to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

[0012] Unless otherwise specified, the positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings. Each drawing described in the following embodiments is a schematic drawing, and the ratios of the size and thickness of each component in each drawing do not necessarily reflect the actual dimensional ratios. Furthermore, the dimensional ratios of each component are not limited to the ratios shown in the drawings.

[0013] In this disclosure, expressions such as "directed in XX direction" and "propagated in XX direction" in relation to light mean that the light forming an image as a whole is directed in XX direction, and the light rays contained in the light forming the image may be tilted relative to XX direction. For example, "light directed in XX direction" only requires that the chief ray of this light is directed in XX direction, and the secondary rays of the light may be tilted relative to XX direction.

[0014] [1. Embodiment] [1.1 Configuration] Fig. 1 is a schematic diagram of a mobile object 100 including an image display device 1 according to the present embodiment. The mobile object 100 in Fig. 1 is an automobile. Hereinafter, the mobile object 100 may be referred to as an automobile 100 simply to facilitate understanding. The image display device 1 in Fig. 1 is a head-up display (HUD) used in the automobile 100.

[0015] The image display device 1 in Fig. 1 is installed in the passenger compartment of the automobile 100 so as to project an image onto a windshield 101 of the automobile 100 from below. In Fig. 1, the image display device 1 is disposed in the dashboard below the windshield 101. When an image is projected from the image display device 1 onto the windshield 101, the image reflected by the windshield 101 is visually recognized by a user D (driver, observer). The user D recognizes the image projected from the image display device 1 as a virtual image Iv. In this way, the image display device 1 displays the virtual image Iv superimposed on a real scene visible through the windshield 101.

[0016] The image display device 1 in FIG. 1 includes a display element 2, an optical system 3, and a control device 4.

[0017] The display element 2 outputs image light L1 that forms an image (video) to display the image. For simplicity, FIG. 1 depicts the image light L1 as directional light, but in reality, the image light L1 enters the optical system 3 as light having an angle corresponding to the viewing angle. The optical axis of the display element 2 is the optical axis of the image light L1. The optical axis of the image light L1 is, for example, the optical axis of light output from the center of the display element 2. Examples of the display element 2 include known displays such as a liquid crystal display, an organic electroluminescence (EL) display, a scanning MEMS mirror, an LCOS (Liquid Crystal on Silicon), a DMD (Digital Mirror Device), and a micro LED. The image generated by the image light L1 visually displays various information, such as road navigation guidance, the distance to a vehicle ahead, the remaining battery charge of the vehicle, and the current vehicle speed.

[0018] The optical system 3 guides the image light L1 output by the display element 2 to a viewing area Ac set for the user's eyes. In the viewing area Ac, the user can view the image formed by the display element 2 without interruption with their own eyes. Hereinafter, the image light output from the optical system 3 may be denoted by the symbol L2 as necessary to distinguish the image light output from the optical system 3 from the image light L1 output by the display element 2 toward the optical system 3.

[0019] 2 is an explanatory diagram of the optical path of image light L2 emitted from the optical system 3 of the image display device 1. In this embodiment, the optical system 3 widens the visual field Ac by a pupil dilation effect. That is, the optical system 3 widens the visual field Ac by replicating the pupil of the image light L1. In this embodiment, the visual field Ac is defined by a rectangular plane. As long as the user D is within the visual field Ac, the image formed by the image light L1 can be viewed even if the position of the user D's eye is shifted in the horizontal and vertical directions in FIG. 2 .

[0020] As shown in FIG. 1 , the optical system 3 includes a light guide member 5 and a projection optical system 6 .

[0021] The light guide member 5 guides the image light L1 that forms an image output from the display element 2 to the user's visual field Ac as an optical image. In this embodiment, the optical image is a virtual image Iv.

[0022] 3 is a perspective view of an example of the configuration of the light guide member 5. In describing the light guide member 5, reference will be made to the orthogonal coordinate system of three axes, XYZ, shown in FIG.

[0023] As shown in FIG. 3 , the light-guiding member 5 includes a main body 50 , an incident region 51 , an auxiliary expansion region 52 , and an exit expansion region 53 .

[0024] The main body 50 is formed of a material that is transparent in the visible light region. The main body 50 is plate-shaped. In this embodiment, the main body 50 is rectangular and plate-shaped. The main body 50 has a first surface 50a and a second surface 50b in the thickness direction of the main body 50. The thickness direction of the main body 50 is the Z-axis direction in FIG. 3. The main body 50 is disposed with the first surface 50a facing the display element 2 and the second surface 50b facing the viewing area Ac. In this case, the first surface 50a is an incident surface through which the image light L1 enters the main body 50, and the second surface 50b is an exit surface through which the image light L2 exits the main body 50. In this embodiment, the light-guiding member 5 is positioned so that the image light L2 exiting the main body 50 is reflected by the light-transmitting member and guided to the viewing area Ac as a virtual image Iv. This allows the user D to see the virtual image Iv superimposed on the real scene visible through the light-transmitting member from the field of view Ac. In the present embodiment, the light-transmitting member is the windshield 101, but the light-transmitting member is not limited to the windshield 101 and may be a member that is transparent (however, not limited to being colorless and transparent) in the visible light range, such as a combiner.

[0025] 4 is a plan view of the light-guiding member 5 as viewed from the second surface 50b side. As shown in FIG. 4, the incident region 51, the auxiliary expansion region 52, and the exit expansion region 53 are formed in the main body 50 of the light-guiding member 5.

[0026] The entrance region 51 allows the image light L1 to enter the main body 50 so that the image light L1 propagates within the main body 50. In this embodiment, the entrance region 51 allows the image light L1, which is incident on the first surface 50a of the main body 50 from a first oblique direction oblique to the normal to the first surface 50a of the main body 50, to enter the main body 50 so that the image light L1 propagates within the main body 50. In this embodiment, the first oblique direction is the direction indicated by D1 in FIG. 3. In this embodiment, the entrance region 51 allows the image light L1 to enter the main body 50 so that the image light L1 propagates within the main body 50 in a predetermined direction (positive direction of the X-axis) perpendicular to the thickness direction of the main body 50. In FIG. 4, the image light propagating in the positive direction of the X-axis is denoted by the symbol L1A. That is, the incident region 51 changes the traveling direction of the image light L1 incident on the incident region 51 and causes the image light L1A to propagate in a predetermined direction within the main body 50. In the present embodiment, the incident region 51 is used for coupling between the display element 2 and the light-guiding member 5. The incident region 51 causes the image light L1 to enter the main body 50 so that the image light L1 propagates within the main body 50 under total reflection conditions. The term "coupling" used here refers to a state in which the image light L1 propagates within the main body 50 of the light-guiding member 5 under total reflection conditions.

[0027] The incident area 51 is configured with a diffractive structure (periodic structure) that diffracts the image light L1. The diffractive structure of the incident area 51 is, for example, a volume hologram element (holographic diffraction grating). The incident area 51 is formed, for example, inside the main body 50.

[0028] FIG. 5 is a schematic diagram of a volume hologram element. The volume hologram element generates a diffraction effect by periodic modulation of the refractive index. In FIG. 5, the Z1 axis direction is the thickness direction of the volume hologram element. The X1 axis direction and the Y1 axis direction are perpendicular to the Z1 axis direction and are perpendicular to each other. The X1 axis direction is the direction in which the refractive index of the volume hologram element does not change. The Y1 axis direction is the direction in which the refractive index of the volume hologram element changes periodically. Gp in FIG. 5 indicates the diffraction period (diffraction pitch) [μm] of the volume hologram element. The diffraction period of the volume hologram element indicates the period of change in the refractive index of the volume hologram element. The diffraction period of the volume hologram element can be defined, for example, as the distance between the maximum refractive index or the minimum refractive index of the volume hologram element.

[0029] 4, the diffraction period of the volume hologram element in the entrance area 51 is represented by Gp1, and the X1, Y1, and Z1 axes of the volume hologram element in the entrance area 51 are represented by the X11, Y11, and Z11 axes, respectively. The direction of the X11 axis (the direction in which the refractive index does not change) of the volume hologram element in the entrance area 51 intersects with the direction of the Y axis at an angle Ga1 in a plane (in the XY plane) perpendicular to the thickness direction of the main body 50.

[0030] The incident region 51 uses a diffraction effect to cause the image light L1 to enter the main body 50 under conditions where the image light L1 is totally reflected by the first surface 50 a and the second surface 50 b. The incident region 51 causes the image light L1 to travel in the positive direction of the X-axis within the main body 50 by being totally reflected by the first surface 50 a and the second surface 50 b.

[0031] The size of the incident area 51 is set so that part or all of the image light L1 from the display element 2 that has passed through the projection optical system 6 is incident on the incident area 51. In this embodiment, as shown in FIG. 4 , the incident area 51 has a rectangular shape.

[0032] The auxiliary expansion region 52 is arranged to be aligned with the incident region 51 in a predetermined direction (the positive direction of the X-axis). The auxiliary expansion region 52 is configured with a diffractive structure (periodic structure) that diffracts the image light L1A. The diffractive structure of the auxiliary expansion region 52 is, for example, a volume hologram element (holographic diffraction grating). The auxiliary expansion region 52 is formed, for example, inside the main body 50. In FIG. 4 , the diffraction period of the volume hologram element of the auxiliary expansion region 52 is represented by Gp2, and the X1-axis, Y1-axis, and Z1-axis of the volume hologram element of the auxiliary expansion region 52 are represented by the X12-axis, Y12-axis, and Z12-axis, respectively. The direction of the X1-axis (the direction in which the refractive index does not change) of the volume hologram element of the auxiliary expansion region 52 intersects with the Y-axis at an angle Ga2 in a plane (in the XY plane) perpendicular to the thickness direction of the main body 50.

[0033] The auxiliary expansion region 52 changes the propagation direction of the image light L1A to a first propagation direction and splits it into multiple beams. The first propagation direction is a direction that intersects a predetermined direction in a plane (XY plane) perpendicular to the thickness direction of the main body 50. For example, the first propagation direction is the positive direction of the Y axis. In FIG. 4 , the image light propagating in the positive direction of the Y axis is designated by the symbol L1B. In this manner, the auxiliary expansion region 52 is configured to split the image light L1A propagating in the predetermined direction into multiple image light beams L1B propagating in the first propagation direction. The auxiliary expansion region 52 splits the image light L1A propagating within the main body 50 of the light-guiding member 5, thereby directing multiple image light beams L1B aligned in a predetermined direction (the positive direction of the X axis) toward the output expansion region 53. In this manner, the auxiliary expansion region 52 expands the pupil of the image light L1 in the predetermined direction (the positive direction of the X axis). In other words, as shown in Figure 4, the auxiliary expansion region 52 splits the image light L1A into multiple, approximately parallel image light L1B that travel toward the output expansion region 53, thereby replicating and expanding the pupil of the image light L1 projected by the projection optical system 6 in a predetermined direction (the positive direction of the X-axis).

[0034] The size of the auxiliary expansion region 52 is set so that the image light L1A from the entrance region 51 enters the auxiliary expansion region 52. As an example, the dimension in the Y-axis direction of the end of the auxiliary expansion region 52 on the entrance region 51 side (the right end in FIG. 4 ) is set so that all of the image light L1A diffracted by the entrance region 51 enters the auxiliary expansion region 52. In the present embodiment, as shown in FIG. 4 , the auxiliary expansion region 52 has a rectangular shape.

[0035] The output expansion region 53 is arranged alongside the auxiliary expansion region 52 in the first propagation direction (positive direction of the Y axis). The output expansion region 53 is configured with a diffractive structure (periodic structure) that diffracts the image light L1B. The diffractive structure of the output expansion region 53 is, for example, a volume hologram element (holographic diffraction grating). In FIG. 4 , the diffraction period of the volume hologram element of the output expansion region 53 is represented by Gp3, and the X1-axis, Y1-axis, and Z1-axis of the volume hologram element of the output expansion region 53 are represented by the X13-axis, Y13-axis, and Z13-axis, respectively. The direction of the X1-axis (the direction in which the refractive index does not change) of the volume hologram element of the output expansion region 53 intersects with the Y-axis at an angle Ga3 in a plane (XY plane) perpendicular to the thickness direction of the main body 50.

[0036] The output extension region 53 changes the traveling direction of the image light L1B to a second propagation direction and splits it into multiple beams. The second propagation direction is the direction from the main body 50 toward the viewing area Ac. In FIG. 3 , the image light propagating in the second propagation direction is denoted by the symbol L1C. The output extension region 53 outputs the multiple image light beams L1C propagating in the second propagation direction from the second surface 50b of the main body 50 in a second inclined direction inclined with respect to the normal to the second surface 50b of the main body 50. In FIG. 3 , the image light propagating in the second inclined direction is denoted by the symbol L2. In this embodiment, the second inclined direction is the direction indicated by D1 in FIG. 3 . Therefore, the first inclined direction and the second inclined direction are parallel to each other. In this manner, the output expansion region 53 is configured to split the image light L1B propagating in the first propagation direction into multiple image light beams L2 that propagate in the second propagation direction and exit from the second surface 50b of the main body 50 in the second inclined direction. The output expansion region 53 splits the image light L1B propagating within the main body 50 of the light-guiding member 5, thereby directing the multiple image light beams L2 aligned in the first propagation direction (positive direction of the Y axis) toward the field of view Ac. In this manner, the output expansion region 53 expands the pupil of the image light L1 in the first propagation direction (positive direction of the Y axis). That is, as shown in FIG. 3 , the output expansion region 53 splits the image light L1B into multiple, substantially parallel image light beams L2 that propagate toward the field of view Ac, thereby replicating and expanding the pupil of the image light L1 projected by the projection optical system 6 in the first propagation direction (positive direction of the Y axis).

[0037] In this way, the auxiliary expansion region 52 and the exit expansion region 53 divide the image light L1 incident from the entrance region 51 into the main body 50 of the light-guiding member 5, thereby replicating and expanding the pupil of the image light L1. In the present embodiment, the auxiliary expansion region 52 and the exit expansion region 53 split the image light L1 incident from the entrance region 51 into multiple image light beams L2 and emit the multiple image light beams L2 into the field of view Ac, thereby replicating and expanding the pupil of the image light L1. The multiple image light beams L2 reaching the field of view Ac may be parallel to one another. The expression "the multiple image light beams L2 are parallel to one another" does not necessarily mean that the multiple image light beams L2 are parallel to one another in the strict sense, but also includes the multiple image light beams L2 being approximately parallel to one another. The multiple image light beams L2 do not need to be parallel to one another in the strict sense, as long as the directions of the multiple image light beams L2 are aligned to the extent that they can be considered parallel in terms of optical design. Having the multiple image light beams L2 parallel to one another improves the uniformity of the pupil arrangement of the image light in the field of view Ac, thereby reducing pupil loss of the image light in the field of view Ac. In this embodiment, as shown in FIG. 2 , the multiple image light beams L2 are reflected by the windshield 101 and head toward the field of view Ac. The light-guiding member 5 is preferably configured so that the multiple image light beams L2 reflected by the windshield 101 and head toward the field of view Ac are parallel to one another. When the windshield 101 has a curved surface as shown in FIG. 2 , even if the multiple image light beams L2 head toward the windshield 101 in the same direction, the angles of incidence and reflection of the image light beams L2 vary depending on the location within the surface of the windshield 101. Therefore, the light-guiding member 5 is configured so that the multiple image light beams L2 are not necessarily parallel to one another until they are emitted from the light-guiding member 5 and enter the windshield 101, but become parallel to one another after being reflected by the windshield 101. On the other hand, when the windshield 101 is flat, the angle of incidence and the angle of reflection of the image light L2 are the same regardless of the location within the surface of the windshield 101 as long as the direction toward the windshield 101 is the same. Therefore, the light-guiding member 5 may be configured so that multiple parallel beams of image light L2 are emitted from the light-guiding member 5.

[0038] The projection optical system 6 projects image light L1 that forms an image output from the display element 2. As a result, the projection optical system 6 causes the image light L1 from the display element 2 to be incident on the light-guiding member 5. As shown in FIG. 1 , the projection optical system 6 is located between the display element 2 and the light-guiding member 5. The projection optical system 6, for example, collimates the image light L1 from the display element 2 and causes it to be incident on the incident area 51. The projection optical system 6 causes the image light L1 to be incident on the incident area 51 as approximately collimated light. The projection optical system 6 is, for example, a biconvex lens.

[0039] The control device 4 can be realized using a semiconductor element or the like. The control device 4 can be configured, for example, by a microcomputer, a CPU, an MPU, a GPU, a DSP, an FPGA, or an ASIC. The control device 4 realizes predetermined functions by reading data and programs stored in the storage device 4a and performing various arithmetic processing. The storage device 4a is a storage medium that stores programs and data necessary to realize the functions of the control device 4. The storage device 4a can be realized, for example, by a hard disk drive (HDD), an SSD, a RAM, a DRAM, a ferroelectric memory, a flash memory, a magnetic disk, or a combination thereof. The storage device 4a stores multiple image data representing a virtual image Iv as an optical image. The control device 4 determines the virtual image Iv to be displayed based on vehicle-related information acquired from an external device. The control device 4 reads the image data of the determined virtual image Iv from the storage device 4a and outputs it to the display element 2.

[0040] In the image display device 1 described above, the light-guiding member 5 is configured so that, when the refractive indexes of the medium on the incident side of the image light relative to the exit extension region 53 and the medium on the exit side of the image light relative to the exit extension region 53 are equal, θ_out is greater than or equal to 15° and less than 45°, and θ_in is greater than θ_out by 14° or more, for at least a portion of the exit extension region 53.

[0041] 3, θ_in is the angle of incidence of image light L1B incident on the output extension region 53 in a predetermined plane P1 that includes the normal N1 of the output extension region 53. θ_out is the angle of emission of image light L1C that is emitted from the output extension region 53 with the highest diffraction efficiency in the predetermined plane P1 that includes the normal N1 of the output extension region 53.

[0042] This can reduce the generation of unnecessary diffracted light (hereinafter also simply referred to as unnecessary light) in the extended emission region 53. This can improve the utilization efficiency of the image light L1 from the display element 2.

[0043] In particular, in this embodiment, the light guide member 5 is configured to satisfy the following formula (1).

[0044]

[0045] In the present embodiment, when the refractive index of the medium on the image light incident side of the output extension region 53 and the refractive index of the medium on the image light exit side of the output extension region 53 are equal in at least a portion of the output extension region 53, satisfying formula (1) can reduce the generation of unnecessary diffracted light (hereinafter simply referred to as unnecessary light) in the output extension region 53. This can improve the utilization efficiency of the image light L1 from the display element 2.

[0046] Here, at least a portion of the output extension region 53 may be any part of the output extension region 53, but may be included in the end of the output extension region 53 on the input region 51 side on the optical path of the image light L1 from the input region 51 to the output extension region 53. In this embodiment, the optical path of the image light L1 from the input region 51 to the output extension region 53 passes through the auxiliary extension region 52. Therefore, the "end of the output extension region 53 on the input region 51 side on the optical path of the image light L1 from the input region 51 to the output extension region 53" is, for example, the end of the output extension region 53 on the input region 51 side in FIG. 4 , not the end on the negative side of the X axis of the output extension region 53, but the end on the negative side of the Y axis. In this embodiment, the end of the output extension region 53 on the input region 51 side may be a region occupying one-quarter of the output extension region 53 in the first propagation direction from the end 53 a of the output extension region 53 on the input region 51 side. On the optical path of the image light L1 from the incident region 51 to the output expansion region 53, the end of the output expansion region 53 on the incident region 51 side is the end of the output expansion region 53 on the side where the image light is incident, and therefore is a portion of the output expansion region 53 where a large amount of image light L1B enters the output expansion region 53. Therefore, by reducing the generation of unnecessary light in the portion where a large amount of image light L1B enters the output expansion region 53, it is possible to efficiently improve the utilization efficiency of the image light L1 from the display element 2. Of course, the light-guiding member 5 may be configured so as to satisfy formula (1) over the entire output expansion region 53 when the refractive indexes of the medium on the input side of the image light relative to the output expansion region 53 and the medium on the output side of the image light relative to the output expansion region 53 are equal.

[0047] Table 1 below shows the lower limit of the incident angle θ_in that satisfies Expression (1). In Table 1, the incident angle θ_in is a value rounded off to the nearest whole number.

[0048]

[0049] 6 is a graph showing an example of the relationship between the incident angle θ_in and the output angle θ_out, where R1 shows the relationship in Table 1, and R2 shows the relationship θ_in=θ_out for reference only.

[0050] When θ_out is at least 15° or more and 45° or less, R1 can be approximated by a straight line, and from this point of view, in this embodiment, the light-guiding member 5 is configured to satisfy the following formula (2): In the following formula (2), the incident angle θ_in is a value rounded off to the nearest integer.

[0051] In the present embodiment, when the refractive index of the medium on the image light incident side of the output extension region 53 and the refractive index of the medium on the image light exit side of the output extension region 53 are equal in at least a portion of the output extension region 53, satisfying formula (2) can reduce the generation of unnecessary light in the output extension region 53. This can improve the utilization efficiency of the image light L1 from the display element 2.

[0052] Next, the above formula (1) will be described with reference to Fig. 7 and Fig. 8. Fig. 7 is an explanatory diagram of the diffraction effect of image light L1B by the extended emission region 53 of the light-guiding member 5. Fig. 8 is an explanatory diagram of the diffraction effect of image light by the extended emission region 53 of the light-guiding member 500 of a comparative example. Diffraction by the extended emission region 53 is expressed by the following formula (3).

[0053]

[0054] In Equation (3), m is the diffraction order, d is the diffraction pitch (diffraction period), λ is the wavelength of the image light L1B, and θm is the emission angle of the m-th diffracted image light (m-th diffracted light) emitted from the emission extension region 53 on a predetermined plane P1 including the normal N1 of the emission extension region 53.

[0055] In formula (3), n_in is the refractive index of the medium on the incident side of the image light relative to the extended output region 53. n_out is the refractive index of the medium on the exit side of the extended output region 53. In this embodiment, the medium on the incident side of the image light relative to the extended output region 53 and the medium on the exit side of the image light relative to the extended output region 53 are both the main body 50 of the light-guiding member 5. The refractive index n_in and the refractive index n_out are equal. Therefore, the refractive index of the main body 50 of the light-guiding member 5 is n ０ Then, the following equation (4) is obtained from the above equation (3).

[0056]

[0057] 8, image light beams L1C, L1D, and L1E traveling in different directions are obtained from image light beam L1B by the output expansion region 53. Here, image light beam L1C is first-order diffracted light, image light beam L1D is −1st-order diffracted light, and image light beam L1E is second-order diffracted light. Image light beam L1B on the output side of the output expansion region 53 is zeroth-order diffracted light.

[0058] The image light L1C is the image light that is emitted with the highest diffraction efficiency from the emission extension region 53. Since the image light L1C is first-order diffracted light, m=1, and the following equation (5) holds.

[0059]

[0060] The image light L1D is unnecessary light. Since the image light L1D is −1st-order diffracted light, m=−1, and the following equation (6) holds.

[0061]

[0062] 7, by eliminating the image light L1D, the amount of image light L1C increases. This is expected to improve the utilization efficiency of the image light L1 from the display element 2. The condition for the image light L1D not to be present is that the image light L1D is not present on the output side of the output extension region 53, which is the condition when θ －１ From the above, the condition for the absence of the image light L1D is given by the following equation (7).

[0063]

[0064] From equations (6) and (7), the following equation (8) is obtained.

[0065] From equations (5) and (8), the following equation (9) is obtained.

[0066]

[0067] Since the image light L1C is the image light that is emitted from the emission extension region 53 with the highest diffraction efficiency, θ １ is replaced with θ_out and transformed, the above equation (1) is obtained.

[0068] The phrase "assuming that the refractive index of the medium on the incident side of the image light with respect to the output expansion region 53 is equal to that of the medium on the exit side of the image light with respect to the output expansion region 53" means, for example, that the output angle sin θ_out is corrected in advance under the condition that the refractive index of the medium on the incident side of the image light with respect to the output expansion region 53 is equal to that of the medium on the exit side of the image light with respect to the output expansion region 53. For example, the output angle sin θ_out when the refractive index of the medium on the incident side of the image light with respect to the output expansion region 53 is different from that of the medium on the exit side of the image light with respect to the output expansion region 53 is set to sin θ_out1, and the output angle sin θ_out when the refractive index of the medium on the incident side of the image light with respect to the output expansion region 53 is equal to that of the medium on the exit side of the image light with respect to the output expansion region 53 is set to sin θ_out2. Using the above refractive indices n_in and n_out, sin θ_out2=(n_out / n_in)×sin θ_out1. That is, sin θ_out2 may be used in place of sin θ_out in equation (1).

[0069] 8 has a similar configuration to the light-guiding member 5 of FIG. 3 , but unlike the light-guiding member 5, no part of the extended-emission region 530 satisfies the above formula (1). Therefore, the image light L1D is present in the light-guiding member 500 of the comparative example. The presence of the image light L1D reduces the amount of the image light L1C, which may be one of the factors reducing the utilization efficiency of the image light L1 from the display element 2.

[0070] FIG. 9 is a graph showing an example of the relationship between the incident angle (θ_in) of the image light L1B into the extended output region 53 and the proportion of −1st-order diffracted light (image light L1D). As an example, when the incident angle is 43°, the proportion of first-order diffracted light is 18%, the proportion of −1st-order diffracted light is 10%, and the proportion of second-order diffracted light is 0.6%. As an example, when the incident angle is 51°, the proportion of first-order diffracted light is 22%, the proportion of −1st-order diffracted light is 0%, and the proportion of second-order diffracted light is 0.2%. Therefore, by eliminating the presence of −1st-order diffracted light (image light L1D), the proportion of first-order diffracted light (image light L1C), i.e., the amount of light, increases. This is expected to improve the utilization efficiency of the image light L1 from the display element 2.

[0071] The diffraction efficiency of the diffractive structure of the output expansion region 53 can vary depending on the diffraction pitch (diffraction period Gp3) and the polarization state of the image light. FIG. 10 is a graph showing an example of the relationship between diffraction pitch and diffraction efficiency. Graph F1 in FIG. 10 shows the change in the maximum value of diffraction efficiency as the polarization state of the image light changes, depending on the diffraction pitch. Graph F2 in FIG. 10 shows the change in the minimum value of diffraction efficiency as the polarization state of the image light changes, depending on the diffraction pitch. In FIG. 10 , the angle of incidence of the image light on the output expansion region 53 is 45°, and the wavelength of the image light is 0.52 μm. In FIG. 10 , the diffractive structure of the output expansion region 53 is a volume hologram element, and this volume hologram element has a thickness of 5 μm, a refractive index of 1.505, and a refractive index modulation amount of 0.03.

[0072] As can be seen from FIG. 10 , when the diffraction pitch is in the range of 1.4 μm to 2.0 μm, both the maximum and minimum values ​​of the diffraction efficiency increase. When the diffraction pitch is below approximately 0.45 μm, the difference between the maximum and minimum values ​​of the diffraction efficiency rapidly increases. This increase in the difference between the maximum and minimum values ​​of the diffraction efficiency leads to an increase in the change in the diffraction efficiency as the image light propagates through the output expansion region 53, which contributes to a decrease in the image quality of the virtual image Iv. This is because pupil expansion involves a change in the polarization state of any image light due to the diffractive structure. If the diffraction pitch is too large, both the maximum and minimum values ​​of the diffraction efficiency decrease. In FIG. 10 , when the diffraction pitch is approximately 3.5 μm, the maximum and minimum values ​​of the diffraction efficiency are approximately half of the peak (the maximum and minimum values ​​of the diffraction efficiency when the diffraction pitch is in the range of 1.4 μm to 2.0 μm).

[0073] A simulation was conducted to measure the change in the maximum and minimum diffraction efficiencies depending on the diffraction pitch when the polarization state of the image light was changed while changing the diffraction structure of the output expansion region 53, as shown in Figure 10.The results confirmed that when the diffraction period of the output expansion region 53 is 1.23 x λ / n or more and 10.0 x λ / n or less, the maximum and minimum diffraction efficiencies can be increased while reducing the difference between the maximum and minimum values.In other words, the utilization efficiency of the image light L1 from the display element 2 can be improved, and degradation of the image quality of the virtual image Iv due to the polarization state of the image light L1 can be reduced.Here, λ is the wavelength of the image light, and n is the refractive index of the output expansion region 53.If the diffraction structure of the output expansion region 53 is a volume hologram element, the refractive index of the output expansion region 53 is the refractive index of the volume hologram element.

[0074] [1.2 Examples and Comparative Examples] Examples and comparative examples of the light guide member 5 will be described below. The following examples are merely a part of possible examples of the light guide member 5.

[0075] Tables 2 to 4 below show the image light parameters and diffractive structure parameters for the entrance region 51, auxiliary extension region 52, and exit extension region 53 of Examples 1 to 4 and Comparative Examples 1 to 3, respectively.

[0076] The parameters of the image light related to the entrance region 51, the auxiliary extension region 52, and the exit extension region 53 include θ_inx [°], θ_iny [°], θ_outx [°], and θ_outy [°].

[0077] θ_inx is the angle of incidence of the image light in the XZ plane of the light-guiding member 5 in FIG. 3 . θ_outx is the angle of emission of the image light in the XZ plane of the light-guiding member 5. This will be described in more detail with reference to FIG. 11 . FIG. 11 is an explanatory diagram of the angle of incidence θ_inx and the angle of emission θ_outx of the image light in the XZ plane of the light-guiding member 5 in FIG. 3 . In FIG. 11 , the diffractive structure 54 represents the diffractive structure of the incident region 51, the auxiliary expansion region 52, or the emission expansion region 53. Image light L11 represents the image light incident on the incident region 51, the auxiliary expansion region 52, or the emission expansion region 53. Image light L12 represents the image light emitted with the highest diffraction efficiency from the incident region 51, the auxiliary expansion region 52, or the emission expansion region 53. In FIG. 11 , θ_inx is the angle of incidence of the image light L11 incident on the diffractive structure 54 in a predetermined plane (XZ plane) including the normal N2 of the diffractive structure 54. In FIG. 11, θ_outx is the emission angle of the image light L12 that is emitted from the diffractive structure 54 with the highest diffraction efficiency in a predetermined plane (XZ plane) that includes the normal N2 of the diffractive structure 54.

[0078] θ_iniy is the angle of incidence of the image light in the YZ plane of the light-guiding member 5 in FIG. 3 . θ_outy is the angle of emission of the image light in the YZ plane of the light-guiding member 5. This will be described in more detail with reference to FIG. 12 . FIG. 12 is an explanatory diagram of the angle of incidence θ_iniy and the angle of emission θ_outy of the image light in the YZ plane of the light-guiding member 5 in FIG. 3 . In FIG. 12 , the diffractive structure 54 represents the diffractive structure of the entrance region 51, the auxiliary expansion region 52, or the exit expansion region 53. Image light L11 represents the image light incident on the entrance region 51, the auxiliary expansion region 52, or the exit expansion region 53. Image light L12 represents the image light emitted from the entrance region 51, the auxiliary expansion region 52, or the exit expansion region 53 with the highest diffraction efficiency. In FIG. 12 , θ_iniy is the angle of incidence of the image light L11 incident on the diffractive structure 54 in a predetermined plane (YZ plane) including the normal N2 of the diffractive structure 54. In FIG. 12, θ_outy is the emission angle of the image light L12 that is emitted from the diffractive structure 54 with the highest diffraction efficiency in a predetermined plane (YZ plane) that includes the normal N2 of the diffractive structure 54.

[0079] The parameters of the diffractive structure for the input region 51 include Ga1 [°] and Gp1 [μm] shown in Fig. 4. The parameters of the diffractive structure for the auxiliary extension region 52 include Ga2 [°] and Gp2 [μm] shown in Fig. 4. The parameters of the diffractive structure for the output extension region 53 include Ga3 [°] and Gp3 [μm] shown in Fig. 4.

[0080]

[0081]

[0082]

[0083] Table 5 below shows the left and right sides of equation (1) and the presence or absence of unwanted light for Examples 1 to 4 and Comparative Examples 1 to 3. The left and right sides of equation (1) in Table 5 are calculated by substituting θ_iny from Table 4 for θ_in in equation (1) and θ_outy from Table 4 for θ_out in equation (1). Note that in Examples 1 to 4 and Comparative Examples 1 to 3, the refractive index n of the main body 50 of the light-guiding member 5 is 1.505. In Examples 1, 3, and 4 and Comparative Examples 1 and 3, the wavelength of the image light L1 is 0.52 μm. In Example 2 and Comparative Example 2, the wavelength of the image light L1 is 0.62 μm.

[0084]

[0085] When comparing Example 1 with Comparative Example 1, the presence of unnecessary light was confirmed in the YZ plane in Comparative Example 1, but not in Example 1. When comparing Example 2 with Comparative Example 2, the presence of unnecessary light was confirmed in the YZ plane in Comparative Example 2, but not in Example 2. When comparing Example 3 with Comparative Example 3, the presence of unnecessary light was confirmed in Comparative Example 3 in a plane tilted 7.12° from the Y-axis direction in the XY plane, but not in Example 3. In Example 4, the presence of unnecessary light was not confirmed in the YZ plane.

[0086] From Table 5, it can be seen that Examples 1 to 4 satisfy Expression (1), while Comparative Examples 1 to 3 do not. Therefore, by having at least a portion of the extended emission region 53 satisfy Expression (1), it is possible to reduce the generation of unnecessary light in the extended emission region 53. This improves the utilization efficiency of the image light L1 from the display element 2. In particular, in Examples 1 to 3, the emission angle θ_outy of the image light in the YZ plane of the light-guiding member 5 is 15° or greater and 45° or less, and the incident angle θ_iny of the image light in the YZ plane of the light-guiding member 5 is 14° or greater than the emission angle θ_outy. Furthermore, Examples 1 to 3 satisfy Expression (2). In Examples 1 to 3, the first and second inclination angles of the image light L1 are not 0°, making them suitable for head-up displays. In Example 4, the first and second inclination angles of the image light L1 are 0°, making them suitable for head-mounted displays.

[0087] [1.3 Effects, etc.] The optical system 3 described above includes a light-guiding member 5 that guides image light L1 that forms an image output from the display element 2 to the visual field Ac of the user D as an optical image (virtual image Iv). The light-guiding member 5 includes a plate-shaped main body 50, an entrance region 51 that is formed in the main body 50 and that allows the image light L1 to enter the main body 50 so that the image light L1 propagates within the main body 50, and an exit extension region 53 that is formed in the main body 50 and has a diffractive structure that splits, in the first propagation direction, the image light L1B propagating in a first propagation direction intersecting the thickness direction of the main body 50 into a plurality of image light beams L1C that propagate in a second propagation direction intersecting the first propagation direction, and outputs the split image light L1B from the light-guiding member 5. In at least a portion of the output extension region 53, when the refractive index of the medium on the image light incident side relative to the output extension region 53 is equal to the refractive index of the medium on the image light exit side relative to the output extension region 53, the output angle θ_out of the image light L1C emitted from the output extension region 53 with the highest diffraction efficiency in a predetermined plane P1 containing the normal N of the output extension region 53 is equal to or greater than 15° and equal to or less than 45°, and the incident angle θ_in of the image light L1B incident on the output extension region 53 in the predetermined plane P1 is greater than the output angle θ_out by 14° or more. This configuration can improve the utilization efficiency of the image light L1 from the display element 2.

[0088] In the optical system 3, the following formula (1) is satisfied.

[0089]

[0090] This configuration can improve the efficiency of use of the image light L1 from the display element 2.

[0091] In the optical system 3, the following formula (2) is satisfied.

[0092]

[0093] This configuration can improve the efficiency of use of the image light L1 from the display element 2.

[0094] In the optical system 3, the main body 50 has a first surface 50a and a second surface 50b in the thickness direction. The entrance region 51 allows the image light L1, which is incident on the first surface 50a from a first oblique direction oblique to the normal to the first surface 50a, to enter the main body 50 so that the image light L1 propagates within the main body 50. The exit extension region 53 allows multiple image light beams L1C, which propagate in a second propagation direction, to exit from the second surface 50b in a second oblique direction oblique to the normal to the second surface 50b. This configuration can improve the utilization efficiency of the image light L1 from the display element 2.

[0095] In the optical system 3, the first tilt direction and the second tilt direction are parallel to each other. This configuration can improve the utilization efficiency of the image light L1 from the display element 2.

[0096] In the optical system 3, the diffractive structure of the output expansion region 53 is a volume hologram element located inside the main body 50. This configuration allows the size of the output expansion region 53 to be easily increased.

[0097] In the optical system 3, the light-guiding member 5 further includes an auxiliary expansion region 52 formed in the main body 50 and having a diffraction structure that splits, in the predetermined direction, the image light L1A propagating in the main body 50 by the entrance region 51 into a plurality of image light beams L1B propagating in a first propagation direction and directs the light beams toward the exit expansion region 53. This configuration enables pupil expansion in a plurality of different directions.

[0098] In the optical system 3, the diffraction pitch of the output extension region 53 is equal to or greater than 1.23×λ / n and equal to or less than 10.0×λ / n, where λ is the wavelength of the image light L1, L1A, L1B, L1C, and L2, and n is the refractive index of the output extension region 53. This configuration can improve the utilization efficiency of the image light L1 from the display element 2 and reduce degradation in the image quality of the optical image (virtual image Iv) due to the polarization state of the image light L1.

[0099] In the optical system 3, at least a portion of the exit extension region 53 is included in the end of the exit extension region 53 on the side of the incident region 51 on the optical path of the image light L1 from the incident region 51 to the exit extension region 53. This configuration can improve the utilization efficiency of the image light L1 from the display element 2.

[0100] In the optical system 3, the light-guiding member 5 is positioned so as to guide the image light L2 emitted from the main body 50 to the viewing area Ac as an optical image (virtual image Iv) by reflecting it off the light-transmitting member (windshield 101). This configuration can be used as a head-up display.

[0101] The optical system 3 further includes a projection optical system 6 that causes the image light L1 to be incident as substantially collimated light on the incident area 51 of the light-guiding member 5. This configuration can further improve the utilization efficiency of the image light L1 from the display element 2.

[0102] The image display device 1 described above includes the optical system 3 and the display element 2. This configuration can improve the utilization efficiency of the image light L1 from the display element 2.

[0103] [2. Modifications] The embodiments of the present disclosure are not limited to the above-described embodiments. The above-described embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved. Modifications of the above-described embodiments are listed below. The modifications described below can be applied in appropriate combinations.

[0104] In one modified example, the diffraction structure of the output expansion region 53 is not limited to a volume hologram element (holographic diffraction grating) and may be a surface relief diffraction grating. When the surface relief diffraction grating is a reflective type, the medium on the side where the image light enters the output expansion region 53 and the medium on the side where the image light exits the output expansion region 53 are both the main body 50 of the light-guiding member 5. When the surface relief diffraction grating is a transmissive type, the medium on the side where the image light enters the output expansion region 53 is the main body 50 of the light-guiding member 5, but the medium on the side where the image light exits the output expansion region 53 is, for example, air. Regardless of whether the surface relief diffraction grating is a reflective type or a transmissive type, it is sufficient that at least a portion of the output expansion region 53 satisfies Equation (1) when the refractive indexes of the medium on the side where the image light enters the output expansion region 53 and the medium on the side where the image light exits the output expansion region 53 are equal.

[0105] FIG. 13 is a schematic diagram of a surface relief diffraction grating. In particular, the surface relief diffraction grating of FIG. 13 is a reflective type. The surface relief diffraction grating generates diffraction action using periodically arranged uneven portions 55. The uneven portions 55 in FIG. 13 are grooves. In FIG. 13, the direction of the Z2 axis is the thickness direction of the surface relief diffraction grating. The directions of the X2 axis and the Y2 axis are perpendicular to the direction of the Z2 axis and perpendicular to each other. The direction of the X2 axis is the direction in which the uneven portions 55 extend. The direction of the Y2 axis is the direction in which the uneven portions 55 are arranged. Gp in FIG. 13 indicates the diffraction period (diffraction pitch) [μm] of the surface relief diffraction grating. The diffraction period Gp3 of the output extension region 53 of FIG. 4 may correspond to the diffraction period Gp in FIG. 13. The angle Ga3 of the output extension region 53 in FIG. 4 may correspond to the angle of the X2 axis of the surface relief diffraction grating in FIG. 13 relative to the Y axis in a plane (XY plane) perpendicular to the thickness direction of the main body 50. The surface relief diffraction grating does not have to be made of the same material as the light-guiding member 5, and may be made of a different material. For example, the light-guiding member 5 may be made of glass, and the surface relief diffraction grating may be made of an ultraviolet-curable resin. In this case, the surface relief diffraction grating can be formed by nanoimprinting. Note that, if the diffractive structure of the output extension region 53 is a surface relief diffraction grating, the refractive index of the output extension region 53 is the refractive index of the material forming the surface relief diffraction grating.

[0106] In one modified example, the diffraction structures of the entrance region 51 and the auxiliary expansion region 52 are not limited to volume hologram elements (holographic diffraction gratings) and may be surface relief diffraction gratings. In another modified example, the entrance region 51 and the auxiliary expansion region 52 may include half mirrors. The light-guiding member 5 does not need to include the auxiliary expansion region 52. That is, the light-guiding member 5 may be configured to perform pupil expansion in one axis rather than two axes. In this case, the entrance region 51 changes the traveling direction of the image light L1 incident on the entrance region 51 and propagates the image light L1B through the main body 50 in the first propagation direction. The exit expansion region 53 may split the image light L1B from the entrance region 51 in the first propagation direction into multiple image light beams L1C propagating in the second propagation direction and emit the split image light beams L1C from the light-guiding member 5.

[0107] In one modified example, the diffraction pitch of the extended emission region 53 is equal to or greater than 1.36×λ / n and equal to or less than 3.82×λ / n, where λ is the wavelength of the image light L1, and n is the refractive index of the main body 50. This improves the utilization efficiency of the image light L1 from the display element 2. In particular, when used as a head-up display, the utilization efficiency of the image light L1 from the display element 2 can be improved.

[0108] In one modified example, in the optical system 3, the range of the incident angle θ_in that is 14° or more greater than the exit angle θ_out or that satisfies formula (1) includes the range determined by the angles of incidence of secondary rays that define the two outer edges of the image light L1B that enter the exit extended region 53 with respect to the predetermined plane P1. In practice, the image light L1B often has a certain degree of spread, and the angles at which the chief ray and secondary rays of the image light L1B enter the exit extended region 53 may differ. Therefore, in order to further increase the utilization efficiency of the image light L1 from the display element 2, it is preferable that the angle of incidence be 14° or more greater than the exit angle θ_out or that satisfy formula (1) for all of the rays contained in the image light L1B. In other words, by setting the range of the incident angle θ_in that is 14° or more larger than the output angle θ_out or that satisfies formula (1) to include the range determined by the angles of incidence of the secondary rays that define the two outer edges of the image light L1B that enters the output extension region 53 with respect to the predetermined plane P1, the generation of unnecessary light is reduced for all of the light rays contained in the image light L1B. In this way, the utilization efficiency of the image light L1 from the display element 2 can be further improved throughout the entire viewing region Ac.

[0109] In one modified example, the light-guiding member 5 does not necessarily have to be positioned so as to guide the image light L2 emitted from the main body 50 to the viewing area Ac as an optical image (virtual image Iv) by reflecting it off the light-transmitting member (windshield 101). For example, the light-guiding member 5 may be positioned so as to be aligned in a straight line with the viewing area Ac. In other words, the optical path from the light-guiding member 5 to the viewing area Ac may be a straight line.

[0110] In one modified example, the projection optical system 6 may include a first optical element and a second optical element as multiple optical elements, rather than a single optical element. The first optical element may be, for example, a cemented lens combining a negative meniscus lens and a biconvex lens, and the second optical element may be a cemented lens combining a positive meniscus lens and a negative meniscus lens. Note that the optical system 3 does not necessarily have to include the projection optical system 6.

[0111] In one modified example, the projection optical system 6 and the incident area 51 do not necessarily need to be aligned in a straight line. In other words, the optical path of the image light L1 from the projection optical system 6 to the incident area 51 does not necessarily have to be a straight line. For example, the image light L1 from the projection optical system 6 may be reflected by a reflector and incident on the incident area 51. In this case, the optical path of the image light L1 from the projection optical system 6 to the incident area 51 is not linear but, for example, L-shaped.

[0112] In one modified example, the image display device 1 may include a plurality of light guide members 5 each corresponding to a wavelength of light contained in the image light L1, thereby making it possible to provide a color image to the user.

[0113] In one modified example, the image display device 1 is not limited to a head-up display used in an automobile, but can also be applied to moving bodies other than automobiles, such as motorcycles, trains, airplanes, construction machinery, ships, etc. Furthermore, the image display device 1 is not limited to being used in moving bodies, and may be used, for example, in amusement facilities, as a wearable terminal such as a head-mounted display (HMD), medical equipment, or a stationary device.

[0114] Furthermore, in the above-described embodiments, the display element 2 and the optical system 3 that allow the user to view a virtual image Iv as an example of an optical image have been described. In this embodiment, the optical image viewed by the display element 2 is not limited to the virtual image Iv, but may be, for example, a real image. For example, the display element 2 may be configured such that a real image is formed between a light-transmitting member such as the windshield 101 and the user D by a pupil-widening optical system 3 similar to that described above. Displaying such a real image is useful, for example, in amusement applications. Even when such a real image is viewed as an optical image, the influence of polarization in the light-guiding member 5 on brightness unevenness can be considered, as in the case of the virtual image Iv described above. According to the display element 2 and the optical system 3 of the present disclosure, brightness unevenness can be suppressed by controlling the polarization state, as in the above-described embodiments, thereby suppressing fluctuations in the image quality of the real image.

[0115] [3. Aspects] As is clear from the above-described embodiments and modifications, the present disclosure includes the following aspects. In the following, reference numerals are given in parentheses only to clarify the correspondence with the embodiments.

[0116] In a first aspect, an optical system (3) includes a light-guiding member (5) that guides image light (L1) that forms an image output from a display element (2) to a visual field (Ac) of a user (D) as an optical image (virtual image Iv). The light-guiding member (5) includes a plate-shaped main body (50), an entrance region (51) formed in the main body (50) that causes the image light (L1) to enter the main body (50) so that the image light (L1) propagates within the main body (50), and an exit extension region (53) formed in the main body (50) and having a diffraction structure that splits image light (L1B) propagating in a first propagation direction intersecting a thickness direction of the main body (50) into a plurality of image light (L1C) propagating in a second propagation direction intersecting the first propagation direction, and outputs the split image light from the light-guiding member (5). In at least a portion of the output extension region (53), when the refractive index of a medium on the incident side of the image light (L1B) relative to the output extension region (53) is equal to the refractive index of a medium on the exit side of the image light (L1B) relative to the output extension region (53), the output angle θ_out of the image light (L1C) emitted from the output extension region (53) with the highest diffraction efficiency in a predetermined plane (P1) including a normal (N) to the output extension region (53) is 15° or more and 45° or less, and the incident angle θ_in of the image light (LIB) incident on the output extension region (53) in the predetermined plane (P1) is greater than the output angle θ_out by 14° or more. This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0117] The second aspect is an optical system (3) based on the first aspect. In this aspect, the following formula (1) is satisfied:

[0118]

[0119] This embodiment can improve the utilization efficiency of the image light (L1) from the display element (2).

[0120] The third aspect is an optical system (3) based on the first or second aspect. In this aspect, the following formula (2) is satisfied.

[0121]

[0122] This embodiment can improve the utilization efficiency of the image light (L1) from the display element (2).

[0123] A fourth aspect is an optical system (3) based on any one of the first to third aspects. In this aspect, the main body (50) has a first surface (50a) and a second surface (50b) in a thickness direction. The entrance region (51) causes the image light (L1) incident on the first surface (50a) from a first oblique direction oblique to the normal to the first surface (50a) to enter the main body (50) so that the image light (L1) propagates within the main body (50). The exit extension region (53) causes the multiple image light beams (L1C) propagating in the second propagation direction to exit from the second surface (50b) in a second oblique direction oblique to the normal to the second surface (50b). This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0124] A fifth aspect is an optical system (3) based on the fourth aspect. In this aspect, the first tilt direction and the second tilt direction are parallel to each other. This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0125] A sixth aspect is an optical system (3) based on any one of the first to fifth aspects. In this aspect, the diffractive structure of the output extension region (53) is a volume hologram element located inside the main body (50). In this aspect, the size of the output extension region (53) can be easily increased.

[0126] A seventh aspect is an optical system (3) based on any one of the first to sixth aspects. In this aspect, the light-guiding member (5) further includes an auxiliary expansion region (52) formed in the main body (50) and having a diffraction structure that splits the image light (L1A) propagating in a predetermined direction within the main body (50) by the entrance region (51) into multiple image light (L1B) propagating in the first propagation direction in the predetermined direction and directs the multiple image light (L1B) toward the exit expansion region (53). This aspect enables pupil expansion in multiple different directions.

[0127] An eighth aspect is an optical system (3) based on any one of the first to seventh aspects. In this aspect, the diffraction pitch of the output extension region (53) is 1.23×λ / n or more and 10.0×λ / n or less, where λ is the wavelength of the image light (L1, L1A, L1B, L1C, L2). n is the refractive index of the output extension region (53). This aspect can improve the utilization efficiency of the image light (L1) from the display element (2) and reduce degradation in image quality of the optical image (virtual image Iv) due to the polarization state of the image light (L1).

[0128] A ninth aspect is an optical system (3) based on any one of the first to eighth aspects. In this aspect, the diffraction pitch of the output extension region (53) is 1.36×λ / n or more and 3.82×λ / n or less, where λ is the wavelength of the image light (L1, L1A, L1B, L1C, L2). n is the refractive index of the output extension region (53). This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0129] A tenth aspect is an optical system (3) based on any one of the first to ninth aspects. In this aspect, the range of the incident angle θ_in that is 14° or more greater than the exit angle θ_out includes a range determined by the incident angles of secondary rays that define the two outer edges of the image light that enters the exit extension region (53) with respect to the predetermined plane (P1). This aspect can further improve the utilization efficiency of the image light (L1) from the display element (2).

[0130] An eleventh aspect is an optical system (3) based on any one of the first to tenth aspects. In this aspect, the at least a portion of the output extension region (53) is included in an end of the output extension region (53) on the side of the incident region (51) on the optical path of the image light (L1) from the incident region (51) to the output extension region (53). This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0131] A twelfth aspect is an optical system (3) based on any one of the first to eleventh aspects. In this aspect, the light-guiding member (5) is positioned so as to guide the image light (L2) emitted from the main body (50) to the field of view (Ac) as the optical image (virtual image Iv) by reflecting the image light (L2) on a light-transmitting member (windshield 101). This aspect can be used as a head-up display.

[0132] A thirteenth aspect is an optical system (3) based on any one of the first to twelfth aspects. In this aspect, the optical system (3) further includes a projection optical system (6) that causes the image light (L1) to be incident on the entrance area (51) of the light-guiding member (5) as substantially collimated light. This aspect can further improve the utilization efficiency of the image light (L1) from the display element (2).

[0133] A fourteenth aspect is an image display device (1) comprising an optical system (3) based on any one of the first to thirteenth aspects and the display element (2). This aspect can improve the utilization efficiency of the image light (L1) from the display element (2).

[0134] The second to thirteenth aspects are not essential.

[0135] As described above, the embodiments have been described as examples of the technology in the present disclosure. For this purpose, the accompanying drawings and detailed description have been provided. Therefore, the components described in the accompanying drawings and detailed description may include not only components essential for solving the problem, but also components that are not essential for solving the problem in order to exemplify the above technology. Therefore, the fact that these non-essential components are described in the accompanying drawings or detailed description should not be interpreted as immediately indicating that these non-essential components are essential. Furthermore, because the above-described embodiments are intended to exemplify the technology in the present disclosure, various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents.

[0136] The present disclosure is applicable to an optical system and an image display device. Specifically, the present disclosure is applicable to an optical system for guiding image light from a display element to a user's visual field as an optical image, and an image display device including the optical system.

[0137] REFERENCE SIGNS LIST 1 Image display device 2 Display element 3 Optical system 5 Light guide member 50 Main body 50a First surface 50b Second surface 51 Incident area 52 Auxiliary expansion area 53 Exit expansion area 6 Projection optical system 100 Mobile body 101 Windshield (light-transmitting member) Ac Field of view area D User Iv Virtual image (optical image) L1, L1A, L1B, L1C, L2 Image light N Normal n Refractive index P1 Predetermined plane θ_in Incident angle θ_out Exit angle λ Wavelength

Claims

1. a light guiding member that guides image light that forms an image output from the display element to a visual field area of ​​a user as an optical image; The light guiding member is A plate-shaped main body; an incidence area formed in the main body portion, the incidence area allowing the image light to be incident into the main body portion so that the image light propagates within the main body portion; an output extension region having a diffraction structure formed in the main body portion and configured to split an image light propagating in a first propagation direction intersecting a thickness direction of the main body portion into a plurality of image lights propagating in a second propagation direction intersecting the first propagation direction in the first propagation direction and output the image light from the light guiding member; Equipped with When the output angle of the image light output from the output extension region is corrected under the condition that the refractive indexes of a medium on the input side of the image light with respect to the output extension region and a medium on the output side of the image light with respect to the output extension region are equal to each other, at least a portion of the output extension region is an emission angle θ_out of the image light emitted from the emission extension region with the highest diffraction efficiency in a predetermined plane including a normal line of the emission extension region is 15° or more and 45° or less; The incident angle θ_in of the image light incident on the output extension region in the predetermined plane is 14° or more larger than the output angle θ_out. optical system.

2. Satisfy the following formula (1): [0010] The optical system of claim 1 .

3. The following formula (2) is satisfied: [0025] The optical system of claim 1 .

4. The main body portion has a first surface and a second surface in a thickness direction, the incidence region causes the image light, which is incident on the first surface from a first inclined direction inclined with respect to a normal line of the first surface, to be incident into the main body portion such that the image light propagates within the main body portion; The emission extension region emits the plurality of image lights propagating in the second propagation direction from the second surface in a second inclined direction inclined with respect to a normal to the second surface. The optical system of claim 1 .

5. The first inclination direction and the second inclination direction are parallel to each other. The optical system according to claim 4 .

6. the diffractive structure of the output expansion region is a volume hologram element located inside the body portion; The optical system according to any one of claims 1 to 5.

7. The auxiliary expansion region is formed in the main body and has a diffraction structure that splits the image light propagating in a predetermined direction within the main body by the incident region into a plurality of image lights propagating in the first propagation direction in the predetermined direction and directs the image light toward the output expansion region. The optical system according to any one of claims 1 to 5.

8. The diffraction pitch of the output expansion region is 1.23×λ / n or more and 10.0×λ / n or less, λ is the wavelength of the image light, n is the refractive index of the output expansion region; The optical system according to any one of claims 1 to 5.

9. The diffraction pitch of the output expansion region is equal to or greater than 1.36×λ / n and equal to or less than 3.82×λ / n; λ is the wavelength of the image light, n is the refractive index of the output expansion region; The optical system according to any one of claims 1 to 5.

10. The range of the incident angle θ_in that is 14° or more larger than the exit angle θ_out includes a range determined by the incident angle of a secondary ray that defines both outer edges of the image light incident on the exit extension area with respect to the specified plane, The optical system according to any one of claims 1 to 5.

11. The at least a portion of the exit extension region is included in an end portion of the exit extension region on the side of the entrance region on an optical path of the image light from the entrance region to the exit extension region. The optical system according to any one of claims 1 to 5.

12. The light guide member is positioned so as to guide the image light emitted from the main body portion to the viewing area as the optical image by reflecting the image light with a light-transmitting member. The optical system according to any one of claims 1 to 5.

13. a projection optical system that causes the image light to be incident on the incident area of ​​the light guiding member as substantially collimated light, The optical system according to any one of claims 1 to 5.

14. An optical system according to any one of claims 1 to 5; The display element; Equipped with Image display device.

Images