Image display device and optical system

Abstract

An image display device (2) that visually confirms an optical image is provided with: a display unit that emits image light that is visually confirmed as a virtual image; and a light guide (13) that guides the image light from the display unit to the outside. The light guide has a pair of main surfaces (13a, 13b) facing each other. The light guide body is provided with: an incident region (21) into which image light from the display unit is incident and which diffracts the image light so as to propagate inside the light guide body by reflection between the pair of main surfaces; and an expansion region (23) which diffracts the propagated image light at a plurality of positions where the image light propagates in the light guide body and emits the diffracted image light as a plurality of image light. The image light incident on the incident region is set to a second polarization state such that the image light propagating to the expansion region is in a first polarization state in which the image light is polarized in a direction parallel or perpendicular to a plane including a normal direction of the main surface in the expansion region and a propagation direction of the propagating image light. The second polarization state is different from the first polarization state.

Description

Technical Field

[0001] The present disclosure relates to an image display device that allows an observer to visually recognize an optical image such as a virtual image, and an optical system used in the image display device. Background Art

[0002] Patent document 1 discloses an optical display system having a plurality of diffractive optical elements (DOEs) for pupil expansion in a virtual reality or mixed reality head mounted display (HMD) device or the like. In the optical display system, a main light beam from an imager is transmitted from a first DOE to a second DOE as a TM polarized light beam or a TE polarized light beam or the like. The second DOE includes a grating feature configured to rotate the polarization state of a light disturbance in addition to performing pupil expansion of the optical light beam. In such a second DOE, the polarization of the disturbance light beam is rotated so as to be orthogonal to the polarization of the main light beam used for image display, thereby reducing light interference.

[0003] Prior Art Literature

[0004] Patent Literature

[0005] Patent Document 1: U.S. Patent No. 9946072 Summary of the invention

[0006] Problem that the invention aims to solve

[0007] The present disclosure provides an image display device and an optical system capable of suppressing a change in image quality in the image display device that enables a virtual image to be visually recognized.

[0008] Means used to solve problems

[0009] The image display device disclosed in the present invention enables visual confirmation of an optical image, and the image display device comprises: a display unit that emits image light to be visually confirmed as an optical image; and a light guide that guides the image light from the display unit to the outside. The light guide has a pair of main surfaces that are opposite to each other. The light guide is provided with: an incident area for the image light from the display unit to be incident, and diffracts the image light so that it propagates inside the light guide through reflection between the pair of main surfaces; and an expansion area that diffracts the propagated image light at multiple positions in the light guide where the image light propagates, and emits it as multiple image lights. The image light incident on the incident area is set to a second polarization state, so that the image light propagated to the expansion area becomes a first polarization state polarized in a direction parallel to or perpendicular to a plane including the normal direction of the main surface in the expansion area and the propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

[0010] The optical system disclosed in the present invention comprises: a light guide that guides image light from a display unit to the outside, and the display unit emits image light that is visually recognized as an optical image; and a polarization setting unit that is arranged between the display unit and the light guide. The light guide has a pair of main surfaces that are opposite to each other. The light guide is provided with: an incident area for image light from the display unit to be incident, and diffracts the image light so that it propagates inside the light guide through reflection between a pair of main surfaces; and an expansion area that diffracts the propagated image light at multiple positions in the light guide where the image light propagates, and emits it as multiple image lights. The polarization setting unit sets the image light incident on the incident area to a second polarization state, so that the image light propagated to the expansion area becomes a first polarization state polarized in a direction parallel to or perpendicular to a plane including the normal direction of the main surface in the expansion area and the propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

[0011] Effects of the Invention

[0012] According to the image display device and the optical system in the present disclosure, it is possible to suppress a change in image quality in the image display device that enables visual recognition of a virtual image. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a diagram for explaining the outline of the head-up display system according to the first embodiment of the present disclosure.

[0014] Figure 2 This is a diagram illustrating the configuration of a head-up display system according to the first embodiment.

[0015] Figure 3 This is a plan view showing a configuration example of a light guide in the image display device according to Embodiment 1.

[0016] Figure 4 This is a diagram for explaining the operation of pupil dilation in the image display device according to Embodiment 1.

[0017] Figure 5 This is a diagram illustrating the polarization state in the image display device according to Embodiment 1.

[0018] Figure 6 This is a diagram for explaining the stability of the polarization state of light propagating through a light guide.

[0019] Figure 7 This is a graph illustrating the relationship between the polarization state of propagating light and the diffraction efficiency.

[0020] Figure 8 This is a diagram showing an example of simulation results when the polarization state of propagating light is unstable.

[0021] Fig. 9 This is a diagram showing an example of simulation results when the polarization state of propagating light is stable.

[0022] Fig.10 This is a diagram showing another example of simulation results when the polarization state of propagating light is unstable.

[0023] Fig.11 This is a diagram showing another example of simulation results when the polarization state of propagating light is stable.

[0024] Fig.12 This is a diagram illustrating the relationship between the diffraction pitch and the diffraction efficiency in the first expansion region.

[0025] Fig.13 This is a diagram illustrating the relationship between the diffraction pitch and the diffraction efficiency in the second extended region. DETAILED DESCRIPTION

[0026] The following describes the embodiments in detail with reference to the accompanying drawings as appropriate. However, sometimes, a more detailed description than necessary is omitted. For example, sometimes, a detailed description of a well-known matter or a repeated description of a substantially identical structure is omitted. This is to avoid the following description from becoming unnecessarily lengthy and to make it easy for those skilled in the art to understand.

[0027] In addition, the applicant provides the drawings and the following description in order to enable those skilled in the art to fully understand the present disclosure, and does not intend to limit the subject matter described in the patent claims thereby.

[0028] (Implementation method 1)

[0029] Embodiment 1 of the present disclosure will be described below with reference to the drawings. In this embodiment, an example of a system to which a pupil dilation type image display device and an optical system are applied will be described.

[0030] 1. Structure

[0031] 1-1. System Overview

[0032] Figure 1 1 is a diagram for explaining the outline of a head-up display (HUD) system 1 according to the present embodiment. Figure 1 As shown, the HUD system 1 includes an image display device 2 and a windshield 5. The HUD system 1 is a system that allows an observer D, such as a passenger, to visually check a virtual image Iv, which is an example of an optical image, in a vehicle 3 or the like provided with the windshield 5. In the HUD system 1, an area in which the observer D's eyes are located so that the observer D can visually check the virtual image Iv is referred to as a visual confirmation area Ac.

[0033] For example Figure 1 As shown, the image display device 2 of the present embodiment includes: a display unit 11 that generates image light L1 to be visually recognized as a virtual image Iv; and a light guide 13 that dilates the pupil (e.g., exit pupil) of the image light L1 from the display unit 11 and guides the dilated image light L2 toward the windshield 5. According to such a pupil dilation type image display device 2, it is easy to ensure a large visual recognition area Ac in the HUD system 1.

[0034] When pupil dilation technology is applied to HUD, for example, the following problem is considered: when the observer D moves his eyes within the visual confirmation area Ac, the brightness of the virtual image Iv changes, and the brightness unevenness is more significant, so that the image quality changes. Therefore, in this embodiment, in order to solve such a brightness unevenness problem and suppress the change of image quality, a structure is adopted to control the polarization in the pupil dilation type image display device 2. The structure of the image display device 2 in the HUD system 1 of this embodiment is described below.

[0035] 1-2. System structure

[0036] Figure 2 The structure of the HUD system 1 according to the present embodiment is exemplified. In the HUD system 1, the image display device 2 is arranged, for example, inside a dashboard (not shown) below a windshield 5 of a vehicle 3. Figure 2 As shown, the image display device 2 of this embodiment includes a display unit 11, a light guide 13, a polarization setting unit 14, and a control unit 15. An optical system 20 including the light guide 13 and the polarization setting unit 14 is an example of an optical system in the image display device 2 of this embodiment.

[0037] The windshield 5 is an example of a light-transmitting member provided in a moving body such as the vehicle 3. The light-transmitting member of the HUD system 1 is not limited to the windshield 5, and may be various members having light-transmitting properties in the visible light region, such as a combiner.

[0038] The display unit 11 has a display surface that displays an image through a plurality of pixels, and is a display element that generates image light L1 that represents the image on the display surface. For example, the display unit 11 includes a liquid crystal display, an organic light-emitting diode (OLED) display, an LCOS (Liquid Crystal On Silicon), a DMD (Digital Mirror Device), a micro LED, or a scanning MEMS mirror.

[0039] The display unit 11 may not have a display surface inside. A display surface such as a screen may be provided outside the display unit 11. The display unit 11 may also include various light sources such as a laser light source or an LED light source that emits monochromatic or multi-color light for generating the image light L1. The image light L1 is composed of, for example, a light beam with a divergence angle corresponding to the viewing angle of the image. For such various light beams, the light paths of the central light beam and the like are appropriately illustrated in the figure.

[0040] The light guide 13 is formed of a material that is transparent in the visible light region and is a light guide member configured to allow incident light to be totally reflected internally. The light guide 13 has a pair of main surfaces 13a and 13b that are opposed to each other. The light guide 13 is not limited to a planar shape and may also be a curved surface shape. The following describes a structural example of the light guide 13 in a planar shape in which the first main surface 13a and the second main surface 13b are parallel. Figure 2 Although one light guide 13 is shown in the figure, the image display device 2 may include a plurality of light guides 13. For example, light guides 13 of different RGB colors may be used.

[0041] The light guide 13 is composed of, for example, a glass or resin plate having mirror-finished surfaces such as the main surfaces 13a and 13b. Hereinafter, the normal direction of the main surfaces 13a and 13b of the light guide 13 is referred to as the Z direction, and two directions parallel to the main surfaces 13a and 13b and orthogonal to each other are referred to as the X and Y directions. The X direction corresponds to the horizontal direction of the virtual image Iv, and the Y direction corresponds to the vertical direction of the virtual image Iv.

[0042] The light guide 13 is arranged in the vehicle 3, for example, with the first main surface 13a on the -Z side facing downward and the second main surface 13b on the +Z side facing upward. Figure 2 As shown, the light guide 13 is arranged obliquely in the vehicle 3. The arrangement of the light guide 13 is restricted, for example, so that the reflected light when sunlight or the like enters the light guide 13 from the outside of the vehicle 3 through the windshield 5 does not reach the visual confirmation area Ac or the like.

[0043] In the image display device 2 of the present embodiment, the light guide 13 is configured so that the image light L1 incident from the display unit 11 on the -Z side and the image light L2 emitted to the windshield 5 on the +Z side are inclined in the Y direction relative to the normal lines of the main surfaces 13a and 13b. In the image display device 2 of the present embodiment, a diffraction structure for realizing the pupil dilation operation is provided in the light guide 13. The details of the structure of the light guide 13 will be described later.

[0044] The polarization setting unit 14 is composed of various optical elements arranged between the display unit 11 and the light guide 13, and sets the image light L1 incident from the display unit 11 to the light guide 13 to a specific polarization state. For example, the polarization setting unit 14 includes one or more optical elements among a half-wave plate, a quarter-wave plate, and a polarizing plate. The polarization setting unit 14 can also be assembled into various optical systems such as a collimator arranged between the display unit 11 and the light guide 13, or can be integrally formed with the display unit 11 or the light guide 13.

[0045] The image display device 2 may not include an optical element serving as the polarization setting unit 14. For example, when the display unit 11 generates the image light L1 in a given polarization state such as linear polarization by using a laser light source or the like, the function of the polarization setting unit 14 may be realized by rotating the image light L1 in the optical axis direction or by aligning the positions of the display unit 11 and the light guide 13.

[0046] The control unit 15 controls the overall operation of the image display device 2, for example. The control unit 15 can be implemented by a circuit composed of semiconductor elements, etc. The control unit 15 can be composed of, for example, a microcomputer, a CPU, an MPU, a GPU, a DSP, an FPGA, or an ASIC. The control unit 15 realizes a predetermined function by reading data and programs stored in an internal memory, etc. and performing various calculations. In addition, the control unit 15 has a storage device 17 such as an internal memory. The control unit 15 can also be an external structure of the image display device 2.

[0047] The storage device 17 is a storage medium for storing programs and data required to realize the functions of the control unit 15. The storage device 17 can be realized by, for example, a hard disk (HDD), an SSD, a RAM, a DRAM, a ferroelectric memory, a flash memory, a magnetic disk, or a combination thereof. A plurality of image data representing a virtual image Iv is stored in the storage device 17. For example, the storage device 17 stores data for displaying the image content of the display unit 11 for displaying various information such as a road driving guidance display, a distance to a vehicle ahead, a remaining battery level of the vehicle, and a current vehicle speed. The storage device 17 is not limited to an internal memory of the control unit 15, but may also be an external structure of the control unit 15.

[0048] 1-3. About light guide

[0049] use Figure 3 The structure of the light guide 13 in the image display device 2 of the present embodiment will be described in detail. Figure 3 It is a plan view showing a configuration example of the light guide 13 in the image display device 2 .

[0050] In the pupil dilation type image display device 2 of the present embodiment, for example, Figure 3As shown, the light guide 13 includes a coupling region 21, a first expansion region 23, and a second expansion region 25. Each region 21, 23, 25 constitutes a diffractive structure element by providing a diffractive structure for realizing various diffraction capabilities in the light guide 13. Figure 3 In the light guide 13 of the structural example, the first expansion region 23 is arranged on the +X side of the coupling region 21 , and the second expansion region 25 is arranged on the +Y side of the first expansion region 23 .

[0051] The diffraction structure of each region 21, 23, 25 in the light guide 13 can be composed of various periodic structures such as volume holograms or diffraction gratings that can realize diffraction of light. Hereinafter, a structural example of setting each diffraction structure of the light guide 13 between the first and second main surfaces 13a and 13b by a volume hologram will be described. The volume hologram is composed of a periodic refractive index distribution in a dielectric film such as interference fringes formed inside the light guide 13. For example, each region 21, 23, 25 can also be a three-dimensional region in the light guide 13.

[0052] The coupling region 21 is an example of an incident region that diffracts the incident image light L1 so as to be optically coupled with the image light L1 from the display unit 11 in the light guide 13. The optical coupling is a state in which the light incident to the coupling region 21 satisfies the total reflection condition and propagates inside the light guide 13. For example, a diffraction capability is set in the coupling region 21 for converting the direction of the incident image light L1 into the image light L11 propagating in the X direction (hereinafter also referred to as "propagating light L11") so as to travel to the first expansion region 23.

[0053] For example Figure 3 As shown, the diffraction structure of the coupling region 21 has a diffraction pitch Gp1 and a tilt angle Ga1 for achieving the above-mentioned diffraction capability. The diffraction pitch Gp1 represents, for example, the period of the diffraction structure such as the period of the interference fringes in the hologram. The tilt angle Ga1 represents, for example, the inclination of the direction in which the interference fringes extend relative to a given reference direction (for example, the Y direction). For example, in Figure 3 In the example, a negative (ie, clockwise) tilt angle Ga1 is set in the XY plane in accordance with the fact that the direction in which the image light L1 is incident from the display unit 11 is tilted toward the −Y side from the Z direction of the normal line of the coupling region 21 .

[0054] The first expansion region 23 is a region for diffracting the image light L11 (i.e., the propagation light L11) that propagates in the first propagation direction by total reflection between the principal surfaces 13a and 13b of the light guide 13. The image light L11 is expanded by splitting and replicating the light beam of the image light L11 through multiple diffractions to emit multiple light beams of image light L12. Hereinafter, the image light L12 of each light beam obtained by splitting at this time is also referred to as "split light L12".

[0055] For example Figure 3 As shown, in the first expansion region 23, a diffraction capability is set to emit each split light L12 after diffraction, which originates from the propagation light L11 having the X direction as the first propagation direction, toward the Y direction as the second propagation direction to the second expansion region 25. The diffraction structure of the first expansion region 23 has a diffraction pitch Gp2 and an inclination angle Ga2 for realizing a relatively large diffraction capability as described above. The diffraction pitch Gp2 of the first expansion region 23 is, for example, narrower than the diffraction pitch Gp1 of the coupling region 21. The inclination angle Ga2 of the diffraction structure of the first expansion region 23 is, for example, a negative value like the inclination angle Ga1 of the diffraction structure of the coupling region 21, for example, an angle greater than the inclination angle Ga1.

[0056] The second expansion region 25 is a region that performs diffraction for expanding the propagating image light L12 in the second propagation direction that intersects the first propagation direction. The first and second propagation directions are not limited to the X and Y directions, respectively, and may be orthogonal as long as they intersect each other. Hereinafter, the image light of each light beam that is split and copied when the image light L12 is expanded in the second propagation direction in the second expansion region 25 is also referred to as "copy light" (in Figure 3 The copied light of the plurality of light beams constitutes the image light L2 as a result of the expansion (see Figure 4 ).

[0057] For example, the second expansion region 25 is set with a diffraction capability (see FIG. 1 ) for causing the diffracted copy light from the image light L12 that is totally reflected inside the light guide 13 and propagates in the Y direction to be emitted to the outside without satisfying the total reflection condition. Figure 4 ). The diffraction capability of such a second expansion region 25 is smaller than the diffraction capability of the first expansion region 23, so the diffraction pitch Gp3 of the second expansion region 25 is, for example, wider than the diffraction pitch Gp2 of the first expansion region 23. In addition, the diffraction structure of the second expansion region 25, for example, exhibits an inclination at an inclination angle Ga3 orthogonal to the second propagation direction (Y direction), and extends in the first propagation direction (X direction).

[0058] In the above, an example of the structure of the light guide 13 in the image display device 2 of the present embodiment is described. The light guide 13 is not limited to the above-mentioned structure example, and various structures can be adopted. For example, the diffraction structure of each region 21, 23, 25 in the light guide 13 is not limited to a volume hologram, and can also be composed of a diffraction grating or the like. In addition, the diffraction structure of each region 21, 23, 25 can also be provided on the first or second main surface 13a, 13b in the light guide 13.

[0059] In addition, Figure 3 , the diffraction structures of the regions 21, 23, and 25 are shown at tilt angles Ga1 to Ga3 in the XY plane. The diffraction structures of the regions 21, 23, and 25 are not particularly limited thereto, and may be tilted in various planes including the Z direction, for example.

[0060] 2. Action

[0061] The operations of the HUD system 1 and the image display device 2 configured as above will be described below.

[0062] In the HUD system 1 ( Figure 2 ), the control unit 15 of the image display device 2 determines the virtual image Iv to be displayed based on various information such as the driving environment acquired in the vehicle 3. The control unit 15 reads the image data of the determined virtual image Iv from the storage device 17 and outputs it to the display unit 11. The display unit 11 generates image light L1 representing an image according to the control performed by the control unit 15, and emits the generated image light L1.

[0063] In the image display device 2 of the present embodiment, the image light L1 from the display unit 11 is incident on the coupling region 21 of the light guide 13 in a state where the image light L1 is set to a specific polarization state by the polarization setting unit 14, for example. In the light guide 13, the incident image light L1 is two-dimensionally split and copied by the pupil expansion operation in the first and second expansion regions 23 and 25 (details will be described later). The image light L2 expanded in this way is emitted from the light guide 13 toward the windshield 5.

[0064] In the HUD system 1, the image light L2 incident on the windshield 5 is reflected and projected to the visual confirmation area Ac. The observer D can recognize the image light L2 projected from the HUD system 1 as a virtual image Iv by positioning his eyes in the visual confirmation area Ac.

[0065] As described above, the HUD system 1 displays the virtual image Iv by superimposing it on the real scene that can be visually confirmed through the windshield 5. According to the pupil dilation type image display device 2, the image light L2 including a plurality of replicas is projected onto the visual confirmation area Ac, so that even if the position of the eye is deviated, the observer D can visually confirm the virtual image Iv as long as he is in the visual confirmation area Ac.

[0066] 2-1. Pupil dilation action

[0067] Regarding the pupil dilation operation in the image display device 2 of this embodiment, the following is used: Figure 4 to illustrate. Figure 4 In FIG. 1 , in the perspective view of the light guide 13 of the image display device 2 , optical paths of central light rays of various image lights L1 , L11 to L13 , and L2 are exemplified.

[0068] In the image display device 2 of the present embodiment, the image light L1 from the display unit 11 is incident on the light guide 13 from the second principal surface 13b of the light guide 13, for example, in a polarization state set by the polarization setting unit 14, obliquely in the Y direction relative to the Z direction of the normal line. The incident image light L1 is refracted in the light guide 13, for example, and reaches the diffraction structure of the coupling region 21. In addition, when the diffraction structure of the coupling region 21 is provided on the second principal surface 13b, the image light L1 is incident on the diffraction structure of the coupling region 21 before being refracted in the light guide 13.

[0069] The diffraction structure of the coupling region 21 diffracts the image light L1 so as to be optically coupled with the incident image light L1. The diffracted image light L11 of the coupling region 21 is transformed into the propagation light L11 that propagates in the X direction toward the first expansion region 23. Due to the diffraction in the coupling region 21, the propagation light L11 has an incident angle θ1 that is greater than the critical angle, which is the minimum angle that causes total reflection in Fresnel reflection of the light guide 13, for example, with respect to the normal (Z direction) of each main surface 13a, 13b in the first expansion region 23 of the light guide 13.

[0070] The propagating light L11 from the coupling region 21 propagates in the X direction each time it is reflected between the first and second main surfaces 13a and 13b of the light guide 13 in the first expansion region 23, thereby passing through a plurality of positions P1 arranged in the X direction in the diffraction structure of the first expansion region 23. Figure 4 In FIG. 4 , four diffraction positions P1 in the first extended region 23 are exemplified, but the number of diffraction positions P1 is not particularly limited. Figure 4 In the example, an example is shown in which the passing position from the -Z side to the +Z side is the diffraction position P1, but in addition to or instead of this, the passing position from the +Z side to the -Z side may also be the diffraction position P1.

[0071] The diffraction structure of the first expansion area 23 sequentially diffracts the propagation light L11 at each diffraction position P1 so that a part of the propagation light L11 is split into split light L12, and the split light L12 travels to the second expansion area 25 and propagates in the Y direction. In this way, by splitting and replicating the propagation light L11 in the first expansion area 23 into a plurality of split lights L12 arranged in the X direction, the image light L1 can be expanded in the X direction.

[0072] The split light L12 is diffracted in the first expansion region 23, so that the incident angle of the split light L12 is greater than the critical angle of the light guide 13, for example, with respect to the normal (Z direction) of each main surface 13a, 13b of the light guide 13 in the second expansion region 2. The multiple split light L12 from the first expansion region 23 passes through the multiple positions P2 arranged along the Y direction in the second expansion region 25 by repeatedly reflecting in the light guide 13 during propagation in the Y direction. Figure 4 In FIG. 1 , three diffraction positions P2 are exemplified for each divided light L12 , but the number of diffraction positions P2 is not particularly limited.

[0073] The diffraction structure of the second expansion area 25 sequentially diffracts each split light L12 at each diffraction position P3, so that a part of each split light L12 is further split and emitted as a copy light L13 indicating the result of the split copy to the outside of the light guide 13. In this way, the image light L1 can be expanded in the Y direction by the plurality of copy lights L13 arranged in the Y direction obtained in the second expansion area 25 according to each split light L12.

[0074] The image light L2 expanded in the X and Y directions can be obtained by the pupil expansion in the first and second expansion regions 23 and 25. The replica light L13 has an incident angle smaller than the critical angle of the light guide 13, for example, with respect to the normal line (Z direction) of the first principal surface 13a of the light guide 13 in the second expansion region 25 due to diffraction in the second expansion region 25.

[0075] As described above, according to the pupil dilation action in the image display device 2 of this embodiment, the light beam of the image light L1 from the display unit 11 can be expanded into a light beam of the image light L2 including a plurality of copied lights L13 copied by diffraction in the first and second expansion regions 23 and 25 of the light guide 13.

[0076] 2-2. Polarization Control

[0077] Regarding the control of the polarization state in the image display device 2 of this embodiment during the pupil dilation operation as described above, the polarization state is controlled using Figure 5 To explain.

[0078] Figure 5 (A) to (C) are graphs illustrating polarization states in the image display device 2 of the present embodiment. The horizontal axis of each graph corresponds to P polarization, and the vertical axis corresponds to S polarization. P polarization is a polarization state of linear polarization parallel to the direction of the electric field of light relative to the incident plane, and S polarization is a polarization state of linear polarization perpendicular to the direction of the electric field. The incident plane is a plane defined as being parallel to the direction of light rays before and after the incident light.

[0079] Figure 5 (A) illustrates the polarization state of the image light L1 incident on the coupling region 21 in the image display device 2 of this embodiment. For example, a plane W ( Figure 4 ) is set as the incident plane to determine the polarization state of the image light L1 when it is incident on the coupling region 21.

[0080] Figure 5 The (B) pair comes from Figure 5 The polarization state of the propagating light L11 in the first expansion region 23 of the image light L1 of (A) is exemplified. For example, the polarization state of the propagating light L11 is defined using the XZ plane as the incident plane related to propagation into the first expansion region 23 (see Figure 4 ). The propagation to the first expansion region 23 is based on the propagation of reflection in the main surfaces 13a and 13b orthogonal to the incident surface in the first expansion region 23, so the polarization state of the propagating light L11 can be determined using the above-mentioned main surfaces 13a and 13b.

[0081] exist Figure 5 In the examples (A) to (C) of FIG. 1 , in the image display device 2 of the present embodiment, as shown in FIG. Figure 5 As shown in (B), the polarization state of the propagating light L11 in the first expansion region 23 is set to P polarization. The polarization state of the propagating light L11 deviates from the polarization state of the image light L1 based on the conversion due to diffraction such as the direction conversion of the image light L1 in the coupling region 21 to the propagating light L11.

[0082] So, for example Figure 5 As shown in (A) and (B), the image display device 2 of this embodiment controls the polarization state of the propagation light L11 in the first expansion area 23 by setting the polarization state of the image light L1 to deviate from the P polarization or the like. As a result, in the image display device 2 of this embodiment, the change in the diffraction efficiency from the propagation light L11 in the first expansion area 23 to the split light L12 can be suppressed, and the problem of uneven brightness can be solved (details will be described later).

[0083] exist Figure 5In the example of (A), the polarization state of the image light L1 is set to linear polarization that deviates from the P polarization by a specific polarization angle α. The polarization angle is represented by the rotation angle of the polarization direction in a plane orthogonal to the optical axis direction of the central light ray of the image light L1. Figure 5 The polarization state of the desired propagation light L11 such as (B) is set by taking into account the amount of change in the polarization angle before and after diffraction in the coupling region 21 .

[0084] The setting of the polarization angle α in the coupling region 21 can be performed, for example, by measuring the polarization state of the propagation light L11 while changing the polarization state of the image light L1, or can be performed based on theoretical calculation. For example, the amount of change can be calculated based on the incident angle, wavelength, polarization state of the image light L1 incident on the coupling region 21, the configuration of the coupling region 21 and the first expansion region 23, the refractive index of the coupling region 21, the diffraction pitch Gp1, and the tilt angle Ga1.

[0085] Figure 5 The (C) pair comes from Figure 5 The polarization state of the split light L12 in the second expansion area 25 of the propagating light L11 (B) is exemplified. For example, the polarization state of the split light L12 is defined by using the YZ plane as the incident plane related to the propagation to the second expansion area 25. In addition, the polarization state of the replica light L13 is defined by using the YZ plane as the incident plane related to the diffraction in the second expansion area 25.

[0086] exist Figure 5 In the example of (C), the polarization state of the split light L12 changes from Figure 5 The linear polarization of (B) changes to elliptical polarization. The polarization state of the split light L12 in the second expansion area 25 may change from the polarization state of the original propagation light L11 due to diffraction from the propagation light L11 in the first expansion area 23 to the split copy of the split light L12, and propagation in the second expansion area 25. In the image display device 2 of this embodiment, the polarization state in the second expansion area 25 may also change. Even if there is such a change, the problem of uneven brightness in the image display device 2 can be solved by controlling the polarization state in the first expansion area 23.

[0087] 2-2-1.About uneven brightness

[0088] According to the intensive research of the inventors of the present application, it is understood that the problem of uneven brightness is caused by the following point: in the conventional pupil expansion type image display device, the polarization state of the propagating light L11 is in an unstable state, so the diffraction efficiency of the split light L12 obtained from the propagating light L11 varies. Therefore, in order to solve this problem, the inventors of the present application have repeatedly conducted intensive research and finally came up with an image display device 2 that can improve the uneven brightness by stabilizing the polarization state of the propagating light L11. Such knowledge is explained below.

[0089] Figure 6 (A) to (C) are diagrams for explaining the stability of the polarization state of the propagating light L11 in the light guide 13. Figure 6 In (A) to (C), various diffraction positions P1 (see Figure 4 ) is illustrated by way of example.

[0090] Figure 6 (A) illustrates a case where the polarization state of the propagating light L11 is unstable. For example, when the polarization state of the original image light L1 is set to P polarization, it is considered that the following situation occurs: the polarization state of the propagating light L11 becomes a linear polarization whose polarization direction is rotated from P polarization by a polarization angle β at the initial stage of propagation from the coupling region 21 to the first expansion region 23 (for example, the number of reflections is 0).

[0091] exist Figure 6 In (A), the case where the initial polarization angle β of the propagating light L11 is approximately 45°, which is relatively large, is exemplified. In such a case, for example, Figure 6 As shown in (A), during the propagation in the first expansion region 23, the polarization state of the k2 propagation light L11 changes between linear polarizations such as the polarization angle β being approximately ±45° via elliptical polarization, etc., whenever Fresnel reflection is repeated. It is considered that there is the following situation: if the diffraction power is relatively large, such an unstable polarization state will affect the diffraction efficiency from the propagation light L11 to the split light L12.

[0092] Figure 7 This is a graph illustrating the relationship between the polarization state of the propagating light L11 and the diffraction efficiency at a certain diffraction position P1 of the first expansion region 23 . Figure 7 The horizontal axis is the polarization angle [°] of the propagating light L11, and the vertical axis is the diffraction efficiency [%]. Figure 7 In the first expansion region 23, the propagation light L11 is diffracted toward the diffraction position P1 (refer to Figure 4 ) The case where the incident angle θ1 = 47° is illustrated.

[0093] exist Figure 7 In the example, the maximum value of the diffraction efficiency when the polarization angle β = ±45° is approximately 90%, and the minimum value is approximately 35%. Although the design of controlling the diffraction efficiency in each part of the first expansion area 23 is sometimes adopted, the variation ratio of the maximum value and the minimum value of the diffraction efficiency is almost unchanged. Figure 6 In the case of (A), the diffraction efficiency at the multiple diffraction positions P1 in the first expansion area 23 varies in such a wide range. Therefore, it is considered that there is a problem that the light amount fluctuates between the multiple split lights L12 corresponding to the diffraction positions P1, resulting in uneven brightness that degrades the image quality of the virtual image Iv.

[0094] 2-2-2. Details of polarization control

[0095] Therefore, the image display device 2 of this embodiment sets the polarization state of the image light L1 to deviate from P polarization or S polarization so that the initial propagation light L11 propagating to the first expansion region 23 has the polarization state of P polarization or S polarization.

[0096] Figure 6 (B) shows an example of the polarization state of the propagating light L11 in the image display device 2 of the present embodiment. Figure 6 (C) shows another example of the polarization state of the propagating light L11 in the image display device 2 of the present embodiment.

[0097] exist Figure 6 In (B), the case where the initial polarization state of the propagating light L11 in the first expansion region 23 is P polarization is exemplified. In this case, even if the propagating light L11 is repeatedly reflected between the main surfaces 13a and 13b of the light guide 13 during propagation in the first expansion region 23, the polarization state of the propagating light L11 will be the same as Figure 6 As shown in (B), the P polarization is kept stable. By stabilizing the polarization state of the k2 propagation light L11, various diffraction positions P1 ( Figure 4 ) to suppress the fluctuation of diffraction efficiency at . Figure 7 In the example shown in FIG. 1 , a diffraction efficiency of approximately 80% is stably obtained in the plurality of split lights L12 at various diffraction positions P1.

[0098] exist Figure 6In (C), the case where the polarization state of the initial propagation light L11 in the first expansion region 23 is S polarization is exemplified. In this case, as in the case of P polarization described above, the polarization state of the propagation light L11 can be stabilized to S polarization for reflection during propagation in the first expansion region 23, and the fluctuation of the diffraction efficiency can be suppressed. In this way, the brightness unevenness caused by the fluctuation of the light amount between the plurality of split lights L12 can be reduced, and the fluctuation of the image quality of the virtual image Iv can be suppressed.

[0099] A specific example of implementing the polarization control in the image display device 2 of the present embodiment as described above will be described below.

[0100] In the first embodiment, the polarization angle α ( Figure 5 (A) is set to "86°", thereby obtaining a polarization state in which the polarization angle of the propagating light L11 is "0°", that is, a polarization state of P polarization, as a result of diffraction in the coupling region 21. At this time, as a result of diffraction in the first expansion region 23, the polarization angle of the split light L12 becomes "59°". Furthermore, as a result of diffraction in the second expansion region 25, the replica light L13 becomes a polarization state in which the polarization angle is within the range of "±59°".

[0101] In the second embodiment, the polarization angle α is set to "-24°" as the polarization state of the image light L1 in the coupling region 21, thereby obtaining a polarization state in which the polarization angle of the propagation light L11 is "90°", that is, a polarization state of S polarization. At this time, the polarization angle of the split light L12 becomes "-6°". Furthermore, the copy light L13 becomes a polarization state in which the polarization angle is within the range of "±6°".

[0102] According to the above embodiments, the polarization state of the final copy light L13 in the second embodiment has a polarization angle closer to P polarization than in the first embodiment. In the HUD system 1, the copy light L13 is emitted from the light guide 13 to the windshield 5, so the first embodiment in which the polarization state of the propagation light L11 is P polarization is advantageous in consideration of the so-called Brewster angle. In the image display device 2 of this embodiment, the setting of the polarization state of the propagation light L11 can be appropriately selected according to various specifications.

[0103] In the image display device 2 of this embodiment, the split light L12 after diffraction of the propagating light L11 in the first expansion region 23 may be in a polarization state deviated from the P polarization or the S polarization in the second expansion region 25 (see Figure 5(C)). In the present embodiment, the diffraction power of the second expansion region 25 is weaker than the diffraction power of the first expansion region 23, so the change of the diffraction efficiency in the second expansion region 25 with respect to the change of the polarization state is smaller than the change of the diffraction efficiency in the first expansion region 23. Therefore, in the image display device 2 of the present embodiment, by controlling the polarization state in the first expansion region 23, even if the polarization state in the second expansion region 25 is not particularly controlled, the brightness unevenness can be effectively reduced.

[0104] In addition, in the image display device 2 of the present embodiment, the pupil dilation of the first expansion area 23 corresponds to the horizontal direction of the virtual image Iv, and the pupil dilation of the second expansion area 25 corresponds to the vertical direction of the virtual image Iv. It is considered that sometimes the brightness unevenness in the horizontal direction is more significantly perceived by the observer D of the virtual image Iv than in the vertical direction. In contrast, the image display device 2 of the present embodiment can effectively reduce the brightness unevenness that is easily perceived by the observer D by suppressing the change of the diffraction efficiency in the first expansion area 23.

[0105] In the image display device 2 of the present embodiment, the polarization state of the propagation light L11 of the first expansion area 23 may not strictly conform to the P polarization or the S polarization, and may be set within the range of an allowable error. For example, even if the polarization state of the propagation light L11 of the first expansion area 23 in the initial propagation deviates from the P polarization or the S polarization by the polarization angle Δα, the deviation of the polarization state during propagation of the first expansion area 23 is also within the range of the polarization angle ±Δα. The change in diffraction efficiency at this time will not be perceived by the observer D as a low image quality of the virtual image Iv, and this degree of error may be allowed.

[0106] For example, in Stevens' power law, brightness changes are felt at the third root, so in the virtual image Iv of the HUD system 1, if a brightness change of about ±5% is allowed, the error in light quantity is about ±14%. Figure 7 As shown, the image display device 2 of this embodiment can also set the polarization state of the propagating light L11 so that the polarization angle Δα of the allowable error relative to the P polarization is within the range of more than -9° and less than +15°. Alternatively, the image display device 2 of this embodiment can also set the polarization state of the propagating light L11 so that the polarization angle Δα of the allowable error relative to the S polarization is within the range of more than -6° and less than +6°.

[0107] In addition, in this embodiment, the polarization state of the propagating light L11 is not necessarily limited to linear polarization, and may be, for example, elliptical polarization with a sufficiently small ellipticity. For example, within the range of the polarization angle Δα of the allowable error, elliptical polarization within a range that may vary with respect to linear polarization due to repeated reflections may be allowed (see Figure 6 (A)).

[0108] The polarization state of the propagating light L11 as described above can be set based on the incident plane (e.g., XZ plane) of the propagating light L11 or the main surfaces 13a, 13b (e.g., XY plane) constituting the reflection plane corresponding to the incident plane. That is, the polarization state of the propagating light L11 in the image display device 2 of the present embodiment can be appropriately set within the range of the allowable error as described above, for example, which can be specified along a direction parallel to or perpendicular to the XY plane.

[0109] 2-3. Simulation results

[0110] The following describes numerical simulations performed by the inventors of the present application on the image display device 2 of the present embodiment.

[0111] The simulation results of the inventors of the present application for the brightness unevenness caused by the polarization state of the propagating light L11 are shown in FIG. Figure 8 to Figure 11 middle.

[0112] Figure 8 An example of simulation results is shown in the case where the polarization state of the propagating light L11 is unstable. Fig. 9 An example of simulation results is shown when the polarization state of the propagating light L11 is stable. Fig.10 Another example of simulation results is shown when the polarization state of the propagating light L11 is unstable. Fig.11 Another example of simulation results is shown when the polarization state of the propagating light L11 is stable.

[0113] exist Figure 8 to Figure 11 In the figure, when the polarization angle of the image light L1 incident on the coupling region 21 is set in various ways, the brightness distribution of the image light L2 after the pupil expansion emitted from the light guide 13 is numerically calculated. The depth in each figure reflects the brightness. The greater the brightness, the lighter it is, and the smaller the brightness, the darker it is. In addition, on the right side of each figure, a histogram of the brightness in the brightness distribution on the left is shown. Figures 8 to 11 In the figure, the polarization angle is measured based on the S polarization.

[0114] exist Figure 8In the example, the image light L1 incident on the coupling region 21 is S polarized at a polarization angle of "90.00°" based on S polarization. In this case, the initial polarization angle of the propagating light L11 in the first expansion region 23 is "20.83°". Therefore, the polarization state of the propagating light L11 deviates from S polarization, such as Figure 8 As shown, in the X direction corresponding to the horizontal direction of the virtual image Iv, it can be seen at a glance that there is brightness unevenness in the brightness distribution.

[0115] exist Fig. 9 In the example of the setting of the image display device 2 of this embodiment, the polarization angle of the image light L1 is set to "30.00°". In this case, the polarization angle of the initial propagation light L11 is "3.30°", and the polarization state of the propagation light L11 is set to be close to S polarization. Fig. 9 As shown, it can be confirmed that compared with Figure 8 In this case, the brightness unevenness in the brightness distribution after pupil expansion is improved.

[0116] exist Fig.10 , an example is shown in which the polarization angle of the image light L1 incident on the coupling region 21 is "60.00°" and the polarization angle of the initial propagation light L11 is "42.04°". In this case, the polarization state of the propagation light L11 is also greatly different from the P polarization and the S polarization, so Fig.10 As shown in FIG. 1 , it is obvious that there is brightness unevenness in the brightness distribution after pupil expansion. For example, a brightness change of about three cycles in the X direction is confirmed.

[0117] exist Fig.11 As an example of the setting of the image display device 2 of this embodiment, the polarization angle of the image light L1 is set to "-88.00°". In this case, the polarization angle of the initial propagation light L11 is "89.91°", and the polarization state of the propagation light L11 is set to be close to P polarization. Fig.11 As shown, it can be confirmed that compared with Fig.10 In this case, the brightness unevenness in the brightness distribution after pupil expansion is improved.

[0118] 2-3-1. About diffraction spacing

[0119] The diffraction pitch confirmed by the inventors of the present application through numerical simulation of the diffraction structure of the light guide 13 of the image display device 2 in this embodiment is calculated using Fig.12 as well as Fig.13 to explain.

[0120] Fig.12The relationship between the diffraction pitch Gp2 and the diffraction efficiency in the first expansion region 23 is exemplified. As a result of numerical simulation of the diffraction efficiency when the polarization state of the propagating light L11 is changed while setting various diffraction pitches Gp2 in the first expansion region 23, Fig.12 .

[0121] exist Fig.12 In the simulation, the wavelength λ of the propagating light L11 is set to "0.52μm", the thickness d between the main surfaces 13a and 13b of the light guide 13 is set to "5μm", the refractive index n of the hologram material is set to "1.505", and the refractive index modulation amount Δn of the hologram is set to "0.02". In addition, the relationship between the incident angle and the emission angle at the diffraction position P1 of the first expansion area 23 is set so that, in the diffraction structure in which the traveling direction of the incident light is set to the X direction and the emitted light is propagated in the Y direction, the incident angle and the emission angle are the same size and change according to the diffraction pitch Gp2.

[0122] exist Fig.12 In the graph, the horizontal axis is the diffraction pitch [μm] and the vertical axis is the diffraction efficiency [%]. Graph E1 shows the maximum value of the diffraction efficiency of the propagating light L11 after the polarization state is changed. Graph E2 shows the minimum value of the diffraction efficiency of the propagating light L11 after the polarization state is changed.

[0123] In the first expansion region 23 of the present embodiment, from the viewpoint of obtaining a relatively large diffraction capability from the propagating light L11 propagating in the X direction to the split light L12 propagating in the Y direction, it is considered to use a diffraction pitch Gp2 of 1.0×λ / n[μm] or less. Fig.12 In the example, a large deviation of more than 30% of the diffraction efficiency occurs between the maximum value graph E1 and the minimum value graph E2 when the polarization state of the propagating light L11 changes. In addition, the narrower the diffraction pitch Gp2, the greater the deviation of the diffraction efficiency.

[0124] In such a case, if the polarization state of the propagating light L11 in the first expansion region 23 is unstable, there is a concern that uneven brightness may be caused. In contrast, in the image display device 2 of the present embodiment, the polarization state of the propagating light L11 in the first expansion region 23 is stabilized, so that even if a narrow diffraction pitch Gp2 of 1.0×λ / n[μm] or less is used, uneven brightness can be reduced. In addition, it is also easy to obtain a greater diffraction capability by using a narrower diffraction pitch Gp2 in the first expansion region 23.

[0125] On the other hand, in the second expansion region 25 of the present embodiment, a relatively small diffraction capability is sufficient to diffract the split light L12 propagating in the Y direction in the light guide 13 into the replica light L13 emitted to the outside of the light guide 13 in a state inclined in the Y direction. Therefore, in the second expansion region 25 of the present embodiment, a diffraction pitch Gp3 wider than the diffraction pitch Gp2 of the first expansion region 23 can be used.

[0126] Fig.13 The relationship between the diffraction pitch Gp3 and the diffraction efficiency in the second extended region 23 is exemplified. Fig.13 In the second expansion area 25, Fig.12 The same numerical simulation.

[0127] exist Fig.13 In the simulation, Fig.12 The wavelength λ of the split light L12, the thickness d of the light guide 13, and the refractive index n of the hologram material are set in the same manner as in the case of , and the refractive index modulation amount Δn of the hologram is set to 0.03. In addition, the incident angle at the diffraction position P2 of the second expansion region 25 is set to "45°", and the emission angle is set to be smaller than the critical angle of the light guide 13 and varies according to the diffraction pitch Gp3 within the emission range. Graph F1 shows the maximum value of the diffraction efficiency of the split light L12 with the polarization state changed. Graph F2 shows the minimum value of the diffraction efficiency of the split light L12 with the polarization state changed.

[0128] according to Fig.13 The simulation results confirmed that, in the present embodiment, the diffraction pitch Gp3 of the second extended region 25 can be set within the range R1 of 1.23×λ / n[μm] to 10.0×λ / n[μm].

[0129] Specifically, at the lower limit of the range R1 of the diffraction pitch Gp3, the curve F1 of the maximum value of the diffraction efficiency of the split light L12 has a minimum value, so if it is lower than the lower limit, the change of the diffraction efficiency caused by the change of the polarization state becomes significant. In this regard, by making the diffraction pitch Gp3 of the second expansion area 25 exceed the lower limit of the above range R1, the change of the diffraction efficiency in the second expansion area 25 with respect to the change of the polarization state of the split light L12 can be reduced, and the image quality degradation of the virtual image Iv can be suppressed.

[0130] Further, in Fig.13Within the range R1 of the diffraction spacing Gp3, the wider the diffraction spacing Gp3, the smaller the difference between the maximum value curve F1 and the minimum value curve F2 of the diffraction efficiency of the split light L12. For example, if the diffraction spacing Gp3 is less than 2.4 μm, it is advantageous in that the variation of the diffraction efficiency with respect to the change of the polarization state can be reduced to less than 1%.

[0131] On the other hand, each of the graphs F1 and F2 of the diffraction efficiency of the split light L12 has a range R2 that becomes a peak value near the diffraction spacing Gp3 of 1.4 μm to 2.0 μm. That is, if the diffraction spacing Gp3 is widened beyond the peak range R2, the diffraction efficiency decreases regardless of the polarization state. In response to this, by making the diffraction spacing Gp3 of the second expansion region 25 lower than the upper limit of the above-mentioned range R1, the diffraction efficiency of the split light L12 can be ensured to be more than half of the peak value, etc.

[0132] 3. Summary

[0133] As described above, the image display device 2 in the present embodiment, which allows a virtual image Iv as an example of an optical image to be visually recognized, includes a display unit 11 and a light guide 13. The display unit 11 emits image light L1 that is visually recognized as a virtual image Iv. The light guide 13 guides the image light L1 from the display unit 11 to the outside. The light guide 13 has a pair of main surfaces 13a and 13b that are opposed to each other. The light guide 13 is provided with a coupling region 21 that is an example of an incident region and first and second expansion regions 23 that are examples of expansion regions. The image light L1 from the display unit 11 is incident on the coupling region 21, and the image light L1 is diffracted so that it propagates inside the light guide 13 by reflection between the pair of main surfaces 13a and 13b. The first expansion region 23 diffracts the propagated image light L11 at a plurality of positions P1 where the image light propagates in the light guide 13, and emits the image light L1 as a plurality of image lights. The image light L1 incident on the coupling region 21 is set to a second polarization state different from the first polarization state (see Figure 5 (A), (B)) so that the image light L11 propagating to the first expansion region 23 becomes near P polarization or near S polarization polarized in a direction parallel to or perpendicular to a plane (XZ plane) including the normal direction (Z direction) of the main surfaces 13a, 13b in the first expansion region 23 and the propagation direction (X direction) of the propagating light L11, near P polarization or near S polarization as an example of the first polarization state.

[0134] According to the above image display device 2, the polarization state of the propagating light L11 in the first expansion region 23 can be stabilized to reduce the change of diffraction efficiency, thereby reducing the brightness unevenness. In this way, the change of the image quality in the image display device 2 that allows the virtual image Iv to be visually confirmed can be suppressed. The propagation direction of the propagating light L11 is determined by the direction of the central light of the light beam of the propagating light L11, for example. In addition, the first polarization state can also be determined based on each main surface 13a, 13b orthogonal to the above-mentioned surface.

[0135] In the image display device 2 of the present embodiment, the coupling region 21 is provided so that the image light L1 is incident at an inclination with respect to the normal direction (Z direction) of the main surfaces 13a and 13b in the coupling region 21, and propagates inside the light guide 13. The polarization state of the image light L1 incident on the coupling region 21 is set according to the change of the polarization state of the image light L1 incident with such an inclination, so that the image light L11 in the first expansion region 23 becomes a specific polarization state, thereby suppressing the change of the image quality in the image display device 2.

[0136] In the image display device 2 of the present embodiment, the image light L1 is incident on the coupling region 21 at an inclination in the second direction (Y direction) that intersects a plane including the first direction (X direction) in which the plurality of positions P1 are arranged in the first expansion region 23 and the normal direction (Z direction) of the principal surfaces 13a and 13b. In this case, the incident plane of the image light L1 in the coupling region 21 is different from the incident plane of the propagation light L11 in the first expansion region 23 (see FIG. 2 ). Figure 4 ). By taking into account the deviation of the polarization state of the propagating light L11 from the polarization state of the image light L1 caused by such a difference in the incident surface, it is possible to reduce the brightness unevenness in the image display device 2.

[0137] In the image display device 2 of the present embodiment, the expansion area includes the first and second expansion areas 23 and 25 having different diffraction pitches Gp2 and Gp3. The diffraction pitch Gp2 of the first expansion area 23 is narrower than the diffraction pitch Gp3 of the second expansion area 25. The image light L1 incident on the coupling area 21 is set to the second polarization state so that the image light L11 propagating to the first expansion area 23 becomes the first polarization state. Thus, the polarization state in the first expansion area 23, which is one of the plurality of expansion areas 23 and 25 with a larger diffraction ability, can be stabilized, thereby suppressing the change in its diffraction efficiency, and easily suppressing the change in the image quality of the virtual image Iv caused by uneven brightness.

[0138] In the image display device 2 of the present embodiment, the first expansion region 23 is provided in the light guide 13 so as to diffract the image light L11 propagating from the coupling region 21 and emit a plurality of image lights L12 toward the second expansion region 25. By stabilizing the polarization state in the first expansion region 23 close to the coupling region 21 among the plurality of expansion regions 23 and 25, it is easy to suppress the variation in the image quality of the virtual image Iv caused by the uneven brightness.

[0139] In the image display device 2 of the present embodiment, when the wavelength λ of the image light L1 and the refractive index n in the light guide 13 are used, the diffraction pitch Gp3 in the second expansion region 25 is greater than or equal to 1.23×λ / n and less than or equal to 10.0×λ / n. With such a relatively wide diffraction pitch Gp3, even if the polarization state in the second expansion region 25 is unstable, the change in diffraction efficiency can be suppressed, and the image quality in the image display device 2 can be improved.

[0140] In the image display device 2 of the present embodiment, the first polarization state includes at least one of P polarization and S polarization. The first polarization state may not be strictly consistent with P polarization and S polarization, and can be appropriately set within the range of allowable error. Such a first polarization state can be used to suppress the change of image quality in the image display device 2.

[0141] In the image display device 2 of the present embodiment, the second polarization state has a polarization angle α after the polarization direction is rotated from P polarization or S polarization. The second polarization state is a polarization state different from P polarization and S polarization. For example, the second polarization state is a linear polarization different from P polarization and S polarization. The second polarization state can also be appropriately set within the range of allowable error. By setting the second polarization state in this way, the change of image quality in the image display device 2 can be suppressed. In addition, the second polarization state can also be circular polarization or elliptical polarization.

[0142] In this embodiment, the image display device 2 may further include a polarization setting unit 14. The polarization setting unit 14 is disposed between the display unit 11 and the light guide 13, and sets the image light L1 incident from the display unit 11 to the coupling region 21 to a second polarization state. By setting the second polarization state by the polarization setting unit 14, it is possible to suppress changes in image quality in the image display device 2. The polarization setting unit 14 includes, for example, at least one of a half-wave plate, a quarter-wave plate, and a polarizing plate.

[0143] In the image display device 2 of this embodiment, the display unit 11 may be configured to emit image light in the second polarization state toward the light guide 13. With such a configuration of the display unit 11, it is also possible to suppress changes in image quality in the image display device 2 by setting the second polarization state.

[0144] In the image display device 2 of the present embodiment, the light guide 13 is configured to guide the image light from the display unit 11 to the light-transmitting member. The image display device 2 constitutes a head-up display that displays a virtual image Iv via a light-transmitting member such as a windshield 5 in the HUD system 1. The image display device 2 of the present embodiment can suppress the change in the image quality of the virtual image Iv in the HUD system 1.

[0145] The optical system 20 of the present embodiment includes a light guide 13 and a polarization setting unit 14. The light guide 13 guides the image light L1 from the display unit 11, which is to be visually recognized as the image light L1, to the outside. The polarization setting unit 14 is arranged between the display unit 11 and the light guide 13. The light guide 13 has a pair of main surfaces 13a and 13b facing each other. The light guide 13 is provided with: an incident area for the image light from the display unit 11 to be incident, and the image light is diffracted so that it is propagated inside the light guide 13 by reflection between the pair of main surfaces; and an expansion area, which diffracts the propagated image light at a plurality of positions in the light guide 13 where the image light propagates, and emits it as a plurality of image lights. The polarization setting unit 14 sets the image light incident on the incident region to a second polarization state different from the first polarization state, so that the image light propagating to the expansion region becomes the first polarization state polarized in a direction parallel to or perpendicular to a plane including the normal direction of the main surface in the expansion region and the propagation direction of the propagated image light. According to the optical system 20 of this embodiment, the polarization state of the image light propagating in the expansion region can be stabilized to reduce the variation of the diffraction efficiency, thereby reducing the brightness unevenness, and suppressing the variation of the image quality of the virtual image Iv.

[0146] (Other Implementations)

[0147] As described above, as an illustration of the technology disclosed in this application, embodiment 1 is described. However, the technology in the present disclosure is not limited thereto, and can also be applied to embodiments that are appropriately changed, replaced, added, omitted, etc. In addition, it is also possible to combine the various constituent elements described in the above embodiments to form a new embodiment. Therefore, other embodiments are exemplified below.

[0148] In the above-mentioned first embodiment, a configuration example in which the first expansion region 23 is arranged on the +X side of the coupling region 21 and the second expansion region 25 is arranged on the +Y side of the first expansion region 23 in the light guide 13 is described. In the light guide 13 of the present embodiment, the configuration of the regions 21, 23, and 25 is not limited to the above-mentioned configuration example. For example, the first expansion region 23 may be arranged on the +Y side of the coupling region 21 and the second expansion region 25 may be arranged on the +X side of the first expansion region 23. In the image display device 2 of the present embodiment, polarization control may be performed to stabilize the polarization state of image light propagating in the one of the two expansion regions 23 and 25 arranged as described above, which has a narrower diffraction pitch.

[0149] In the above-mentioned embodiments, an example in which two expansion regions are provided in the light guide 13 is described. In the present embodiment, the number of expansion regions provided in the light guide 13 is not limited to two, and may be one, or three or more.

[0150] In addition, in each of the above-mentioned embodiments, the image display device 2 is described in which the image light L1 incident on the light guide 13 and the image light L2 emitted from the light guide 13 are inclined in the Y direction relative to the normal line of the light guide 13. In the image display device 2 of this embodiment, the above-mentioned image lights L1 and L2 may not be inclined as described above, for example, they may be inclined in a direction different from the Y direction. In addition, one or both of the above-mentioned image lights L1 and L2 may be parallel to the normal line of the light guide 13.

[0151] In addition, in each of the above-mentioned embodiments, the HUD system 1 applied to the vehicle 3 is described, but in the present embodiment, the object to which the HUD system 1 is applied is not limited to the vehicle 3. For example, the object of the HUD system 1 in the present embodiment may also be various mobile bodies such as trains, two-wheeled vehicles, ships, and aircraft, or may be an entertainment machine that is not accompanied by a mobile body. The image display device 2 of the present embodiment may also be configured in the HUD system 1 to emit the image light L2 toward a light-transmitting member provided on various objects.

[0152] In addition, in each of the above-mentioned embodiments, an image display device 2 and an optical system 20 are described that allow a virtual image Iv as an example of an optical image to be visually confirmed. In the present embodiment, the image display device 2 allows the optical image to be visually confirmed to be not limited to the virtual image Iv, and may be, for example, a real image. For example, the image display device 2 of the present embodiment may also be configured to form an image between a light-transmitting component such as a windshield 5 and an observer D through the same pupil dilation type optical system 20 as described above. Such display of a real image is useful, for example, for entertainment purposes. In the case of visually confirming such a real image as an optical image, similarly to the case of the above-mentioned virtual image Iv, it is believed that the polarization in the light guide 13 is affected by the uneven brightness. According to the image display device 2 and the optical system 20 of the present embodiment, the uneven brightness is suppressed by controlling the polarization state similarly to the above-mentioned embodiments, thereby suppressing changes in the image quality of the real image.

[0153] In addition, in each of the above-mentioned embodiments, the image display device 2 included in the HUD system 1 is described, but in the present embodiment, the image display device 2 may not be included in the HUD system 1. In the present embodiment, the image display device 2 may also be various image display devices such as HMD or electronic mirrors that allow visual confirmation of optical images such as virtual images or real images.

[0154] As described above, the embodiments have been described as examples of the technology in the present disclosure. Therefore, the drawings and detailed description are provided.

[0155] Therefore, the components recorded in the drawings and detailed descriptions include not only components necessary for solving the problem, but also components not necessary for solving the problem in order to illustrate the above-mentioned technology. Therefore, it should not be directly determined that these non-essential components are necessary based on the fact that they are recorded in the drawings and detailed descriptions.

[0156] In addition, the above-mentioned embodiments are for illustrating the technology in the present disclosure, and therefore various changes, substitutions, additions, omissions, etc. can be made within the scope of the patent claims or their equivalents.

[0157] (Summary of methods)

[0158] Various aspects of the present disclosure are listed below.

[0159] The first method involved in the present disclosure is an image display device for visually confirming an optical image, comprising: a display unit that emits image light to be visually confirmed as an optical image; and a light guide that guides the image light from the display unit to the outside. The light guide has a pair of main surfaces facing each other. The light guide is provided with: an incident area for image light from the display unit to be incident, and diffracts the image light so that it propagates inside the light guide through reflection between the pair of main surfaces; and an expansion area that diffracts the propagated image light at multiple positions in the light guide where the image light propagates, and emits it as multiple image lights. The image light incident on the incident area is set to a second polarization state so that the image light propagated to the expansion area becomes a first polarization state polarized in a direction parallel to or perpendicular to a plane including the normal direction of the main surface in the expansion area and the propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

[0160] In a second aspect, in the image display device of the first aspect, the incident region is provided so that the image light enters at an inclination with respect to the normal direction of the principal surface in the incident region and propagates inside the light guide.

[0161] In a third aspect, in the image display device of the second aspect, the inclination at which the image light is incident in the incident region is inclined in a second direction intersecting a plane including the first direction arranged at a plurality of positions in the expansion region and the normal direction of the main surface.

[0162] In a fourth aspect, in any one of the image display devices of the first to third aspects, the expansion region includes a first expansion region and a second expansion region having diffraction pitches different from each other. The diffraction pitch of the first expansion region is narrower than the diffraction pitch of the second expansion region. The image light incident on the incident region is set to the second polarization state so that the image light propagating to the first expansion region becomes the first polarization state.

[0163] In a fifth aspect, in the image display device of the fourth aspect, the first expansion region is provided in the light guide so as to diffract the image light propagating from the incident region and emit the plurality of image lights toward the second expansion region.

[0164] In a sixth aspect, in the image display device of the fourth or fifth aspect, when the wavelength λ of the image light and the refractive index n in the light guide are used, the diffraction pitch in the second expansion region is greater than or equal to 1.23×λ / n and less than or equal to 10.0×λ / n.

[0165] In a seventh aspect, in any one of the image display devices of the first to sixth aspects, the first polarization state includes one of P polarization and S polarization.

[0166] In an eighth aspect, in any one of the image display devices of the first to seventh aspects, the second polarization state is polarization having a polarization angle obtained by rotating the polarization direction from P polarization or S polarization and being different from P polarization and S polarization.

[0167] In a ninth aspect, the image display device according to any one of the first to eighth aspects further comprises a polarization setting unit disposed between the display unit and the light guide body and setting the image light incident from the display unit to the incident region to a second polarization state.

[0168] According to a tenth aspect, in the image display device according to the ninth aspect, the polarization setting unit includes at least one of a half-wave plate, a quarter-wave plate, and a polarizing plate.

[0169] In an eleventh aspect, in the image display device according to any one of the first to ninth aspects, the display unit is configured so that the display unit emits the image light in the second polarization state toward the light guide.

[0170] In a twelfth aspect, in the image display device of any one of the first to eleventh aspects, the light guide is arranged to guide image light from the display unit toward the light-transmitting member, and the image display device constitutes a head-up display that displays a virtual image via the light-transmitting member.

[0171] The 13th method is an optical system, comprising: a light guide that guides image light from a display unit to the outside, and the display unit emits image light that is visually confirmed as an optical image; and a polarization setting unit that is arranged between the display unit and the light guide. The light guide has a pair of main surfaces that are opposite to each other. The light guide is provided with: an incident area for image light from the display unit to be incident, and diffracts the image light so that it propagates inside the light guide through reflection between the pair of main surfaces; and an expansion area that diffracts the propagated image light at multiple positions in the light guide where the image light propagates, and emits it as multiple image lights. The polarization setting unit sets the image light incident on the incident area to a second polarization state, so that the image light propagated to the expansion area becomes a first polarization state polarized in a direction parallel to or perpendicular to a plane including the normal direction of the main surface in the expansion area and the propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

[0172] In a fourteenth aspect, the optical system according to the thirteenth aspect has any one of the features of the second to tenth aspects.

[0173] Industrial Applicability

[0174] The present disclosure can be applied to an image display device that allows an observer to visually recognize an optical image, and can be applied to, for example, a HUD, an HMD, an electronic mirror, and the like.

Claims

1. An image display device for visually confirming an optical image, the image display device comprising: A display unit that emits image light to be visually recognized as the optical image; and a light guide body for guiding the image light from the display unit to the outside, The light guide has a pair of main surfaces facing each other. The light guide is provided with: an incident region into which the image light from the display unit is incident and which diffracts the image light so as to propagate inside the light guide body by reflection between the pair of main surfaces; and The expansion region diffracts the propagated image light at a plurality of positions where the image light propagates in the light guide body, and emits the image light as a plurality of image lights. The image light incident on the incident area is set to a second polarization state so that the image light propagating to the expansion area becomes a first polarization state polarized along a direction parallel to or perpendicular to a plane including a normal direction of the main surface in the expansion area and a propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

2. The image display device according to claim 1, wherein: The incident region is provided so that the image light is incident at an inclination with respect to a normal direction of the principal surface in the incident region and propagates inside the light guide.

3. The image display device according to claim 2, wherein: The inclination at which the image light is incident in the incident region is inclined in a second direction intersecting a plane including a first direction in which the plurality of positions are arranged in the expansion region and a normal direction of the principal surface.

4. The image display device according to any one of claims 1 to 3, wherein: The expansion region includes a first expansion region and a second expansion region having different diffraction pitches from each other, The diffraction pitch of the first expansion region is narrower than the diffraction pitch of the second expansion region. The image light incident on the incident region is set to the second polarization state so that the image light propagating to the first expansion region becomes the first polarization state.

5. The image display device according to claim 4, wherein: The first expansion region is provided in the light guide so as to diffract the image lights propagating from the incident region and emit the plurality of image lights toward the second expansion region.

6. The image display device according to claim 4, wherein: When the wavelength λ of the image light and the refractive index n in the light guide are used, the diffraction pitch in the second expansion region is greater than or equal to 1.23×λ / n and less than or equal to 10.0×λ / n.

7. The image display device according to any one of claims 1 to 3, wherein: The first polarization state includes one of P polarization and S polarization.

8. The image display device according to any one of claims 1 to 3, wherein: The second polarization state is a polarization state having a polarization angle obtained by rotating the polarization direction from P polarization or S polarization and different from P polarization and S polarization.

9. The image display device according to any one of claims 1 to 3, wherein: The image display device further includes a polarization setting section disposed between the display section and the light guide body and configured to set the image light incident from the display section to the incident region to the second polarization state.

10. The image display device according to claim 9, wherein: The polarization setting section includes at least one of a half-wavelength plate, a quarter-wavelength plate, and a polarization plate.

11. The image display device according to any one of claims 1 to 3, wherein: The display unit is configured such that the display unit emits the image light in the second polarization state toward the light guide.

12. The image display device according to any one of claims 1 to 3, wherein: The light guide is configured to guide the image light from the display unit toward a light-transmitting member, and the image display device constitutes a head-up display that displays the virtual image via the light-transmitting member.

13. An optical system comprising: a light guide that guides image light from a display unit that emits the image light to be visually recognized as an optical image to the outside; and a polarization setting unit, arranged between the display unit and the light guide, The light guide has a pair of main surfaces facing each other. The light guide is provided with: an incident region into which the image light from the display unit is incident and which diffracts the image light so as to propagate inside the light guide body by reflection between the pair of main surfaces; and The expansion region diffracts the propagated image light at a plurality of positions where the image light propagates in the light guide body, and emits the image light as a plurality of image lights. The polarization setting unit sets the image light incident on the incident area to a second polarization state so that the image light propagating to the expansion area becomes a first polarization state polarized along a direction parallel to or perpendicular to a plane including a normal direction of the main surface in the expansion area and a propagation direction of the propagated image light, and the second polarization state is different from the first polarization state.

Images