Image display device

Abstract

The present invention provides an image display device that, even when non-parallel light such as diverging light is input therein, suppresses the occurrence of inclination of a playback image caused by a difference in the angle of input, that enables high-image-quality playback, and that can suppress the occurrence of misalignment at boundaries having different numbers of reflections. The present inventions is characterized by comprising a transparent medium (101) which has an optical refractive index of not less than 1.3 and an in-coupling HOE (103) and out-coupling HOE (104) which are provided to a part of the transparent medium (101) without an air layer therebetween, wherein: the in-coupling HOE (103) is an optical element that deflects video information of input non-parallel light to parallel light of a prescribed angle which exceeds the critical angle of the transparent medium (101), and causes said video information to propagate inside the transparent medium (101); the out-coupling HOE (104) is an optical element that plays back, as convergent light, parallel light, or diverging light in air, light which has been input from the inside of the transparent medium (101); and a light-diffusing object which is the source of video information that is input to the in-coupling HOE (103) is disposed substantially in a plane formed by an object point group for which light that propagates in the transparent medium becomes parallel light at a wavelength selected according to a Bragg diffraction condition in accordance with the angle of view.

Description

Image display device

[0001] The present invention relates to an image display device, and more particularly to an image display device that displays an image using a light guide plate having a holographic optical element.

[0002] Conventionally, there have been image display devices in which incoupling and outcoupling holographic optical elements (hereinafter referred to as HOEs) are arranged on parallel flat plates and propagate light. Image display devices have been proposed that suppress color unevenness with a simple configuration while expanding the display angle of view to display more information (see, for example, Patent Document 1). Hereinafter, incoupling HOEs will be referred to as In-HOEs, and outcoupling HOEs will be referred to as Out-HOEs. In this technology, light from an image forming unit enters an In-HOE via a refractive optical system such as a lens, which has a complex structure, is expensive, and requires high-precision alignment. In such conventional technology, the light that enters an In-HOE is, in principle, parallel light.

[0003] Japanese Patent Application Laid-Open No. 2022-92719

[0004] When non-parallel light, such as divergent light, is incident on an In-HOE and deflected into parallel light within a light guide plate, the angle of the incident light beam changes depending on the location of the In-HOE, thereby changing the angle of deflection. Placing a point light source with the same wavelength as the recording wavelength at a position on the optical axis of the In-HOE equivalent to the focal length of the hologram lens reproduces parallel light within the substrate. However, placing a point light source, such as a laser, with only components of the same wavelength as the recording wavelength at a position shifted in the propagation direction reduces the intensity of the reproduced light within the substrate. This is due to the influence of Bragg diffraction that occurs in volume holograms. Volume holograms, also known as volume holograms, Lippmann holograms, and thick holograms, are characterized by the recording of interference fringes due to changes in the refractive index of the hologram recording material. Replacing the point light source with a light source containing wavelengths at least around the recording wavelength, such as a white light source, reproduces a wavelength corresponding to the position of the point light source in the propagation direction. In other words, wavelengths longer than the recording wavelength are reproduced in the propagation direction, and wavelengths shorter than the recording wavelength are reproduced in the opposite direction. Hologram lenses also have wavelength dependency, and their focal length is proportional to the recording wavelength / reproduction wavelength.

[0005] 33, the object S is placed near the focal length position on the optical axis 21 of the In-HOE 20, and a person is looking into the Out-HOE 30 (or a positive lens is placed there), and an image is formed near the focal plane, the following problem occurs. For the sake of simplicity, the Out-HOE is assumed to have the function of emitting in-medium parallel light in the air in the normal direction of the light guide plate.

[0006] The emitted light, whose object point is an object S on the In-HOE optical axis 21 (during hologram recording), has an object point at the hologram lens focal position, so the propagating light within the substrate (light guide plate) 10 becomes parallel light, which becomes parallel light when reproduced by the Out-HOE 30, and the reproduced image (image P) is formed at the focal position of the eye (imaging lens). However, the emitted light, whose object point is a position shifted toward the propagation direction, selects a long wavelength and has a short focal point, so the substrate-propagated light tends to converge, which tends to converge when reproduced by the Out-HOE, and the reproduced image approaches the light guide plate. On the other hand, the emitted light, whose object point is a position shifted opposite the propagation direction, selects a short wavelength and has a long focal point, so the substrate-propagated light tends to diverge, which tends to diverge when reproduced by the Out-HOE, and the reproduced image moves away from the light guide plate. In other words, the observed image is tilted, which creates the problem of blurring depending on the location.

[0007] Furthermore, when attempting to reproduce a relatively large image using a thin light guide plate, the problem of image discontinuity arises. If the thickness of the light guide plate is t and the internal propagation angle is φ, the number of reflections of parallel light increases with each distance of t tan φ in the propagation direction, and images emitted from the Out-HOE are observed with different numbers of reflections every 2t tan φ. However, if light of a wavelength selected by the Bragg condition propagates at an angle different from that at the time of recording, image quality degradation occurs at the image boundaries where the number of reflections differs, such as images not connecting, double images appearing, or gaps appearing. Furthermore, due to these issues, the practical application of image reproduction devices with a simple configuration in which non-parallel light is incident on an In-HOE has not progressed to date.

[0008] The present invention aims to solve the above problems and to provide an image display device that can suppress the occurrence of tilt in the reproduced image due to differences in the incident angle even when non-parallel light such as divergent light is incident, thereby enabling high-quality reproduction and suppressing the occurrence of misalignment at the boundary between different numbers of reflections.Furthermore, the present invention realizes a device that can reproduce images at infinity or at a floating position on a transparent light guide plate in a compact housing, and an image display device that can provide binocular stereoscopic vision.

[0009] The image display device of the present invention comprises a transparent medium having an optical refractive index of 1.3 or more, and an incoupling holographic optical element and an outcoupling holographic optical element arranged in a part of the transparent medium without an air layer between them, wherein the incoupling holographic optical element is an optical element that deflects image information of non-parallel light incident thereon into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates within the transparent medium, and the outcoupling holographic optical element is an optical element that reproduces light incident from within the transparent medium as converging light, parallel light, or diverging light in air, and wherein a light diffusing object that is the source of the image information incident on the incoupling holographic optical element is roughly positioned on a plane formed by a group of object points at which light propagating within the transparent medium becomes parallel light at a wavelength selected by a Bragg diffraction condition according to the angle of view.

[0010] In the image display device of the present invention, the light diffusing object that is the source of the image information incident on the incoupling holographic optical element is preferably a planar diffusion screen, a spatial light modulation element, or a self-luminous display element, and the surface thereof is preferably given an inclination angle.

[0011] In the image display device of the present invention, it is preferable that the light diffusing object that is the source of the video information incident on the incoupling holographic optical element is a hologram image that is virtually emitted from a position that does not actually exist.

[0012] In the image display device of the present invention, it is preferable to dispose a concave lens between the incoupling holographic optical element and a light diffusing object that is the source of the image information incident on the optical element.

[0013] In the image display device of the present invention, it is preferable to dispose a mirror between the incoupling holographic optical element and a light diffusing object that is the source of the image information incident on the optical element.

[0014] In the image display device of the present invention, it is preferable that the angle of the mirror can be adjusted so that the light diffusing object surface that is the source of the image information entering the incoupling holographic optical element and the surface of the image information observed from the outcoupling holographic optical element are parallel.

[0015] In the image display device of the present invention, it is preferable that the number of mirrors disposed between the incoupling holographic optical element and the outcoupling holographic optical element is an even number.

[0016] In the image display device of the present invention, it is preferable that the light diffusing object that is the source of the video information that enters the incoupling holographic optical element is all or a part of the information displayed on the display, the display including at least one of a GPS (Global Positioning System), an acceleration sensor, and a gyro sensor and capable of recognizing its own position and angle, and that the video information that enters at least the incoupling holographic optical element has been subjected to image processing including angle conversion, image inversion, size conversion, and distortion conversion based on the position and angle information so that the image that can be observed from the outcoupling holographic optical element remains parallel to the display.

[0017] In the image display device of the present invention, the incoupling holographic optical element is preferably configured by holographic optical elements having two different optical axes, arranged in a direction perpendicular to the propagation direction, with the distance between two intersections where the holographic optical element surface and each optical axis intersect being 58 mm or more and 75 mm or less, and two types of parallax images are preferably input as video information, centered on a plane formed by object point groups where the light propagating within each transparent medium becomes parallel light.

[0018] In the image display device of the present invention, the outcoupling holographic optical element is an optical element that reconstructs light entering from within the transparent medium as parallel light in air, and preferably reconstructs an image with a sense of localization at infinity.

[0019] In the image display device of the present invention, it is preferable that the outcoupling holographic optical element is an optical element that reproduces light entering from within the transparent medium as divergent light in air, and reproduces an image with a sense of localization at a predetermined depth position.

[0020] In the image display device of the present invention, it is preferable that the outcoupling holographic optical element is an optical element that reconstructs light entering from within the transparent medium as convergent light in air, and reconstructs an image that is floating for the observer.

[0021] According to the present invention, it is possible to provide an image display device that can suppress the occurrence of tilt in the reproduced image due to differences in the angle of incidence, even when non-parallel light such as divergent light is incident, thereby enabling high-quality reproduction and suppressing the occurrence of misalignment at boundaries with different numbers of reflections.

[0022] 1 is a schematic diagram of the main parts of an image display device according to an image display mode of the present invention. FIG. 1 is a diagram for supplementary explanation of the principle of application of a reflection hologram in an arrangement in which it appears to be a transmission type. FIG. 2 is a schematic diagram of an example of a layer configuration when coloring in the first embodiment. FIG. 3 is a diagram for explaining an embodiment in which a hologram image is propagated. FIG. 4 is a diagram for explaining an embodiment in which a propagating image is colorized. FIG. 5 is a diagram for explaining an embodiment in which the propagation direction is changed. FIG. 6 is a diagram for explaining an example of an image display device in which In-HOE and Out-HOE are multilayered. FIG. 7 is a diagram for explaining an example of a situation in which a person actually observes in the first embodiment. FIG. 8 is a diagram for explaining an application example of the first embodiment. FIG. 9 is a diagram for explaining an example of an image display device according to the first embodiment with improved portability. FIG. 10 is a diagram for explaining an example of an image display device according to the second embodiment that is compact using a reflecting mirror. FIG. 11 is a diagram for explaining an example of an image display device according to the third embodiment that is compact using a concave lens optical system. FIG. 12 is a diagram for explaining an example of the configuration of an image display device capable of stereoscopic vision according to the fourth embodiment. FIG. 13 is a diagram for explaining an example of actual observation of the image display device according to the fourth embodiment. FIG. 14 is a diagram for explaining an example in which the present invention is applied to an astronomical observation application. FIG. 15 is a diagram for explaining an example in which the present invention is applied to AR display in a museum showcase. FIG. 16 is a diagram for explaining an example in which the present invention is applied to AR display on a flat-lay display. FIG. 10 is a diagram illustrating another example in which the present invention is applied to AR display on a flat-lay display. FIG. 11 is a diagram illustrating an image display device according to a ninth embodiment. FIG. 12 is a schematic diagram of an image display device according to a tenth embodiment, which is an example of an image display device of the present invention. FIG. 13 is a schematic diagram illustrating light propagation through (A) an incoupling HOE and (B) an outcoupling HOE in the tenth embodiment. FIG. 14 is a diagram illustrating the effect of the thickness of a light guide plate on the angle of incidence of a hologram. FIG. 15 is a schematic diagram of an image display device according to an eleventh embodiment, which is an example of an image display device of the present invention. FIG. 16 is a schematic diagram illustrating light propagation through (A) an incoupling HOE and (B) an outcoupling HOE in the eleventh embodiment. FIG. 17 is a diagram illustrating variations in the positional relationship between the incident-side and exit-side HOEs with respect to the light guide plate. FIG. 18 is a schematic diagram of a modified image display device according to an image display form of the present invention. FIG. 19 is a diagram illustrating an example of a device discrimination means in an image display device of the present invention.FIG. 1 is a diagram showing an example of an embodiment of an image display device of the present invention. FIG. 2 is a diagram showing an example in which the present invention is applied to an automobile head-up display. FIG. 3 is a diagram showing an example in which the present invention is applied to AR display of an in-vehicle meter display. FIG. 4 is a schematic diagram of an image display device (double-sided video display) according to a thirteenth embodiment, which is an example of the image display device of the present invention. FIG. 5 is a diagram showing an example in which the image display device (double-sided video display) according to the thirteenth embodiment, which is an example of the image display device of the present invention, is applied to AR display of an in-vehicle meter display. FIG. 6 is a diagram explaining the problem of the present invention.

[0023] Hereinafter, embodiments of the image display device of the present invention will be described with reference to the drawings. However, the present invention is not limited to or restricted by the following examples. Note that the drawings referred to below are schematic, and the dimensional ratios of objects depicted in the drawings may differ from the dimensional ratios of actual objects. The dimensional ratios of objects may also differ between drawings.

[0024] In the present invention, the light diffusing object that is the source of the image information entering the In-HOE is arranged on a plane formed by a cloud of object points at which light propagating through a transparent medium becomes parallel light at a wavelength selected according to the Bragg diffraction condition corresponding to the angle of view. Since incident light from an object point offset from the optical axis of the HOE propagates through the substrate at a Bragg diffraction wavelength corresponding to the angle of deviation of the optical axis, the positions of object points at which parallel light propagates through the substrate at each wavelength are determined, and by connecting these point clouds, a surface is formed in which the counter-propagation side moves away from the HOE surface and approaches the HOE surface as it progresses toward the propagation side. While this surface is strictly a curved surface, it can be approximately considered a plane. This is defined as "a surface formed by a cloud of object points at which light propagating through a transparent medium becomes parallel light." As a more specific example, the light diffusing object that is to enter the In-HOE is tilted.

[0025] First Embodiment The main components of a first embodiment of an image display device 100 to which the present invention is applied will be described with reference to FIG. 1 . The light guide plate (transparent medium) 101 is a transparent acrylic parallel plate with an optical refractive index of approximately 1.5, measuring 10 mm thick, 150 mm long, and 110 mm wide. A holographic recording medium 102 is attached to one side of the light guide plate either directly or via an OCA (Optical Clear Adhesive). An In-HOE is recorded in a region 103 of the holographic recording medium, and an Out-HOE is recorded in another region 104. A surface protective substrate (protective layer) 105 is provided on the surface of the holographic recording medium.

[0026] The In-HOE is a volume hologram recorded at a wavelength of 532 nm, which has the function of deflecting divergent light incident from a position 185 mm away in the normal direction into parallel light at an angle of 60° with the normal, so that it is reflected in the direction of incidence within the light guide plate medium.The effective area is 60 mm in the propagation direction (the vertical direction) and 100 mm in the width direction (the horizontal direction) perpendicular to that, with the center of the HOE being the center of the area.

[0027] The Out-HOE is a volume hologram recorded at a wavelength of 532 nm, and has the function of reflecting parallel light propagating at 60° within the medium and deflecting it in the normal direction as parallel light.The effective area is 60 mm in the propagation direction (the vertical direction) and 100 mm in the width direction perpendicular to that (the horizontal direction).

[0028] At first glance, the Out-HOE appears to reproduce, in a transmissive manner, the light propagating inside the light guide plate at the angle of total reflection, but in reality, it hardly reproduces the transmissive component, and the light that strikes the thin film of the Out-HOE's surface protective substrate is totally reflected at the interface with air and then immediately diffracted back into the air. Strictly speaking, the angle of the light ray changes if there is a difference between the refractive index of the HOE and the refractive index of the light guide plate, but because this change in angle is extremely small, it is shown in the drawings as traveling in a straight line.

[0029] Similarly, a reflection hologram can also be applied to an In-HOE in an arrangement that appears to be a transmission type at first glance. Figure 2 is a schematic explanatory diagram showing the propagation of light when a reflection hologram is used in the In-HOE of (A) region 103 and the Out-HOE of (B) region 104. The In-HOE is an optical element that deflects incident non-parallel light into parallel light at a predetermined angle exceeding the critical angle of the transparent medium 101, and propagates the parallel light within the transparent medium 101. The In-HOE deflects incident light L1 within the In-HOE, and the deflected parallel light L2 is totally reflected at the interface between the In-HOE (protective layer 105) and air, causing it to enter the transparent medium 101 and propagate therethrough.

[0030] The Out-HOE in region 104 is an optical element that reproduces, as convergent light, light incident from within the transparent medium 10 (propagated light resulting from the parallel light propagating through the transparent medium 101). The Out-HOE totally reflects the propagated light (parallel light L2) at the interface between the Out-HOE (protective layer 105) and air, and deflects the totally reflected light within the Out-HOE to reproduce it as convergent light L3.

[0031] In FIG. 1, if an object is placed near the so-called focus of the In-HOE and viewed from the direction of emission of the Out-HOE, a diffraction image of the object can be observed. In this case, in the case of a monochromatic HOE of 532 nm (green), an image of almost only green is visible. This object can be any light diffuser that forms an image, including three-dimensional objects such as figurines, self-luminous display devices, backlit LCD displays, and projection images backlit or frontlit on a diffusion screen. In this example, a smartphone screen was used, and the smartphone was tilted approximately 10° to 20° so that the propagation direction approached the light guide plate. This eliminated the out-of-focus and boundary discontinuities that occurred when the light was not tilted, and enabled continuous image reproduction.

[0032] This can be explained by the fact that, as mentioned above, light incident on the In-HOE from a location other than the focal point propagates at a different wavelength due to the Bragg diffraction condition nλ = 2d sin θ (λ is the wavelength, d is the spacing between the interference fringes, θ is the angle between the interference fringes and the incident light, and n is a natural number), resulting in a phenomenon in which the reconstructed image is tilted; however, this problem can be resolved by providing a tilt angle on the object side. Taking into account the Bragg diffraction condition, the propagation angle within the light guide plate was set to 60°, and the focal position at the selected wavelength was calculated to obtain a tilt of 19.8°, which roughly agreed with the calculated result. The smaller the propagation angle, the smaller the tilt angle.

[0033] Furthermore, because this tilt angle depends on other physical quantities, such as physical changes due to shrinkage of interference fringes and the position on the object side, high-precision alignment was achieved by measuring the boundary continuity and focus in actual reconstructed images rather than determining it from simulation results, and then fine-tuning it to the least noticeable angle.

[0034] Furthermore, if the thickness of the light guide plate can be made large relative to the image area to be displayed, that is, if the propagation direction length of the display area is smaller than 2t tan φ, the boundary due to the difference in the number of reflections within the light guide plate disappears, so the problem of "image continuity" mentioned above does not occur, and if it is only a problem of focus or image tilt, it will not be sensitively recognized, so there is essentially no problem even if the tilt angle is close to 0. It is desirable that the actual tilt angle be in the direction shown in the figure, but an angle that feels most natural can be selected based on design requirements and visual confirmation.

[0035] So far, we have described an example of reproducing a single color, for example, green at 532 nm. However, both In-HOE and Out-HOE can also reproduce multicolor or full-color images if multiple interference fringes are recorded. The multiple interference fringes can be obtained by multiple or sequential exposure of a single layer of holographic recording material, or by laminating holographic layers of interference fringes recorded individually on multiple recording layers to prevent air from entering. Figure 3 shows an example of a stack of red (R), green (G), and blue (B) HOEs. When laminating, the recording material layers can be directly bonded together or can be bonded together using a transparent adhesive or pressure-sensitive adhesive. However, it is essential that there is no air between the layers. Air or other foreign matter would cause total reflection before the light reaches the layer, preventing light propagation. It is also important that the refractive indices of the different materials are as close as possible and that there is minimal light scattering or absorption. Because Bragg diffraction selects wavelengths in the same way for each color, selecting the appropriate parameters eliminates the need to individually change the tilt angle for each color.

[0036] The light-diffusing object that is the source of the video information incident on the In-HOE may be a hologram image virtually emitted from a position that does not actually exist. An embodiment in which a hologram image is propagated will be described using FIG. 4 . An image display device 700 is composed of two light guide plates. An In-HOE (H1) 703 and an Out-HOE (H2) 704 are formed on a portion of a first light guide plate (transparent plate) 701, and an In-HOE (H3) 708 and an Out-HOE (H4) 709 are formed on a portion of a second light guide plate (transparent plate) 706. The first light guide plate 701 and the second light guide plate 706 are arranged so as to be separated from each other by air or an object having an optical refractive index lower than that of the light guide plates, and the Out-HOE (H2) 704 and the In-HOE (H3) 708 are arranged so as to face each other and be close to each other. It is important that a low-refractive-index material exists between the first and second light guide plates to cause total reflection, and the gap distance can be short. The focal lengths of H2 and H3 are roughly the same; more precisely, the distance between the hologram image and the HOE is matched, taking into account the difference in refractive index between the media. Video information incident on H1 is deflected and emitted from H2, but it is possible to reconstruct the image as a divergent beam emanating from a finite distance beyond the first light guide plate 701. The beam enters H3, is deflected, and can be observed from H4. With regard to the second light guide plate 706, the video information incident on the In-HOE is not a real image but a hologram image 711. Propagating the hologram image as a virtual image plane through another light guide plate in this way offers various benefits, as described below.

[0037] A) Compactification of the playback device and improvement of light utilization efficiency. Generally, to reduce various aberrations, it is better to place the light-diffusing object, which is the source of the video information entering the In-HOE, farther from the In-HOE. For example, if the distance from the In-HOE to the light-diffusing object is 2 m, the playback device becomes too large and cannot be made compact. Placing a typical display far away from the In-HOE reduces the amount of light entering the In-HOE, and only a dark image can be propagated. This is because typical displays are designed to be visible from a wide angle, so moving the display farther away reduces the amount of light entering a limited area. In fact, in an experiment where the focal length of H1 was 200 mm and the focal lengths of H2 and H3 were 2 m, for example, a system that relay-propagated the hologram image was able to propagate an image approximately 100 times brighter than a system that placed the display 2 m away, even including the loss of efficiency due to coupling.

[0038] B) Ease of Propagation Performance Compensation When colorizing a propagation image, the three colors (red, green, and blue) can be recorded on a single HOE, or each color can be propagated through a separate light guide plate. In the image display device 750 of the embodiment shown in Figure 5, the portion propagating from H1 to H2 is propagated through separate light guide plates 701R, 701G, and 701B for red, green, and blue. This successfully suppresses fogging by other colors, known as crosstalk. Furthermore, when high-precision image propagation is required, the effects of shrinkage, expansion, and distortion of the holographic recording material must be compensated for according to the color and angle of view. Stacking the light guide plates in this manner offers the advantage of making this compensation easier. Specifically, by providing an adjustment mechanism that allows each layer to be shifted relative to the other layers or to be angled rather than parallel, and a post-adjustment fixing mechanism, high-quality images can be easily obtained. Even if it is desired to restrict the angle of view of the image ultimately seen from H4, it is possible to provide different views for each by restricting the aperture sizes of H1, H2, and H3, for example, thereby increasing the degree of freedom in design.

[0039] C) Wavelength Dispersion Control In the image display device 800 of the embodiment shown in Figure 6, the propagation direction from H1 to H2 is reversed from the propagation direction from H3 to H4. That is, light entering the In-HOE 803 of the first light guide plate 801 from the display D exits the Out-HOE 804, and the resulting hologram image enters the In-HOE 808 of the second light guide plate 806, where it is deflected. The image exiting the Out-HOE 809 reaches the viewer's eye. By reversing the propagation direction in this way, the wavelengths that meet the Bragg diffraction conditions are limited, resulting in a narrowband reproduction wavelength. Because the wavelength of light that meets the Bragg diffraction conditions varies depending on the angle of view in the propagation direction, particularly when attempting to propagate an image with a large field of view (FOV) for reproduction, the image appears bluish at one end of the screen in the propagation direction and reddish at the other end. This results in the problem of image distortion. However, by performing this reverse relay, wavelength selectivity and angle selectivity are simultaneously compensated for, enabling high-quality propagation images. As a result, the components propagating to H3 and H4 propagate as parallel light, even without tilting the display surface for input to H1. Although the angle of view in the propagation direction becomes narrower, as in the image display device 900 of the embodiment illustrated in Figure 7, for example, by stacking three different pairs of In-HOE and Out-HOE and arranging them staggered so as to be continuous in the propagation direction, as in 901, 902, and 903, image propagation with little color change can be achieved over a wide range.

[0040] D) Ease of use and cost reduction: The distance between the In-HOE and the light-diffusing object or display through which the light is incident is often determined by the constraints of the product or device. For example, suppose there are five desired distances: 150 mm, 180 mm, 200 mm, 230 mm, and 250 mm. Meanwhile, suppose there are five desired imaging positions (depth localization planes) for the reproduced image: 200 mm, 500 mm, 5 m, 10 m, and infinity. To accommodate all of these desired positions, 5 x 5 = 25 possible combinations of In-HOEs and Out-HOEs must be created. However, if we were to relay a 2m hologram image using H2 and H3, we could create a total of 10 lenses: five lenses with an Out-HOE focal length of 2m and In-HOE focal lengths of five types (150mm, 180mm, 200mm, 230mm, 250mm), and five lenses with an In-HOE focal length of 2m and Out-HOE focal lengths of five types (200mm, 500mm, 5m, 10m, infinity).By combining these lenses, we would be able to handle all cases, contributing to flexibility and cost reduction.

[0041] In addition, in light guide plate-type image propagation in which parallel light is incident on an In-HOE, stacking multiple light guide plates has already been proposed, but the purpose, means, and effects are different from those of the present invention, so an explanation will be provided. For example, International Publication No. 2020 / 080117 proposes stacking for the purpose of displaying color images. Furthermore, JP-A-2018-533069 proposes that an image propagated by a light guide plate is first emitted into the air and then re-entered into another light guide plate for the purpose of so-called "pupil expansion" in order to expand the visible area, but the light is deflected in a direction different from the propagation direction, for example, in a perpendicular direction. High-quality image propagation cannot be achieved with this idea alone.

[0042] In the above-described embodiment, the purpose and number of layers for forming multiple layers through the hologram image are given as examples of three layers for separating red, green, and blue for colorization, and three layers for widening the angle of view, but these may be combined to form a configuration with a greater number of layers, or a configuration with a smaller number of layers.

[0043] (Application Example) An example of an arrangement in which the image display device 100 described in FIG. 1 is applied to a handheld transparent display will be described using FIGS. 8, 9, and 10. FIG. 8 is a diagram illustrating an example of a situation in which a person actually observes the first embodiment. The image display device 100 is positioned with a smartphone (display) D tilted relative to its light guide plate, and is observed from the side facing the smartphone display. FIG. 9 shows an example of a housing that allows the positional relationship between the light guide plate with HOE and the smartphone D to be fixed in a predetermined state. The housing 110 is designed so that the HOE can be fixed at an appropriate tilt angle by inserting the smartphone into a slot. In this case, the image of the smartphone D will appear at infinity, as if it were observed by placing the smartphone D approximately 185 mm away from the eye. Note that if both the In-HOE 103 and the Out-HOE 104 are formed to support full color, a full-color image will be displayed. However, if either is formed in a single color, only that color will be displayed. Because a single color tends to achieve higher diffraction efficiency, it is possible to display text information in a single color, such as green, for high contrast. Creating an In-HOE and Out-HOE large enough to cover the area visible to both eyes allows for comfortable viewing with both eyes, and by selecting the appropriate angle of view, the image can be magnified. Since the image is positioned at infinity, it reduces eye fatigue. It also has the effect of preventing eye diseases such as myopia. For those with presbyopia, who have difficulty focusing on close objects due to aging, it offers the advantage of making it easier to read large text without wearing reading glasses. Furthermore, while the invisible short-wavelength light emitted by LED light sources used in smartphone displays and other devices is said to be harmful to the eyes, the HOE acts as an optical filter, effectively blocking blue light. Needless to say, because it is a transparent display, it can be used for AR (Augmented Reality) and MR (Mixed Reality), which combine the real world with the display. In addition, by placing a shielding plate behind the eyes of the light guide plate, the contrast increases, making it possible to use it for VR (Virtual Reality) applications.

[0044] FIG. 10 illustrates an embodiment with improved portability. A housing 115 holding an image display device 100 (transparent light guide plate 101, such as an acrylic plate) and a housing 116 holding a smartphone D are connected by a hinge 117, allowing them to rotate open and close. A link mechanism 118 allows the device to be fixed in two states: a compactly stored state as shown in FIG. 10(A) and an open state as shown in FIGS. 10(B) and (C). In the state shown in FIG. 10(A), the display of the smartphone D is protected by the acrylic plate, and the screen can be viewed directly through the in-HOE 103. In the states shown in FIGS. 10(B) and (C), the in-HOE 103 and the smartphone D are held at an appropriate angle and distance, allowing images to be observed through the out-HOE 104. Furthermore, because there is space between the in-HOE 103 and the smartphone D for fingers to fit in, text input and tapping can be performed without any problems, as if the user is supporting only the smartphone D, without being aware of the housing 115. The smartphone D can be held with both hands so that it is horizontally oriented as shown in Figure 10(B), or it can be held vertically as shown in Figure 10(C). When displaying text on a smartphone, white text on a black background is easier to read than black text on a white background. Furthermore, if the image display device is made of a single color, displaying only that color reduces blurring due to color dispersion. In the case of a self-luminous display, since it is inefficient to display light of wavelengths that do not propagate images through holograms, emitting only light of wavelengths that propagate images through holograms on a black background is preferable, as it reduces energy consumption. A dedicated mode suitable for hologram propagation display, which emits only easily visible colors, can also be created as a smartphone application.

[0045] Second Embodiment (Miniaturization Variation 1) A second embodiment, which uses a light guide plate with the same In-HOE and Out-HOE as in the first embodiment described above but with a more compact configuration, will be described with reference to FIG. 11 . This embodiment uses two plane mirrors (mirrors 120A and 120B) so that the optical path length, inclination, and other characteristics are substantially the same as those in FIG. 8 . If the plane mirror reflects on the back surface of the substrate, both the image on the surface of the substrate and the image on the mirror surface will be visible. Therefore, a front-surface mirror is desirable. However, if a mirror or a protective layer must be used on the back surface of the substrate, it is possible to reduce overlapping images by using a mirror with anti-reflection measures (such as an AR coating or a moth-eye structure) on the incident surface. Not only has the overall device been made more compact, but the light guide plate and display have been parallelized, reducing the sense of incongruity. In this case, the housing that secures the main components is not made airtight, but rather a space for fingers is provided between the mirror 120B and the display, making the overall device lighter so that it can be held and operated with one or both hands, even if the display also serves as an input device, such as a game terminal or smartphone.

[0046] [Third Embodiment (Miniaturization Variation 2)] A third embodiment using a light guide plate with the same In-HOE and Out-HOE as in the first embodiment described above, but with a more compact configuration, will be described with reference to FIG. 12 . The projection source, screen, display, etc., can be roughly positioned near the focal plane of the In-HOE, thereby enabling a correspondingly smaller configuration. As shown in the lower diagram of FIG. 12 , by making the In-HOE focal length a single focus and placing a concave lens 130 on its optical axis, the working distance can be shortened due to the lens composition effect. The requirement is that the concave lens 130 be positioned on the optical axis of the HOE closer to the substrate than the In-HOE focal position. While the figure shows a biconcave lens made of glass or resin, any lens with negative power can be used, including plano-concave lenses, meniscus lenses, Fresnel lenses, and hologram lenses. Furthermore, this effect can be imparted to mirror 120A or 120B in combination with the second embodiment described in FIG. 11 .

[0047] [Fourth Embodiment (Further Miniaturization and Stereoscopic Variations)] Shortening the focal length of the In-HOE (f = 185 mm) used in the first to third embodiments naturally allows for a smaller device size. Furthermore, it is preferable in that the amount of light entering the In-HOE increases, enabling brighter image reproduction. However, bringing it closer to the eye can sometimes result in the right and left eye images not fusing, making it difficult to see. For this reason, a fourth embodiment that can also be used effectively with stereoscopic vision is shown in FIGS. 13 and 14 . In the image display device 200 of this embodiment, the Out-HOE 204 is not divided, but the In-HOE 203 is divided into a right eye 203R and a left eye 203L and mounted on the light guide plate 201. The distance between the lens optical axes of the right eye 203R and the left eye 203L is set equal to the human interpupillary distance. Images for the left and right eyes are displayed on the display near the focal points of the In-HOE for the left and right eyes, or more precisely, on the plane formed by the object point cloud where light propagating through a transparent medium becomes parallel light at a wavelength selected according to the Bragg diffraction condition corresponding to the angle of view. In this case, if different parallax images are displayed for the left and right eyes, stereoscopic vision with a three-dimensional effect can be achieved. The interpupillary distance of a human being is generally between 58 mm and 75 mm, with an average of 62 to 64 mm, but this varies from person to person, and the distance may be changed or made adjustable for children, adults, women, and men.

[0048] Fig. 14 shows an example in which the light guide plate with HOE (image display device 200) shown in Fig. 13 can be used in combination with a smartphone. By inserting the smartphone into a slot in the housing 110A, it can be fixed at an appropriate tilt angle. The smartphone is removable, and an opening is provided to facilitate removal.

[0049] [Fifth Embodiment (Astronomical Observation Application)] FIG. 15(A) shows the concept of an example in which the present invention is applied to an astronomical observation application, and FIG. 15(B) shows its basic structure. An In-HOE 103 is arranged on a light guide plate 101 via two mirrors M in front of a smartphone D. An image from the smartphone D is incident on the In-HOE 103 and observed through an Out-HOE 104 arranged at a different position on the same light guide plate 101. The focal length of the In-HOE 103 is set to match the optical path length from the smartphone D to the In-HOE 103 via the two mirrors M. An Out-HOE 104 is also arranged on the same light guide plate 101, and is capable of displaying an image at infinity. The infinity distance makes it suitable for astronomical observation. Conventionally, there have been applications that can display the location of stars in real time, assuming that the smartphone's position and angle are known using the smartphone's GPS, acceleration sensor, gyro sensor, etc. However, while conventional applications are useful for roughly viewing the star's position, the smartphone will obscure the star when attempting to view its position with the naked eye. In this embodiment, star information can be displayed while viewing the exact location of the star. Here, it is preferable that the smartphone screen and the Out-HOE are parallel and that an even number of mirrors are used for reflection. This is because tilting the screen will cause misalignment when using a direct-view application, and an odd number of mirrors will result in image inversion. However, this is not the case if the application's video information is displayed after image processing. For example, when light is incident on the In-HOE without using a single mirror, as in the structure shown in FIG. 15(C), if the present invention is applied and the smartphone is tilted relative to the In-HOE, strictly speaking, the orientation recognized by the smartphone's various sensors and the image at infinity seen from the Out-HOE will not be parallel. However, by acquiring information indicating that the smartphone is tilted by a predetermined angle and correcting this through image processing, the application's functionality can be used as is. It is also possible to create a structure as shown in Figure 15(D) in which the image is reflected by a single mirror, but in this case, in addition to correcting the angle of the smartphone, it is necessary to mirror-invert the displayed image.As such, many variations are possible, but by automatically or manually transferring information such as the angle between the virtual image plane and the smartphone, the number of mirrors, and the screen size to the application, a dedicated "image processing mode" can be created to create AR at infinity.The light guide plate is detachable, and if different In-HOE / Out-HOE specifications are attached, the unit can also be used for AR at finite distances other than infinity.

[0050] [Sixth Embodiment (AR Display in a Museum Showcase)] FIG. 16A shows an example in which the present invention is applied to a showcase for exhibits in a museum or art gallery, providing a display unit. The right diagram in FIG. 16A shows the interior of the showcase base. An Out-HOE 104 is placed on a portion of the transparent plate (transparent glass plate or transparent plastic plate) constituting the showcase, and an In-HOE 103 and a combination of a projector Pr and a screen S (a display unit is also possible) are placed in a hidden portion of the showcase base or ceiling. By looking into the Out-HOE 104 (display unit), the image on the screen S can be viewed at a predetermined depth position. By displaying additional information about the exhibit at the same depth as the object and in close proximity, viewers can view the additional information superimposed on the actual exhibit. The additional information can be an explanation of the exhibit or supplementary information. For example, in the case of a stone monument with unclear engraved characters, the outline of the characters can be displayed as additional information. In the case of a sculpture or other structure missing a portion, the missing portion can be visualized to deepen the viewer's understanding and interest. This display does not need to be placed on all surfaces. The advantage of having it on only some surfaces is that it allows for some areas to be viewed without additional information, while others can be viewed together with the additional information. Considering that viewers may be tall or short, the field of view in the propagation direction can be widened, or multiple Out-HOEs can be placed at different heights. The display does not necessarily have to be generated on the transparent plate that constitutes the showcase housing. It can also be a stand-alone type, as shown in FIG. 16(B), where the display device is placed away from the exhibit to provide an explanation of the exhibit. The internal structure of the display device shown in FIG. 16(B) is shown in FIG. 16(C). A combination of a projector Pr and a screen S is placed within the display device's base, and image information reflected by a mirror M is arranged to enter the In-HOE 103. By looking into the Out-HOE 104 (display unit), the image on the screen S can be viewed at a specified depth, which is useful for applications such as reading supplementary information about paintings displayed on the wall in a museum from a short distance away.Although projection mapping can be used to display information superimposed on exhibits, it requires direct illumination of the exhibits with strong light, making it difficult to apply to exhibits that require consideration of light-induced deterioration. Furthermore, applying the Pepper's Ghost display method requires the use of a large-scale device equipped with angled half mirrors. Unlike these other methods, the display of this embodiment has the advantage of eliminating concerns about deterioration of exhibits or space requirements, and allowing viewers to view additional information only when they want to see it.

[0051] [Seventh Embodiment (AR Display on a Flat Display, Part 1)] Figure 17 shows an example of an AR unit to which the present invention is applied to a showcase that can be viewed from above. Examples of such showcases include showcases for exhibits in museums and art galleries, and showcases in stores selling jewelry, luxury watches, and art objects. The AR unit has an In-HOE 103 and an Out-HOE 104 attached to a portion of a light guide plate 101. The In-HOE 103 is boxed so that image light from a smartphone or similarly shaped display D can enter through a mirror M while blocking other ambient light. When looking through the top of the Out-HOE 104, for example, an image from a smartphone D can be displayed at 1x1 magnification at a position approximately 150 mm below the Out-HOE 104. Forming at least three sliding members on the bottom of this AR unit allows it to be moved freely on the showcase, which is preferable. Using sensors or cameras built into the smartphone or installed separately, items in the display case can be recognized along with their location, and information about them can be displayed next to, on the surface of, or inside the object. With a smartphone, related information can be obtained via the internet, which has the advantage of being displayed in real time.

[0052] [Eighth Embodiment (AR Display on a Flat Display, Part 2)] Figure 18 shows an example of the seventh embodiment, in which AR explaining the internal structure of an object can be applied. The depth position at which the display can be displayed can be set arbitrarily, but when the working distance (WD) is short, the depth of focus is not high, so a certain depth position is generally designed. For example, suppose a smartphone screen can be displayed 180 mm behind the screen. If the foreground of a real object is positioned 180 mm away, a so-called projection mapping effect can be achieved. If the real object is moved about 30 mm closer to the In-HOE, an explanation of the interior can be provided. A structure that can move the real object can be provided, and its depth position information can be read by a sensor or camera, and different information or images can be displayed depending on the depth. For example, to show the internal structure of a mechanical device as shown in the figure, the component holding the mechanical device can be moved up and down, and when the surface coincides with the WD, information explaining the material of the surface display is displayed. Then, by raising the mechanical device position by a few millimeters, a substrate that is hidden from the outside and located a few millimeters below the surface is displayed, and information about the substrate is displayed. In this way, a virtual cross-section of the inside of an object can be taken, and information or explanations can be displayed in real time. In this embodiment, the height of the AR unit is fixed and the height of the object is changed, but the height of the AR unit may be variable relative to a fixed object. Furthermore, for an object observed from the side rather than the up and down direction, either the AR unit or the object may be moved horizontally relative to the object.

[0053] [Ninth Embodiment (Floating Image Display)] A ninth embodiment will be described with reference to FIG. 19 . In the image display device 300 of this embodiment, the displayed depth was infinite in the first to fourth embodiments. However, by setting the focal length of the Out-HOE 304 to a finite value, an image floating in mid-air can be displayed. If an object is placed at the approximate focal position of the In-HOE, a floating image is generated at the approximate focal position of the Out-HOE 304. The size of the floating image is proportional to the focal length of the Out-HOE 304 divided by the focal length of the In-HOE 303. The amount of levitation is approximately the focal length of the Out-HOE. When the present invention is applied, an object placed at the approximate focal position of the In-HOE is tilted at an angle such that the propagation direction approaches the light guide plate, as in the case described above.

[0054] Tenth Embodiment Fig. 20 is a schematic diagram of an image display device 500 using reflective holographic optical elements (HOEs) as the In-HOE and Out-HOE. Fig. 21 is a schematic explanatory diagram showing (A) the In-HOE and (B) the propagation of light in the Out-HOE. In this embodiment, the In-HOE 20 is disposed on the image light incident surface side of a light guide plate 10, which is a transparent medium having an optical refractive index of 1.3 or more, and the Out-HOE 30 is disposed on the image light exit surface side of the light guide plate 10, each without an air gap between them. Strictly speaking, if there is a difference between the refractive index of the HOEs 20 and 30 and the refractive index of the light guide plate 10, the angle of the light ray will change. However, because this change in angle is minimal, the light ray is shown as traveling straight in the drawings (same below).

[0055] The In-HOE 20 is an optical element that deflects incident non-parallel light into parallel light at a predetermined angle exceeding the critical angle of the transparent medium 10, and propagates through the transparent medium 10. The In-HOE 20 deflects the incident light L1 within the In-HOE 20, and the deflected parallel light L2 is totally reflected at the interface between the In-HOE 20 and air, and is incident on the transparent medium 10 to propagate therethrough.

[0056] The Out-HOE 30 is an optical element that reproduces, as convergent light, light incident from within the transparent medium 10 (propagated light that is the parallel light propagated within the transparent medium 10). The Out-HOE 30 totally reflects the propagated light (parallel light L2) at the interface between the Out-HOE and air, and deflects the totally reflected light within the Out-HOE 30 to reproduce it as convergent light L3.

[0057] At first glance, the HOE that propagates as shown in Figure 20 appears to be a transmissive HOE, but it is noteworthy that in this embodiment, it is configured as a reflective HOE. A transmissive HOE typically creates a hologram by interfering light from the same side of the HOE medium, resulting in interference fringes that are nearly perpendicular to the HOE medium and low wavelength selectivity (high chromatic dispersion). On the other hand, a reflective HOE creates a hologram by interfering light from the opposite side of the HOE medium, resulting in interference fringes that are nearly parallel to the HOE medium surface and high wavelength selectivity (low chromatic dispersion). When white light (light that is not a single wavelength (e.g., laser)) is incident, if the HOE is configured with low wavelength selectivity, other colors will be diffracted in directions that should not be deflected, resulting in significant color breakup and blurring when the image is reproduced. In contrast, if what appears to be a transmissive HOE is configured as a reflective HOE, that is, if the diffraction toward the incident light direction is reduced to as close to zero as possible and only the diffraction toward the reflected light direction is allowed, the light diffracted toward the reflected light direction is immediately 100% totally reflected at the interface between the HOE and air and propagates toward the incident light direction. In this way, despite having the characteristics of a reflective HOE, the HOE can be placed on the image light incident surface side of the light guide plate, unlike conventional configurations.

[0058] In the configuration of Figure 20, the Out-HOE 30 is attached to the front side of the transparent medium (light guide plate) 10, and although it appears to be a transmissive type at first glance, it also reproduces light as a reflective type. Light propagating within the light guide plate is totally reflected at the air-side interface, and this light reflected at the interface with air becomes input light to the Out-HOE. When this input light (parallel light) is incident on the HOE's optical axis at a waveguide propagation angle of 60°, it is focused at the focal position of the HOE, just like a positive lens, and convergent light is reproduced.

[0059] If it can be placed on the image light incident surface side of the light guide plate, it is possible to avoid design constraints due to focal length and the thickness of the light guide plate. When light rays from a point light source are introduced into the light guide plate, the angle of incidence of the light rays to the hologram changes depending on the thickness and distance of the light guide plate (the distance from the position where the light rays perpendicular to the light guide plate and passing through the point light source intersect with the light guide plate). If the angle of incidence to the hologram changes, the playback angle of the hologram also changes. The change in the playback angle is almost equivalent to the change in the angle of incidence. Figure 22 is a diagram explaining the effect of the thickness of the light guide plate on the angle of incidence of the hologram. Let t be the distance in the thickness direction of the light guide plate and r be the distance from directly below the point light source, and t = 0, t Ａ , t Ｂ (0 < t Ａ <t Ｂ ), r = r １ , r ２ (0 < r １ <r ２ ) and light is emitted from a point light source at a height H from the surface at t = 0. １ , r ２ The incident angles at the point θ1, θ2, t = t Ａ The incident angle to the surface is θ1 Ａ , θ2 Ａ , t=t Ｂ The incident angle to the surface is θ1 Ｂ , θ2 Ｂ (θ is a positive value only). In this case, θ2 > θ2 Ａ >θ2 Ｂ , θ1>θ1 Ａ >θ1 ＢIt can be seen that the thicker the light guide plate, the greater the change. To eliminate or mitigate this effect, conventional configurations require different holograms to be prepared depending on the thickness of the light guide plate. The incident angle changes more significantly as the distance r increases and the height H of the point light source decreases (the light source is closer to the light guide plate surface). Therefore, for holograms with a large distance r or holograms with a close point light source, the influence of the substrate thickness increases, making it increasingly necessary to create holograms that are tailored to the thickness. In contrast, the configuration of this embodiment places an In-HOE or Out-HOE in front of the light guide plate (on the front side of the light guide plate) without using a light guide plate, eliminating the influence of the focal length or the thickness of the substrate and eliminating the need to create holograms tailored to the substrate thickness.

[0060] When using two HOEs to extract an image from a different location on the light guide plate, as in the present invention, various distortions on the light input side can lead to degradation of the reproduced image quality. In this case, if the thickness of the light guide plate is known, it is possible to minimize various distortions by taking that thickness into account. However, if the thickness of the light guide plate on which the HOE is disposed varies, multiple dedicated HOEs must be manufactured to accommodate these differences in thickness. For example, this would result in multiple products being manufactured for 3 mm thick glass, 5 mm thick glass, and other applications, which would not only be cumbersome but could also result in performance issues when combined with products of different specifications. Therefore, by positioning the HOE so that it is located immediately after the light input, even though it is a reflective HOE, this problem can be solved, allowing products of the same specifications to be attached to light guide plates of any thickness.

[0061] Each component will be described below, but the present invention is not limited to these.

[0062] (Transparent Medium) The transparent medium is made of a material with an optical refractive index of 1.3 or greater. It can be a parallel plate, or it can be a cylinder, a portion of a cylinder, or a portion of a hollow sphere. In the present invention, since total reflection is utilized, the transparent medium must be made of a transparent material with an optical refractive index of 1.3 or greater. Examples of materials with an optical refractive index of 1.3 or greater include acrylic resin, polycarbonate resin, polyethylene terephthalate resin, optical glass BK7, white plate glass, and blue plate glass. Since an optical refractive index greater than 1.6 increases propagation angle deflection and glass interface reflection, the optical refractive index is preferably 1.3 or greater and 1.6 or less. Hologram materials include photopolymers, silver halide materials, and dichromated gelatin. However, hologram materials generally have an optical refractive index of approximately 1.3 to 1.7, and avoiding an optical interface with the transparent medium (base material) prevents the formation of unnecessary holograms. Therefore, it is preferable to use transparent medium and hologram material with similar optical refractive indices. Furthermore, if the transparency is reduced, a loss of light intensity occurs during propagation, so high transparency is preferable.

[0063] (Holographic Optical Element (HOE)) In the present invention, the same In-HOE and Out-HOE can be used. In the tenth embodiment, the In-HOE and Out-HOE are both reflective, but they may be either reflective or transmissive, or one may be reflective and the other transmissive. Each HOE is bonded to the base material of the transparent medium without an air gap between them. Note that in the configuration of the present invention, there is no need to create an optimal hologram in accordance with the substrate thickness of the transparent medium; as long as a hologram is provided, any substrate can be used.

[0064] The focal lengths of the In-HOE and Out-HOE are preferably within the range of 5 mm to 300 mm. A short focal length tends to result in a poorer sense of floating of the image, but a larger field of view and increased image sharpness. A long focal length tends to result in a superior sense of floating of the image, but a narrower field of view, increased instability, and a loss of image sharpness. Furthermore, a long focal length results in a larger device. It is desirable that the ratio of the hologram sizes be approximately the same as the ratio of the focal lengths.

[0065] Furthermore, when a protective layer is provided on the surface of the hologram material, the distance from the interface with air on the side where parallel light within the medium is totally reflected to the In-HOE or Out-HOE is preferably 0.5 mm or less. This is because, while there is no problem with distortion if parallel light is incident, when diffused or condensed light is incident, the longer the distance from the air interface to the HOE, the greater the distortion.

[0066] [Eleventh Embodiment] Fig. 23 is a schematic diagram of an image display device 600 that uses transmission-type holographic optical elements (HOEs) as the In-HOE and Out-HOE. Fig. 24 is a schematic explanatory diagram showing the propagation of light in (A) the In-HOE and (B) the Out-HOE. In this embodiment, the In-HOE 40 is disposed on the surface opposite the image light incident surface of the light guide plate 10, which is a transparent medium having an optical refractive index of 1.3 or more, and the Out-HOE 50 is disposed on the surface opposite the image light exit surface of the light guide plate 10, each without an air layer between them.

[0067] The In-HOE 40 is an optical element that deflects incident non-parallel light into parallel light at a predetermined angle exceeding the critical angle of the transparent medium 10, and propagates the parallel light within the transparent medium 10. The In-HOE 40 deflects the incident light L1 within the In-HOE 40, and the deflected parallel light L2 is totally reflected at the interface between the In-HOE 40 and air, causing it to enter the transparent medium 10.

[0068] The Out-HOE 50 is an optical element that reproduces convergent light when the propagated light, which is the parallel light propagated through the transparent medium 10, enters the Out-HOE 50. The Out-HOE 50 totally reflects the propagated light (parallel light L2) at the interface between the Out-HOE 50 and air, and deflects the totally reflected light within the Out-HOE 50 to reproduce it as convergent light L3.

[0069] 23 appears to be a reflective HOE, but it is noteworthy that in this embodiment it is a transmissive HOE. If the HOE used for incoupling from the light guide plate side is a transmissive HOE, it can be used preferably for laser reproduction because it does not have high wavelength selectivity.

[0070] [Modifications of HOE Arrangement] There are various variations on which side of the light guide plate the In-HOE and Out-HOE are located. Figure 25 shows variations in the positional relationship between the incident and exit-side HOEs relative to the light guide plate. Assuming that all reflective edge-lit HOEs are used, all of the variations shown in the figure are possible. a) and h) are orthodox, conventional configurations. b), c), f), and g) can be achieved by total reflection with a thin protective film on the In-HOE side immediately after deflection by the HOE, while b), d), e), and g) can be achieved by total reflection with a thin protective film on the Out-HOE side just before light enters the HOE. Configurations e), f), g), and h) can be selected when it is desired to introduce image light from the same side as the observer, and configurations a), b), c), and d) can be selected when it is desired to introduce image light from the opposite side of the observer, depending on the configuration of the device. In the cases of a), b), c), and d), the In-HOE and Out-HOE must be bonded separately from both sides of the light guide plate, which complicates the process and increases costs, whereas in the cases of c), d), g), and h), the light guide plate can be bonded from one side, the process is simple, and the In-HOE and Out-HOE can be manufactured from a single sheet, which reduces manufacturing costs, and there is no change in the total thickness of the medium through which light propagates, and there is no change in the optical path length due to boundaries or edge scattering, which also offers great performance benefits.

[0071] Up to this point, we have explained an example in which a reflective edge-lit HOE is used, but it is also possible to use a transmissive edge-lit HOE. That is, b) and g) are orthodox, conventional configurations, but a), d), e), and h) can be realized as a reflective configuration by having the light pass through a thick light guide plate on the In-HOE side and then undergo total reflection by a thin protective film immediately after deflection in the HOE, and a), c), f), and h) can be realized by using light that is totally reflected by a thin protective film immediately before entering the HOE on the Out-HOE side. The advantages and disadvantages in terms of configuration, layout, manufacturing, and process are the same as those described above.

[0072] (Display Modification) The "object" positioned at a position where light enters the In-HOE has been described using a smartphone display screen as an example, but it may also be a smartwatch, a portable game console, a tablet, a laptop computer, a touch panel, a PC monitor, etc. Furthermore, the "object" itself may not be a display, but may be a light diffuser or a screen, and projection may be performed on a transmissive screen using backlighting and a reflective screen using frontlighting. By narrowing the viewing angle compared to a normal display and matching it with the angle of view at which light should enter the In-HOE, light utilization efficiency can be improved, making it possible to brighten the image observed from the Out-HOE. Specifically, this can be achieved by using a display component with a diffusion function that has a narrower diffusion angle. Furthermore, in addition to the two-dimensional displays described above, curved displays are also acceptable. This can be achieved by displaying on a self-luminous flexible display such as an organic electroluminescence (EL) display, or by projecting onto a curved screen. The key feature of the present invention is that the reproduction wavelength selected by the Bragg diffraction conditions is corrected by the tilt of the image. This can be achieved approximately by tilting the entire planar object while maintaining its flatness. However, strictly speaking, the amount of floating relative to the image height is not linear but rather curved. Therefore, as shown in Figure 26, a slight curvature can be applied to the two-dimensional display to enable more precise image reproduction. The shape of the display D' of the image display device 100', which is a modified example shown in Figure 26, represents a cross-section including the optical axis, but it is also effective to apply a curvature in the depth direction of the page. Furthermore, the "object" placed at the position where light enters the In-HOE may be something like a figurine with a slightly uneven surface. In this case, the image observed from the Out-HOE will also be a three-dimensional image.

[0073] The imaging device of the present invention can be broken down into: A: a device in which an In-HOE and an Out-HOE are integrated with a light guide plate; B: an image display device (smartphone, screen, etc.); and C: a component that can fix A and B while maintaining their positional and angular relationships. By standardizing the focal length and external shape of the In-HOE, it becomes possible to use different types of A interchangeably without changing B and C. That is, by preparing a stereoscopic vision device as described in the fourth embodiment shown in FIG. 27(A1), a device that localizes an image at infinity as described in the first embodiment shown in FIG. 27(A2), and a device that displays an aerial image as described in the ninth embodiment shown in FIG. 27(A3), and selecting one of these and replacing it with C, it can be used for different purposes. The shape can also be designed to prevent incorrect insertion, front and back, top and bottom, or left and right. Furthermore, it is possible to provide a shape for distinguishing the type, the focal length or the propagation angle of the In-HOE (for example, a different notch shape depending on the type, as shown in the figure), or to provide an electronic memory device. In this case, by providing a detection mechanism in C or B, it becomes possible for the image display device in B to determine which type of A device has been attached and to display an appropriate image or to display a warning when a device with a different focal length has been attached.

[0074] It is also possible to replace B with a different type without changing A and C. For example, instead of using a dedicated display device, C is designed to be easily attached and detached, assuming that you will only attach your everyday smartphone when using this device and then remove it after use. Also, since the displays you want to attach vary in size and thickness, the structure is such that any display can be attached and fixed at a specified angle to a certain extent.

[0075] The present invention can be used for a variety of applications, including head-up displays, AR smart glasses, and VR glasses. Examples of head-up displays include head-up displays for drivers of automobiles, airplanes, and trains, and head-up displays that can be attached to helmets, hats, glasses, face shields, etc., for hands-free viewing while driving motorcycles or bicycles, hiking, running, walking, etc. An example of an embodiment is shown in FIG. 28 . This figure shows an image display device 400 in which an acrylic plate with an in-HOE and an out-HOE can be clipped onto a hat, allowing the smartwatch screen to be viewed see-through when the smartwatch is attached to the holder. Information that is essential to see while playing sports or driving is often text, such as the time (hours) or short messages, and even this size is sufficient.

[0076] An example of application of the present invention to a head-up display for an automobile, an aircraft, or the like is shown in Figure 29. Figure 29(A) is a schematic diagram of a type in which an image propagates inside a windshield (front glass). An In-HOE 103 is formed in optically tight contact with a portion of the windshield 150 behind the dashboard. Image light incident from the In-HOE propagates through the windshield by total reflection and is emitted by the Out-HOE 104, thereby reaching the eyes of the driver 151. The Out-HOE 104 is designed to emit light not in its normal direction but at an inclined angle. Since the image can be localized at any depth, including infinity, the driver can obtain various visual information while driving without having to shift their gaze to an instrument panel or the like. FIG. 29(B) has the same purpose, but instead of propagating the image through the windshield, the image light emitted from the Out-HOE 104 of the light guide plate described in the first to fourth embodiments is specularly reflected off the windshield surface so that it reaches the driver's eyes. Both projection optical systems have the advantage of being compact and capable of projecting images at any depth, even close to infinity. Although the windshield has a slightly curved structure, in both cases of FIGS. 29(A) and 29(B), optical system compensation and image compensation tailored to the shape of the windshield make it possible to display image information without creating a sense of incongruity. While the image display unit can of course be configured as a dedicated projection optical system, it is also possible to have only the optical system components other than the illustrated display D pre-installed in the vehicle, and then have the driver's own display-equipped device, such as a smartphone or tablet, installed at position D after installation.

[0077] [Twelfth Embodiment (In-Vehicle Meter Display)] Figure 30 is a diagram showing an example in which the present invention is applied to an AR display on an in-vehicle meter display. The AR unit has a simple structure with a holding mechanism H that allows a light guide plate 101, to which an In-HOE 103 and an Out-HOE 104 are attached, to be fitted and attached to the top of a meter display D at a predetermined angle and distance. When this AR unit is attached to the meter display D and an image is displayed on the portion facing the In-HOE 103, the image can be displayed on a separate portion, the Out-HOE 104. The AR unit may be integrated with the meter display in advance, or may be attachable and detachable afterward. It is also possible to provide a function on the display side that detects the type and position of the AR unit attached, and display an appropriate image in the corresponding display area depending on the installation location and type. By placing a light guide plate in front of an existing meter display in an area that is not normally used, such as a blind spot caused by the steering wheel or the edge, an agent or the like can be displayed in a raised position. It is also possible to display information at a distance, such as outside the vehicle.

[0078] [Thirteenth Embodiment] Figure 31 shows a thirteenth embodiment of the present invention, in which a double-sided image display device is configured in which different moving images can be viewed from both sides. A pair of In-HOE (601A) and Out-HOE (602A) formed on a single sheet of hologram recording material and another pair of In-HOE (601B) and Out-HOE (602B) formed on a single sheet of hologram recording material are stacked relative to a light guide plate such that the Out-HOEs overlap but the In-HOEs do not overlap (here, they are orthogonal, shifted by 90°). In Figure 31(A), the bottom left diagram shows the X-X cross section (light propagation through 601A and 602A), and the bottom right diagram shows the Y-Y cross section (light propagation through 601B and 602B).

[0079] Display DA and display DB (such as a liquid crystal panel, organic EL display, or projection screen) are placed near the focal point of each In-HOE (in FIG. 31 , on the front side of the light guide plate). In this case, an In-HOE is bonded to the surface of the light guide plate closer to the light source. By applying the present invention, the In-HOE is reflective, but propagates toward the Out-HOE at an angle exceeding the critical angle inside the light guide plate. The Out-HOEs propagate the images of the corresponding displays so that they can be observed from air. However, the present invention is applied only to display 602A, and display 602B is formed as an orthodox reflective type. In this way, the image displayed on display DA is observed from the display side, as in the state shown in FIG. 31(C), but remains transparent without any image being reproduced from the opposite side of the display. Furthermore, the image displayed on display DB is observed from the opposite side of the display, as in the state shown in FIG. 31(B), but remains transparent without any image being reproduced from the display side. Furthermore, when displays DA and DB are turned on simultaneously, different images are simultaneously reproduced on both sides, as shown in Figure 31(D). In other words, independent images can be displayed on both sides at independent times. This is because the volume edge-lit hologram has sufficient angular selectivity, so that light propagating from different, orthogonal directions will not reproduce each other.

[0080] It is preferable to provide an anti-reflection layer on both the front and back surfaces, as this will prevent the reflected image on the back surface from being seen. In other words, by providing an anti-reflection film as an anti-reflection treatment or forming a moth-eye structure, it is possible to prevent the image reproduced on the back surface from being reflected back at the interface with air.

[0081] By doing this, for example, by installing it on the window glass of a taxi or bus, it will be possible to display advertisements or warnings with different content simultaneously or at independent times, such as an image that passengers can see from inside and an image that non-passengers can see from outside.

[0082] The above is an example of propagating images at different angles onto a single light guide plate, but it is also possible to display images with different depth positions, provided that they are output to the same side.

[0083] 32 shows a fourteenth embodiment in which the present invention is applied to an image display device that uses a portion of an in-vehicle meter display as an image display, as described in the twelfth embodiment, to enable video images to be viewed from the same direction but at different depths. A transparent light guide plate is placed in front of the meter display, separated by a WD. The light guide plate includes a pair of In-HOE (601A) and Out-HOE (602A) formed on a single sheet of hologram recording material, and another pair of In-HOE (601B) and Out-HOE (602B) formed on a single sheet of hologram recording material. The two pairs of In-HOEs are stacked at an angle (here, offset by 90° so that they are perpendicular to each other) so that the Out-HOEs overlap but do not overlap. These two pairs of In-HOE and Out-HOE are formed so that the propagation directions are different. By displaying a processed floating image on the meter display in the portion DA facing the In-HOE (601A), an agent image can be displayed floating approximately 30 mm from a distance on the same light guide plate. Furthermore, by displaying another processed image required for driving on the portion DB facing the In-HOE (601B) on the same data display, a speedometer can be displayed 30 meters in the background. Because the angles of the propagating light are orthogonal, even if the Out-HOE is placed in the same location, there is no interference between the images, and two images can be observed simultaneously or sequentially at independent timings. The back of the Out-HOE may be located above the dashboard, or it may be used as a head-up display by allowing the driver to see the outside scenery through the windshield. While an example of displaying the image above the meter display has been described, it can also be placed in a different orientation. For example, it can be placed in front of the interior panel of the vehicle and images of the underside of the hood, which is normally a blind spot from the driver's seat, can be obtained from a camera mounted at the front of the vehicle and displayed in real time with a sense of depth, and if an abnormality is detected, a warning mark will be displayed in the location where it appears.

[0084] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0085] 100, 200, 300, 400, 500, 600, 700, 750, 800, 900 Image display device 10, 101, 201 Light guide plate 20, 40 Incoupling holographic optical element (In-HOE) 21, 41 Optical axis of incoupling HOE 30, 50 Outcoupling holographic optical element (Out-HOE) 31, 51 Optical axis of outcoupling HOE 102 Holographic recording medium 103 In-HOE (In-HOE region) 104 Out-HOE (Out-HOE region) 105 Surface protection substrate (protective layer) 110, 110A, 115, 116 Housing 117 Hinge portion 118 Link mechanism portion 120A, 120B Mirror 150 Windshield 151 Driver 130 Concave lens 203, 303 In-HOE 203R In-HOE for right eye 203L In-HOE for left eye 204, 304 Out-HOE 701, 706, 801, 806 Light guide plate 702, 707, 802, 807 Holographic recording medium 703, 708, 803, 808 In-HOE (In-HOE region) 704, 709, 804, 809 Out-HOE (In-HOE region) 705, 710, 805, 810 Surface protective substrate (protective layer) 711, 811, 906 Hologram image (intermediate image) 712, 812, 905 Observation image L1 Incident light L2 Deflected light L3 Converging light S Object P Image D, D' Display (smartphone, meter display) M Mirror S Screen Pr Projector H Holding mechanism

Claims

1. An image display device comprising: a transparent medium having an optical refractive index of 1.3 or more; and an incoupling holographic optical element and an outcoupling holographic optical element arranged in a portion of the transparent medium without an air layer between them, wherein the incoupling holographic optical element is an optical element that deflects image information of non-parallel light incident thereon into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates within the transparent medium, and the outcoupling holographic optical element is an optical element that reproduces light incident from within the transparent medium as converging light, parallel light, or diverging light in air, and wherein a light diffusing object that is the source of the image information incident on the incoupling holographic optical element is roughly positioned on a plane formed by a group of object points that will cause light propagating within the transparent medium to become parallel light at a wavelength selected by the Bragg diffraction condition according to the angle of view.

2. The image display device according to claim 1, wherein the light diffusing object that is the source of the image information incident on the incoupling holographic optical element is either a flat diffusion screen, a spatial light modulation element, or a self-luminous display element, and an inclination angle is given to the surface thereof.

3. The image display device according to claim 1, wherein the light diffusing object that is the source of the image information incident on the incoupling holographic optical element is a hologram image that is virtually emitted from a position that does not actually exist.

4. An image display device according to claim 1 or 2, characterized in that a concave lens is disposed between the incoupling holographic optical element and a light diffusing object that is the source of the image information incident on the optical element.

5. An image display device according to claim 1 or 2, characterized in that a mirror is disposed between said incoupling holographic optical element and a light diffusing object that is the source of the image information incident on said optical element.

6. An image display device as described in claim 5, characterized in that the angle of the mirror can be adjusted so that the surface of the light diffusing object that is the source of the image information entering the incoupling holographic optical element and the surface of the image information observed from the outcoupling holographic optical element are parallel.

7. An image display device according to claim 5, characterized in that the number of mirrors arranged between the incoupling holographic optical element and the outcoupling holographic optical element is an even number.

8. An image display device according to claim 1, characterized in that the light diffusing object that is the source of the image information that enters the incoupling holographic optical element is all or a part of the object displayed on the display, the display includes at least one of a GPS (Global Positioning System), an acceleration sensor and a gyro sensor and is capable of recognizing its own position and angle, and the image information that enters at least the incoupling holographic optical element has been subjected to image processing including angle conversion, image inversion, size conversion and distortion conversion based on the position and angle information so that the image that can be observed from the outcoupling holographic optical element remains parallel to the display.

9. The image display device according to claim 1 or 2, characterized in that the incoupling holographic optical element is configured by holographic optical elements having two types of optical axes, arranged in a direction perpendicular to the propagation direction, with the distance between two intersection points where the holographic optical element surface and each optical axis intersect being 58 mm or more and 75 mm or less, and two types of parallax images are input as video information, centered on a plane formed by object point groups where the propagating light within each transparent medium becomes parallel light.

10. An image display device according to claim 1 or 2, characterized in that the outcoupling holographic optical element is an optical element that reconstructs light entering from within the transparent medium as parallel light in air, and reconstructs an image with a sense of positioning at infinity.

11. An image display device according to claim 1 or 2, characterized in that the outcoupling holographic optical element is an optical element that reproduces light entering from within the transparent medium as divergent light in air, and reproduces an image with a sense of localization at a predetermined depth position.

12. An image display device according to claim 1 or 2, characterized in that the outcoupling holographic optical element is an optical element that reconstructs light entering from within the transparent medium as convergent light in air, thereby reconstructing an image that appears floating for the observer.

Images