Image display device and double-sided image display device

Abstract

To provide an image display device having a simple structure and a high degree of freedom in design.SOLUTION: And an in-coupling HOE20 and an out-coupling HOE30 disposed on a part of the transparent medium 10 without an air layer, wherein the in-coupling HOE20 and the out-coupling HOE30 are provided with a protection layer on a side opposite to the transparent medium 10, and the protection layer is transparent and has a thickness less than 0. 5mm. 1.3, the in-coupling HOE20 is an optical device that deflects light incident from an interface with air into parallel light having a predetermined angle exceeding a threshold angle of the transparent medium 10 and propagates the parallel light in the transparent medium, and the out-coupling HOE30 is an optical device or an image hologram that reconstructs light incident from the inside of the transparent medium 10 into air, in which the in-coupling HOE20 totally reflects the deflected parallel light at an interface between a protection layer of the in-coupling HOE20 and air and propagates the parallel light in the transparent medium 10.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[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. [Background technology]

[0002] Conventionally, there have been image display devices that arrange incoupling and outcoupling holographic optical elements (hereinafter referred to as HOEs) on parallel plates and propagate light. An image display device has been proposed that can display more information by expanding the display angle of view while suppressing color unevenness with a simple configuration (see, for example, Patent Document 1). Hereinafter, incoupling HOEs will be referred to as In-HOEs and outcoupling HOEs as Out-HOEs. Another proposed holographic image display device uses the Out-HOE as a recorded edge-lit hologram image, a light source placed near the focal position of the In-HOE, converts light from the light source into parallel light within the medium using the In-HOE, and uses that light as illumination light to reconstruct the edge-lit hologram (see, for example, Patent Document 2).

[0003] Representative examples of these conventional techniques are shown in Figure 13. Figure 13(A) shows an example in which video information 70 enters a reflective In-HOE 80, propagates through a substrate (light guide plate) 10 as parallel light exceeding the critical angle, and is emitted into air by a reflective Out-HOE 90 from the surface opposite the light source (video information). Figure 13(B) shows an example in which light from a point light source 71 is propagated through the substrate 10 as parallel light exceeding the critical angle by the In-HOE 80, and is then emitted into air from the surface opposite the point light source 71. The HOE here is a thick holographic optical element, also known as a volume type, Lippmann type, or volume type. Even when white light is used as a light source, the diffracted light is wavelength-selected based on the Bragg diffraction conditions of the interference fringes recorded as refractive index modulation, and is used for optical elements and image reconstruction. For recording materials on which HOEs are recorded, those that diffract light in the same direction as the incident light are defined as reflective, and those that diffract light in the opposite direction are defined as transmissive. Reflective types have higher wavelength and angle selectivity than transmissive types, and because the properties of reflective and transmissive types are different, they cannot be used interchangeably.

[0004] In the conventional example shown in Figure 13, it is a design requirement that the image display device and light source that will enter the In-HOE must be located on the side opposite the viewer side of the substrate. For example, if the device is to be used on the front glass of an amusement machine or automobile instrument panel, it is not practical to project light or images from the viewer side through the front glass, and there is a demand for placement on the rear side. Based on the principle that the incoming light is reflected toward the light guide plate by the In-HOE and the light propagated within the light guide plate is reflected by the Out-HOE, the layout shown in Figure 13 has traditionally been used, but this has had the following problems A) to E). A) Optical path differences caused by steps result in performance degradation Even if the layer thickness difference is less than 1 mm, when light propagates at an angle φ within the medium, an optical path difference of 2 × (layer thickness difference) × (tan φ) occurs. The larger φ is, the more areas the light does not reach, resulting in significant image degradation. B) Edge scattering causes performance degradation This is also related to A) above, but total reflection does not occur at the step area, causing light leakage and a decrease in light utilization efficiency, and light scattering at that area can cause unnecessary scattered light to propagate within the medium, resulting in performance degradation. C) The process is complicated and costs increase. It is necessary to attach the film to both sides of the light guide plate with high precision, which makes the manufacturing process more complicated and more expensive than attaching it to one side. D) Functional limitations arise If the thickness of the part where light enters the In-HOE from the air and the part where it exits the Out-HOE into the air changes, the optical function also changes. For example, if non-parallel light is incident on the In-HOE, distortion will increase. Also, if the Out-HOE is an image hologram, as the distance from the Out-HOE to the air interface increases, the image will be positioned further back, which will limit the image expression capabilities. E) There are limitations to the design freedom. If the HOE (or its protective layer) is on the observer's side, it can be easily touched by the observer and damaged or peeled off, so even if there is a design reason to place it on the back side, optical constraints sometimes limit the degree of freedom.

[0005] To address issues A) and B), patent document 3, for example, has proposed optically bonding two flat transparent media plates to both sides of the display. However, this method could not be easily attached to existing glass or acrylic materials for commercialization. Furthermore, the design flexibility was limited, making it impossible to create a double-sided display using holograms, allowing different images to be viewed simultaneously from both sides of the transparent media. Patent document 4 also discloses a system that uses a transparent screen to allow different images to be viewed from the front and back of the transparent media. For such applications, a light guide plate-type edge-lit HOE could be used to display a compact, bright image with reduced unwanted images. However, the lack of design flexibility for the placement of the In-HOE and Out-HOE prevented this from being realized. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-92719 [Patent Document 2] International Publication No. 2018 / 221091 [Patent Document 3] Japanese Patent Application Publication No. 2023-175794 [Patent Document 4] Japanese Patent Publication No. 2023-068348 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above-mentioned problems, and has an object to provide an image display device with a simple structure and high degree of freedom in design. [Means for solving the problem]

[0008] The first 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 and the outcoupling holographic optical element have a protective layer on the side opposite the transparent medium, the protective layer being a transparent layer having a thickness of less than 0.5 mm, the incoupling holographic optical element is an optical element that deflects light entering from an interface with air into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates the light through the transparent medium, and the outcoupling holographic optical element is an optical element or image hologram that reconstructs light entering from within the transparent medium in air, and the incoupling holographic optical element totally reflects the deflected parallel light at the interface between the protective layer of the incoupling holographic optical element and the air, causing the deflected parallel light to propagate through the transparent medium.

[0009] Furthermore, a second 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 disposed in a part of the transparent medium without an air layer therebetween, the incoupling holographic optical element and the outcoupling holographic optical element each having a protective layer on the side opposite the transparent medium, the protective layer being a transparent layer having a thickness of less than 0.5 mm, the incoupling holographic optical element being an optical element that deflects light entering from an interface with air into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates through the transparent medium, and the outcoupling holographic optical element being an optical element or image hologram that reproduces light entering from within the transparent medium in air, and the outcoupling holographic optical element is characterized in that the light entering from within the transparent medium is totally reflected at the interface between the protective layer of the outcoupling holographic optical element and the air, and then diffracted and reproduced in air.

[0010] In the image display device of the present invention, it is preferable that the incoupling holographic optical element and the outcoupling holographic optical element are disposed on the same surface of the transparent medium and are provided on the same hologram material.

[0011] In the image display device of the present invention, the transparent medium is preferably any one of a parallel plate, a cylinder, a part of a cylinder, and a part of a hollow sphere.

[0012] The double-sided image display device of the present invention includes at least two sets of image display devices each combining an incoupling holographic optical element and an outcoupling holographic optical element, At least one set of the image display devices is either the first image display device of the present invention or the second image display device of the present invention, characterized in that an incoupling holographic optical element and an outcoupling holographic optical element constituting the image display device are combined with a single transparent medium having an optical refractive index of 1.3 or more, and are arranged at different angles so that different images can be selectively observed from the front and back of the same part of the transparent medium.

[0013] In the double-sided image display device of the present invention, it is preferable that the light incident surfaces of the incoupling holographic optical elements of the two sets of image display devices are on the same plane.

[0014] In the double-sided image display device of the present invention, it is preferable that the image display portion of the transparent medium is subjected to anti-reflection treatment or moth-eye treatment. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide an image display device having a simple structure and a high degree of freedom in design.The image display device of the present invention can also be used as an optical deflection device. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a main part of an image display device according to a first embodiment of the present invention, which is an example of the image display device of the present invention. [Figure 2] FIG. 1 is a schematic diagram of a main part of an image display device according to a second embodiment of the present invention, which is an example of the image display device of the present invention. [Figure 3] FIG. 10 is a schematic diagram of an image display device according to a third embodiment, which is an example of an image display device of the present invention. [Figure 4] 10A and 10B are schematic explanatory diagrams showing the propagation of light in an incoupling HOE and an outcoupling HOE in the third embodiment. [Figure 5] 10A and 10B are diagrams illustrating the influence of the thickness of the light guide plate on the angle of incidence of a hologram. [Figure 6] FIG. 10 is a schematic diagram of an image display device according to a fourth embodiment, which is an example of an image display device of the present invention. [Figure 7] 10A and 10B are schematic explanatory diagrams showing the propagation of light in an incoupling HOE and an outcoupling HOE in a fourth embodiment. [Figure 8] 10A to 10C are diagrams showing variations in HOE arrangement in the image display device of the present invention. [Figure 9] (A) Schematic explanatory diagram showing the propagation of light when evaluating an incoupling HOE and (B) an outcoupling HOE. The upper diagram is a schematic explanatory diagram for a reflective HOE, and the lower diagram is a schematic explanatory diagram for a transmissive HOE. [Figure 10] 10A and 10B are diagrams illustrating image propagation by an image display device according to a third embodiment. [Figure 11] FIG. 10 is a schematic diagram of an image display device (double-sided display of recorded holograms) according to a fifth embodiment, which is one example of the image display device of the present invention. [Figure 12] FIG. 10 is a schematic diagram of an image display device (double-sided moving image display) according to a sixth embodiment, which is an example of an image display device of the present invention. [Figure 13] FIG. 1 is a diagram illustrating a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] [First embodiment] An image display device 100 according to a first embodiment of the present invention will be described with reference to FIG. 1. Light guide plate 101 is a transparent acrylic parallel plate with an optical refractive index of approximately 1.5, measuring 10 mm thick, 100 mm long, and 100 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) transparent optical adhesive. A reflective In-HOE is recorded in a region 103 of the holographic recording medium, and a reflective 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.

[0019] The In-HOE is a volume hologram recorded in red, green, and blue wavelengths. When divergent light is incident on the In-HOE, which is positioned 38 mm away from the normal, it deflects the light into parallel light at an angle of 60° to the normal within the light guide plate medium, reflecting it back toward the incident direction.The effective area is 60 mm in the propagation direction (the vertical direction) (2t·tanφ=2×10×tan60°=34.6 mm or more), 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.

[0020] The Out-HOE is a volume-type edge-lit image hologram recorded in the same red, green, and blue wavelengths as the In-HOE, and reproduces an image in the direction of reflecting parallel light propagating at a 60° angle within the medium. At first glance, the Out-HOE appears to be reproducing light propagating through the light guide plate at a total reflection angle in a transmission-type manner, but in reality, it hardly reproduces the transmission-type component. Light that strikes the thin film of the Out-HOE's surface protection 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 that of the light guide plate. However, because this change in angle is minimal, it is shown as traveling in a straight line in the drawings.

[0021] In this way, by placing the edge-lit hologram, which is an Out-HOE, on the surface that appears to be transmissive, the following effects are produced when there is a design constraint that the illumination light source must be placed farther away from the observer. a) Image reconstruction can be performed on the surface closest to the observer. In the case of an arrangement such as that shown in Figure 13(B), the image is reconstructed at a depth equivalent to the thickness of the light guide plate. Although it is possible to position the image at a floating position using the hologram recording optical system, reconstructing the image at a distance from the recording medium will result in image blurring proportionally increasing as the point light source quality of the illumination deteriorates. Therefore, from the perspective of image quality degradation, it is better to position the hologram recording medium as close to the front as possible. b) Because the In-HOE and image hologram medium can be attached to the same surface, it is easy to process them later on existing light guide plates. For example, when attaching them to window glass or transparent doors, it is extremely difficult to process the front and back surfaces in position, but the fact that it can be installed on just one side is a major advantage. c) Furthering on b), the In-HOE and the image hologram can be formed on the same material, as explained above. A) Optical path differences caused by steps result in performance degradation B) Edge scattering causes performance degradation C) The process is complicated and costs increase. Solve the problem.

[0022] [Second embodiment] An image display device 200 according to a second embodiment of the present invention will be described with reference to FIG. 2. This embodiment is essentially similar to the first embodiment, but differs in that the attachment surfaces of the In-HOE (region 103) and the reflective edge-lit hologram (region 104) are located on the opposite side of the light guide plate (substrate). In the first embodiment, the hologram recording material is attached to the viewer's side of the light guide plate. If the viewer can touch this surface with their hands, there is a concern that it may be scratched or peeled off. However, the second embodiment has the advantage of increased durability. Although the image is reproduced on the surface farther from the viewer, if the thickness of the light guide plate does not significantly affect the image reproduction, the configuration can be determined by prioritizing durability. Similarly, the performance of the In-HOE also depends on the distance to the light source-side air interface (described later in the third embodiment). Therefore, applying the present invention has the great advantage of allowing the arrangement to be selected taking into account design requirements and differences in performance. As a variation of the In-HOE, it may be arrayed perpendicular to the light propagation direction, as described in Patent Document 2.

[0023] [Third embodiment] Fig. 3 is a schematic diagram of an image display device 300 that uses reflective holographic optical elements (HOEs) as the In-HOE and Out-HOE. Fig. 4 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 the light guide plate 10, which is a transparent medium with 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, the angle of the light ray changes if there is a difference between the refractive index of the HOEs 20 and 30 and that of the light guide plate 10. However, because this change in angle is minimal, the light ray is shown as traveling straight in the drawings (same below).

[0024] The In-HOE 20 is an optical element that deflects incident non-parallel light into parallel light at a predetermined angle that exceeds 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, causing it to enter the transparent medium 10 and propagate therethrough.

[0025] The Out-HOE 30 is an optical element that regenerates light entering from within the transparent medium 10 (the propagating light that is the parallel light propagated within the transparent medium 10) as convergent light. The Out-HOE 30 then totally reflects the propagating 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 regenerate it as convergent light L3.

[0026] At first glance, the HOE shown in Figure 3 appears to be a transmissive HOE. However, it is noteworthy that this embodiment uses 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 poor 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 poor wavelength selectivity (low chromatic dispersion). When white light (light that is not a single wavelength, such as a laser) is incident, an HOE with poor wavelength selectivity diffracts other colors in directions that should not be deflected, resulting in significant color breakup and blurring when the image is reconstructed. In contrast, if what appears to be a transmissive HOE is configured as a reflective HOE, that is, if diffraction toward the incident light direction is reduced to as close to zero as possible and only 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.

[0027] In the configuration of FIG. 3, Out-HOE 30 is bonded to the front side of the transparent medium (light guide plate) 10 and appears to be a transmissive type at first glance, but it is also reproduced in a reflective type. The light propagating in the light guide plate is totally reflected at the air-side interface, and the interface reflection light with this air becomes the input light to the Out-HOE. When this input light (parallel light) is incident at a waveguide propagation angle of 60° with respect to the optical axis of the HOE, it forms a focus at the focal position of the HOE like a positive lens, and the converging light is reproduced.

[0028] If it can be arranged on the image light incident surface side of the light guide plate, design constraints due to the focal length and the thickness of the light guide plate can be avoided. When introducing the light rays from a point light source into the light guide plate, the incident angle of the light rays to the hologram changes under the influence of the thickness of the light guide plate and the distance (the distance from the position where the light ray perpendicular to the light guide plate and passing through the point light source intersects the light guide plate). If the incident angle to the hologram changes, the reproduction angle at the hologram changes. The change in the reproduction angle is approximately the same as the change in the incident angle. FIG. 5 is a diagram for explaining the influence of the thickness of the light guide plate on the incident angle of the hologram. Let the distance in the thickness direction of the light guide plate be t and the distance from directly below the point light source be r, and when t = 0, t A , t B (0 < t A < t B ), r = r1, r2 (0 < r1 < r2), and assume that light is emitted from a point light source at a position of height H from the surface of t = 0. Let the incident angles at the r1 and r2 points on the surface of t = 0 be θ1 and θ2, and the incident angles on the surface of t = t A be θ1 A , θ2 A , and the incident angles on the surface of t = t B be θ1 B , θ2 B (θ is a positive quantity only). At this time, 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 closer it is to the light guide plate surface). Therefore, for holograms with a large distance r or 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 (on the front side of the light guide plate) without using a light guide plate, eliminating the influence of the focal length or substrate thickness and eliminating the need to create holograms tailored to the substrate thickness.

[0029] When using two HOEs to extract images from different locations 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. If the thickness of the light guide plate is known, it is possible to minimize these distortions by taking that thickness into account. However, if the thickness of the light guide plate on which the HOE is mounted varies, multiple dedicated HOEs must be manufactured for each thickness. For example, this would require the production of multiple products for 3mm thick glass and 5mm thick glass, which would not only be cumbersome, but could also result in performance issues when combined with different specifications. Therefore, by positioning the HOE immediately at the light input position, even though it is a reflective type, this problem can be solved, allowing products of the same specifications to be attached to light guide plates of any thickness.

[0030] Each component will be described below, but the present invention is not limited to these.

[0031] (Transparent medium) The transparent medium is made of a material with an optical refractive index of 1.3 or higher. 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, total reflection is utilized, so the transparent medium must be made of a transparent material with an optical refractive index of 1.3 or higher. Examples of materials with an optical refractive index of 1.3 or higher include acrylic resin, polycarbonate resin, polyethylene terephthalate resin, optical glass BK7, white plate glass, and blue plate glass. A refractive index greater than 1.6 increases propagation angle deviation and glass interface reflection, so the optical refractive index is preferably 1.3 or higher and 1.6 or lower. Hologram materials include photopolymers, silver halide materials, and dichromated gelatin. 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.

[0032] (Holographic Optical Element (HOE)) In the present invention, the In-HOE and Out-HOE can be the same. In the third 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. Note that in the configuration of the present invention, there is no need to create an optimal hologram to match the substrate thickness of the transparent medium; as long as the hologram is supplied, it can be used with any substrate.

[0033] 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.

[0034] Furthermore, when a protective layer is provided on the surface of the hologram material, it is preferable that 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 be 0.5 mm or less. If parallel light is incident, there is no problem with distortion, but if diffused or focused light is incident, the longer the distance from the air interface to the HOE, the greater the distortion.

[0035] [Fourth embodiment] Fig. 6 is a schematic diagram of an image display device 400 that uses transmission-type holographic optical elements (HOEs) as the In-HOE and Out-HOE. Fig. 7 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 with 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 gap between them.

[0036] The In-HOE 40 is an optical element that deflects incident non-parallel light into parallel light at a predetermined angle that exceeds the critical angle of the transparent medium 10, and propagates the light through 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.

[0037] The Out-HOE 50 is an optical element that regenerates 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 then totally reflects the propagated light (parallel light L2) at the interface between the Out-HOE 50 and the air, and deflects the totally reflected light within the Out-HOE 50 to regenerate convergent light L3.

[0038] At first glance, the HOE that propagates light as shown in Figure 6 appears to be a reflective HOE, but what is noteworthy is 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 regeneration because it does not have high wavelength selectivity.

[0039] [HOE placement variations] Whether an image should be displayed on the front or back side of the transparent medium (light guide plate) depends on the needs of the application, installation location, etc., but with this invention, the image display position does not need to be fixed to one side, which is a great advantage in that it increases the degree of freedom in product design. Figure 8 shows variations in HOE arrangements in the image display device of this invention.

[0040] Assuming that all configurations are reflective edge-lit HOEs, a) and h) in Figure 8 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 used if you want to input image light from the same side as the observer, and configurations a), b), c), and d) can be used if you want to input image light from the opposite side to the observer, so you can choose the configuration that best suits your device. In the cases of a), b), c), and d), the In-HOE and Out-HOE must be bonded separately to 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 to one side, the process is simple, and the In-HOE and Out-HOE can be manufactured from a single sheet, which reduces manufacturing costs.There is also a significant performance advantage in that 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.

[0041] Also, when configuring a transmissive edge-lit HOE, b) and g) are orthodox, conventional configurations. A reflective-type configuration can be achieved 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, while a), c), f) and h) can be achieved by using light that is totally reflected by a thin protective film just 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.

[0042] [Application of Use (1) Evaluation Method] In conventional holograms reproduced on a substrate, when evaluating the performance of the hologram in a finished product bonded to a substrate, it is not possible to evaluate the performance of the substrate separately. For example, if only a hologram is supplied and a problem occurs with the hologram reproduction light at the supply destination, even if the problem is with the substrate to which the supplied hologram is bonded, it is impossible to distinguish between a problem with the hologram and a problem with the substrate without performing non-destructive testing. Problems caused by the substrate include, for example, air bubbles, defects, birefringence, and scattering within the substrate. In the image display device of the present invention, by directing the reproduction light to the air interface side, hologram evaluation can be performed independently of the substrate side, making it possible to perform non-destructive evaluation that is separate from problems with the substrate side.

[0043] Figure 9 is a schematic diagram showing the propagation of light during evaluation of (A) In-HOE and (B) Out-HOE. The upper diagram is for a reflective HOE, and the lower diagram is for a transmissive HOE. Evaluation is performed by matching a prism 60 as shown in the diagram. This makes it easy to evaluate the performance of the HOE alone after deflection, without being affected by the substrate performance. In the case of Out-HOE, the same method as In-HOE can be used by performing reverse playback.

[0044] [Application (2) Authenticity Verification] Holograms are sometimes used to determine authenticity. For example, if the image display device of the present invention is incorporated into a particular brand-name product (such as a watch), it is easy to determine whether the brand-name product is genuine or not. As with the evaluation method described above, by optically contacting the prism 60 and checking that light is not transmitted through that portion, it is easy to determine whether the product satisfies the features of the present invention. This makes it possible to determine whether the product is genuine (whether or not the image display device of the present invention is used). [Example]

[0045] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0046] [Example 1] (Transparent medium) The transparent medium was an acrylic light guide plate, which was 10 mm thick, 150 mm long, and 110 mm wide, with an optical refractive index nd of 1.49.

[0047] (In-HOE) The In-HOE used a Lippmann hologram with a recording wavelength of 532 nm. This In-HOE had a photopolymer average optical refractive index nd = 1.500, a focal length of 53 mm, an effective area of ​​60 mm in the propagation direction and 100 mm in the width direction, and the HOE optical axis at the center of the area. This hologram functions similarly to a positive lens. When divergent light is input to the HOE, collimated light is propagated into the light guide plate. Furthermore, when a point light source is placed at a position equivalent to the focal position and incident on the HOE, collimated light is reconstructed. This collimated light propagates at a 60° angle relative to the HOE's optical axis. As shown in Figure 4(A), the reconstructed light is generated on the side of the input light, reflected at the air-side interface of the HOE, enters the light guide plate, and propagates through the light guide plate by total internal reflection.

[0048] (Out-HOE) The Out-HOE uses a Lippmann hologram with the same recording wavelength of 532 nm as the In-HOE. When parallel light is input into this Out-HOE, convergent light is reproduced. As shown in Figure 4(B), when parallel light is incident at an angle of 60° to the optical axis of the HOE, the light is focused at the focal position of the HOE, just like a positive lens.

[0049] (Image display device) The In-HOE and the Out-HOE were bonded to one surface of the light guide plate without any air gap. When the optical axis of the Out-HOE was roughly aligned on a line passing through the optical axis of the In-HOE and horizontal to the propagation direction, the parallel light propagating within the light guide plate was reflected at the interface between the HOE and the air, and the propagating light became the input light, regenerating convergent light. In this example, the distance between the optical axis 21 of the In-HOE and the optical axis 31 of the Out-HOE was set to 69 mm. In this example, the distance was set to 69 mm to match the position where the light emitted from the optical axis coincides with the optical axis on the extraction side when propagated. However, the In-HOE and the Out-HOE can be closer or farther apart as long as they do not interfere with each other.

[0050] (Image propagation) The HOE used functions equivalent to a lens with a focal length of +53 mm, but the image plane tilts because the reconstructed wavelength is selected for each angle of view. In the case of propagation from a light source originating from object S as shown in Figure 10, when light emitted from object S enters the In-HOE, the wavelength of the propagating light within the light guide plate reconstructed by the HOE changes depending on the position of object point S. This is due to wavelength selection by Bragg diffraction; on the axis, the reconstructed wavelength is the same as the recording wavelength, but as it moves away from the propagation side, the reconstructed wavelength shifts to longer wavelengths and to shorter wavelengths on the counter-propagation side. The light then propagates through the waveguide and reaches the Out-HOE, where it is reconstructed. This reconstructed light forms an image P of the object with the same effect as the positive lens described above, but the lens action causes the focal length to be inversely proportional to the wavelength due to the effect of the hologram. Therefore, a reconstructed image P originating from an object point S located farther in the propagation direction (where long wavelengths are selected) will be imaged closer to the HOE, while a reconstructed image P originating from an object point S located farther in the counter-propagation direction will be imaged farther away from the HOE, resulting in a tilt of the image plane. This image plane tilt can be corrected by pre-tilting the object. The correction direction is to move the object on the propagation side closer to the HOE and the object on the counter-propagation side farther away from the HOE. The object can be a real object or painting, or a video or still image projected onto a screen. In this configuration, an image approximately 1:1 in size is formed at the same height as the object position. This can be recognized as a hollow image. It is assumed that the light source originating from the object contains at least the recording wavelength and wavelengths just before and after it.

[0051] As described above, the present invention can provide an image display device with a simple structure and a high degree of freedom in design. Being able to move away from the concept of a reflective HOE being located behind the light guide plate and a transmissive HOE being located in front of the light guide plate increases usability and design freedom, offering significant benefits. For example, it becomes possible to freely choose whether to place the light source unit behind or in front of the light guide plate, depending on the required characteristics of the product. It is also desirable to have greater freedom in terms of whether the image display device should be kept out of reach of people or whether it is acceptable to touch it, and whether the image should be positioned in front or behind.

[0052] [Fifth embodiment] Figure 11 shows a fifth embodiment of the present invention, in which a double-sided image display device is constructed in which different hologram images can be viewed from both sides. A light guide plate is configured with a pair of In-HOE (501A) and image hologram (502A) formed on a single sheet of hologram recording material, and another pair of In-HOE (501B) and image hologram (502B) formed on a single sheet of hologram recording material. These are stacked at an angle (here, offset by 90° and perpendicular to each other) so that the image hologram areas of each pair overlap, but the In-HOEs do not overlap. In Figure 11(A), the bottom left diagram shows the XX cross section (light propagation through 501A and 502A), and the bottom right diagram shows the YY cross section (light propagation through 501B and 502B).

[0053] Light sources, for example, LED-A and LED-B, are placed near the focal points of each In-HOE (on the front side of the light guide plate in Figure 11(A) and on the back side of the light guide plate in Figures 11(B), (C), and (D)). In this case, the In-HOE is attached to the light guide plate closer to the light source, but by applying the present invention, even though it is a reflective type, light propagates inside the light guide plate at an angle exceeding the critical angle in the direction of the image hologram. The image hologram is a reflective edge-lit hologram that reconstructs an image in air by illuminating with light whose angle matches the propagation angle within the medium. However, the present invention is applied only to 502A, and 502B is formed as an orthodox reflective type. In this way, when only LED-A is turned on, the image hologram of A is reconstructed on the light source side, as in the state of Figure 11(C), but no image is reconstructed on the side opposite the light source, and the area remains transparent. Furthermore, when only LED-B is turned on, the image hologram of B is reconstructed in the direction opposite the light source, as shown in Figure 11(D), but no image is reconstructed on the light source side, which remains transparent. When LED-A and LED-B are turned on simultaneously, different images are reconstructed on both sides simultaneously, as shown in Figure 11(E). 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 reconstruct each other.

[0054] 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 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.

[0055] In this example, the two axes are perpendicular to each other, but other angles are also possible. Furthermore, by combining not only two sets but also three or more sets at different angles, it is possible to switch between multiple types of display on one side instead of just one type.

[0056] [Sixth embodiment] Figure 12 shows a sixth 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 on a light guide plate so that the Out-HOEs overlap but the In-HOEs do not overlap, at an angle (here, they are orthogonal, shifted by 90°). In Figure 12(A), the bottom left diagram shows the XX cross section (light propagation through 601A and 602A), and the bottom right diagram shows the YY cross section (light propagation through 601B and 602B).

[0057] 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 front of the light guide plate in Figure 12). In this case, the In-HOE is attached to the surface of the light guide plate closer to the light source. By applying the present invention, even though it is a reflective type, the light is propagated 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 the 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 can be observed from the display side as in Figure 12(C), but no image is reproduced from the opposite side of the display, and the display remains transparent. Similarly, the image displayed on display DB can be observed from the opposite side of the display as in Figure 12(B), but no image is reproduced from the display side, and the display remains transparent. Furthermore, when display DA and display DB are turned on simultaneously, different images are simultaneously reproduced on both sides, as shown in Figure 12(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.

[0058] By applying this, it is possible to mix the fifth and sixth embodiments. For example, it is possible to propagate a moving image seen from the front side in the −45° direction within the light guide plate, illumination light for displaying a 3D holographic still image seen from the back side in the −15° direction, illumination light for displaying a 3D holographic still image seen from the front side in the +15° direction, and a moving image seen from the back side in the +45° direction, so that different types of images can be selectively observed from both sides.

[0059] 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.

[0060] The image display device of the present invention can also be used as an optical deflection device. For example, in the first embodiment, the image hologram 104 can be replaced with an OUT-HOE, on which no image is recorded, that has the function of emitting parallel light within a medium into the air as parallel light, convergent light, or divergent light. In this case, for example, if a transparent light guide plate is arranged in a flashlight so that the IN-HOE faces the direction of light emission, in addition to the zero-order transmitted light that travels straight, light of a specific color is irradiated in a specific direction from the OUT-HOE arranged in another part of the light guide plate, and the device can be used as a lighting device without image information.

[0061] 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. [Explanation of symbols]

[0062] 100, 200, 300, 400 Image display device 10, 101 Light guide plate (substrate) 102 Holographic recording media 103 In-HOE area 104 Out-HOE area 105 Surface protection base material (protective layer) 20, 40, 80 In-coupling holographic optical element (In-HOE) 21, 41 Optical axis of incoupling HOE 30, 50, 90 Outcoupling Holographic Optical Element (Out-HOE) 31, 51 Optical axis of outcoupling HOE 60 Prism 70 Video Information 71 Point light source 95 Out-HOE (Edge Lid Hologram) L1 incident light L2 polarized light L3 Convergent light S object P statue

Claims

1. 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 disposed in a part of the transparent medium without an air layer therebetween; the incoupling holographic optical element and the outcoupling holographic optical element each have a protective layer on a side opposite to the transparent medium; The protective layer is a transparent layer having a thickness of less than 0.5 mm, the incoupling holographic optical element is an optical element that deflects light incident from an interface with air into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates the parallel light through the transparent medium; The outcoupling holographic optical element is an optical element or an image hologram that reproduces light incident from within the transparent medium in air, The image display device is characterized in that the incoupling holographic optical element causes the deflected parallel light to be totally reflected at the interface between the protective layer of the incoupling holographic optical element and air, and propagates the deflected parallel light into the transparent medium.

2. 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 disposed in a part of the transparent medium without an air layer therebetween; the incoupling holographic optical element and the outcoupling holographic optical element each have a protective layer on a side opposite to the transparent medium; The protective layer is a transparent layer having a thickness of less than 0.5 mm, the incoupling holographic optical element is an optical element that deflects light incident from an interface with air into parallel light at a predetermined angle exceeding the critical angle of the transparent medium and propagates the parallel light through the transparent medium; The outcoupling holographic optical element is an optical element or an image hologram that reproduces light incident from within the transparent medium in air, The outcoupling holographic optical element is characterized in that light entering from within the transparent medium is totally reflected at the interface between the protective layer of the outcoupling holographic optical element and air, and then diffracted and reproduced in air.

3. 3. The image display device according to claim 1, wherein the incoupling holographic optical element and the outcoupling holographic optical element are disposed on the same surface of the transparent medium and are provided on the same hologram material.

4. 3. The image display device according to claim 1, wherein the transparent medium is any one of a parallel plate, a cylinder, a part of a cylinder, and a part of a hollow sphere.

5. At least two sets of image display devices are provided, each combining an incoupling holographic optical element and an outcoupling holographic optical element; At least one of the sets of image display devices is either the image display device according to claim 1 or the image display device according to claim 2, A double-sided image display device characterized in that an incoupling holographic optical element and an outcoupling holographic optical element that constitute the image display device are combined with a single transparent medium having an optical refractive index of 1.3 or more and are arranged at different angles, so that different images can be selectively observed from the front and back of the same part of the transparent medium.

6. 6. A double-sided image display device according to claim 5, wherein the light incident surfaces of the incoupling holographic optical elements of the two sets of image display devices are on the same plane.

7. 6. The double-sided image display device according to claim 5, wherein the image display portion of the transparent medium is subjected to anti-reflection treatment or moth-eye treatment.

Images