Hologram light guide plate with plurality of layers and head mount display using hologram light guide plate

It is an objective of this disclosure to protect a highly transparent hologram light guide plate from water vapor and ultraviolet ray, thereby suppressing deterioration of the hologram light guide plate even when employed in a head mount display used in outdoor environments. A hologram light guide plate according to this disclosure comprises a protection layer that protects a hologram layer and an intermediate layer that is placed between a glass layer and the protection layer, wherein the glass layer and the hologram layer form a transfer layer that transfers image light. The intermediate layer causes the image light to transfer only in the transfer layer in a section from an input area of the image light to an output area of the image light.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japanese Patent Application No. 2019-138527 filed Jul. 29, 2019, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Technical Field

The present disclosure relates to a hologram light guide plate that transfers images to eyes, and also relates to a head mount display using the hologram light guide plate.

2. Description of the Related Art

As a wearable device, head mount display is expected to be employed in various applications, in that it can provide network information on the Internet within a part of field of vision. A component that transfers image light at the field of vision of the head mount display is referred to as a light guide plate.

In order to always display images within a part of field of vision, it is required for the light guide plate to have very high transparency. Conventionally, various schemes are proposed for light guide plate, such as a light guide plate using prism, a light guide plate using diffraction by means of groove structure, a light guide plate using reflective mirror array, or a hologram light guide plate using diffraction by means of refractive index modulation. Among those schemes, the hologram light guide plate is promising as a technique that can achieve transparency at a same level as typical eye glasses. US2017/0363811A1 (PTL1) is one of documents proposing a holographic light guide plate.

SUMMARY OF DISCLOSURE

It is commonly known that a hologram material, which implements refractive index modulation, degrades due to water vapor or ultraviolet ray. The hologram light guide plate described in PTL1 does not consider suppressing deterioration due to water vapor or ultraviolet ray. Then it is conceivable that a protection layer may be provided to protect the hologram material.

A hologram light guide plate confines image light within the plate to transfer the light. Accordingly, during the image light transfers, it is necessary to confine the image light within a transfer layer. However, if a protection layer that shuts out water vapor or ultraviolet ray is provided at outside of the hologram light guide plate, the image light may also transfer to the protection layer during the transfer process. Consequently, the image could be blurred when the image light reaches eyes of user.

This disclosure is made in the light of the technical problem above. It is an objective of this disclosure to protect a highly transparent hologram light guide plate from water vapor and ultraviolet ray, thereby suppressing deterioration of the hologram light guide plate even when employed in a head mount display used in outdoor environments.

A hologram light guide plate according to this disclosure comprises a protection layer that protects a hologram layer and an intermediate layer that is placed between a glass layer and the protection layer, wherein the glass layer and the hologram layer form a transfer layer that transfers image light. The intermediate layer causes the image light to transfer only in the transfer layer in a section from an input area of the image light to an output area of the image light.

With the hologram light guide plate according to this disclosure, it is possible to achieve a head mount display usable in outdoor environments for long time, by implementing a highly transparent hologram light guide plate whereas protecting the hologram layer from water vapor and ultraviolet ray.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a schematic diagram of a hologram light guide plate1according to an embodiment 1 of this disclosure. The hologram light guide plate1includes an input area2, an output area3, and a monitor area4. The input area2is an area into which image light is inputted. The output area3is an area from which the image light is outputted. The monitor area4is an area that is used for monitoring parallelism of the hologram light guide plate1by means of external light.

The right part ofFIG. 1illustrates a structure of a cross section15of the hologram light guide plate1. The hologram light guide plate1is formed by a hologram layer6, glass layers7and8, an intermediate layer9, a water vapor barrier layer10, an ultraviolet ray barrier layer11, a surface augment layer12, and a dry area13.

The glass layer7is placed at one side of the hologram layer6, and the glass layer8is placed at another side of the hologram layer6. In the embodiment 1, the glass layer7is a layer placed at a side into which the image light is inputted, and the glass layer8is a layer placed at a side external from a user61(refer toFIG. 6below).

The hologram layer6has a functionality to transfer the image light to eyes by means of refraction index modulation. The hologram layer6is formed from a hologram material (photo polymer). Monomer or oligomer reacts under photopolymerization by irradiating interference light with a wavelength around 405 nm, and then changes its structure into polymer, thereby achieving refractive index modulation in compliance with the interference light. Irradiating the interference light with a predetermined distribution to achieve refractive index modulation is typically referred to as light recording. In other words, light recording is to fix an interference state of the light within the hologram layer6. The input area2and the output area3are areas where light recording is performed by a predetermined interference light. The monitor area4is an area where light recording is not performed. The monitor area4does not modulate external light.

The glass layers7and8are transparent plates placed at outside of the hologram layer6. A cost effective material may be advantageously used as the glass layers, such as BK7 from Schott Corporation. A transparent optical resin may be used as the glass layers. In that case, in order to achieve refractive index modulation in the hologram layer6, a material may be used which transparency is high around 405 nm (the wavelength used for light recording). For example, Zeonex from Zeon Corporation or APEL from Mitsui Chemicals Corporation may be used. When increasing FOV (Field Of View), which indicates the size of image light, a material may be used which refractive index is high in accordance with such FOV.

The image light transfers within the glass layers7and8. Accordingly, a material may be used as the material of the glass layers which anisotropy is very small within the glass layers7and8, so as to avoid unnecessary modulations. In addition, each of the external surfaces of the glass layers7and8(the surfaces contacting with the intermediate layer9) preferably keeps very high parallelism to each other. Since it is difficult to achieve parallelism below several seconds, it is desirable to monitor the angle of the glass layers by means of external light via the monitor area4. Especially when using transparent optical resin, it is beneficial to keep a wide range of the monitor area4so as to monitor whether a distortion has occurred. By recording amount of reflected light during the inspection process, it is possible to utilize the monitor area4so as to monitor the deterioration state of the hologram layer6. The attenuated amount of light indicates the deterioration.

The water vapor barrier layer10has a functionality that prevents water vapor from proceeding from external environments into the hologram layer6. A transparent resin is preferable as the water vapor barrier layer10. Polyvinylidene chloride may be preferable.

The ultraviolet ray barrier layer11is transparent against visible light (430 to 650 nm), and has a functionality that prevents ultraviolet ray (up to 430 nm), which destructs the hologram layer6, from proceeding from external environments. For example, by forming a multilayer of SiO2and TiO2, the ultraviolet ray barrier layer11is achieved. As the ultraviolet ray barrier layer11, a material may be used which transmittance against visible light (i.e. image light) is higher than that against ultraviolet ray.

The surface augment layer12is transparent, and has a functionality that prevents damages to the surface of the hologram light guide plate1. For example, by forming the surface augment layer12using silicone hard coat, it is possible to both achieve high transparency and hardness.

The water vapor barrier layer10, the ultraviolet barrier layer11, and the surface augment layer12work as a protection layer that protects the hologram layer6. The protect layer (i.e. the three layer structure of the water vapor barrier layer10, the ultraviolet barrier layer11, and the surface augment layer12) is formed at both sides of the hologram light guide plate1. These three layers forming the protect layer are placed in this order from the inner side to the external side of the hologram light guide plate1.

The intermediate layer9is placed between the glass layer7and the protect layer, and also is placed between the glass layer8and the protect layer. The intermediate layer9has a functionality that prevents the image light from spreading into the protect layer. This functionality is achieved by configuring the refractive index of the glass layers7and8smaller than that of the intermediate layer9. The intermediate layer9may also have a functionality that achieves close contact between the protect layer and the glass layers7and8. Thermosetting transparent silicone resin may be used as the intermediate layer9. Transparent silicone resin is advantageous in adjusting its refractive index by adjusting the composition of its material. The intermediate layer9may also be an air layer.

The intermediate layer9and the protect layer are formed so that the surface area sizes of them are slightly larger than those of the stacked body of glass layer7/hologram layer6/glass layer8. By binding together the redundant portion of the intermediate layer9and the protect layer at the end edge of the hologram light guide plate1, an inner space is formed between the stack body of glass layer7/hologram layer6/glass layer8and the end edge of the hologram light guide plate1. This inner space is referred to as a dry area13. The dry area13is a part of the intermediate layer9. The dry area13absorbs water moisture. In order to sufficiently dry so that no moisture is left, highly hygroscopic material is preferable for the dry area13, such as silica gel. Other hygroscopic materials may also be employed.

FIG. 2is a schematic diagram illustrating a scene where a frame is mounted to the hologram light guide plate1. Frames20and21are mounted around the hologram light guide plate1. The frames20and21have functionalities for: (a) improving the appearance so that the protect layer is not viewable to users; (b) removing unnecessary light caused by incident light spreading from the side surface; (c) preventing the incident light from proceeding into the opposite surface; (d) supporting the hologram light guide plate1. Therefore, the frames20and21are formed by opaque materials, and are formed into a shape surrounding the side surface of the hologram light guide plate1. The frame21has a shape covering the side opposite to the input area2. The frames20and21could be cost-effectively achieved by manufacturing them from molding thermoplastic resin.

FIG. 3is a diagram illustrating a functionality of the intermediate layer9. The image light incident from the input area2transfers by reflecting within the glass layer7, the glass layer8, and the hologram layer6, until emitting from the output area3.

FIG. 3illustrates a behavior of light at a boundary between the glass layer8and the intermediate layer9. Since the refractive index of the intermediate layer9is relatively lower than that of the glass layer8, the image light33totally reflects at the boundary between the glass layer8and the intermediate layer9. The internal distortion of the protect layer is generally large. Thus when the image light transmits through the protect layer, the image light is randomly modulated and the image is deteriorated. The intermediate layer9reflects the image light, thereby preventing the image light from transmitting into the protect layer. Accordingly, the intermediate layer9prevents deterioration of the image.

It is assumed that the refractive index of the glass layer8is N1, and the refractive index of the intermediate layer9is N2. In this case, the critical angle θc (the angle between the image light and the normal line31) at which the image light totally reflects at the boundary between the glass layer8and the intermediate layer9is described by Equation 1 below.
N1·Sinθc=N2  (1)

It is assumed that an angle θFis an angle (the angle formed by the reflected light with respect to the boundary between the glass layer8and the intermediate layer9) that represents a FOV corresponding to the size of the image light. θFis described by Equation 2 below. The unit in Equation 2 is “degree”.
θF<90−θc(2)

For example, if the glass layer8is formed of BK7, N1 is approximately 1.52 at visible light region. If it is requested to configure θFat 20 degree, N2=1.42 and θc=70 degree from Equations 1 and 2. When increasing the image size (i.e. increasing θF), the refractive index N2 is set at or below 1.42 according to the relationship above. θFis more readily increased by selecting materials with high refractive index N1. As discussed above, by adjusting the refractive indexes N1 and N2, it is possible to achieve desired size of the image light.

FIG. 4is a schematic diagram illustrating a structure of a head mount display62. The head mount display62at least includes the hologram light guide plate1, the frames20and21, and an optical engine41.

The image light (arrow mark) generated by the optical engine41is inputted into the input area2of the hologram light guide plate1, and is outputted from the output area3into eyes of the user. In the section from the input area2to the output area3, the image light totally reflects at the boundary between the glass layers and the intermediate layer9, as described withFIG. 3. Accordingly, glass layer7/hologram layer6/glass layer8work as a transfer layer that transfers the image light within the hologram light guide plate1.

The optical engine41includes an image light source42, an imaging lens43, a light guide prism44, and a monitor opening45.

The image light source42is an optical device that generates the image light. Recently developed devices may be employed as the image light source42, such as OLED (Organic Light Emitting Diode) or micro LED. It is also possible to combine the illumination optical system with LCOS (Liquid Crystal On Silicon), transmissive liquid crystal, DMD (Digital Micromirror Device), and the like. OLED or micro LED are beneficial in decreasing the size of the optical engine41.

The imaging lens43has a lens functionality that transfers, to the eyes of the user, the image light generated by the image light source42within desired FOV range. In head mount display, the imaging lens43is designed so that the image is a virtual image. Details of the design for the imaging lens43are not described in this document, since it is a commonly known technique.

The light guide prism44is an optical element that guides the image light into the hologram light guide plate1. InFIG. 4, the image light is incident onto the hologram light guide plate1perpendicularly. However, the incident angle could be oblique. This is advantageous in unnecessitating the input area2, because there exists a condition where it is not necessary to perform refractive index modulation to the input area2by finely design the inclination of the optical axis, for example

It is preferable for the light guide prism44to have a prism structure that has a surface in parallel with the hologram light guide plate1. By monitoring the angle of the light guide prism44using external light via the monitor opening45and by inspecting the parallelism of the monitored angle using the monitor area4, it is possible to finely confirm whether the image light is incident onto the hologram light guide plate1at a predetermined angle.

FIG. 5is a flowchart illustrating a process for manufacturing the head mount display62. Hereinafter, each step inFIG. 5will be described.

A hologram substrate is prepared, which is configured by the hologram layer6and the glass layers7and8(S501). Then the parallelism of the glass layers7and8(whether the external surfaces of both layers are placed in parallel to each other) is checked using the monitor area4, thereby confirming that the parallelism is at desired value (S502).

The input area2and the output area3of the hologram layer6are recorded so that those areas have a desired refractive index distribution (S503). In this process, if the ultraviolet ray barrier layer11exists, it is impossible to irradiate interference light having wavelength around 405 nm onto the hologram layer6. Accordingly, it is necessary to form the ultraviolet ray barrier layer11after the recording.

In order to avoid unintentional recording in the hologram layer6, overall area of the hologram layer6is completely recorded using light having wavelength around 405 nm (S504: post cure).

The protect layer is formed including the intermediate layer9(S505). The hologram layer6is degraded when exposed to ultraviolet ray or water vapor after the post cure. The protect layer is to prevent such degradation.

The frames20and21are mounted (S506). The parallelism of the glass layers7and8is inspected again (S507). During the manufacturing process, the parallelism could be degraded due to unnecessary deflation of the hologram layer6, for example Thus the parallelism is inspected again in S507. The parallelism inspection may be performed via the monitor area4, as in S502. If the parallelism is degraded, then the device in the process is discarded as a defective product.

The optical engine41is mounted (S508). The parallelism is reviewed in each of the monitor opening45and the monitor area4(509). Finally, the optical engine41and the hologram light guide plate1are mounted onto the frame (S510). Then manufacturing process for the head mount display62is completed.

FIG. 6is a diagram illustrating a scene where a user61equips the head mount display62. When the user61equips the head mount display62as glasses, the user sees an image64in the space produced by the image light outputted from the outpour area3of the hologram light guide plate1. The head mount display62includes a camera63. The camera63monitors external environments. Image processing of the external environment provides appropriate views to the user.

FIG. 7is a system block diagram of the head mount display62. The head mount display62includes a controller71, the camera63, the hologram light guide plate1, and the optical engine41. The hologram light guide plate1includes the hologram layer6, the glass layers7and8, the intermediate layer9, the ultraviolet ray barrier layer11, the water vapor barrier layer10, the surface augment layer12, and the frames20and21.

When the user61equipping the head mount display61performs a predetermined action, the controller71activates the camera63, and then performs image analysis processing to determine the information provided to the user. The controller71activates the optical engine41to produce the image light. The image light transfers via the hologram light guide plate1to provide the user61with the information. The hologram light guide plate1includes the ultraviolet ray barrier layer11and the water vapor barrier layer10so that ultraviolet ray or water vapor will not proceed into the hologram layer6. The intermediate layer9, which refractive index is lower than that of the glass layers7and8, is provided at external sides of the glass layers7and8, so that the image light reflects at inner surfaces of the hologram light guide plate1to transfer within the hologram light guide plate1.

The hologram light guide plate1according to the embodiment 1 includes the intermediate layer9between the glass layers7and8and the protect layer. In the section between the input area2and the output area3, the image light totally reflects at the boundary surface between the intermediate layer9and the glass layers7and8. Accordingly, the image light transfers through only the hologram layer6and the glass layer (7or8) within that section. Therefore, it is possible to prevent the image light from leaking out into the protect layer before reaching the eye of the user61, thereby preventing degradation of the image quality.

The hologram light guide plate1according to the embodiment 1 protects the hologram layer6from ultraviolet ray by means of the protect layer, thereby it is possible to increase the transparency of the hologram light guide plate1. Even if ultraviolet ray transmits through a highly transparent portion, the hologram layer6is protected by the protect layer. Thus there is no risk of unnecessary recording, thereby achieving the high transparency of the hologram light guide plate1. Further, it is possible to prevent the image light from leaking out into the protect layer by means of the intermediate layer9. Thus there is no risk of degradation of image quality. Accordingly, it is possible to achieve a head mount display which is usable for long time in outdoor environments.

The hologram light guide plate1according to the embodiment 1 monitors external light incident onto the monitor area4when the external light reflects from the hologram light guide plate1, thereby monitoring the parallelism of the glass layers7and8. Further, via the monitor opening45, it is possible to monitor the mount angle of the light guide prism44with respect to the hologram light guide plate1. By using both of them, it is possible to precisely monitor the image quality.

In an embodiment 2 of this disclosure, a modification of the hologram light guide plate1will be described. In the embodiment 2, same reference numerals will be assigned to components which are same as those in the embodiment 1, and repetitive description thereof may be omitted. Therefore, differences from the embodiment 1 will be mainly described below.

FIG. 8is a side sectional view illustrating a configuration of a modified example of the hologram light guide plate1. Comparing with the hologram light guide plate1inFIG. 1, the configuration inFIG. 8is different fromFIG. 1in that no protect layer is provided at the side of the glass layer7. Further, the glass layer7includes a small groove81, and the frame21includes a small protrusion84. A screw85is attachable to the frames20and21. By aligning the small groove81with the small protrusion84and fastening the screw85, the glass layer7and the intermediate layer9are pressed with each other, thereby prevent water vapor from proceeding between the glass layer7and the intermediate layer9.

According to the configuration inFIG. 8, when the user61in an outdoor environment equips the head mount display62, ultraviolet ray does not reach the hologram layer6. Thus it is not necessary to protect the hologram layer6at the side of the user61from ultraviolet ray. Accordingly, the protect layer at the side of the user61may be omitted. This is cost-effective.

FIG. 9is a configuration example where the hologram light guide plate1is integrated with glasses. The hologram light guide plate1inFIG. 9includes a lens91in addition to the configuration inFIG. 1. This configuration integrates the glasses with the hologram light guide plate1so as to keep normal field of view for a user with poor eyesight. Accordingly, it is possible to integrate the glasses with the head mount display62.

A lens of glasses typically has a meniscus structure. However, in order to integrate the hologram light guide plate1with the lens of glasses, it is necessary to keep flatness of the surface at the side of the user61so that the image light is not disturbed. Therefore, the lens91has a convex structure. The surface where the lens91contacts with the intermediate layer9is flat, and the opposite side is a convex surface. The lens91has a refractive index higher than that of the glass layer8. Thus by placing the intermediate layer9between the glass layer8and the lens91, it is possible to prevent the image light from proceeding into the lens91. The protect layer may be placed at external side of the lens92by techniques as in typical glasses.

FIG. 10is a configuration example of the hologram light guide plate1in which the water vapor barrier layer10is placed at a side of the hologram light guide plate1. In this case, the water vapor barrier layer10protrudes into a part of the hologram layer6, thereby preventing water vapor from reaching the hologram layer6. For example, the water vapor barrier layer10is placed at a side surface of the hologram light guide plate1, and a part of the water vapor barrier layer10is protruded, so that a top surface (most protruded surface) of the protruded portion contacts with the side surface of the hologram layer6. Accordingly, it is possible to absolutely barrier the hologram layer6from water vapor.

Further, in the configuration ofFIG. 10, the protect layer is placed at the side of the glass layer8only. This is cost-effective as inFIG. 8. The protect layer may also be placed at both sides.

Modification of Disclosure

The disclosure is not limited to the embodiments described above, and includes various modifications. For example, the above-described embodiments have been described in detail for easy understanding of the disclosure, and are not necessarily limited to those having all the configurations described above. A part of the configuration of a certain embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of a certain embodiment. A part of the configuration of each embodiment can be added, deleted, or replaced with another configuration.

In the embodiments above, the light guide prism44may be directly in contact with the glass layers7or8of the hologram light guide plate1. Direct contact means that there is no other material layer between the light guide prism44and the glass layers7or8. Accordingly, there is no material such as air layer between the light guide prism44and the glass layers7or8. Thus it is possible to avoid disadvantages such as degradation of image quality due to unintentional refraction of the image light.

REFERENCE SIGNS LIST