Patent Application
20230028757
The present invention relates to a technology in which a light guide plate and a diffraction element are combined, and particularly relates to an image display technology that is compact, lightweight, and capable of performing an augmented reality display.
Augmented reality image display devices allow a user to view not only a projected image but also surroundings at the same time. The projected image can be superimposed on a real world that is perceived by the user. Applications of such displays include video games and wearable devices such as glasses. The user can visually recognize the image that is supplied from a projector in the state of being superimposed on the real world by wearing an image display device in the form of eyeglasses or goggles in which a translucent light guide plate and the projector are integrated.
An example of such image display devices is described in “PTL 1” to “PTL 3”. In these patent literatures, a light guide plate includes a plurality of diffraction gratings having a corrugated shape formed on a glass substrate. Light beam emitted from a projector is coupled to the light guide plate by a diffraction grating for incidence, and propagates inside the light guide plate while being totally internally reflected. The light beam propagates in total internal reflection in the light guide plate while being converted into a plurality of light beams replicated by another diffraction grating, and finally exits from the plate light guide. Some of the emitted light beams form an image on a retina through a pupil of a user, and the image is recognized as an augmented reality image overlaid on an image of a real world.
In the light guide plate using such corrugated diffraction gratings, a wavenumber vector K of the light beam emitted from the projector becomes K0 according to Snell's law as the light is refracted in the light guide plate. The diffraction grating for incidence further causes conversion into a wavenumber vector K1 that allows the total internal reflection and propagation inside the light guide plate. The wavenumber vector changes, such as K2, K3, and so on, each time the light beam is diffracted repeatedly due to a diffraction effect of one or a plurality of other diffraction gratings provided in the light guide plate.
Assuming that a wavenumber vector of the light beam finally emitted from the light guide plate is K′, |K′|=|K|, where K′=K in a case where the projector is present on the opposite side of an eye across the light guide plate. On the other hand, in a case where the projector is present on the opposite side of the eye across the light guide plate, the light guide plate has the same effect as a reflective mirror in terms of the wavenumber vector. Assuming that a normal vector of the light guide plate corresponds to a z direction, comparison of wavenumber vector components x, y, and z can be expressed as Kx′=Kx, Ky′=Ky, and Kz′=−Kz.
The light guide plate functions to guide light waves while replicating a light beam emitted from the projector into a plurality of light beams such that the plurality of emitted light beams are perceived by the user as image information equivalent to an original image. At this time, a group of the replicated light beams spatially spreads while having a wavenumber vector equivalent to a light beam having the image information and emitted from the projector. A part of the group of the replicated light beams enters the pupil and forms an image on the retina together with external environment information to be visually recognized, and it is possible to provide the user with augmented reality information added to the external environment information.
Light beams having the image information vary in wavenumber vector magnitude depending on wavelengths thereof. Since the corrugated diffraction grating has a constant wavenumber vector, the diffracted wavenumber vector K1 varies depending on a wavelength of an incident light beam, and light propagates in the light guide plate at different angles. A refractive index of the glass substrate constituting the light guide plate is approximately constant relative to the wavelength, and a range of a condition for guiding light while causing total internal reflection varies depending on a wavelength of an incident light beam. Therefore, it is necessary to stack a plurality of light guide plates that differ per wavelength on each other in order to allow the user to perceive an image with a wide viewing angle. Generally, it is considered that, as for the number of light guide plates, the number of light guide plates respectively corresponding to red light (R), green light (G), and blue light (B), or a range of two to four plates, one more or less than the mentioned number, is considered adequate.
An image display device described in “PTL 1” is an image display device configured to expand input light in two dimensions, and includes three linear diffraction gratings. One diffraction grating is configured for incidence, and the other two diffraction gratings for exit are typically arranged to be overlaid on one another on a front surface and a back surface of the light guide plate, and function as diffraction gratings for replication and exit, respectively. In addition, “PTL 1” describes an example in which the diffraction grating for exit is formed on one surface by periodic structures of pillar-shaped photonic crystals.
An image display device described in “PTL 2” discloses a technology in which optical structures have a shape including a plurality of straight sides in order to solve a problem that an image projected by the photonic crystal in “PTL 1” has high brightness in the central portion of the field of view.
In image display devices described in “PTL 3” and “PTL 4”, three diffraction gratings also serving as an incident diffraction grating, a deflecting diffraction grating, and an exit diffraction grating are arranged without regions being overlaid on each other in a light guide plate. “PTL 3” discloses an overhanging triangular diffraction grating in order to increase diffraction efficiency of an incident diffraction grating.
“PTL 5” and “PTL 6” disclose a technology in which two reflective volume holograms respectively for incidence and exit are used as diffraction gratings formed on a light guide plate. In these, the reflective volume hologram is obtained by forming diffraction gratings corresponding to a plurality of wavelengths in a space in a multiplexed manner, and diffracts light beams of a plurality of wavelengths at the same angle, which is different from the corrugated diffraction grating in “PTL 1” to “PTL 3”. Therefore, a user can recognize an RGB image with one light guide plate. Meanwhile, a wide viewing angle can be achieved since the light beam is replicated in the two-dimensional direction in the light guide plate in the corrugated diffraction grating. However, the reflective volume hologram provides the function of only one-dimensional replication, and thus, has a feature that a viewing angle is relatively narrow.
PTL 1: JP 2017-528739 A
PTL 2: WO 2018/178626 A1
PTL 3: WO 2016/130342 A1
PTL 4: WO 99/52002 A1
PTL 5: JP 2007-94175 A
PTL 6: JP 2013-200467 A
Hereinafter, a light guide plate having a corrugated diffraction grating will be described as the light guide plate. In addition, in order to facilitate understanding, image inversion by a lens effect of an eye and an effect in which an image projected on a retina is further inverted through processing in a brain before being perceived are left out of description, and a relation between a pixel position and brightness will be discussed regarding a projected image that is projected on a front screen from an image light source arranged on the same side as the eye with respect to the light guide plate. The actually visible image is vertically inverted image thereof.
Regarding a substrate material, “PTL 1” discloses a technology in which the light guide plate is made of a glass material as described with reference to
The photonic crystal obtained by improving the problem that the brightness is high at the central portion of the projected image described in “PTL 2” is configured such that the optical structures have the shape including the plurality straight sides in order to solve the problem that the image projected by the photonic crystal, which is linear rather than the pillar shape, has high brightness in the central portion of the field of view. In “PTL 2”, a central stripe-like portion having higher brightness is improved as described in line 34 of page 1 and line 8 of page 2. Note that the content of WO 2016/020643 cited in “PTL 2” is the same as that of “PTL 1”. In “PTL 2”, the central stripe portion having higher brightness, which is the problem, is not explicitly disclosed in the drawings and the like.
The incident diffraction grating disclosed in
Generally, in an image display element, a light beam having image information is coupled by an incident diffraction grating provided in a light guide plate so as to have a wavenumber that enables the light to be guided in total internal reflection inside the light guide plate, and propagates in the light guide plate. A part of the light beam intersecting an exit diffraction grating is diffracted and emitted from the light guide plate with a wavenumber equivalent to the original image light beam. The image information that is provided to a user has traveling angle information, that is, a wavenumber, depending on a pixel position in the original image information. The image information for one pixel needs to exit from a specific position in the light guide plate, determined by a traveling angle, a distance between the light guide plate and the user's pupil, and a size of the user's pupil, in order to exit from the light guide plate and arrive at the user's pupil.
As described above, the light beam is replicated and emitted with spatial spread in the light guide plate, and thus, the light beam visually recognized by the user decreases as the spatial spread increases, and the visually recognized brightness thereof decreases. Meanwhile, an exit position visually recognized by the user changes depending on the pixel position in the original image information, and thus, the brightness inevitably changes depending on a pixel position in the image display device using the light guide plate.
In the prior arts, it is suitable to use a method of directly etching a glass substrate to prepare a light guide plate, a nanoimprint method suitable for formation of a pattern having a high aspect ratio, or the like. When the photonic crystals of “PTL 1” and “PTL 2” based on the “PTL 1” are used and refractive indices of the substrate and the photonic crystal are the same, it is necessary to set an aspect ratio, which is a ratio between a representative length such as a diameter of a bottom surface to the height thereof, to about 2 or more.
Here, in a case where glass is used for the light guide plate as disclosed in “PTL 1” or the like, there are problems in processing cost and weight at the time of being worn by the user. Therefore, the problems can be solved by using plastic for the light guide plate. Note that the terms “resin” and “plastic” are used synonymously in the present specification and the like. The plastic means a material made of a polymer compound, and is a concept that does not include glass but includes a resin, polycarbonate, an acrylic resin, and a photocurable resin.
When plastic is used for the light guide plate, the diffraction grating can be formed by an injection molding technology or the like that has been proven as a method for manufacturing an optical disk medium. Since an aspect ratio of a surface-relief pattern formed by the injection molding technology or the like does not exceed 1, it is difficult to apply the injection molding technology or the like because the accuracy of pattern transfer decreases when an attempt is made to obtain an aspect ratio of 2 or more. This is a problem caused by the essential principle of the manufacturing method in which a molten polycarbonate resin, acrylic resin, polyolefin resin, or the like has high viscosity, and the resin does not accurately enter irregularities having a high aspect ratio formed with a nanometer period. In addition, the incident diffraction grating of “PTL 3” uses the overhung triangular diffraction grating, and thus, it is difficult to apply the injection molding technology or the like because a matrix (stamper) and the light guide plate are not peelable.
In addition, the plastic light guide plate has a smaller mechanical strength (Young's modulus) than the conventional glass light guide plate, and thus, a deformation thereof caused by the environmental temperature or atmospheric pressure increases. Although details will be described later, it is effective to adopt a transmissive optical configuration in which an image source and the user are located on opposite sides with the light guide plate interposed therebetween in order to reduce the influence of the deformation on the image information. Therefore, it is desirable to adopt a configuration capable of avoiding a decrease in brightness of the image information visually recognized by the user even in the transmissive optical configuration.
In this manner, it is necessary to adopt a configuration in consideration of the manufacturing method and the brightness of the image information in order to apply the plastic light guide plate to the image display element. Therefore, an object of the present invention is to improve brightness of image information that is visually recognized by a user while using plastic for a light guide plate.
A preferred aspect of the present invention is an image display element including: a substrate made of resin; an incident diffraction grating that diffracts incident light; and an exit diffraction grating that emits the light, the incident diffraction grating being formed on a first surface of the substrate, the exit diffraction grating being formed on a second surface on a side opposite to the first surface of the substrate, and the exit diffraction grating being formed on one surface.
Another preferred aspect of the present invention is an image display device on which the above image display element is mounted, the image display device being configured such that image light is incident from the second surface side of the substrate, and the image light can be visually recognized from the first surface side of the substrate.
Another preferred aspect of the present invention is an image display method that is performed using an image display element including a substrate made of resin, an incident diffraction grating formed on a first surface of the substrate, and an exit diffraction grating formed on a second surface on a side opposite to the first surface of the substrate, the exit diffraction grating being formed on one surface. In this method, image light is incident on the incident diffraction grating, the image light reflected and diffracted by the incident diffraction grating propagates inside the substrate, and an image is display by allowing a user to visually recognize the image light reflected and diffracted by the exit diffraction grating and emitted from the first surface.
It is possible to improve the brightness of the image information that is visually recognized by the user while using the plastic for the light guide plate.
Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not construed as being limited to the description of the embodiments to be described below. Those skilled in the art can easily understand that specific configurations can be changed without departing from the spirit or gist of the present invention.
In a configuration of the invention to be described below, the same reference signs will be commonly used for the same parts or parts having similar functions in different drawings, and the redundant description thereof will be omitted in some cases.
When there are a plurality of elements having the same or similar functions, the same reference sign will be sometimes described with different subscripts. When it is unnecessary to distinguish between these plural elements, however, the subscripts will be sometimes omitted in the description.
The notations such as “first”, “second”, and “third” in the present specification and the like are given to identify components, and do not necessarily limit the number, the order, or the content thereof. Further, a number for identifying a component is used for each context, and a number used in one context does not necessarily indicate the same configuration in another context. Further, a component identified by a certain number is not prevented from also serving a function as a component identified by another number.
Positions, sizes, shapes, ranges, and the like of the respective components illustrated in the drawings and the like do not always indicate actual positions, sizes, shapes, ranges and the like in order to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to the positions, sizes, shapes, ranges, and the like disclosed in the drawings and the like.
The publications, patents, and patent applications cited in the present specification constitute a part of the description of the present specification as they are.
Components expressed by the singular in the present specification are intended to include the plural unless clearly indicated in the context.
In the embodiments to be described hereinafter, an exit diffraction grating is formed on a surface opposite to an incident diffraction grating when a light guide plate made of plastic is applied. With this configuration, highly efficient reflective diffraction can be used for diffraction toward user's eyes, and thus, brightness of image information is improved.
The wavenumber vectors kR and kT are obtained by applying K to the propagating light beam kprop based on the principle of diffraction. Therefore, kR and kT are obtained by inverting only the sign of a vector component in a z direction in the drawing. Therefore, the user can visually recognize the image information by viewing any of the light beams of the reflective diffraction and the transmissive diffraction. However, the both form images inverted with respect to x and y directions, and thus, it is necessary to perform image inversion processing by an image source (not illustrated) as necessary.
When a structure period of the exit diffraction grating 102 is P, the magnitude of the wavenumber vector K is 2π/P. Assuming that a width and a height of a convex portion of the exit diffraction grating 102 are w and h, respectively, an aspect ratio is represented by h/w. In the case of the light guide plate 100 prepared by an injection molding method or the like, it is difficult to obtain favorable molding when the aspect ratio h/w exceeds approximately 1. In such a case, reflective diffraction efficiency is greater than transmissive diffraction efficiency. An incident angle θ of the light beam propagating inside the light guide plate 100 to the exit diffraction grating 102 is about 40 to 80 degrees.
When an aspect ratio of a corrugated pattern that is transferred to the surface of the light guide plate 100 is low, it is easy to form the corrugated pattern with a proven plastic molding technology such as the injection molding method. Therefore, in a preferred mode of the present embodiment, a diffraction grating having a two-dimensional mesh pattern is proposed as the exit diffraction grating 102. As a result, the aspect ratio of the corrugated pattern that is transferred to the surface of the light guide plate becomes 1 or less, and the light guide plate 100 suitable to use the plastic molding technology such as the injection molding method can be provided.
A photonic crystal or a diffraction grating described in PTL 1 spatially applies phase modulation to incident light through surface irregularities. The magnitude of the phase modulation increases in proportion to a difference in refractive index between a surface structure and air and a height of the surface irregularities.
As will be described later, not a transmissive diffraction grating of “PTL 3” but a reflective diffraction grating is adopted regarding the incident diffraction grating of the present embodiment, which can contribute to a reduction in the aspect ratio by utilizing reflection having a large deflection effect with respect to refraction.
In this manner, it is possible to provide the diffraction grating having a low aspect ratio and to provide the light guide plate that can be achieved by the plastic molding technology, such as the injection molding method, is safe and lightweight, and enables high image brightness.
Note that the description will proceed with a coordinate system that has an optical axis direction and the surface of the light guide plate as a Z axis and an X-Y plane, respectively, in the description of the present specification. In addition, when a pupil of the user is approximated to a circle, an exit position in the light guide plate visually recognized by the user depending on a pixel position also becomes a circle. Hereinafter, the exit position will be referred to as an exit circle.
Among image light beams visually recognized by the user's pupil 400, a light beam 301 corresponding to the center of a field of view (display image) travels straight in the x-z plane and reaches the user's pupil 400 as illustrated in the drawing. Diffraction in the y direction, which is caused by an effect of the light guide plate 100, is not explicitly illustrated, but the diffraction occurs at least once by each of the incident diffraction grating 101 and the exit diffraction grating 102.
On the other hand, among the image light beams visually recognized by the user's pupil 400, a light beam 302 corresponding to the periphery of the field of view (display image) travels in the right direction in the drawing in a case where there is no diffraction in the x direction. Meanwhile, in order for the user to recognize this light beam as a projected image, a light beam having the same angle needs to reach the user's pupil 400 through a path indicated as a visually recognized light beam 304 in the drawing. An exit circle 303 is a virtual circle on the exit diffraction grating 102 that is obtained by translating the user's pupil 400 in the direction of the visually recognized light beam. Only the light beam 304 emitted from the exit circle 303 on the exit diffraction grating 102 is recognized as the projected image by the user, and the other light beams are not recognized. In this manner, the diffraction effect in the x direction is required for the exit diffraction grating 102.
Assuming that a grating pitch is P, the magnitude of the wavenumber vector of the diffraction grating is expressed by K=2π/P as described with reference to
ko=ki
k
o
x
=k
i
x+0+(K/√3)−(K/√3)=kix
k
o
y
=k
i
y
+K−(K/2)−(K/2)=kiy
koz=kiz
Next, a simulation method for analyzing an image display element of the embodiment will be briefly described. A ray tracing method proposed by G. H. Spencer, et al. in 1962 [G. H. Spencer and M. V. R. K. Murty, “General Ray-Tracing Procedure”, J. Opt. Soc. Am. 52, p. 672 (1962)] is a method of paying attention to particulates of light to trace paths thereof and calculating an image or the like that is observed at a point, and continues to be vigorously improved mainly in the field of computer graphics. Monte Carlo ray tracing method based on the ray tracing method [I. Powell “Ray Tracing through systems containing holographic optical elements”, Appl. Opt. 31, pp. 2259-2264 (1992).] is a method of stochastically processing path splitting by diffraction and reflection to prevent an exponential increase in calculation amount, and is suitable for a simulation of a light guide plate which repeats diffraction and propagation in total internal reflection. In the Monte Carlo beam tracing method, reflection and refraction can be faithfully reproduced, but it is essential to develop a suitable model for the diffraction.
A diffraction model, which covers a range of wavelengths (about 400 to 700 nm) over the entire visible light spectrum and an incident angle range suitable for a viewing angle (about 40°) of a projected image, is essential for a light guide plate for an application as a head-mounted display, and the calculation amount becomes enormous with a commercially available simulator. Here, in view of the fact that visually recognized light beams are some of all light beams, an algorithm is used in which the calculation amount is reduced to 1/1000 or less, the algorithm for stopping, in advance, a calculation of a light beam guided to a region that is not visually recognized. Regarding angle and wavelength dependencies of diffraction efficiency obtained by the diffraction grating, calculation results obtained by a finite differential time domain (FDTD) method are tabulated in advance and referred to.
Hereinafter, a configuration of the image display element of the embodiment will be described.
<1. Overall Configuration of Image Display Element>
The light guide plates 100a and 100b have different pattern periods P1 and P2, respectively, and cover different ranges of wavelengths. P1 is, for example, 360 nm, and P2 is, for example, 460 nm. Note that the number of the light guide plates 100 is arbitrary, and one or a plurality of (three or more) light guide plates 100 may be provided in accordance with wavelengths of light to be handled. A pattern period of each light guide plate is desirably changed in accordance with to a wavelength to be handled.
With the configuration of
The incident diffraction grating 101 is configured using a reflective diffraction grating. What causes incident light to be reflected and diffracted, that is, to be reflected to the light source side and propagate inside the light guide plate 100 is referred to as the reflective diffraction grating. Therefore, the incident diffraction grating 101 is formed at a position on a surface of the light guide plate 100 far from the projector 300. A reason for adopting this configuration will be described in detail later with reference to
The exit diffraction grating 102 is formed on a surface of the light guide plate 100 opposite to the surface on which the incident diffraction grating 101 is located. A reason for adopting this configuration will be described in detail later with reference to
In the present embodiment, the exit diffraction grating 102 is basically formed only on one surface of the light guide plate 100. That is, in the example of
<2. Configuration of Exit Diffraction Configuration>
The exit diffraction grating 102 will be described with reference to
In the mesh-type diffraction grating, a relation among a pattern duty, the diffraction efficiency, and the aspect ratio was simulated. When a pattern pitch of the diffraction grating is p and a pattern width is w, the duty is expressed by w/p. In the simulation, the pattern pitch P was 460 nm, the pattern height was 70 nm, the wavelength of the light beam was 550 nm, the thickness of the light guide plate was 1.0 mm, and the refractive index of the light guide plate was 1.58. A viewing angle of the projected image is 40 degrees.
According to the simulation results, it has been found that first-order diffraction efficiency η1 has a maximum value of about 4.2% at w/p=0.5, and has a characteristic of decreasing as w/p approaches 0 or 1. When the diffraction efficiency of about 0.6% is obtained, w/p of the mesh-type diffraction grating of the present embodiment needs to be determined in a range of 0.15 or more and 0.85 or less. In addition, the efficiency was good in a range where w/p is 0.3 or more and 0.7 or less, and the efficiency was best in a range where w/p is 0.4 or more and 0.6 or less.
Regarding the aspect ratio of the pattern, the aspect ratio increases as w/p approaches 1 or 0 since the pattern height is fixed at 70 nm. When the aspect ratio of the pattern being 1 or less is set as a reference for adaptation of an injection molding method or the like, w/p of the mesh-type diffraction grating of the present embodiment needs to be determined in the range of 0.15 to 0.85. In addition, manufacturing becomes the easiest with the minimum aspect ratio when w/p=0.5.
From the above, in principle, it can be said that the diffraction efficiency of the mesh-type diffraction grating is maximum and the aspect ratio of the pattern is minimum when w/p=0.5, that is, w=p−w.
<3. Configuration of Incident Diffraction Grating>
The incident diffraction grating 101 will be described with reference to
In
An image light beam is similarly incident from the left, and the left half of the drawing represents the substrate (Sub). In the reflective diffraction grating, the maximum diffraction efficiency is obtained under a condition that reflection by a blaze surface and diffraction by a periodic structure are tuned in phase. As can be seen from the drawing, it is found that this condition is satisfied with a corrugated pattern having a lower aspect ratio than that of the transmissive type. At this time, a height of the corrugated pattern is about 250 nm, and the aspect ratio is about 0.57. In a prototype element described hereinbefore, a triangular corrugated pattern having a pattern height of 374 nm could be favorably transferred. It can be said that the incident diffraction grating suitable for the light guide plate made of plastic in the present embodiment is a reflective incident diffraction grating.
<4. Study on Diffraction Efficiency of Light Guide Plate>
In the drawing, an image light beam 501 emitted from the projector 300 is diffracted by the incident diffraction grating 101, propagates in total internal reflection through the light guide plate 100, is diffracted at a point 131 in an exit circle (not illustrated) on the exit diffraction grating 102, and is visually recognized by the user. When the user's pupil 400T is located on the same side as the exit diffraction grating 102 with respect to the light guide plate 100, the visually recognized image light beam is transmitted and diffracted at the point 131. Thus, the transmissive diffraction efficiency is dominant in the visually recognized light intensity. On the other hand, when the user's pupil 400R is located on the opposite side of the exit diffraction grating 102 with respect to the light guide plate 100, the visually recognized image light beam is reflected and diffracted at the point 131. Thus, the reflective diffraction efficiency is dominant in the visually recognized light intensity.
A pitch Pprop of the propagation in total internal reflection inside the light guide plate 100 is uniquely determined by a pitch of the incident diffraction grating 101, a refractive index and a thickness of the light guide plate 100, a wavelength and an incident angle of the image light beam 501. The image light beam intersects with the exit diffraction grating N times (=ΔY/Pprop) until reaching the point 131. Thus, when a light intensity of the image light beam 501 is I0 and zero-order reflective diffraction efficiency and first-order transmissive diffraction efficiency of the exit diffraction grating 102 are respectively ηR0 and ηT1, a light intensity IT visually recognized by the pupil 400T of the user is approximated as
I
T
=I
0·(ηR0)N−1·(ηT1) (1).
Similarly, when first-order reflective diffraction efficiency of the exit diffraction grating is ηR1, a light intensity IR visually recognized by the pupil 400R of the user is approximated as
I
R
=I
0·(ηR0)N−1·(ηR1) (2).
The above discussion regarding the diffraction efficiency relates to the diffraction efficiency at the point 131 in
From the above study, it can be said that a positional relation in which the pupil 400R of the user is located on the opposite side of the exit diffraction grating 102 with respect to the light guide plate 100 is desirable from the viewpoint of the brightness visually recognized by the user regarding the arrangement of the light guide plate 100 and the exit diffraction grating 102 illustrated in
From the examples of
<5. Study on Influence of Inclination of Light Guide Plate>
Next, a case where first-order reflective diffraction is performed by the exit diffraction grating 102 to allow the user to visually recognize image light is considered. In this case, the exit diffraction grating 102 is arranged on a surface of the light guide plate 100 on the opposite side of the user's pupil 400.
The influence of a relative inclination between the two light guide plates 100a and 100b will be described with reference to
Note that the relative inclination between the light guide plates 100a and 100b is desirably suppressed to about 3 degrees or less since an angle condition for a light beam that is guided in total internal reflection inside the light guide plate 100 is actually affected. In this case, the user can be provided with the image information with higher brightness as the transmissive diffraction efficiency of the exit diffraction grating 102 increases.
Note that the projector 300 is arranged on the left side of the light guide plate 100 in
The diffraction efficiency when light propagating through the light guide plate 100 is diffracted by the exit diffraction grating 102 and emitted from the light guide plate 100 was calculated by the FDTD method. The reflective diffraction efficiency was 3.5% and the transmissive diffraction efficiency was 2.8% under a condition that a wavelength is 550 nm, a refractive index of the light guide plate 100 is 1.58, a pattern period of the diffraction grating is 460 nm, a width of a convex portion is 150 nm, a height of the convex portion is 70 nm, and light corresponding to a central pixel of a projected image is coupled by incident diffraction and propagates in total internal reflection inside the light guide plate 100. An aspect ratio of a corrugated pattern is 0.47.
It is a matter of course that a manufacturing process is simpler in a case where the incident diffraction grating 101 and the exit diffraction grating 102 are simultaneously formed only on one surface of the light guide plate 100. In the configuration of
In the present embodiment, the exit diffraction gratings 102a and 102b are formed on the surfaces opposite to the incident diffraction gratings 101a and 101b, respectively, as illustrated in
<6. Study on Ideal Image Display Element>
A desirable configuration of an image display element will be studied by putting the findings described in the above embodiment together.
In a case where plastic is used as a light guide plate, a reflective diffraction grating having a low aspect ratio is preferable since it is difficult to form a pattern having a high aspect ratio in an incident diffraction grating having high diffraction efficiency as described with reference to
Then, the reflective incident diffraction grating 101 reflects light to the inside of the light guide plate 100, and thus, is arranged on a surface (second surface) of the light guide plate on a side opposite to an incident surface (first surface) of an image light beam as illustrated in
In addition, in a case where a plurality of the light guide plates 100 are used, a transmissive optical configuration in which light is emitted to the side (second surface) opposite to the incident surface (first surface) of the light beam is desirable in order to reduce a shift of a visually recognized pixel position as described in
In addition, the light guide plate 100 can have a configuration in which the user visually recognizes light in which first-order reflective diffraction is dominant, and thus, the visually recognized brightness can be increased with a low aspect ratio as described with reference to
According to the present embodiment, the exit diffraction grating is formed on the surface on the opposite side of the incident diffraction grating in the light guide plate (image display element) having the surface-relief grating, and thus, the exit diffraction efficiency can be increased to 4% or more. In addition, when the mesh-type exit diffraction grating illustrated in
A conventionally used light guide plate, such as a nanoimprint method or etching, is prepared by a surface processing technology based on a semiconductor processing technology. On the other hand, since the plastic molding technology, such as the injection molding method, is a three-dimensional molding technology of introducing resin into a mold and solidifying the resin, it is easy to form the diffraction gratings on both the surfaces of the light guide plate.
In the drawing, stampers 700 and 701 respectively having surfaces on which a surface of a diffraction grating that needs to be formed is formed in inverted concave-convex forms are fixed to a fixed portion 710 and a movable portion 720 of a mold, respectively. When molten resin 740 is injected from a resin flow path 730 using such a mold and the movable portion 720 of the mold is moved in the right direction in the drawing to apply pressure, the resin 740 can be formed into a shape along a shape of a cavity 750, and a desired light guide plate can be formed through a cooling process. This method is a general method, and the light guide plate in which the diffraction gratings are formed in concave and convex shapes on both the surfaces can be made of plastic using the two stampers.
In the drawing, the image display device of the present embodiment includes the light guide plate 100, the projector 300, and a display image control unit 1400. In addition, as an image formation method, widely known image formation devices can be used, for example, an image formation device including a reflective or transmissive spatial light modulator, a light source, and a lens, an image formation device including organic and inorganic electro luminescence (EL) element arrays and a lens, an image formation device including a light emitting diode array and a lens, an image formation device including a combination of a light source, a semiconductor micro electro mechanical systems (MEMS) mirror array, and a lens, and the like.
In addition, a device that causes resonant movement of a light emitting diode (LED), a laser light source, and a tip of an optical fiber by a MEMS technology, lead zirconate titanate (PZT), or the like can also be used. Among them, the most general device is the image formation device including the reflective or transmissive spatial light modulator, the light source, and the lens. Here, examples of the spatial light modulator include a transmissive or reflective liquid crystal display device such as a liquid crystal on silicon (LCOS), and a digital micromirror device (DMD). As the light source, a white light source from which light is separated into RGB components may be used, or LEDs or laser sources corresponding to the respective colors may be used.
Furthermore, the reflective spatial light modulator can be configured using a liquid crystal display and a polarization beam splitter which reflects and guides a part of light from the light source to the liquid crystal display and allows passage of a part of the light reflected by the liquid crystal display to be guided to a collimating optical system that uses a lens. Examples of a light emitting element constituting the light source can include a red light emitting element, a green light emitting element, a blue light emitting element, and a white light emitting element. The number of pixels may be determined according to specifications required for the image display device, and examples of specific values of the number of pixels can include 320×240, 432×240, 640×480, 1024×768, and 1920×1080 in addition to 1280×720 mentioned above.
In the image display device of the present embodiment, the projector 300 is formed to be integrated with the light guide plate 100 by being positioned such that a light beam that has image information and has been emitted from the projector 300 is emitted to each of the incident diffraction gratings 101 of the light guide plate 100.
In addition, a display image control unit (not illustrated) performs a function of controlling the operation of the projector 300 and appropriately providing the image information to the user's pupil 400.
In the embodiments described above, the incident and exit diffraction gratings and the exit diffraction grating are integrally molded using the material having the same refractive index as that of the waveguide by the injection molding method or the like in the light guide plate (image display element) having the surface-relief grating, and thus, it is possible to make the light guide plate using plastic and achieve the safe and lightweight light guide plate. That is, the light guide plate that has favorable performance with the surface irregularities having the aspect ratio of 1 or less can be prepared by the injection molding method, and it is possible to achieve the improvement in safety and reduction in weight by making the light guide plate using plastic.
In addition, the incident diffraction grating 101 and the exit diffraction grating 102 are formed on opposing surfaces of the light guide plate 100, and the brightness of the visually recognized image can be improved by improving the diffraction efficiency using the reflective diffraction. The case where the image information is provided to the user has been described in the present embodiment, and the image display device of the present embodiment can include various sensors such as a touch sensor, a temperature sensor, and an acceleration sensor configured to acquire information regarding the user and the external environment, and an eye tracking mechanism configured to measure movement of the user's eye.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-002824 | Jan 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/038160 | 10/8/2020 | WO |
