JP7096084B2 - Manufacturing method of light guide plate, light guide plate module, image display device and light guide plate - Google Patents

Description

The present invention relates to an image display element and an apparatus capable of displaying augmented reality in a compact and lightweight manner by combining a light guide plate and a diffractive element.

In an augmented reality image display device, the user can see not only the projected image but also the surroundings at the same time. The projected image can overlap the real world perceived by the user. Other uses for these displays include video games and wearable devices such as eyeglasses. The user can visually recognize the image supplied from the projector by superimposing it on the real world by wearing glasses or a goggle-shaped image display device in which a translucent light guide plate and a projector are integrated.

Some of these image display devices are described in "Patent Document 1" to "Patent Document 3". In these patent documents, the light guide plate is composed of a plurality of concave-convex-shaped diffraction gratings formed on a glass substrate. The light rays emitted from the projector are coupled to the light guide plate by the diffraction grating for incident light, and propagate inside the light guide plate while being totally reflected. The light beam is totally reflected and propagated in the light guide plate while being converted into a plurality of light rays duplicated by yet another diffraction grating, and finally emitted from the light guide plate. A part of the emitted light beam is imaged on the retina through the user's pupil and recognized as an augmented reality image superimposed on the image in the real world.

In a light guide plate using such a concave-convex diffraction grating, the wave vector K of the light beam emitted from the projector is refracted into the light guide plate, and the wave number vector becomes K0 according to Snell's law. Further, it is converted into a wave vector K1 capable of total reflection propagation inside the light guide plate by the diffraction grating for incident. It is subjected to diffraction action by another one or a plurality of diffraction gratings provided on the light guide plate, and the wave number vector changes each time diffraction is repeated as in K2, K3, ....

If the wave vector of the light ray finally emitted from the light guide plate is K', then | K'| = | K |, and if the projector is on the opposite side of the eye via the light guide plate, K'= It becomes K. On the other hand, when the projector is on the opposite side of the eye through the light guide plate, the light guide plate has the same function as the reflection mirror with respect to the wave vector, and the normal vector of the light guide plate is taken in the z direction, and the x and y of the wave vector are taken. , Z components are compared and can be expressed as Kx'= Kx, Ky'= Ky, Kz'= −Kz.

The function of the light guide plate is to guide the light rays emitted from the projector while replicating them to a plurality of rays so that the user can recognize the plurality of light rays emitted as image information equivalent to the original image. The magnitude of the wave vector differs depending on the wavelength of the light beam. Since the concave-convex diffraction grating has a constant wave vector, the diffracted wave vector K1 differs depending on the wavelength of the incident light beam, and propagates in the light guide plate at different angles. The refractive index of the glass substrate constituting the light guide plate is substantially constant with respect to the wavelength, and the range of conditions for guiding light while totally reflecting is different depending on the wavelength of the incident light beam. Therefore, in order for the user to recognize an image having a wide viewing angle, it is necessary to stack a plurality of light guide plates different for each wavelength. Generally, it is considered that the optimum number of light guide plates is the number corresponding to each of R, G, and B, or about plus or minus one.

The image display device described in "Patent Document 1" is an image display device for enlarging input light in two dimensions, and includes three linear diffraction gratings. One is an incident diffraction grating, and the other two diffraction gratings are typically arranged on the front surface and the back surface of the light guide plate so as to overlap each other, and serve as a diffraction grating for duplication and emission.

In the image display devices described in "Patent Document 2" and "Patent Document 3", three diffraction gratings for incident, X-direction duplication, and Y-direction duplication and emission overlap in a light guide plate. It is placed without any problems. "Patent Document 3" discloses an overhung triangular diffraction grating in order to increase the diffraction efficiency of the incident diffraction grating.

"Patent Document 4" and "Patent Document 5" disclose techniques for using two reflective volumetric holograms, one for incident and the other for emission, as a diffraction grating formed on a light guide plate. In these, the volumetric hologram is formed by forming multiple diffraction gratings corresponding to a plurality of wavelengths in the space, and unlike the concave-convex diffraction gratings of "Patent Document 1" to "Patent Document 3", a plurality of diffraction gratings are formed. Diffracts light rays of wavelength at the same angle. Therefore, the RGB image can be recognized by the user with one light guide plate. On the other hand, in the concave-convex diffractive grids of "Patent Document 1" to "Patent Document 3", a wide viewing angle can be realized because the light rays are duplicated in the two-dimensional directions of X and Y, whereas "Patent Document 4" is used. Since "Patent Document 5" provides only the function of one-dimensional duplication in the X direction, it has a feature that the viewing angle is relatively narrow.

Special Table 2017-528739 US2016 / 0231566A1 Publication WO99 / 52002A1 Gazette Japanese Unexamined Patent Publication No. 2007-94175 Japanese Unexamined Patent Publication No. 2013-200467 Re-table 2014/156167

"Patent Document 1" discloses a technique of using a glass material for a light guide plate as described in item 0038 of the substrate material. As for the diffraction grating, as described in paragraph 0017, the technique of forming the surface of the waveguide (= glass plate) by etching is disclosed.

Regarding the diffraction grating in "Patent Document 2", as described in the section 0044, the surface relief grating SRG (= uneven diffraction grating) is used for etching and / or etching the substrate in order to produce a desired periodic microstructure on the substrate. Alternatively, it discloses a technique manufactured by an appropriate "microfabrication process" including deposition. Regarding the substrate, it is said that the optical component itself or the manufacturing master of a mold for manufacturing the optical component may be used. In FIGS. 5B to 5C of "Patent Document 2", an overhanging triangular diffraction grating suitable for an incident diffraction grating is shown.

Patent Document 3 discloses a technique of using a glass substrate as described on page 8 of the substrate. Fig. 1 of Patent Document 3 shows the configuration of the light guide plate.

The light guide plate disclosed in these techniques uses a glass substrate, and the diffraction grating is formed by a "microfabrication process" such as etching or lithography, which is widely used in semiconductor processing. In the etching process of the "microfabrication process", an uneven shape can be formed on the surface of the glass substrate by cutting using plasma particles such as Ga in a vacuum device.

Since the light guide plate formed by the etching process of the glass substrate includes a vacuum process and the like, there is a limit to the reduction of the manufacturing cost. In addition, as seen in recent examples of eyeglasses, glass materials are heavy, so plastic lenses have become the mainstream for users to wear them comfortably.

In the light of the object of the present invention, the problem of the prior art is the reduction of cost and weight by using the light guide plate made of glass.

A preferred aspect of the present invention is a light guide plate comprising a first member made of a first resin and a second member made of a second resin formed on at least a part of the first member. be. In this light guide plate, the first resin and the second resin are different resins, a first diffraction grating composed of a first pattern is formed on the first member, and a second diffraction grating is formed on the second member. A second diffraction grating consisting of a pattern is formed. The first pattern and the second pattern are different patterns.

Another preferred aspect of the present invention is to include the plurality of light guide plates described above, each of the plurality of light guide plates having a different first pattern corresponding to a predetermined wavelength, and the plurality of light guide plates are laminated. It is a formed light guide plate module.

Another preferred aspect of the present invention is an image display device comprising the light guide plate and a projector for projecting an image, wherein the emitted light of the projector is arranged to irradiate a second pattern. ..

Another preferred aspect of the invention is the first step of preparing a substrate which is made of polycarbonate or acrylic resin and has a first diffraction grating pattern on the surface, and photocuring on top of the first diffraction grating pattern. A light guide plate comprising a second step of arranging the resin, a third step of transferring the second diffraction grating pattern to the photocurable resin, and a fourth step of curing the photocurable resin. It is a manufacturing method of.

According to the present invention, it is possible to reduce the cost and weight by using the light guide plate made of glass.

Schematic side view showing the definition of the shape parameter of the diffraction grating. Schematic side view showing the definition of the shape parameter of the diffraction grating. The flow chart which shows the manufacturing method of the light guide plate of an Example. The schematic diagram which shows the manufacturing method of the light guide plate of an Example. The schematic diagram which shows the manufacturing method of the light guide plate of an Example. The schematic diagram which shows the manufacturing method of the light guide plate of an Example. The schematic diagram which shows the manufacturing method of the light guide plate of an Example. The schematic diagram which shows the manufacturing method of the light guide plate of an Example. External photograph of the prototype element. Diffraction pattern image of the evaluation result of the prototype element. Diffraction pattern image of the evaluation result of the prototype element. Graph diagram of the evaluation result of the prototype element. Beam pattern image of the evaluation result of the prototype element. The graph which shows the measurement result of the primary diffraction efficiency and the comparison result of a diffraction model. The graph which shows the relationship between the pitch of a diffraction grating and the pixel ratio. The graph which shows the relationship between the pitch of a diffraction grating and the pixel ratio. The graph which shows the relationship between the pitch of a diffraction grating and the pixel ratio. The graph which shows the other simulation result which shows the relationship between the diffraction grating pitch and the pixel ratio. The graph which shows the other simulation result which shows the relationship between the diffraction grating pitch and the pixel ratio. The graph which shows the other simulation result which shows the relationship between the diffraction grating pitch and the pixel ratio. The image figure which shows the simulation result which shows the pixel ratio in the case of a viewing angle of 25 degrees. The image figure which shows the simulation result which shows the pixel ratio in the case of a viewing angle of 25 degrees. The figure which shows the result which summarized the suitability for utilizing the mass production process technique of an optical disk for the light guide plate of an Example. The graph which shows the relationship between the height of the emission diffraction grating obtained by the electromagnetic field calculation and the diffraction efficiency. The graph which shows the result of having calculated the relationship between the diffraction efficiency of an emission diffraction grating and the brightness of a projected image. The graph which shows the result of having calculated the relationship between the diffraction efficiency of an emission diffraction grating and the image quality of a projected image. The side view which shows the whole process of forming the light guide plate of an Example which utilized the mass production process of an optical disk. A side view showing the process of creating an incident diffraction grating of an example. Calculation result image diagram of the image projected on the retina when the diameter of the incident beam is 4 mmφ. The graph of the calculation result summarizing the RMS value of the luminance when the diameter of the incident beam is 4 mmφ. The graph of the calculation result summarizing the maximum value of the luminance when the diameter of the incident beam is 4 mmφ. Calculation result image diagram of the image projected on the retina when the diameter of the incident beam is 1 mmφ. The graph of the calculation result summarizing the RMS value of the luminance when the diameter of the incident beam is 1 mmφ. The graph of the calculation result summarizing the maximum value of the luminance when the diameter of an incident beam is 1 mmφ. The graph which shows the relationship between the thickness of a light guide plate and the interval of the point where a light ray and an emission diffraction grating intersect. The graph of the calculation result which shows the relationship between the thickness of a light guide plate and the diffraction efficiency of the emission diffraction grating which minimizes the luminance maximum and the luminance unevenness. The sectional view which shows the structure of the light guide plate of an Example. An image diagram showing a simulation result of a transmission type incident diffraction grating. An image diagram showing a simulation result of a reflection type incident diffraction grating. An image diagram showing a simulation result that visualizes the intensity distribution of light rays propagating inside the light guide plate. An image diagram showing a simulation result that visualizes the intensity distribution of light rays propagating inside the light guide plate. The schematic diagram which shows the improvement method of the luminance unevenness of a projected image by an Example. The schematic diagram which shows the improvement method of the luminance unevenness of a projected image by an Example. Graph diagram explaining how to reduce luminance unevenness The schematic diagram which shows the light guide plate of an Example which reduces the luminance unevenness. Schematic diagram showing another embodiment using the light guide plate of the embodiment. The schematic diagram which shows the light guide plate of an Example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the description of the embodiments shown below. It is easily understood by those skilled in the art that a specific configuration thereof can be changed without departing from the idea or purpose of the present invention.

In the configuration of the invention described below, the same reference numerals may be used in common among different drawings for the same parts or parts having similar functions, and duplicate description may be omitted.

When there are a plurality of elements having the same or similar functions, they may be described by adding different subscripts to the same code. However, if it is not necessary to distinguish between multiple elements, the subscript may be omitted for explanation.

Notations such as "first", "second", and "third" in the present specification and the like are attached to identify components, and do not necessarily limit the number, order, or contents thereof. is not. Further, the numbers for identifying the components are used for each context, and the numbers used in one context do not always indicate the same composition in the other contexts. Further, it does not prevent the component identified by a certain number from functioning as the component identified by another number.

The position, size, shape, range, etc. of each configuration shown in the drawings and the like may not represent the actual position, size, shape, range, etc. in order to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to the position, size, shape, range and the like disclosed in the drawings and the like.

The following examples disclose techniques that enable the light guide plate to be made of plastic. Further, as described above, a diffraction grating for incident use and other diffraction gratings are formed on the light guide plate. In the present specification, the former will be referred to as an incident diffraction grating and the latter as an outgoing diffraction grating.

Optical discs such as CD (Compact Disc), DVD (Digital Versaille Disc), and BD (Blu-ray (registered trademark) Disc), which are widely used, use a plastic material typified by polycarbonate as a substrate. The surface of the substrate is formed with irregularities having a pitch of about 320 nm (25 GB capacity BD) to 740 nm (4.7 GB capacity DVD) and a depth of about 50 to 100 nm as guide grooves for light spots. The substrate is mass-produced by an injection molding method that does not include a vacuum process, and is supplied at low cost. In this specification, the indications "A to B" are used to mean "A or more and B or less". Further, in the present specification, "resin" means a material made of a polymer compound, and is a concept that does not contain glass but includes resin, polycarbonate, acrylic resin, and photocurable resin.

The results of our studies on the pitch and depth of the diffraction grating required for the light guide plate are described below. The pitch of the diffraction grating is common to the incident diffraction grating and the emission diffraction grating, and 200 to 700 nm, preferably 300 to 600 nm is suitable. Regarding the depth of the diffraction grating, it is necessary to consider that the suitable diffraction efficiency differs between the incident diffraction grating and the emission diffraction grating. Generally, there is a relationship that the diffraction efficiency of a diffraction grating increases as the depth increases. It is desirable that the incident diffraction grating has a primary diffraction efficiency of 10% to 80%, preferably 30% to 70% in order to efficiently bond the incident light rays to the light guide plate. On the other hand, since the emission diffraction grating carries out replication of light rays and emission from the light guide plate over a wide area, the primary diffraction efficiency is 0.1% to 10%, preferably 0.5% to 5.0%. It is desirable to have.

It is desirable that the depth of the incident diffraction grating is 50 to 500 nm, preferably 100 to 400 nm, depending on the appropriate value of the diffraction efficiency. On the other hand, the depth of the emission diffraction grating is preferably 10 to 200 nm, preferably 20 to 100 nm. It should be noted that the pitch and depth required for the emission diffraction grating are substantially the same as the pitch and depth of the optical disc provided at low cost by using the injection molding method. Therefore, it can be seen that the emission diffraction grating can be inexpensively formed as a concave-convex diffraction grating on a plastic substrate by utilizing the manufacturing technique of the optical disc.

In addition, optimization of the incident diffraction grating and the emission diffraction grating is realized by setting at least one of their shape, aspect, height, duty, and patterning area area to be appropriate for each grating.

For DVDs and BDs, multi-layer optical discs with increased recording capacity have been put into practical use. The first recording layer is formed by using the plastic substrate formed by the above-mentioned injection molding, and the second recording layer is formed on the first recording layer by a pattern transfer technique using a UV resin by a 2P (Photo-Polymation) method. ing. The 2P method can form a diffraction grating having a larger depth than the injection molding method. As will be described later, according to the studies by the inventors, it has been found that a diffraction grating having a depth of 0.3 μm, which is a suitable condition for an incident diffraction grating, can be satisfactorily formed by the 2P method.

Next, the area of the diffraction grating of the light guide plate will be described. Although described in "Patent Document 1" to "Patent Document 3", the area of the incident diffraction grating is smaller than the area of the exit diffraction grating in light of the respective functions described above, and the area of the incident diffraction grating < <Exit diffraction grating.

By forming the light guide plate by the following process, it is possible to provide a light guide plate that realizes cost reduction by utilizing the manufacturing technique of the optical disk and weight reduction by plasticization.
(1) Create a plastic substrate with a large area emission diffraction grating by the injection molding method.
(2) An incident diffraction grating is formed in a part of the area above it by the 2P method.
(3) One or more of these are built into the frame to form a light guide plate.

In the following examples, a light guide plate having a concave-convex diffraction grating as a light guide plate will be described in more detail. First, in order to facilitate the understanding of the following examples, the definitions of the shape parameters of the diffraction grating are summarized.

FIG. 1A is a definition of the shape parameter of the incident diffraction grating and FIG. 1B is the definition of the shape parameter of the emission diffraction grating, and is a view of the substrate from the side. As shown in FIG. 1A, the incident diffraction grating is defined by the pitch P, the height H, and the blaze angle θ _B. Further, as shown in FIG. 1B, the exit diffraction grating is defined by the pitch P, the height H, the convex portion width A, and the duty D = A / P. The example shown here shows typical shapes of an incident diffraction grating and an outgoing diffraction grating, and in this embodiment, a sinusoidal diffraction grating or the like can be used in addition to these.

FIG. 2 is a flow chart showing a process of creating an image display element (light guide plate) according to the first embodiment of the present invention.
3A to 3E are diagrams schematically showing each stage of the production process. The process of creating the embodiment will be described with reference to FIGS. 2 and 3A to 3E.

First, a master die 301 for an incident diffraction grating and a master die 302 for an exit diffraction grating are prepared by an EB drawing method or the like (S101, FIG. 3A). The mother die 302 for the emission diffraction grating is transferred to a Ni stamper and used so as to be compatible with the injection molding process.

Next, using the mother die 302 for the emission diffraction grating, a substrate 304 having the emission diffraction grating 303 formed on the surface using a plastic material by an injection molding method is produced (S102, FIG. 3B). As the plastic material constituting the substrate 304, polycarbonate, which is widely used, is preferable, but acrylic resin such as polymethyl methacrylate resin (PMMA) and other transparent resin materials can also be used. Note that FIG. 3B also shows an example of the arrangement of the formed emission diffraction grating 303 on the DVD-RAM disk 305 used as the substrate.

Next, an antireflection coating 306 for enhancing the visibility of the outside world is formed on the surface of the substrate 304 opposite to the diffraction grating by a sputtering method or the like (S103, FIG. 3C).

Next, the incident diffraction grating 307 is formed on a part of the substrate 304 formed by injection molding by the 2P method using the matrix 301 for the incident diffraction grating (S104, FIG. 3D). An incident diffraction grating is formed on the substrate 304 with a UV resin, and the thickness of the UV resin for that purpose is about 2 to 10 μm. FIG. 3D shows both a side view and a top view. As shown in FIG. 3D, the incident diffraction grating 307 is formed so as to be superimposed on the exit diffraction grating 303. Instead of the 2P method, a nanoimprint method or the like using a dry sheet as a stamper may be used.

Subsequently, if necessary, in order to improve the wavelength separation performance and the diffraction efficiency of the incident diffraction grating 307, a reflection coating 308 is applied on the incident diffraction grating 307 by a mask sputtering method or the like (S105, FIG. 3E). From the above, it is possible to form an inexpensive and weighable light guide plate 300 made of plastic.

FIG. 4 shows an external photograph of regions A, B, and C of the prototype element on a DVD-RAM disk. The uneven pattern of the DVD-RAM disk corresponds to the emission diffraction grating, and the prototype element corresponds to the incident diffraction grating. Here, a pattern is transferred by the 2P method using a commercially available blaze-type diffraction grating as a matrix on a polycarbonate substrate having a periodic uneven pattern on the surface created by the injection molding method, and a reflective coating is formed by the mask sputtering method. did. The selected blaze type diffraction grating had a pitch of 833 nm, and the height of the uneven pattern of the triangle measured by AFM (Atomic Force Microscope) was 374 nm. As the polycarbonate substrate, a DVD-RAM substrate 401 having a diameter of 80 mm formed by an injection molding method was selected.

The uneven pattern formed as the guide groove of the light spot on the DVD-RAM substrate 401 acts as a rectangular diffraction grating. The pitch of the diffraction grating is 1230 nm, and the pattern height is about 70 nm. The three element regions A, B, and C in the figure are rectangles having a side of 12.7 mm, and (1) a region A in which only the grating 2P resin is formed, and (2) a region B in which only the reflective coating is formed, respectively. And (3) a waveguide check region C in which a reflective coating is formed on a 2P resin of a grating. The grating 2P resin corresponds to the incident diffraction grating 307 described in FIG. 3D.

The reflective coating is a five-layer optical filter prepared to improve the diffraction efficiency with respect to the red laser (wavelength 635 nm) used for the evaluation. The film composition is selected from dielectric materials widely used as optical films for optical disks, and polycarbonate substrate (refractive index 1.58, film thickness 600 μm (same below)) / ZnS-SiO ₂ (2.33, 88 nm) / SiO ₂ (1.47, 80 nm) / ZnS-SiO ₂ (2.33, 88 nm) / SiO ₂ (1.47, 80 nm) / ZnS-SiO ₂ (2.33, 88 nm). Below, from the evaluation results of this prototype element, the feasibility of a plastic light guide plate in which an incident diffraction grating is formed on an emission diffraction grating made of an injection-molded substrate, which is the main point of this embodiment, by a 2P method or the like is shown.
(1) Performance of Incident Diffraction Grating Formed by 2P Method FIGS. 5A and 5B show diffraction patterns when a laser beam is vertically incident on the DVD-RAM substrate 401 described with reference to FIG. FIG. 5A is a pattern in the region where the 2P resin of the grating is not formed. FIG. 5B is a pattern in the region where the 2P resin of the grating is formed.

As can be seen in FIG. 5A, the diffraction pattern of the DVD-RAM appears in the region where the grating that serves as the incident diffraction grating is not formed. On the other hand, it can be seen that the diffraction pattern of the DVD-RAM disappears and the diffraction pattern of the incident diffraction grating appears in the region A where the incident diffraction grating formed on the 2P method is present, as shown in FIG. 5B. This shows that the image display element of this embodiment can overwrite the diffraction grating pattern.
(2) Spectral characteristics of the reflective coating FIG. 6 shows the results showing the wavelength (horizontal axis) dependence of the reflectance and the transmittance (vertical axis) of the reflective coating. This is an evaluation of the region B in which the optical thin film is laminated only on a part of the region by the mask sputtering method. As can be seen in the figure, the design value shown by the dotted line and the measured value shown by the solid line are in good agreement, and the wavelength-dependent reflective film is formed by coating the plastic light guide plate with an optical thin film. It was shown to be possible.
(3) Coupling Characteristics of Incident Diffraction Grating with Reflection Coating FIG. 7 shows a pattern of transmitted laser light when a laser beam is incident on an incident diffraction grating (region C) having a reflection coating formed. As can be seen in the figure, it can be seen that the duplicated beams are aligned and emitted at equal intervals with respect to the incident beam. This is because the laser beam is diffracted by the incident diffraction grating and totally reflected and propagated in the light guide plate, and the beam is duplicated while hitting the incident diffraction grating twice, three times, and four times, and the incident beam. It shows the state of emitting with the wave number of. Although this is one-dimensional, it is a result showing that the basic functions of the light guide plate such as propagation, duplication, and emission of the incident beam have been achieved. The range of the region C is shown by a dotted line as the element size.
From the above, it was shown that the image display element of this embodiment satisfies the basic function as a light guide plate for virtual reality.

Here, the projected image by the image display element of this embodiment is investigated by simulation, and a specific configuration suitable for this embodiment is described.
The ray tracing method proposed by GH Spencer et al. In 1962 [GH Spencer and MBTK Murty, “General Ray-Tracing Procedure”, J. Opt. Soc. Am. 52, p.672 (1962).] It is a method to calculate the image observed at a certain point by tracking the path focusing on the particle nature, and it is being energetically improved mainly in the field of computer graphics.

The Monte Carlo ray tracing method based on the ray tracing method [I. Powell “Ray Tracing through sysytems containing holographic optical elements”, Appl. Opt. 31, pp.2259-2264 (1992).] This is a method to prevent the exponential increase in the amount of calculation by treating the ray tracing probabilistically, and is suitable for the simulation of a light guide plate that repeats diffraction and total reflection propagation. Although the Monte Carlo ray tracing method can faithfully reproduce reflections and refractions, it is essential to develop a suitable model for diffraction.

For a light guide plate for a head-mounted display, a diffraction model corresponding to the wavelength range (about 400-700 nm) over the entire visible light range and the incident angle range corresponding to the viewing angle of 40 ° of the projected image is indispensable. Hereinafter, the diffraction model used in this embodiment will be described.

In the following description, it is assumed that the optical axis is in the z direction, the normal vector of the light guide plate is in the z direction, and at least a diffraction grating is formed on the front surface or the back surface. Also, for the sake of simplification of the explanation, it is assumed that the projector optical system and the pupil are located on the opposite side of the light guide plate. When both are on the same side with respect to the light guide plate, in the following discussion of the wave vector, the light beam emitted from the light guide plate may be inverted by the xy plane mirror.

Wave vector of light rays incident on the light guide plate

year,
Wave vector of the emitted ray

And.

The information of the projected image is composed of intensity, wavelength, and pixel information, and the wavelength and pixel information are determined by the angle information of light rays, that is, the wave vector by the lens action of the eye. In order to store the wavelength and pixel information of the projected image, the light guide plate is required to have the following functions (Equation (1)).

Diffraction of a ray is represented by adding the product of the diffraction order and the wave vector of the grating to the wave vector of the ray. Regarding the diffraction when the incident ray and the diffraction grating intersect the _nth time, if the wave vector of the ray changes from kn to _k'n , the following relationship holds (Equation (2)).

Here, mn is the diffraction order and K _n is the wave vector of the diffraction grating at the _nth intersection.

When the light beam is diffracted at the Nth intersection with the diffraction grating and emitted from the light guide plate, the following relationship holds from Eq. (1) (Equation (3)).

This represents the basic operation of the light guide plate. When equation (2) is expressed by each component of x, y, and z, it becomes the following (equations (4) to (7)).

Here, the subscripts of x, y, and z represent the x, y, and z components of each wave vector.

here,

At this time, the wave vector becomes an imaginary number, which indicates that no diffracted ray is generated under the specified conditions.

The generation of diffracted rays is defined as follows (Equation (8)).

Diffraction efficiency

Then, according to the law of conservation of energy, the following (Equation (9)) is obtained.

Diffraction efficiency considering the presence or absence of diffracted light and the dependence of the incident angle, where θ _n is the angle of incidence in the plane where the wave vector K _n of the diffraction grating and the z-axis extend.

Is modeled as follows (Equation (10)).

Here, η ⁰ _mn is the diffraction efficiency in the vertical incident of the diffraction order mn at the _nth intersection. The second term on the right side of Eq. (10) corresponds to the presence or absence of the generation of diffracted rays, the third term corresponds to the energy conservation law, and the fourth term corresponds to the incident angle dependence with γ as a constant.

On the other hand, as commercial software that adopts the Monte Carlo ray tracing method, there is LightTools (trademark) for designing illumination optical systems, but since angle dependence cannot be handled, the diffraction efficiency is as follows (Equation (11)). ).

Here, a commercially available element # 47-551 manufactured by Edmund with a density of 1,800 threads / mm was used as the same blaze-type diffraction grating as the incident diffraction grating, and the relationship between the incident angle and the diffraction efficiency at a wavelength of 532 nm was measured.

FIG. 8 shows the measurement result of the primary diffraction efficiency and the comparison result of the diffraction model based on the equation (10). From the experimental result 801 shown by the black circle, the range of the incident angle at which both the positive and negative primary diffracted light are generated is plus or minus 3 °. As can be seen in the figure, the + 1st-order diffracted light is generated at an incident angle of -3 ° or more, coexists with the -1st-order diffracted light up to + 3 °, and the -1st-order diffracted light disappears at + 3 ° or more. The conventional model 802 in the figure shows the diffraction model of Eq. (11), and since the diffraction efficiency is constant regardless of the incident angle, when positive and negative primary diffracted light coexist depending on the incident angle,- It cannot represent the decrease in diffraction efficiency when the primary light disappears and when the angle of incidence increases. On the other hand, in the model 803 of the equation (10), it can be seen that the angle dependence of the diffraction efficiency of the blazed diffraction grating can be simply expressed.

The parameter conditions suitable for the image element of this embodiment are described by the Monte Carlo simulator that implements the model of Eq. (10). In the following, the following simulation conditions are used unless otherwise specified. The wavelength of the RGB light source is R = 635 nm, G = 550 nm, B = 460 nm, the wavelength spread of the light source is 20 nm for each color, the aspect ratio of the projected image is 16: 9, the number of pixels is 1280 x 720, and the beam incident on the light guide plate. The diameter is 4 mm, the diameter of the pupil of the eye is 2 mm, and the distance between the eye and the light source plate is 17 mm. Regarding the light guide plate, the basic configuration is described in Patent Document 1, the refractive index of the light guide plate plasticized according to this embodiment is 1.58, the thickness of the light guide plate is 0.6 mm, and the size of the incident diffraction grating. Is 5 mm x 5 mm, and the size of the emission diffraction grating is sufficiently large. Also, ignore the reflection of light at the edges of the eyes and light guide plate.

9A-9C show the relationship between the pitch of the exit diffraction grating and the pixel range of the projectable image. Here, the pixel ratio on the vertical axis indicates the ratio of pixels in which the path of the light rays reaching the pupil by diffraction by the input / output diffraction grating and total reflection is 1 or more, and when the pixel ratio is smaller than 1, one of the projected images. It means that there will be a gap in the part. Since the pixel ratio can be obtained only by the presence or absence of a path, there is an advantage that the characteristics of the light guide plate can be grasped without depending on the diffraction efficiency of each diffraction grating or the thickness of the light guide plate.

FIG. 9A is a result when the viewing angle is 40 degrees. It can be seen that there is a difference in the range in which the pixel ratio is 1 depending on the wavelength of each color of RGB. This corresponds to the fact that the diffraction grating has a strong wavelength dependence. By combining the two light guide plates corresponding to BG and GR, which have the conditions indicated by the two arrows in the figure, the diffraction grating pitch of about 390 nm, and 470 nm, the pixel ratio of the RGB image becomes 1, and the projection of the image becomes 1. It turns out that it is possible.

FIG. 9B is a result when the viewing angle is 50 degrees. It can be seen that the range of the pitch of the diffraction grating at which the pixel ratio is 1 becomes narrower as the viewing angle increases. The median value of the pitch of the diffraction grating in which the pixel ratio is 1 for each color is about 360, 440, and 520 nm, respectively. It can be seen that full-screen image projection is possible by combining three light guide plates corresponding to each of B, G, and R, which have the diffraction grating pitch indicated by the three arrows in the figure.

FIG. 9C is a result when the viewing angle is 70 degrees. It can be seen that as the viewing angle increases, the pixel ratio decreases and the maximum value becomes less than 1. This means that a light guide plate having one diffraction grating pitch cannot project an image of any of RGB colors on the entire surface. In this case, by sharing the area of the display image with two or more light guide plates and setting R + G + B ≧ 1, it is possible to project a full-screen image. By combining the four light guide plates indicated by the four arrows in the figure, full-screen image projection becomes possible. The four light guide plates correspond to, for example, a short wavelength region of B, a long wavelength region of B and a short wavelength region of G, a long wavelength region of G and a short wavelength region of R, and a long wavelength region of R. In any case, the visible wavelength region is shared by a plurality of diffraction gratings having different diffraction grating pitches in the range of the diffraction grating pitch of 300 nm to 550 nm.

Hereinafter, similarly, in the light guide plate of the present embodiment, the pitch and the number of diffraction gratings can be appropriately set according to the viewing angle of the projected image. Since the purpose of this embodiment is to provide the light guide plate at low cost, the refractive index of the plastic material used as the substrate material is about 1.7 at the maximum. When the refractive index of the substrate material increases, it is possible to maximize the image range to be displayed by selecting a diffraction grating pitch that is approximately inversely proportional to it. At this time, the range in which the pixel ratio becomes 1 also becomes large, and as a result, by selecting a substrate material having a large refractive index, it is possible to project an image with a large viewing angle. When limited to plastic materials, the maximum viewing angle that can be handled in this embodiment is about 80 degrees. In order to provide the light guide plate at low cost, it is desirable that the number of light guide plates is one. The relationship between the viewing angle and the pixel ratio at this time will be described in FIG. 10 and below.

10A to 10C are other simulation results showing the relationship between the emission diffraction grating pitch and the pixel ratio, and FIGS. 10A, B, and C show the case where the viewing angles are 30, 25, and 20 degrees, respectively. The arrows shown in the figure are conditions of the diffraction grating pitch suitable for projecting an RGB image with one light guide plate, and substantially correspond to the intersection of the characteristics of R and B. In the case of this example, when the refractive index of the substrate is 1.58, it is about plus or minus 25 nm (400 nm to 450 nm) centered on 425 nm. As can be seen in FIGS. 10A to 10C, according to this embodiment, it can be seen that an image can be projected up to approximately 25 degrees with one light guide plate. However, when the viewing angle becomes large as shown in FIGS. 10B to 10C, a part of the red image (R) or the blue image (B) may not be displayed.

11A and 11B are simulation results showing the pixel ratio when the viewing angle is 25 degrees. Here, the pixel ratio for each wavelength is displayed by luminance. In the figure, the area with high brightness is shown in white and the area with low brightness is shown in gray.

FIG. 11A shows the results of the substrate having a refractive index of 1.58 and a diffraction grating pitch of 425 nm. The white area 1101 in the figure indicates that all RGB images can be displayed. In the figure, the upper gray area 1102 shows the area where the blue image (B) is not displayed, and the lower gray area 1103 shows the area where the red image (R) is not displayed.

FIG. 11B shows the results of the substrate having a refractive index of 1.70 and a diffraction grating pitch of 395 nm, and it can be seen that an RGB image can be projected in all regions 1104. This corresponds to the fact that by increasing the refractive index of the substrate as described above, the angle range of the wave vector of the light beam that can be guided by total internal reflection in the light guide plate becomes large. At this time, it is preferable to select a diffraction grating pitch that is substantially inversely proportional to the refractive index, preferably inversely proportional to the square root of the refractive index. The results shown here determine the diffraction grating pitch based on the latter relationship.

In a light guide plate for a head-mounted display, the diffraction efficiency of the incident diffraction grating has a primary diffraction efficiency in the range of 10% to 80%, preferably in the range of 30% to 70% from the viewpoint of the brightness and power consumption of the projected image. Is desirable. On the other hand, since the emission diffraction grating carries out duplication of light rays and emission from the light guide plate over a wide area, it is 0.1% to 10%, preferably 0.2 to 7.5%, and more preferably 0. It is desirable to have a primary diffraction efficiency of 5-5%. The diffraction efficiency of the diffraction grating is determined by the pitch, height, and shape of the formed periodic uneven pattern. Mass-produced optical discs such as DVDs and BDs In order to create a light guide plate using process technology, it is necessary to discuss the suitability of process technology regarding the pitch, height, and shape of the diffraction grating. There is.

Polycarbonate materials are used in optical discs such as DVDs and BDs that are widely used. On the surface of the polycarbonate substrate formed by the injection molding method, a periodic uneven pattern with a depth of 10 to 100 nm is emitted at a pitch of 320 nm (25 GB capacity BD) to 740 nm (4.7 GB capacity DVD) as a guide groove for light spots. It is formed as a guide groove of a spot or as coded data. By coating the surface of a polycarbonate substrate with an optical thin film by a sputtering method, reproduction-only and rewritable optical discs are mass-produced, and the substrates are supplied at low cost.

In addition, some optical discs have a double-layered data surface to increase the recording capacity, and almost all DVDs and BDs of movies have two-layered data. In the second layer, an uneven pattern is formed on a polycarbonate substrate with a UV effect resin by a 2P (Photo Polymerization) method. By utilizing the mass production process of optical discs, light guide plates can be manufactured at low cost. At this time, the weight of polycarbonate is 1.2 compared to the specific density of glass of 2.5, so the weight can be reduced to 1/2 or less.

FIG. 12 summarizes the suitability for utilizing the mass production process technique of the optical disc for the light guide plate of this embodiment. The parameters of the light guide plate in the figure are based on the simulation results shown above. Appropriate values of these parameters change depending on the aspect ratio and viewing angle of the projected image, and it is clearly stated in the light guide plate of this embodiment that the appropriate parameters are not limited to the numerical values in the figure. Keep it. In the figure, regarding the material, thickness, refractive index, emission and incident diffraction grating pitch, it can be seen that the light guide plate of this embodiment is within the range of the conditions although it is mass-produced as an optical disk, and has high compatibility. .. This is because both use visible light as a light source and employ a diffraction grating (unevenness pattern) that is close to the diffraction limit in order to maximize image quality and recording capacity.

The emission diffraction grating can be realized with the same rectangular cross-sectional shape as the guide groove of the optical disk, and the height of the pattern is also the same, so that the injection molding process can be used as it is. Regarding the incident diffraction grating, the blaze type diffraction grating having a triangular cross-sectional shape with a height of about 200 to 400 nm is suitable for the light guide plate of this embodiment in order to improve the light utilization efficiency, and therefore the suitability of the injection molding method. It is judged to be slightly inferior. On the other hand, since the 2P method uses a UV curable resin having a low viscosity, the transferability of the pattern is excellent, and it can be said that the 2P method is suitable for forming an incident diffraction grating. The above-mentioned prototype element has a blaze-type diffraction grating formed by a 2P method on a polycarbonate optical disk substrate produced by an injection molding method, which has the basic functions of a light guide plate of waveguide and light beam duplication. Is shown. Therefore, it has been proved that it is possible to utilize the 2P method, which is a process technique of an optical disk, for forming an incident diffraction grating in the figure.

The brightness and image quality of the projected image are mainly dominated by the diffraction efficiency of the exit grating. Next, the relationship between the diffraction efficiency and the height of the uneven pattern will be described in more detail.

FIG. 13 shows the relationship between the height of the emission diffraction grating obtained by the electromagnetic field calculation (FDTD method; Fine Diffractive Time Domain method) and the diffraction efficiency. Here, the wavelength is 550 nm, the pitch of the pattern of the diffraction grating is 440 nm, and the shape of the grating is rectangular, corresponding to green, which is the center of the RGB wavelength band.

The height of the grating acts on the light beam as a phase difference. Reflecting this, the results show a damped vibration type periodic response. It can be seen that the practical region for the emission grating is the range of the height up to 200 nm in which the phase difference is approximately λ / 4 or less, and the maximum value of the diffraction efficiency is about 7%. From FIG. 13, the standard of the primary diffraction efficiency of the emission diffraction grating is about 0.1% to 7.5%, preferably about 0.5% to 5.0%. As described above, when the process technology of the optical disk is applied, it can be said that the height of the uneven pattern of the diffraction grating is about 100 nm, that is, the diffraction efficiency is about 4% or less.

14A and 14B are the results of calculating the relationship between the diffraction efficiency of the emission diffraction grating and the luminance and luminance unevenness of the projected image. The wavelength and the pitch condition of the emission diffraction grating were the same as above, and the angle of view of the projected image was 40 degrees. The thickness of the light guide plate corresponds to "Patent Document 1", that is, a general glass substrate thickness of 1.0 mm, a DVD substrate thickness of 0.6 mm, and a Blu-ray Disc cover layer thickness. There are three types with a diameter of 0.1 mm.

FIG. 14A shows the relationship between the diffraction efficiency of the emission grating and the brightness at the center of the projected image. The vertical axis in the figure is proportional to the light utilization efficiency. The three points indicated by the arrows in the figure are the conditions for maximizing the central brightness, and the diffraction efficiencies are 0.7% and 3.5, respectively, when the substrate thickness is 0.1 mm, 0.6 mm, and 1.0 mm, respectively. %, 5.5%.

It can be seen that the maximum brightness can be obtained with a large diffraction efficiency as the thickness of the light guide plate becomes thicker. This reflects the geometric condition that the number of intersections of the light beam and the emission grating is inversely proportional to the thickness of the light guide plate.

FIG. 14B shows the relationship between the diffraction efficiency of the emission diffraction grating and the RMS (Root Mean Square) value of the brightness of the projected image. The vertical axis in the figure is the relative value standardized by the central brightness. The three points indicated by the arrows in the figure are the conditions under which the RMS value of the luminance is the minimum, that is, the luminance unevenness is the minimum, and the diffraction efficiency is the same when the substrate thickness is 0.1 mm, 0.6 mm, and 1.0 mm, respectively. It is 0.3%, 1.7%, and 2.3%.

As described above, it can be seen that the conditions in which the central brightness and the RMS value of the brightness are the best do not match. Therefore, it is preferable to appropriately select the diffraction efficiency of the emission diffraction grating according to the application in which the luminance is prioritized and the application in which the luminance unevenness is prioritized.

As an example of the selection method, the diffraction efficiency is set between the condition where the brightness at the center of the image is maximized and the condition where the brightness unevenness is minimized at a predetermined light guide plate thickness. For example, under the condition that the angle of view of the projected image is 40 degrees, when the substrate thickness is 0.1 mm, the diffraction efficiency of the emission diffraction grating presented in this embodiment is between 0.3% and 5.5%. be. According to this embodiment, the appropriate diffraction efficiency of the emission diffraction grating can be determined by using the simulation method presented above according to the conditions such as the angle of view of the projected image and the diameter of the beam incident on the light guide plate. can.

From the above, it was found that in the light guide plate, in addition to the pitch and diffraction efficiency of the diffraction grating, the thickness of the substrate plays an important role in the image quality of the projected image. At the same time, it was shown that the central luminance and the luminance unevenness have a trade-off relationship with the diffraction efficiency of the emission grating. The purpose of this embodiment is to provide a light guide plate at low cost by utilizing the process technology of an optical disc.

In the explanation of FIG. 12, it was stated that the numerical values in the figure do not limit the technical scope of the present invention, but the reason is that the thickness of the substrate is an important parameter that affects the image quality. At the same time, as the light guide plate of this embodiment, if the purpose is to increase the diffraction efficiency of the emission diffraction grating and the height of the uneven pattern does not meet the condition range of the process technology, the substrate can be made thinner to effectively diffract. It was shown that it can be solved by increasing the efficiency.

FIG. 15 is a schematic diagram showing the entire process of forming a light guide plate corresponding to a plurality of wavelengths by utilizing the mass production process of an optical disc.
In (step 1), a master die 301 for an incident diffraction grating and a master die 302 for an emission diffraction grating are prepared from a Si master created by an EB drawing method or the like. Subsequently, in (step 2), the polycarbonate substrate 304 on which the emission diffraction grating 303 is formed is produced by the mother mold 302 for the emission diffraction grating and the injection molding method. In (step 3), a mother die 301 for an incident diffraction grating and an incident diffraction grating are formed by the 2P method to produce one light guide plate 1500. Finally, in (step 4), a plurality of light guide plates 1500 corresponding to each color of RGB are combined as needed to complete the light guide plate of this embodiment.

FIG. 16 is a schematic diagram showing the process of creating an incident diffraction grating of this embodiment. In the upper half of FIG. 16, first, a master (mother mold) 1601 of an incident diffraction grating in which an uneven pattern is formed on Si or the like in advance by using an EB drawing method or the like is prepared. The mother die 301R, which is a resin stamper for the 2P method, is prepared as follows. After dropping the UV curable resin 1602 selected in consideration of the transferability of clay and the pattern onto the master 1601, the transparent substrate 1603 is used to pressurize (A). Then, (B) exposure with ultraviolet rays and (C) peeling are sequentially performed to prepare the resin stamper 301R. As the UV exposure apparatus used here, a conveyor type exposure apparatus or the like can be selected.

The lower half of FIG. 16 shows a process of forming an incident diffraction grating 307R on a polycarbonate substrate 304R on which an emission diffraction grating is previously formed by an injection molding method. In order to improve the peelability of the resin stamper 301R, it is advisable to form a Ti film or the like as a metal peeling layer on the surface of the resin stamper in advance by about 30 nm.

After dropping the UV curable resin 1604 onto the substrate 304R on which the emission diffraction grating 303 is formed, (D) pressurization, (E) exposure with ultraviolet rays, and (F) peeling are sequentially performed to form an incident diffraction grating 307R. As the UV curable resin 1604 used here, a UV curable resin different from the above-mentioned UV curable resin 1602 can be used in consideration of the refractive index in addition to the peelability and transferability.

Since the refractive index of general UV curable resin is about 1.5 and the difference from the refractive index of polycarbonate is small, the phase difference generated by the uneven pattern of the base is small, and diffraction is suppressed to 1/10 or less. Will be done. However, when giving priority to image quality, it is preferable that the diffraction due to the uneven pattern of the emission diffraction grating 303, which is the base, is as close to zero as possible. In this case, it is effective to select a UV resin having a refractive index close to that of the substrate material.

Further, since the above-mentioned reflection coating is formed along the uneven pattern of the emission diffraction grating 303 formed later, it also has the effect of effectively reducing the influence of the uneven pattern of the underlying emission diffraction grating. Further, it is also effective to make a flat surface in the region where the incident diffraction grating 307R is formed without forming the emission diffraction grating 303 in advance. Furthermore, considering the adhesion between the UV resin of the incident diffraction grating 307 and the substrate material, a method of forming a narrow-pitch uneven pattern under conditions that do not diffract with respect to the visible light wavelength, for example, at about 240 nm or less is also effective. Have.

In the above-mentioned prototype element, the same UV resin was used for both. After that, the above-mentioned antireflection coating, antireflection coating, etc. are appropriately formed for the purpose of enhancing the see-through property when worn by the user. Further, it is easily applicable to preliminarily apply water-repellent, oil-repellent, and scratch-preventing treatments such as SiO ₂ nanoparticles, which are widely used in optical discs, to the surface of the substrate produced by injection molding.

Next, it is shown that the thickness of the light guide plate of this embodiment has an appropriate value according to the diameter of the beam incident on the light guide plate. Parameters other than the diameter of the incident beam and the diffraction efficiency of the exit diffraction grating are the same as above.

FIG. 17 shows the calculation result of the image projected on the retina when the diameter of the incident beam is 4 mmφ. Here, the primary diffraction efficiency of the emission diffraction grating is set to 1.7%. This is the result of changing the thickness T of the light guide plate to (a) 2.0 mm, (b) 1.0 mm, (c) 0.6 mm, (d) 0.3 mm, and (e) 0.1 mm. ..

As can be seen in FIG. 17A, when the thickness T of the light guide plate is 2.0 mm, it can be seen that the image duplication action by the emission diffraction grating is separated as a diffraction pattern. It can be seen that by making the light guide plate thinner, the interval between the diffraction patterns becomes smaller due to the geometrical reason described above, and the diffraction patterns overlap when the substrate thickness is 0.6 mm or less.

18A and 18B show the results of summarizing the RMS value and the maximum value of the luminance as the quality of the projected image.
As can be seen in FIG. 18A, it can be seen that the RMS value of the luminance is good as 10% or less in the range of the substrate thickness of 0.5 mm to 1.0 mm. On the other hand, as seen in FIG. 18B, the maximum brightness increases as the substrate thickness becomes thinner, and becomes maximum when the thickness of the light guide plate is 0.2 mm. Similar to the above diffraction efficiency, regarding the thickness of the light guide plate, there is a trade-off relationship between the maximum luminance and the uneven luminance, and it is possible to select a suitable value according to the application.

FIG. 19 shows the calculation result of the image projected on the retina when the diameter of the incident beam is 1 mmφ. This is the result of changing the thickness T of the light guide plate to (a) 2.0 mm, (b) 1.0 mm, (c) 0.6 mm, (d) 0.3 mm, and (e) 0.1 mm. ..

Since the size of the projection optical system can be considered to be roughly proportional to the diameter of the incident beam, the results shown here are for the case where the light guide plate of this embodiment is applied to a small head-mounted display. Here, the primary diffraction efficiency of the emission diffraction grating is set to 0.85%. Since the propagation angle of light rays is determined by the pitch of the diffraction grating, the angle of the incident light, and the wavelength, if these are constant, the number of times the light rays intersect the exit diffraction grating before being projected onto the retina is inversely proportional to the substrate thickness. Therefore, by setting the diffraction efficiency of the emission diffraction grating in substantially proportional to the substrate thickness, the light utilization efficiency can be made substantially constant.

As can be seen in FIG. 19A, when the thickness of the light guide plate is 2.0 mm, it can be seen that the image duplication action by the emission diffraction grating is separated as a diffraction pattern. It can be seen that by making the light guide plate thinner, the interval between the diffraction patterns becomes smaller, and the diffraction patterns overlap when the thickness of the light guide plate is 0.6 mm or less.

20A and 20B show the results of summarizing the RMS value and the maximum value of the luminance. As seen in FIG. 20A, it can be seen that the RMS value of the luminance is good as 10% or less in the range of the substrate thickness of 0.2 mm to 0.5 mm. On the other hand, as seen in FIG. 20B, the maximum brightness increases as the thickness of the light guide plate becomes thinner. From these relationships, it is also possible to select a suitable thickness of the light guide plate according to the application.

As described above, the light guide plate of this embodiment is a head mount having high brightness and small brightness unevenness by appropriately selecting the thickness of the light guide plate and the diffraction efficiency of the emission diffraction grating according to the diameter of the incident beam. A light guide plate for a display can be provided. Here, the operation of this example is presented when the angle of view is as wide as 40 degrees. In this case, in the range where the diameter of the incident beam is in the range of 1 mm to 4 mm, the appropriate thickness of the light guide plate is 0.2 mm to 1.0 mm, and there is a roughly proportional relationship between the two. According to this embodiment, when the diameter of the incident beam exceeds the range illustrated here, the appropriate thickness of the light guide plate can be easily obtained by using this proportional relationship. More specifically, it is possible to obtain an appropriate value by using the simulation method presented in this embodiment as in the above.

As shown in FIGS. 17 to 20 of Example 4, the thickness of the light guide plate and the diffraction efficiency of the emission diffraction grating suitable for this embodiment are the diameter of the beam which is a group of light rays including the image information incident on the light guide plate. It was shown that there is an appropriate value according to. The wave number of the light beam propagating while totally reflecting inside the light guide plate is determined by the wavelength, the angle of incidence, the refractive index of the light guide plate, and the pitch of the incident diffraction grating, and is expressed by equations (2) to (7). When the diffraction order _mn = 1, the propagation angle θ _p0 when the light ray containing the image information is perpendicularly incident on the incident diffraction grating of the light guide plate is expressed by the following equation (12).

Here, n _s is the refractive index of the substrate (light guide plate), P is the pitch of the diffraction grating, and λ is the wavelength of the light beam. According to the above example, assuming that the wavelength is 550 nm, the pitch of the diffraction grating is 440 nm, and the refractive index of the light guide plate is 1.58, θ _p0 is 52.3 °. When the thickness of the light guide plate is t, the distance L between the points where the light beam and the emission diffraction grating intersect is given by the following equation (13).

FIG. 21 is an example showing the relationship between the thickness of the light guide plate (board) and the beam spacing L. Excluding the effect of the wavelength spread of the light source, when the diameter of the beam incident on the light guide plate is smaller than L, the image information recognized by the user is a discontinuous bright spot as shown in FIG. 17 (a). It becomes a pattern. That is, FIG. 21 shows that the image quality deteriorates as the thickness of the light guide plate increases, assuming that the diameter of the beam is constant.

Here, in order to show the relationship between the thickness of the light guide plate, the diffraction efficiency of the emission diffraction grating, and the quality of the image recognized by the user, the case where the diameter of the incident beam is equal to L will be considered. By performing the same calculations as illustrated in FIGS. 17 to 20, the diffraction efficiency of the exit diffraction grating that maximizes the brightness with respect to the thickness of the light guide plate and the brightness unevenness (luminance RMS value) are minimized. The diffraction efficiency of the emission diffraction grating was determined for each.

The results are shown in FIG. As shown in FIG. 13, the maximum value of the diffraction efficiency when the emission diffraction grating is rectangular is 7.5%. Further, from the viewpoint of utilizing the optical disc process of the present embodiment, the lower limit of the thickness of the light guide plate is realistic from the viewpoint of feasibility of a cover layer thickness of 0.1 mm for the Blu-ray Disc. In the figure, the area shown by the diagonal line is the range of the diffraction efficiency from the maximum brightness to the minimum brightness unevenness, the maximum value of the diffraction efficiency is 7.5% or less, and the lower limit of the thickness of the light guide plate is 0. The range is 1 mm. That is, the shaded area in FIG. 22 shows in more detail the range of the thickness of the light guide plate and the diffraction efficiency of the emission diffraction grating suitable for this embodiment.

From FIG. 22, it can be seen that the thickness of the light guide plate suitable for this embodiment is in the range of 0.1 mm to 4.0 mm when the diffraction efficiency of the emission diffraction grating is appropriately set according to the application. As described above, the preferred range of the diffraction efficiency of the emission diffraction grating is 0.1% to 10%, preferably 0.2, from the viewpoint of performing the duplication of light rays and the emission from the light guide plate over a wide area. Although it is desirable to have a primary diffraction efficiency of% to 7.5%, as shown in FIG. 22, the thickness of the light guide plate is in the range of 0.1 mm to 4.0 mm, which is in the range of suitable diffraction efficiency. The calculation result is 0.2% to 7.5%, and it can be seen that it corresponds to the above range. It is more preferably 0.5% to 5.0% because the duplication of light rays and the emission from the light guide plate are carried out over a wide area. In this case, the thickness of the substrate can be reduced to about 0.5 mm to 3 mm.

As described above, each parameter suitable for this embodiment is shown for the light guide plate having a rectangular shape as the cross-sectional shape of the emission diffraction grating using polycarbonate as the material of the light guide plate. In this embodiment, an acrylic resin or a plastic material having a high refractive index generally used for spectacle lenses (refractive index ≧ 1.70) can be used as a material for the light guide plate. Further, as the cross-sectional shape of the emission diffraction grating, any shape such as a sine wave shape, a triangle, or a trapezoid can be used. It is considered that the influence of the refractive index of the light guide plate and the cross-sectional shape of the emission diffraction grating is a region shift of about plus or minus 20% with respect to the range suitable for carrying out this embodiment shown in FIG.

By the way, the light guide plate duplicates the light rays including the incident image information while totally reflecting and guiding the light rays inside, and emits the light rays that can be seen by the user. Strength is required. This is because if the light guide plate bends and the parallelism between the incident diffraction grating and the exit diffraction grating is impaired, the image information recognized by the user is distorted by the angle between the two. In the above, the practical lower limit of the thickness of the light guide plate suitable for this embodiment is 0.1 mm, which is the thickness of the cover layer of the Blu-ray Disc. It is inevitable that the 0.1 mm thick plastic material will bend under its own weight.

The configuration of the light guide plate of this embodiment to avoid this is shown below. The calculations so far have been obtained for the case where an emission diffraction grating is formed on one side surface of the light guide plate. At this time, the interval L at which the light beam and the emission diffraction grating intersect is expressed by Eq. (13). When D << L with respect to the diameter D of the incident beam, the visually recognized image has a discontinuous bright spot pattern. As shown in FIG. 17, the image quality is improved because L is proportionally reduced by reducing the thickness t of the light guide plate.

FIG. 23 is an example showing the configuration of the light guide plate of this embodiment. Here, only the emission diffraction grating is shown, and an example in which one plastic substrate and four emission diffraction gratings are integrated is shown. In the figure, 11 is a substrate, 12, 14, 16 and 18 are emission diffraction gratings, and 13, 15 and 17 are transparent spacer layers.

According to this configuration, the spacing between the exit diffraction gratings 12, 14, 16 and 18 coincides with the spacing between the spacer layers, which is 0.1 mm thick. The thickness t of the light guide plate in (Equation 13) is 1/2 of the distance 2t moved in the thickness direction of the light guide plate before the light ray and the emission diffraction grating intersect. Therefore, in this configuration, the minimum value of t can be matched with the thickness of the spacer layer, and it can be seen from the above discussion that it is effective in improving the quality of the image.

The configuration of the light guide plate of this embodiment shown in FIG. 23 is basically the same as the standard of the multilayer Blu-ray Disc, and it is easy to create it by using the same process. At this time, the emission diffraction grating 12 is formed on the substrate 11 by the injection molding method, and the spacer layer 13 and the emission diffraction grating 14 are formed on the spacer layer 13 by the 2P method. By repeating this, the spacer layer 15, the emission diffraction grating 16, the spacer layer 17, and the emission diffraction grating 18 may be sequentially formed. Further, in the multi-layer ROM medium of Blu-ray Disc or DVD, after forming each information holding layer (corresponding to the emission diffraction grating of this embodiment) on which the diffraction grating pattern is formed, a dielectric reflective film, a thin metal thin film, or the like is formed. It is formed and the reflectance is controlled to a predetermined value.

In the case of the light guide plate of this embodiment, the diffraction efficiency can be controlled by forming a dielectric reflective film or a thin metal thin film on the emission diffraction grating. At this time, since the diffraction efficiency of the emission diffraction grating is substantially proportional to the reflectance of the reflective film formed from the wave theory, the film thickness can be easily designed. This embodiment is an extension of the concept of the thickness of the light guide plate. By forming two or more layers of emission diffraction gratings formed on a transparent plastic substrate, emission diffraction is performed while maintaining mechanical strength. By reducing the spacing in the thickness direction of the grating, the quality of the image recognized by the user is improved. The upper limit of the number of layers of the emission diffraction grating that can be formed by this embodiment is approximately eight layers.

24A and 24B are simulation results showing an incident diffraction grating suitable for this embodiment. Here, the calculation was performed assuming that the wavelength of the light source was 550 nm, the pitch of the diffraction grating was 440 nm, and the refractive index of the substrate was 1.58. From the viewpoint of light utilization efficiency, the incident diffraction grating is required to have high diffraction efficiency.

The overhanging triangular diffraction grating described in "Patent Document 2" is an excellent technique capable of coupling incident light to a light guide plate with high efficiency by transmission diffraction. On the other hand, it is difficult to create an overhanging shape by the method for creating an incident diffraction grating shown in FIGS. 16A and 16B. Here, we examined a blaze-type diffraction grating that can be produced by the injection molding method, the 2P method, or the like and has a normal triangular shape that does not overhang.

FIG. 24A is a simulation result of the same transmission type diffraction grating as in “Patent Document 2”. The image ray 2401 is configured to be incident from the left, and the right half of the figure represents the substrate 2402. On the surface of the substrate 2402, a triangular uneven pattern showing a diffraction grating can be seen. In the transmission type diffraction grating, the maximum diffraction efficiency can be obtained under the condition that the refraction due to the blaze plane and the diffraction due to the periodic structure are phase-tuned. As shown in the figure, in order to realize this, the height of the uneven pattern needs to be large, the angle of the pattern needs to be 70 to 80 degrees, and the aspect ratio obtained by dividing the height of the pattern by the cycle needs to be 10 or more. be. In general, in the transfer of uneven patterns by the 2P method, if the aspect ratio exceeds 2, problems such as peelability occur and the yield at the time of mass production decreases. It can be seen that the transmission type diffraction grating shown here is not suitable as the incident diffraction grating of this embodiment.

FIG. 24B is a simulation result of a reflection type diffraction grating. Similarly, the image ray 2401 is configured to be incident from the left, and the left half of the figure represents the substrate 2402. On the surface of the substrate 2402, a triangular uneven pattern showing a diffraction grating can be seen. In the reflection type diffraction grating, the maximum diffraction efficiency can be obtained under the condition of phase tuning by reflection by the blaze surface and diffraction by the periodic structure. As can be seen in the figure, it can be seen that this condition is satisfied by the uneven pattern having a lower aspect ratio than the transmissive type. The height of the uneven pattern at this time is about 250 nm, and the aspect ratio is about 0.57. In the above-mentioned prototype element, it was possible to transfer a triangular uneven pattern having a pattern height of 374 nm satisfactorily. It can be said that it is preferable to use a reflection type diffraction grating for the light guide plate of this embodiment.

From the results shown in FIGS. 14A and 14B, it was shown that when the diffraction efficiency of the emission diffraction grating is constant, there is a trade-off relationship between the brightness of the projected image and the brightness unevenness. Here, the configuration of an emission diffraction grating suitable for the light guide plate of this embodiment, which has high luminance and small luminance unevenness, is shown. Below, the calculation was performed assuming that the wavelength of the light source was 550 nm, the pitch of the diffraction grating was 440 nm, the refractive index of the substrate was 1.58, and the thickness of the substrate was 0.6 mm.

25A and 25B are simulation results that visualize the intensity distribution of the light rays propagating inside the light guide plate. In the figure, the incident diffraction grating is arranged on the lower side in the y direction, and the pupil corresponding to the user's eye is arranged on the upper side in the y direction. FIG. 25A shows the intensity distribution of light rays reaching the center of the projected image. The emission circle O in the figure indicates the region where the light ray reaching the pupil was last diffracted on the emission diffraction grating. The region with high brightness on the straight line from the incident diffraction grating toward the y direction indicates the main ray group (hereinafter referred to as the main ray group) diffracted by the incident diffraction grating and propagating inside the light guide plate. The propagation direction of the main ray group is indicated by arrow 251.

As can be seen in FIG. 25A, it has a characteristic that the intensity is gradually attenuated by the propagation of the main ray group. The low-luminance ray group spreading around the main ray group is a ray group whose traveling direction is deflected in the xy plane by being diffracted by the emission diffraction grating. Under this condition, it can be seen that the exit circle O and the pupil coincide with each other in the xy plane. Therefore, it is a part of the strong main ray group that reaches the pupil and is recognized as an image.

FIG. 25B shows the intensity distribution of light rays reaching the lower right corner of the projected image. Here, in order to facilitate understanding, the effect of recognizing the image inverted by the lens action of the eye and the image projected on the retina by processing it in the brain and further inverting it is omitted, and it is recognized at the lower right corner of the visual field. The image is displayed as having a wave number vector toward the lower right corner.

As can be seen in the figure, the main ray group travels from the incident diffraction grating toward the upper right. Further, it can be seen that the emission circle O and the pupil P do not match in the xy plane, and the ray that reaches the pupil and is recognized as an image is deflected in the upper left direction from the main ray group. Therefore, the intensity of the light beam that reaches the pupil P and is recognized as an image is lower than that in the case of FIG. 25A. The above is one of the reasons why the luminance unevenness occurs when the image is projected using the light guide plate. In addition, the diameter of the incident video beam, the crosstalk between the RGB light guide plates, the -1st order diffraction of the incident diffraction grating, and the scattering due to the non-uniform width of the pitch of the diffraction grating and the height of the uneven pattern cause uneven brightness. It is a big cause. In this embodiment, a method for improving the luminance unevenness due to the disagreement between the main ray group and the emission circle shown in the figure is provided below.

26A and 26B are schematic diagrams showing a method for improving the luminance unevenness of the projected image according to the present embodiment. In the figure, the solution is shown in an exemplary manner by superimposing the simulation results of FIGS. 25A and 25B. In the figure, 100 is a light guide plate, and 210 and 220 are diffraction efficiency enhancing regions provided in a part of the emission diffraction grating. As shown in the figure, the traveling direction of the main ray group propagating in the light guide plate differs depending on the position of the visually recognized image.

Utilizing this, in the case of FIG. 26B, the main light group can be strongly diffracted in the direction of the emission circle by the diffraction efficiency enhancement region 210, so that the intensity of the light rays reaching the emission circle can be increased, and the luminance unevenness can be increased. Can be improved.

As shown in FIG. 26A, it is preferable that the diffraction efficiency enhancing region is formed in a region that does not coincide with the main ray group toward the center of the image. This can be realized by controlling the duty ratio and height of the unevenness forming the grating when creating a master with the above-mentioned EB drawing device or the like.

The light guide plate described in FIG. 3 of "Patent Document 1" includes a waveguide and three linear diffraction gratings H0, H1 and H2. The incident light beam illuminates the diffraction grating H0, and this light is coupled to the waveguide. The two lattices H1 and H2 formed in the waveguide overlap each other. Utilizing the findings of "Patent Document 1", this embodiment can also be realized by forming only the diffraction grating H1 in the diffraction efficiency enhancing region 210 and forming only the diffraction grating H2 in the diffraction efficiency enhancing region 220. In an EB drawing device, it is easy to pattern H1 and H2 in the same plane to form a diffraction grating having two wave vector, and this book aims to provide a light guide plate at low cost by an injection molding method. In the embodiment, it can be said that the light guide plate having the emission diffraction grating formed on one surface is suitable in that the process of the optical disk is utilized.

FIG. 27 is another schematic diagram showing a method for improving the luminance unevenness of the projected image according to the present embodiment. Here, a solution focusing on the intensity attenuation in the y direction due to the propagation of the main ray group in FIG. 25A is provided. FIG. 27 shows the result of calculating the relationship between the pixel position in the y direction of the projected image and the brightness under the same conditions as above. As can be seen in the figure, when the emission diffraction grating has a uniform diffraction efficiency in the plane, the pixel position in the y direction increases according to the propagation length from the incident diffraction grating to the emission circle (projected image). According to the upper side), the luminance attenuation of the image occurs (characteristic 2701). On the other hand, when this attenuation is gradual, the luminance unevenness of the projected image can be improved by correcting the diffraction efficiency of the emission diffraction grating in the plane by linear approximation (characteristic 2702).

FIG. 28 is a schematic diagram showing a light guide plate of this embodiment that reduces luminance unevenness of an image formed by a projector 281 that is an image forming means. In the figure, 300 is a light guide plate, 307 is an incident diffraction grating, and 303 is an emission diffraction grating. An enlarged view of the uneven pattern of the emission diffraction grating is also shown. As can be seen in the figure, the luminance unevenness shown in FIG. 27 can be improved by giving a distribution to the diffraction efficiency by changing the duty of the uneven pattern of the emission diffraction grating in the y direction (light propagation direction). It will be possible.

When semiconductor process technology is applied to create a master, the resist is exposed to a predetermined pattern by an EB drawing method or a reduced exposure method, and then cleaning and etching are performed in sequence to form an uneven pattern on the surface of the Si substrate. .. At this time, the thickness of the resist is uniformly applied by a spin coating method or the like, and the electrons and photons used for exposure have an absorptivity distribution in the thickness direction of the resist, so that the uneven pattern is uniform. It is preferable to have a high height. As described above, the diffraction efficiency of the diffraction grating is controlled according to the phase difference given by the unevenness pattern, which can also be controlled by changing the duty of the unevenness pattern. From the above, it can be said that a method of in-plane modulation of the duty of the uneven pattern is suitable as a method of imparting an in-plane distribution of diffraction efficiency suitable for the light guide plate of the present embodiment.

FIG. 29 is another embodiment using the light guide plate of this embodiment. The light guide plate 300 of this embodiment can project an image even when the user's eyes are on the opposite side of the projector 281 due to the symmetry of diffraction by the diffraction grating. In this case, the user can easily recognize the image by mirror-inverting the projection pattern of the liquid crystal element in the projector (not shown).

FIG. 30 is a schematic diagram showing the configuration of the image display device of this embodiment. The light having the image information emitted from the projector 281 in the figure is delivered to the user 400 by the action of the light guide plates 300B, 300G, and 300R of B, G, and R, and realizes augmented reality. Each of the light guide plates B, G, and R is a combination of the light guide plates of the embodiment shown in FIG. 3 as shown in FIG. 15, and the pitch and depth of the formed diffraction grating correspond to each color. It is optimized.

In FIG. 30, the image display device of this embodiment includes light guide plates 300B, 300G, 300R, a projector 281 as an image forming means, and a display image control unit (not shown). Here, as shown in FIG. 15, the light guide plate module 3001 integrates the light guide plates of R, G, and B corresponding to the color display. The image forming means includes, for example, an image forming apparatus composed of a reflective or transmissive spatial light modulator, a light source and a lens, an image forming apparatus using an organic and inorganic EL (Electro Luminescence) element array and a lens, and light emission. A widely known image forming apparatus such as an image forming apparatus using a diode array and a lens and an image forming apparatus combining a light source and a semiconductor MEMS mirror array and a lens can be used. Further, it is also possible to use an LED or a laser light source in which the tip of the optical fiber is resonantly moved by MEMS technology, PZT or the like. Of these, the most common is an image forming apparatus consisting of a reflective or transmissive spatial light modulator, a light source, and a lens. Here, examples of the spatial light modulator include a transmissive or reflective liquid crystal display device such as LCOS (Liquid Crystal On Silicon) and a digital micromirror device (DMD), and the white light source is separated into RGB as the light source. It is also possible to use an LED or a laser corresponding to each color. Further, the reflection type spatial light modulator reflects a part of the light from the liquid crystal display device and the light source and guides the light to the liquid crystal display device, and passes a part of the light reflected by the liquid crystal display device. It can be configured to consist of a polarizing beam splitter that leads to a collimating optical system using a lens.

Examples of the light emitting element constituting the light source 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 based on the specifications required for the image display device, and as specific values of the number of pixels, in addition to the 1280x720 shown above, 320 × 240, 432 × 240, 640 × 480. 1024 × 768 and 1920 × 1080 can be exemplified.

In the image display device of this embodiment, the light rays including the image information emitted from the projector 281 are positioned so as to be applied to the incident diffraction gratings of the light guide plates 300B, 300G, and 300R, and integrated with the light guide plate module 3001. It is formed by being transformed.

Further, the display image control unit (not shown) controls the operation of the projector 281 and functions to provide the user 400 with image information as appropriate.

In this embodiment, the case of providing image information to the user is shown, but the image display device of this embodiment also includes a touch sensor, a temperature sensor, an acceleration sensor, etc. for acquiring information on the user and the outside world. It is possible to equip various sensors and an eye tracking mechanism for measuring the movement of the user's eyes.

According to the above-described embodiment, the light guide plate (image display element) having the concave-convex diffraction grating described in "Patent Document 1" to "Patent Document 3", which is advantageous for realizing a wide viewing angle, and the light guide plate (image display element) thereof are used. It is possible to reduce the cost and weight of the image display device that has been used and provide it to the user.

According to the above-described embodiment, it is possible to provide a light guide plate that realizes cost reduction by utilizing the optical disc manufacturing technique and weight reduction by plasticization. As a guide, the cost is about 1/10 to 1/100, and the weight is about 1/2 or less. In addition, due to the plasticization of the light guide plate, when the user wears it, if it receives an impact due to an unexpected external force, the plastic has the property of being less likely to scatter than glass, so it is ancillary that safety is improved. The effect is also realized.
Further, as shown in FIG. 1, the head-up display (HUD) for in-vehicle use described in Patent Document 6 has almost the same structure as the head-mounted display of the present invention, and provides appropriate information to the driver. Can be presented. Therefore, the light guide plate of the present invention can be easily applied to a head-up display.

300: Light guide plate 307: Incident diffraction grating 303: Emission diffraction grating

Claims (15)

A first member made of a first resin forming a waveguide ,
A second member made of a second resin formed on at least a part of the first member, and a second member.
Equipped with
The first resin and the second resin are different resins.
A first diffraction grating composed of a first pattern is formed on the first member as an emission diffraction grating .
A second diffraction grating composed of a second pattern is formed on the second member as an incident diffraction grating .
The first pattern and the second pattern are different patterns.
Light guide plate. The first pattern and the second pattern are
At least one of the shape, aspect, height, duty ratio , patterning area area is different,
The light guide plate according to claim 1. The first resin is polycarbonate or acrylic resin,
The second resin is a photocurable resin.
The light guide plate according to claim 1. The pitch of the first pattern is 400 nm to 450 nm.
The light guide plate according to claim 1. The first resin is polycarbonate,
The pitch of the first pattern is 360 nm to 520 nm.
The height of the first pattern is 20 nm to 100 nm.
The light guide plate according to claim 1. The second resin is a photocurable resin,
The second pattern is a triangle,
The pitch of the second pattern is 360 nm to 520 nm.
The height of the second pattern is 100 nm to 400 nm.
The light guide plate according to claim 1. The first member constitutes a substrate having a thickness of 0.1 mm to 4.0 mm.
The primary diffraction efficiency of the first diffraction grating is 0.1% to 7.5%.
The light guide plate according to claim 1. The primary diffraction efficiency of the first diffraction grating is 0.5% to 5.0%.
The light guide plate according to claim 7. A diffraction efficiency enhancing region is provided in a part of the first diffraction grating.
The light guide plate according to claim 1. The first diffraction grating gives a distribution to the diffraction efficiency by changing the duty ratio of the first pattern along the light propagation direction.
The light guide plate according to claim 1. The primary diffraction efficiency of the second diffraction grating is 30% to 70%.
The light guide plate according to claim 1. A plurality of the light guide plates according to claim 1 are provided, and each of the plurality of light guide plates has the first pattern different from each other corresponding to a predetermined wavelength, and is formed by laminating the plurality of the light guide plates.
Light guide plate module. The first pattern of the plurality of light guide plates is set to different pitches in the range of a pitch of 300 nm to 550 nm.
The light guide plate module according to claim 12. The light guide plate according to claim 1 and
Equipped with a projector that projects images
The emitted light of the projector is arranged so as to illuminate the second pattern.
Image display device. The first step of preparing a substrate which is formed of polycarbonate or acrylic resin and has a first diffraction grating pattern on the surface, and
A second step of arranging the photocurable resin on the first diffraction grating pattern, and
A third step of transferring the second diffraction grating pattern to the photocurable resin, and
The fourth step of curing the photocurable resin and
A method for manufacturing a light guide plate, which comprises.

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