JP2020008599A - Light guide plate, light guide plate module, image display device, and method of manufacturing light guide plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce cost and weight by using a light guide plate made of glass. A light guide plate including 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. 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 the pattern is formed. The first pattern and the second pattern are different patterns. [Selection diagram] FIG. 16

Description

  The present invention relates to an image display device and a device that combine a light guide plate and a diffractive element, are small and light, and can display augmented reality.

  With an augmented reality image display device, a user can simultaneously see the surroundings as well as the projected image. The projected image may overlap the real world perceived by the user. Other uses for these displays include video games and wearable devices such as glasses. The user can visually recognize an image supplied from the projector over the real world by wearing glasses or a goggle-shaped image display device in which the translucent light guide plate and the projector are integrated.

  Some of such image display devices are described in Patent Literature 1 to Patent Literature 3. In these patent documents, the light guide plate is composed of a plurality of uneven diffraction gratings formed on a glass substrate. Light emitted from the projector is coupled to the light guide plate by the incident diffraction grating, and propagates 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 beams duplicated by another diffraction grating, and finally emitted from the light guide plate. A part of the emitted light beam is imaged on the retina via the user's pupil, and is recognized as an augmented reality image overlapping a real world image.

  In such a light guide plate using an uneven diffraction grating, the wave vector K of the light beam emitted from the projector is refracted into the light guide plate and the wave vector becomes K0 according to Snell's law. Further, the light is converted into a wave vector K1 capable of total reflection propagation inside the light guide plate by the diffraction grating for incidence. The diffracted light is affected by one or more other diffraction gratings provided on the light guide plate, and the wave vector changes each time the diffraction is repeated as indicated by K2, K3,.

  Assuming that the wave vector of the light beam finally emitted from the light guide plate is K ', | K' | = | K |, and when 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 via the light guide plate, the light guide plate has the same action as the reflection mirror with respect to the wave number vector, the normal vector of the light guide plate is taken in the z direction, and the wave number vector x, y , Z components can be expressed as Kx ′ = Kx, Ky ′ = Ky, Kz ′ = − Kz.

  The function of the light guide plate is to guide the light beam emitted from the projector while replicating the light beam into a plurality of light beams, so that the emitted light beams are recognized by the user as image information equivalent to the original image. The magnitude of the wave number vector varies depending on the wavelength of the light beam. Since the concavo-convex type diffraction grating has a constant wave number vector, the diffracted wave number 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 conducting light while total reflection differs depending on the wavelength of the incident light beam. For this reason, in order to allow a user to recognize an image with a wide viewing angle, it is necessary to stack a plurality of light guide plates that are different for each wavelength. Generally, it is considered that the number of light guide plates is optimal for each of R, G, and B, or about plus or minus one.

  The image display device described in Patent Literature 1 is an image display device for expanding input light in two dimensions, and includes three linear diffraction gratings. One is a diffraction grating for incidence, and the other two diffraction gratings are typically arranged on the front and back surfaces of the light guide plate so as to overlap each other, and perform the functions of a diffraction grating for duplication and an emission.

  In the image display devices described in “Patent Document 2” and “Patent Document 3”, three diffraction gratings for incidence, X-direction duplication, and Y-direction duplication and emission have overlapping regions in the light guide plate. Are located without. Patent Document 3 discloses an overhanging triangular diffraction grating to increase the diffraction efficiency of an incident diffraction grating.

  Patent Literature 4 and Patent Literature 5 disclose techniques using two reflection-type volume holograms for incidence and emission as diffraction gratings formed on a light guide plate. In these, the volume hologram is formed by multiplexing diffraction gratings corresponding to a plurality of wavelengths in a space, and is different from the uneven diffraction gratings of “Patent Document 1” to “Patent Document 3”. Diffracts light rays of wavelengths at the same angle. Therefore, the RGB image can be recognized by the user with one light guide plate. On the other hand, the concave and convex diffraction gratings of Patent Documents 1 to 3 can realize a wide viewing angle because light rays are duplicated in two-dimensional directions of X and Y, whereas Patent Documents 4 and 5 disclose light beams. In Patent Document 5, since only the function of one-dimensional duplication in the X direction is provided, the viewing angle is relatively narrow.

JP-T-2017-528739 US2016 / 0231566A1 WO99 / 52002A1 JP 2007-94175 A JP 2013-200467 A JP-A-2014 / 156167

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

  Patent Document 2 relates to a diffraction grating, and as described in paragraph 0044, a surface relief grating SRG (= concavo-convex type diffraction grating) is used for etching and / or etching a substrate to produce a desired periodic fine structure on the substrate. Or a technique manufactured by a suitable "microfabrication process" including deposition. It is said that the substrate may be an optical component itself or a production master such as a mold for producing the optical component. FIGS. 5B to 5C of “Patent Document 2” show an overhanging triangular diffraction grating suitable for an incident diffraction grating.

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

  The light guide plate disclosed in these techniques uses a glass substrate, and the diffraction grating is formed by a “fine processing process” such as etching or lithography widely used in semiconductor processing. In the etching process of the “fine processing 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 glass substrate etching process includes a vacuum process and the like, there is a limit in reducing the manufacturing cost. Further, as seen in recent examples of glasses, glass materials are heavy, and plastic lenses have become mainstream for users to wear comfortably.

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

  One preferred aspect of the present invention is a light guide plate including 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. is there. In this light guide plate, the first resin and the second resin are different resins, a first member is provided with a first diffraction grating having a first pattern, and a second member is provided with a second diffraction grating. A second diffraction grating consisting of a pattern is formed. The first pattern and the second pattern are different patterns.

  Another preferable aspect of the present invention includes a plurality of the light guide plates described above, each of the plurality of light guide plates having a first pattern different from each other corresponding to a predetermined wavelength, and stacking the plurality of light guide plates. It is a formed light guide plate module.

  Another preferred aspect of the present invention is an image display device including the light guide plate described above and a projector that projects an image, wherein the light emitted from the projector is arranged to irradiate the second pattern. .

  Another preferred aspect of the present invention is a first step of preparing a substrate formed of polycarbonate or acrylic resin and having a first diffraction grating pattern on the surface, and a step of photo-curing on the first diffraction grating pattern. A light guide plate comprising: a second step of disposing a resin; a third step of transferring a 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, cost and weight can be reduced by using a glass light guide plate.

FIG. 3 is a schematic side view showing the definition of the shape parameter of the diffraction grating. FIG. 3 is a schematic side view showing the definition of the shape parameter of the diffraction grating. 5 is a flowchart showing a method for manufacturing the light guide plate of the embodiment. FIG. 7 is a schematic view illustrating a method for manufacturing the light guide plate of the example. FIG. 7 is a schematic view illustrating a method for manufacturing the light guide plate of the example. FIG. 7 is a schematic view illustrating a method for manufacturing the light guide plate of the example. FIG. 7 is a schematic view illustrating a method for manufacturing the light guide plate of the example. FIG. 7 is a schematic view illustrating a method for manufacturing the light guide plate of the example. Photo of the appearance of the prototype device. Diffraction pattern image of evaluation result of prototype element. Diffraction pattern image of evaluation result of prototype element. FIG. 9 is a graph showing evaluation results of a prototype element. Beam pattern image of evaluation result of prototype element. FIG. 6 is a graph showing a comparison result between a measurement result of the first-order diffraction efficiency and a diffraction model. FIG. 4 is a graph showing the relationship between the pitch of the diffraction grating and the pixel ratio. FIG. 4 is a graph showing the relationship between the pitch of the diffraction grating and the pixel ratio. FIG. 4 is a graph showing the relationship between the pitch of the diffraction grating and the pixel ratio. FIG. 9 is a graph showing another simulation result showing the relationship between the diffraction grating pitch and the pixel ratio. FIG. 9 is a graph showing another simulation result showing the relationship between the diffraction grating pitch and the pixel ratio. FIG. 9 is a graph showing another simulation result showing the relationship between the diffraction grating pitch and the pixel ratio. FIG. 9 is an image diagram showing a simulation result indicating a pixel ratio in a case where a viewing angle is 25 degrees. FIG. 9 is an image diagram showing a simulation result indicating a pixel ratio in a case where a viewing angle is 25 degrees. FIG. 9 is a table showing results summarizing suitability for utilizing the optical disc mass production process technology for the light guide plate of the embodiment. FIG. 4 is a graph showing the relationship between the height of an output diffraction grating and diffraction efficiency obtained by electromagnetic field calculation. FIG. 9 is a graph showing the result of calculating the relationship between the diffraction efficiency of an output diffraction grating and the brightness of a projected image. FIG. 9 is a graph showing the result of calculating the relationship between the diffraction efficiency of an output diffraction grating and the image quality of a projected image. FIG. 4 is a schematic side view showing the entire process of forming the light guide plate of the embodiment utilizing the mass production process of the optical disc. FIG. 3 is a schematic side view showing a process of producing the incident diffraction grating of the embodiment. FIG. 11 is a diagram illustrating a calculation result image of an image projected on the retina when the diameter of an incident beam is 4 mmφ. FIG. 7 is a graph showing calculation results summarizing RMS values of luminance when the diameter of an incident beam is 4 mmφ. FIG. 9 is a graph of a calculation result summarizing the maximum value of luminance when the diameter of an incident beam is 4 mmφ. FIG. 9 is a diagram illustrating a calculation result image of an image projected on the retina when the diameter of an incident beam is 1 mmφ. FIG. 9 is a graph showing calculation results summarizing RMS values of luminance when the diameter of an incident beam is 1 mmφ. FIG. 9 is a graph of a calculation result summarizing the maximum value of luminance when the diameter of an incident beam is 1 mmφ. FIG. 4 is a graph showing the relationship between the thickness of a light guide plate and the distance between points where light rays and an output diffraction grating intersect. FIG. 9 is a graph of a calculation result showing a relationship between the thickness of the light guide plate and the diffraction efficiency of the output diffraction grating at which the luminance becomes maximum and the luminance unevenness becomes minimum. Sectional drawing which shows the structure of the light-guide plate of an Example. FIG. 9 is an image diagram showing a simulation result of a transmission type incident diffraction grating. FIG. 7 is an image diagram showing a simulation result of a reflection type incident diffraction grating. The image figure showing the simulation result which visualized the intensity distribution of the light ray which propagates inside the light guide plate. The image figure showing the simulation result which visualized the intensity distribution of the light ray which propagates inside the light guide plate. FIG. 4 is a schematic diagram illustrating a method for improving unevenness in luminance of a projected image according to the embodiment. FIG. 4 is a schematic diagram illustrating a method for improving unevenness in luminance of a projected image according to the embodiment. FIG. 4 is a graph illustrating a method for reducing luminance unevenness. The schematic diagram which shows the light-guide plate of the Example which reduces luminance unevenness. The schematic diagram which shows another Example using the light-guide plate of an Example. 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. Note that the present invention is not construed as being limited to the description of the embodiments below. It is easily understood by those skilled in the art that the specific configuration can be changed without departing from the spirit or spirit of the present invention.

  In the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description thereof is not repeated in some cases.

  When there are a plurality of elements having the same or similar functions, the same reference numerals may be given different subscripts for explanation. However, when there is no need to distinguish a plurality of elements, the description may be omitted with suffixes omitted.

  Notations such as “first”, “second”, and “third” in this specification and the like are used to identify constituent elements, and necessarily limit the number, order, or content thereof. is not. Also, numbers for identifying components are used for each context, and numbers used in one context do not necessarily indicate the same configuration in another context. Also, this does not prevent a component identified by a certain number from also having a function of a component identified by another number.

  The position, size, shape, range, or the like of each component illustrated in the drawings and the like is not accurately represented in some cases in order to facilitate understanding of the present invention. For this reason, the present invention is not necessarily limited to the position, size, shape, range, and the like disclosed in the drawings and the like.

  In the following embodiments, a technology that enables plasticization of the light guide plate will be disclosed. In addition, as described above, the light guide plate is formed with a diffraction grating for incidence and other diffraction gratings. In the present specification, description will be made below by unifying the former as an incident diffraction grating and the latter as an output diffraction grating.

  In optical disks such as CDs (Compact Discs), DVDs (Digital Versatile Discs), and BDs (Blu-ray (registered trademark) Discs) that are widely used, a plastic material typified by polycarbonate is used as a substrate. Irregularities having a pitch of about 320 nm (25 GB capacity BD) to about 740 nm (4.7 GB capacity DVD) and a depth of about 50 to 100 nm are formed on the surface of the substrate as light spot guide grooves. Substrates are mass-produced by an injection molding method that does not include a vacuum process, and are supplied at low cost. In this specification, the expression “A to B” is used in the meaning of “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 include glass but includes resin, polycarbonate, acrylic resin, and photocurable resin.

  The results of a study by the inventors on the pitch and depth of the diffraction grating required for the light guide plate will be described below. The pitch of the diffraction grating is common to the incident diffraction grating and the output diffraction grating, and is 200 to 700 nm, preferably 300 to 600 nm. With respect to the depth of the diffraction grating, it is necessary to consider that the preferred diffraction efficiency differs between the incident diffraction grating and the output diffraction grating. Generally, there is a relationship that the diffraction efficiency of a diffraction grating increases as the depth increases. The incident diffraction grating desirably has a first-order diffraction efficiency of 10% to 80%, preferably 30% to 70% in order to efficiently couple incident light rays to the light guide plate. On the other hand, the output diffraction grating has a first-order diffraction efficiency of 0.1% to 10%, preferably 0.5% to 5.0% in order to replicate light rays and emit light from the light guide plate over a wide area. It is desirable to have

  Depending on the appropriate value of the diffraction efficiency, it is desired that the depth of the incident diffraction grating is 50 to 500 nm, preferably 100 to 400 nm. On the other hand, the depth of the output diffraction grating is desirably 10 to 200 nm, preferably 20 to 100 nm. It should be noted that the pitch and depth required for the output diffraction grating are approximately equal to the pitch and depth of an optical disk that is provided at low cost by using an injection molding method. Therefore, it can be seen that by utilizing the manufacturing technology of the optical disk, it is possible to form an output diffraction grating at low cost as a concavo-convex diffraction grating on a plastic substrate.

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

  For DVDs and BDs, multilayer optical discs with increased recording capacity have been put to practical use. The first recording layer is formed by using the plastic substrate formed by the above-described injection molding, and the second recording layer is formed thereon by a pattern transfer technique using a UV resin by a 2P (Photo-Polymarization) method. ing. The 2P method can form a diffraction grating having a greater depth than the injection molding method. As will be described later, it has been found from studies by the inventors that a diffraction grating having a depth of 0.3 μm, which is a suitable condition for the incident diffraction grating, can be formed favorably 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”, in view of the respective functions described above, the area of the incident diffraction grating is smaller than the area of the output diffraction grating, and the area of the incident diffraction grating < <This is an output diffraction grating.

By forming the light guide plate by the following steps, it is possible to provide a light guide plate that realizes low cost by utilizing the manufacturing technology of the optical disc and lightening by using plastic.
(1) A plastic substrate having a large-area output diffraction grating is prepared by injection molding.
(2) An incident diffraction grating is formed in a part of the region above by the 2P method.
(3) One or more of these are incorporated in a frame to form a light guide plate.

  In the following embodiments, the light guide plate having a concave-convex type diffraction grating as an example will be described in further detail. First, definitions of the shape parameters of the diffraction grating will be summarized to facilitate understanding of the following embodiments.

FIG. 1A shows the definition of the shape parameters of the incident diffraction grating, and FIG. 1B shows the definition of the shape parameters of the output diffraction grating. As shown in Figure 1A, the input diffraction grating is defined by a pitch P, height H, and the blaze angle theta _B. As shown in FIG. 1B, the output diffraction grating is defined by a pitch P, a height H, a convex portion width A, and a duty D = A / P. The example shown here shows a typical shape as the incident diffraction grating and the outgoing diffraction grating. In this embodiment, a sinusoidal diffraction grating or the like can be used in addition to these.

FIG. 2 is a flowchart showing a process of manufacturing the image display device (light guide plate) according to the first embodiment of the present invention.
3A to 3E are diagrams schematically showing each stage of the creation process. With reference to FIG. 2 and FIGS. 3A to 3E, a description will be given of a manufacturing process of the embodiment.

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

  Next, a substrate 304 having an emission diffraction grating 303 formed on the surface thereof using a plastic material by an injection molding method using the emission diffraction grating master 302 (S102, FIG. 3B). As a plastic material forming the substrate 304, a widely used polycarbonate is preferable, but a transparent resin material other than an acrylic resin such as polymethyl methacrylate resin (PMMA) can also be used. FIG. 3B also shows an example of the arrangement of the formed output diffraction grating 303 on the DVD-RAM disk 305 used as the substrate.

  Next, if necessary, an anti-reflection coating 306 for improving 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, an incident diffraction grating 307 is formed on a part of the substrate 304 formed by injection molding by using a 2P method using a matrix 301 for the incident diffraction grating (S104, FIG. 3D). An incident diffraction grating is formed of UV resin on the substrate 304, and the thickness of the UV resin is about 2 to 10 μm. FIG. 3D shows a side view and a top view together. As shown in FIG. 3D, the incident diffraction grating 307 is formed so as to overlap the output diffraction grating 303. Instead of the 2P method, a nanoimprint method using a dry sheet as a stamper may be used.

  Subsequently, a reflection coating 308 is formed on the incident diffraction grating 307 by a mask sputtering method or the like to improve the wavelength separation performance and the diffraction efficiency of the incident diffraction grating 307 as needed (S105, FIG. 3E). Thus, the inexpensive and light-weight plastic light guide plate 300 can be formed.

  FIG. 4 shows a photograph of the appearance of regions A, B, and C of a prototype device on a DVD-RAM disk. The concavo-convex pattern of the DVD-RAM disk corresponds to the output diffraction grating, and the prototype device corresponds to the incident diffraction grating. Here, a pattern is transferred by a 2P method using a commercially available blazed diffraction grating as a master on a polycarbonate substrate having a periodic uneven pattern on the surface formed by injection molding, and a reflective coating is formed by a mask sputtering method. did. The selected blazed diffraction grating had a pitch of 833 nm, and the height of the triangular uneven pattern 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 concavo-convex pattern formed as a light spot guide groove 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 each having a side of 12.7 mm, and (1) a region A where only the 2P resin of the grating is formed, (2) a region B where only the reflection coating is formed, and And (3) a waveguide check region C in which a reflective coating is formed on the 2P resin of the grating. The 2P resin of the grating corresponds to the incident diffraction grating 307 described with reference to FIG. 3D.

The reflective coating is a five-layer optical filter prepared to increase the diffraction efficiency with respect to the red laser (wavelength 635 nm) used for evaluation. The film configuration is selected from dielectric materials widely used as optical films for optical discs, polycarbonate substrate (refractive index 1.58, film thickness 600 μm (the same applies hereinafter)) / ZnS-SiO ₂ (2.33, 88 nm) / SiO ₂ (1.47, _{80nm) / ZnS-SiO 2 (} 2.33,88nm) / was _{SiO 2 (1.47,80nm) / ZnS-} SiO 2 (2.33,88nm). Hereinafter, the feasibility of a plastic light guide plate in which an incident diffraction grating is formed by a 2P method or the like on an emission diffraction grating formed by an injection-molded substrate, which is the gist of the present embodiment, will be described based on the evaluation results of this prototype device.
(1) Performance of Incident Diffraction Grating Formed by 2P Method FIGS. 5A and 5B show diffraction patterns when laser light is perpendicularly incident on the DVD-RAM substrate 401 described in FIG. FIG. 5A shows a pattern in a region where the 2P resin of the grating is not formed. FIG. 5B is a pattern in a region where the 2P resin of the grating is formed.

As shown in FIG. 5A, a diffraction pattern of the DVD-RAM appears in a region where no grating serving as an incident diffraction grating is 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 is formed thereon by the 2P method, as shown in FIG. 5B. As a result, it was shown that the image display element of the present embodiment can overwrite the diffraction grating pattern.
(2) Spectral Characteristics of Reflective Coating FIG. 6 is a result showing the wavelength (horizontal axis) dependency of the reflectance and the transmittance (vertical axis) of the reflective coating. This is an evaluation of the area B where the optical thin film is laminated only in a part of the area by the mask sputtering method. As can be seen in the figure, the design values indicated by the dotted line and the measured values indicated by the solid line are in good agreement, and the coating with the optical thin film on the plastic light guide plate makes it possible to form the wavelength-dependent reflective film. It has been shown that it is possible.
(3) Coupling Characteristics of Incident Diffraction Grating with Reflective Coating FIG. 7 shows a pattern of transmitted laser light when a laser beam is incident on the incident grating (area C) on which the reflective coating is formed. As can be seen from the figure, it can be seen that the duplicate beams are emitted at regular intervals with respect to the incident beam. This is because the laser beam is diffracted by the incident diffraction grating, propagates through the light guide plate in total reflection, and the beam hits the incident diffraction grating twice, three times, and four times, and is duplicated. FIG. 7 shows a state in which the light is emitted in a state where the wave number is preserved. This is a result indicating that the basic function of the light guide plate, which is one-dimensional, but propagates, duplicates, and emits the incident beam has been achieved. Note that the range of the region C is indicated by a dotted line as the element size.
From the above, it was shown that the image display element of the present embodiment satisfies the basic function as a light guide plate for virtual reality.

Here, a projection image by the image display device of the present embodiment is examined by simulation, and a specific configuration suitable for the present embodiment will be 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)] This is a method of calculating the image observed at a certain point by tracking the path by paying attention to the particle properties, and has been energetically improved mainly in the field of computer graphics.

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

  In light guide plates for head-mounted displays, a diffraction model that supports the wavelength range (approximately 400 to 700 nm) over the entire visible light range and the incident angle range that corresponds to the viewing angle of 40 ° of the projected image is essential. Hereinafter, the diffraction model used in the present embodiment will be described.

  In the following description, it is assumed that the optical axis is in the z direction, the light guide plate has a normal vector in the z direction, and at least a diffraction grating is formed on the front surface or the back surface. For simplicity of description, it is assumed that the projector optical system and the pupil are located on opposite sides 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 number vector, the light emitted from the light guide plate may be subjected to an inversion operation using an xy plane mirror.

  Let the wave vector of the light ray incident on the light guide plate be

age,
The wave vector of the outgoing 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 the light beam, that is, the wave number vector by the lens action of the eye. In order to preserve the wavelength and pixel information of the projected image, the light guide plate is required to have the following operation (Equation (1)).

The diffraction of a light ray is represented by adding the inner product of the diffraction order and the wave vector of the diffraction grating to the wave vector of the light ray. The diffraction when the light beams with the diffraction grating incident intersects the n-th, when the wave vector of the light beam consists of k _n to k _'n, the following relationship holds (equation (2)).

Here, _mn is the diffraction order, and _Kn is the wave vector of the diffraction grating at the n-th 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 is established from Expression (1) (Expression (3)).

This represents the basic function of the light guide plate. When Expression (2) is represented by x, y, and z components, the following Expressions (4) to (7) are obtained.

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

  here,

When, the wave vector becomes an imaginary number, which indicates that no diffracted light beam is generated under the specified conditions.

Defines the generation of diffracted light as follows (Equation (8)).

Diffraction efficiency

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

Assuming that the incident angle in the plane _where the wave vector K _n of the diffraction grating and the z-axis extend is θ _n , the diffraction efficiency in consideration of the presence or absence of the diffracted light and the incident angle dependence

Is modeled as follows (Equation (10)).

Here, eta ⁰ _mn is the n-th intersection diffraction efficiency at normal incidence diffraction order m _n of. The second term on the right side of equation (10) corresponds to the presence or absence of generation of a diffracted light beam, the third term corresponds to the law of conservation of energy, and the fourth term corresponds to the incident angle dependence with γ as a constant.

  On the other hand, there is LightTools (trademark) for designing an illumination optical system as commercially available software that employs the Monte Carlo ray tracing method. However, since the angle dependence cannot be treated, the diffraction efficiency becomes as follows (Equation (11)). ).

  Here, the relationship between the incident angle at a wavelength of 532 nm and the diffraction efficiency was measured using a commercial element # 47-551 of 1,800 lines / mm density manufactured by Edmund as the same blazed diffraction grating as the incident diffraction grating.

  FIG. 8 shows a measurement result of the first-order diffraction efficiency and a comparison result of the diffraction model based on the equation (10). From the experimental results 801 indicated by black circles, the range of the incident angle at which both positive and negative first-order diffracted lights are generated was ± 3 °. As shown 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 disappears at + 3 ° or more. The conventional model 802 in the figure shows the diffraction model of the equation (11). Since the diffraction efficiency is constant irrespective of the incident angle, when the positive and negative first-order diffracted lights coexist according to the incident angle,- When the primary light disappears and when the incident angle increases, the diffraction efficiency cannot be reduced. 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 represented.

  A parameter condition suitable for the image element of this embodiment will be described using a Monte Carlo simulator in which the model of Expression (10) is mounted. 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 × 720, and the beam incident on the light guide plate is 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 guide plate is 17 mm. The light guide plate has a basic configuration described in Patent Document 1. The refractive index of the light guide plate made of plastic according to the present 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 × 5 mm, and the size of the output diffraction grating is sufficiently large. Also, the reflection of light at the eyes and at the edges of the light guide plate is ignored.

  9A to 9C show the relationship between the pitch of the output diffraction grating and the pixel range of the image that can be projected. Here, the pixel ratio on the vertical axis indicates the ratio of pixels in which the path of light rays reaching the pupil due to diffraction by the input / output diffraction grating and total reflection is 1 or more. This means that the part is missing. Since the pixel ratio can be obtained only by the presence or absence of the 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 and the thickness of the light guide plate.

  FIG. 9A shows the result when the viewing angle is 40 degrees. It can be seen that there is a difference in the range where 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 dependency. By combining two light guide plates corresponding to BG and GR consisting of the conditions indicated by two arrows in the figure, a diffraction grating pitch of about 390 nm, and 470 nm, the pixel rate of the RGB image becomes 1, and the image projection is performed. It turns out that it is possible.

  FIG. 9B shows the result when the viewing angle is 50 degrees. It can be seen that as the viewing angle increases, the range of the pitch of the diffraction grating where the pixel ratio becomes 1 becomes narrower. The median value of the pitch of the diffraction grating having a pixel ratio of 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 B, G, and R, respectively, having a diffraction grating pitch indicated by three arrows in the figure.

  FIG. 9C shows the result when the viewing angle is 70 degrees. It can be seen that the pixel ratio decreases as the viewing angle increases, 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 onto the entire surface. In this case, the area of the display image is shared by two or more light guide plates, and by setting R + G + B ≧ 1, it is possible to project an image of the entire screen. By combining four light guide plates indicated by four arrows in the figure, image projection of the entire screen 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 300 nm to 550 nm.

  Hereinafter, similarly, in the light guide plate of this embodiment, the pitch and the number of diffraction gratings can be appropriately set according to the viewing angle of the image to be projected. 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 substantially in inverse proportion thereto. At this time, the range in which the pixel ratio is 1 is also large. As a result, by selecting a substrate material having a large refractive index, an image can be projected 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 to reduce the number of light guide plates to one. The relationship between the viewing angle and the pixel ratio at this time will be described below with reference to FIG.

  10A to 10C show another simulation result showing the relationship between the output diffraction grating pitch and the pixel ratio, and FIGS. 10A, 10B, and 10C show the cases where the viewing angles are 30, 25, and 20 degrees, respectively. The arrow shown in the figure is a condition of the diffraction grating pitch suitable for projecting an RGB image with one light guide plate, and substantially corresponds to the intersection of the R and B characteristics. In the case of this example, when the refractive index of the substrate is 1.58, it is about ± 25 nm (400 nm to 450 nm) centering on 425 nm. As can be seen from FIGS. 10A to 10C, according to the present 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 increases as shown in FIGS. 10B to 10C, a part of the red image (R) or the blue image (B) may not be displayed.

  FIGS. 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, a high luminance area is shown in white, and a low luminance area is shown in gray.

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

  FIG. 11B shows the result when the refractive index of the substrate is 1.70 and the diffraction grating pitch is 395 nm. It can be seen that RGB images can be projected in all the 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 reflection in the light guide plate is increased. At this time, it is good 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 first-order diffraction efficiency in the range of 10% to 80%, and preferably in the range of 30% to 70% from the viewpoint of the brightness of the projected image and power consumption. It is desirable. On the other hand, the output diffraction grating performs 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. Discs such as DVDs and BDs, which are mass-produced as optical disks, need to discuss the suitability of the process technology for the pitch, height, and shape of the diffraction grating in order to create a light guide plate using process technology. There is.

  In optical disks such as DVDs and BDs, which are widely used, polycarbonate materials are used. On the surface of the polycarbonate substrate formed by the injection molding method, a periodic uneven pattern having a pitch of 320 nm (25 GB capacity BD) to 740 nm (4.7 GB capacity DVD) and a depth of 10 to 100 nm is formed as a light spot guide groove. It is formed as spot guide grooves or coded data. By coating an optical thin film on the surface of a polycarbonate substrate by a sputtering method, a read-only or rewritable optical disk is mass-produced, and the substrate is supplied at a low cost.

  Some optical discs have a two-layer data surface to increase the recording capacity, and almost all movie DVDs and BDs have two layers. In the second layer, a concavo-convex pattern is formed on a polycarbonate substrate by a 2P (Photo Polymerization) method using a UV effect resin. By utilizing the mass production process of optical disks, a light guide plate can be produced at low cost. At this time, the weight of polycarbonate is 1.2 compared to the specific gravity of glass of 2.5, so the weight can be reduced to less than 1/2.

  FIG. 12 summarizes the suitability for utilizing the optical disk mass production process technology for the light guide plate of the present embodiment. The parameters of the light guide plate in the figure are based on the simulation results shown above. These parameters change their proper values depending on the aspect ratio and the viewing angle of the image to be projected, and clearly indicate that the proper parameters for the light guide plate of the present embodiment are not limited to the numerical values in the figure. Keep it. In the figure, regarding the material, thickness, refractive index, and pitch of the outgoing and incoming diffraction gratings, the light guide plate of this embodiment is within the range of the condition of the mass-produced optical disc, and it can be seen that the suitability is high. . This is because both employ visible light as a light source and employ a diffraction grating (concavo-convex pattern) close to the diffraction limit in order to maximize image quality and recording capacity.

  The output 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 equivalent, so that the injection molding process can be used as it is. As for the incident diffraction grating, a blazed 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 enhance the light use efficiency. 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 pattern transferability is excellent, and it can be said that the 2P method is suitable for forming an incident diffraction grating. The prototype element described above has a blazed diffraction grating formed by a 2P method on an optical disk substrate made of polycarbonate by injection molding, and it has the basic function of a light guide plate that guides and replicates light rays. Is shown. Therefore, it has been proved that it is possible to utilize the 2P method, which is a process technology of an optical disk, for forming the incident diffraction grating in the figure.

  The brightness and image quality of the projected image are mainly governed by the diffraction efficiency of the output 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 output diffraction grating and the diffraction efficiency obtained by electromagnetic field calculation (FDTD method; Finite Differential Time Domain method). Here, the wavelength is set to 550 nm, the pitch of the pattern of the diffraction grating is set to 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 result shows a damped oscillation type periodic response. It can be seen that a practical area for the output grating is a height range up to 200 nm where 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 first-order diffraction efficiency of the output diffraction grating is about 0.1% to 7.5%, and 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 show the results of calculating the relationship between the diffraction efficiency of the output diffraction grating, the luminance of the projected image, and the luminance unevenness. The wavelength and the pitch condition of the output 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 the thickness of the general glass substrate of 1.0 mm, the thickness of the DVD substrate of 0.6 mm, and the thickness of the cover layer of the Blu-ray Disc corresponding to "Patent Document 1". There are three types with a thickness of 0.1 mm.

  FIG. 14A shows the relationship between the diffraction efficiency of the output grating and the luminance at the center of the projected image. The vertical axis in the figure is proportional to the light use efficiency. The three points indicated by arrows in the figure are the conditions under which the center luminance is maximized, and the diffraction efficiencies are 0.7% and 3.5 when the substrate thickness is 0.1 mm, 0.6 mm and 1.0 mm, respectively. % And 5.5%.

  It can be seen that when the thickness of the light guide plate is increased, the maximum luminance can be obtained with a large diffraction efficiency. This reflects a geometric condition that the number of times the light beam and the output grating intersect is inversely proportional to the thickness of the light guide plate.

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

  Thus, it can be seen that the conditions for the RMS values of the center luminance and the luminance do not match the best conditions. Therefore, it is preferable to appropriately select the diffraction efficiency of the output diffraction grating according to the application in which luminance is prioritized and the application in which luminance unevenness is prioritized.

  As an example of the selection method, for a given thickness of the light guide plate, the diffraction efficiency is set between a condition where the luminance at the center of the image is maximum and a condition where the luminance unevenness is minimum. 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, in the range of the diffraction efficiency of the output diffraction grating presented in this embodiment, the range is from 0.3% to 5.5%. is there. According to this embodiment, it is possible to determine the appropriate diffraction efficiency of the output diffraction grating by using the simulation method presented above according to conditions such as the angle of view of the projected image and the diameter of the beam incident on the light guide plate. it can.

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

  In the description of FIG. 12, it has been described that the numerical values in the figure do not limit the technical scope of the present invention, but this is because the thickness of the substrate is an important parameter that affects the image quality. At the same time, if the light guide plate of this embodiment is used for increasing the diffraction efficiency of the output diffraction grating and the height of the concave / convex pattern does not conform to the condition range of the process technology, the substrate can be effectively thinned by making it thinner. It was shown that it could be solved by increasing the efficiency.

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

  FIG. 16 is a schematic view showing the process of making the incident diffraction grating of this embodiment. In the upper half of FIG. 16, first, a master (matrix) 1601 of an incident diffraction grating in which a concavo-convex pattern is formed on Si or the like in advance using an EB drawing method or the like is prepared. A mother die 301R serving as a resin stamper for the 2P method is prepared as follows. After the UV curable resin 1602 selected in consideration of the transfer properties of the clay and the pattern is dropped on the master 1601, the (A) pressure is applied using the transparent substrate 1603. Next, (B) exposure with ultraviolet light and (C) peeling are sequentially performed to prepare a 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 output diffraction grating has been formed in advance by an injection molding method. In order to enhance the releasability of the resin stamper 301R, a Ti film or the like as a metal release layer having a thickness of about 30 nm is preferably formed on the surface of the resin stamper in advance.

  After the UV curable resin 1604 is dropped on the substrate 304R on which the output diffraction grating 303 is formed, (D) pressurization, (E) exposure with ultraviolet rays, and (F) separation 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-described UV curable resin 1602 can be used in consideration of the refractive index in addition to the releasability and transferability.

  The refractive index of general UV curable resin is about 1.5, and the difference from the refractive index of polycarbonate is 1.58, so the phase difference generated by the uneven pattern of the base is small, and the diffraction is suppressed to 1/10 or less. Is done. However, when priority is given to image quality, it is preferable that the diffraction due to the concave / convex pattern of the output diffraction grating 303 serving as a 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.

  In addition, since the above-mentioned reflective coating is formed along the concave / convex pattern of the output diffraction grating 303 formed later, it also has an effect of effectively reducing the influence of the concave / convex pattern of the underlying output diffraction grating. It is also effective to form a flat surface without forming the output diffraction grating 303 in advance in the area where the incident diffraction grating 307R is formed. Further, in consideration of the adhesion between the UV resin of the incident diffraction grating 307 and the substrate material, a method of forming a concavo-convex pattern having a narrow pitch of, for example, about 240 nm or less and not diffracting the wavelength of visible light is also effective. Have.

In the aforementioned prototype device, the same UV resin was used for both. Thereafter, an anti-reflection coating or the like is appropriately formed for the purpose of enhancing the see-through property when the user wears the above-mentioned reflective coating. Further, it is also possible to easily apply a water-repellent, oil-repellent, or scratch-preventive treatment using SiO ₂ nanoparticles or the like, which are widely used in optical disks, to the surface of the substrate formed 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 output diffraction grating are the same as above.

  FIG. 17 shows a calculation result of an image projected on the retina when the diameter of the incident beam is 4 mmφ. Here, the primary diffraction efficiency of the output diffraction grating was set to 1.7%. This is a 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 from FIG. 17 (a), when the thickness T of the light guide plate is 2.0 mm, the image duplication effect of the output 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 reasons described above, and the diffraction patterns overlap when the substrate thickness is 0.6 mm or less.

FIGS. 18A and 18B show the results obtained by summing the RMS value and the maximum value of the luminance as the quality of the projected image.
As can be seen from FIG. 18A, the RMS value of the luminance is 10% or less when the substrate thickness is in the range of 0.5 mm to 1.0 mm, which is good. On the other hand, as can be seen in FIG. 18B, the maximum luminance increases as the substrate thickness decreases, and reaches a maximum when the thickness of the light guide plate is 0.2 mm. Similarly to the above diffraction efficiency, regarding the thickness of the light guide plate, there is a trade-off between the maximum luminance and the luminance unevenness, and it is possible to select an appropriate value according to the application.

  FIG. 19 shows a calculation result of an image projected on the retina when the diameter of the incident beam is 1 mmφ. This is a 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 substantially proportional to the diameter of the incident beam, the results shown here are obtained when the light guide plate of this embodiment is applied to a small head-mounted display. Here, the primary diffraction efficiency of the output diffraction grating is set to 0.85%. Since the propagation angle of the light beam is determined by the pitch of the diffraction grating, the angle and the wavelength of the incident light, when these are constant, the number of times the light beam intersects with the output diffraction grating before being projected onto the retina is inversely proportional to the substrate thickness. Therefore, by setting the diffraction efficiency of the output diffraction grating substantially in proportion to the substrate thickness, the light use efficiency can be made substantially constant.

  As can be seen from FIG. 19 (a), when the thickness of the light guide plate is 2.0 mm, the duplication of the image by the output 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 light guide plate thickness is 0.6 mm or less.

  FIGS. 20A and 20B show the results obtained by summarizing the RMS value and the maximum value of the luminance. As can be seen from FIG. 20A, the RMS value of the luminance is good at 10% or less when the substrate thickness is in the range of 0.2 mm to 0.5 mm. On the other hand, as shown in FIG. 20B, the maximum luminance increases as the thickness of the light guide plate decreases. From these relations, 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 the present embodiment has a high brightness and a small brightness unevenness by appropriately selecting the thickness of the light guide plate and the diffraction efficiency of the output diffraction grating according to the diameter of the incident beam. A light guide plate for a display can be provided. Here, the operation of the present embodiment has been presented for a case where the angle of view is relatively wide at 40 degrees. In this case, when 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 the two have a roughly proportional relationship. According to the present embodiment, when the diameter of the incident beam exceeds the range exemplified here, an appropriate thickness of the light guide plate can be easily obtained by utilizing this proportional relationship. More specifically, an appropriate value can be obtained by using the simulation method presented in the present embodiment as in the above.

As shown in FIGS. 17 to 20 of the fourth embodiment, the thickness of the light guide plate and the diffraction efficiency of the output diffraction grating suitable for the present embodiment are determined by the diameter of the beam, which is a group of light rays including image information incident on the light guide plate. It is shown that there is an appropriate value according to. The wave number of a light beam that propagates while totally reflecting inside the light guide plate is determined by the wavelength, the incident angle, the refractive index of the light guide plate, and the pitch of the incident diffraction grating, and is expressed by Expressions (2) to (7). Assuming that the diffraction order m _n = 1, the propagation angle θ _p0 when the light beam including the image information is vertically incident on the incident diffraction grating of the light guide plate is expressed by the following equation (12).

Here, _ns 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, when 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 interval L between points where the light beam and the output diffraction grating intersect is represented by the following equation (13).

  FIG. 21 is an example showing the relationship between the thickness of the light guide plate (substrate) and the beam interval L. Considering the effect of the spread of the wavelength of the light source, if the diameter of the beam incident on the light guide plate is smaller than L, the video information recognized by the user is a discontinuous bright spot as shown in FIG. It becomes a pattern. That is, FIG. 21 shows that the image quality is degraded when the thickness of the light guide plate is increased on the assumption that the beam diameter is constant.

  Here, in order to show the relationship between the thickness of the light guide plate, the diffraction efficiency of the output diffraction grating, and the quality of the image recognized by the user, a case where the diameter of the incident beam is equal to L will be considered. A calculation similar to that illustrated in FIGS. 17 to 20 is performed to minimize the diffraction efficiency of the output diffraction grating and the luminance unevenness (luminance RMS value) at which the luminance becomes maximum with respect to the thickness of the light guide plate. The diffraction efficiency of the output diffraction grating was determined.

  FIG. 22 shows the results. As shown in FIG. 13, the maximum value of the diffraction efficiency when the output diffraction grating is rectangular is 7.5%. Further, from the viewpoint of utilizing the process of the optical disk of the present embodiment, the lower limit of the thickness of the light guide plate is realistically 0.1 mm in thickness of the cover layer of the Blu-ray Disc from the viewpoint of feasibility. In the figure, the shaded area is the range of the diffraction efficiency from the maximum luminance to the minimum luminance 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%. .1 mm. That is, the shaded area in FIG. 22 shows the thickness of the light guide plate and the range of the diffraction efficiency of the output diffraction grating suitable for the present embodiment in more detail.

  FIG. 22 shows that when the diffraction efficiency of the output diffraction grating is appropriately set according to the application, the thickness of the light guide plate suitable for this embodiment is in the range of 0.1 mm to 4.0 mm. As described above, the preferable range of the diffraction efficiency of the output diffraction grating is from 0.1% to 10%, preferably from 0.2%, from the viewpoint that the light beam is replicated and emitted from the light guide plate over a large area. It is desirable to have a first-order diffraction efficiency of 0.1% to 7.5%. However, as shown in FIG. The calculated result is from 0.2% to 7.5%, which is in agreement with the aforementioned range. The ratio is more preferably 0.5% to 5.0% in order to replicate light rays and emit light from the light guide plate over a wide area. In this case, the substrate thickness can be reduced to about 0.5 mm to 3 mm.

  The parameters suitable for the present embodiment have been described above for a light guide plate having a rectangular shape as a cross-sectional shape of an exit diffraction grating using polycarbonate as a material of the light guide plate. In this embodiment, an acrylic resin or a plastic material having a high refractive index (refractive index ≧ 1.70) generally used for an eyeglass lens can be used as a material for the light guide plate. Further, as the cross-sectional shape of the output diffraction grating, any shape such as a sine wave shape, a triangle, and a trapezoid can be used. The influence of the refractive index of the light guide plate and the cross-sectional shape of the output diffraction grating is considered to be about ± 20% of the area shift from the range suitable for implementing this embodiment shown in FIG.

  Since the light guide plate replicates the light beam including the image information incident thereon while performing total internal reflection light guide, and emits a light beam that can be visually recognized by the user, at least the mechanical guide that does not bend greatly due to its own weight is used. Strength is required. This is because if the light guide plate is bent and the parallelism between the incident diffraction grating and the output diffraction grating is deteriorated, image information recognized by the user is distorted by the angle between the two. In the above description, it is assumed that the thickness of the light guide plate suitable for the present embodiment is the thickness of the cover layer of the Blu-ray Disc of 0.1 mm, which is a practical lower limit. It is inevitable that a plastic material having a thickness of 0.1 mm bends under its own weight.

  In order to avoid this, the configuration of the light guide plate of the present embodiment will be described below. The calculations so far are obtained in the case where the output diffraction grating is formed on one surface of the light guide plate. At this time, an interval L at which the light beam and the output diffraction grating intersect is represented by Expression (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 the thickness t of the light guide plate is reduced, L is reduced in proportion, so that the image quality is improved as shown in FIG.

  FIG. 23 is an embodiment showing the configuration of the light guide plate of this embodiment. Here, only the output diffraction grating is shown, and an example is shown in which one plastic substrate and four output diffraction gratings are integrated. 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 interval between the output diffraction gratings 12, 14, 16, and 18 corresponds to the interval between the spacer layers, and the thickness is set to 0.1 mm. The thickness t of the light guide plate in (Equation 13) is の of the distance 2t moved in the thickness direction of the light guide plate until the light beam and the output diffraction grating intersect. Therefore, in this configuration, the minimum value of t can be made to match 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 the present embodiment shown in FIG. 23 is basically the same as the standard of the multi-layer Blu-ray Disc, and can be easily manufactured by using a similar process. At this time, the output diffraction grating 12 is formed on the substrate 11 by the injection molding method, and the spacer layer 13 and the output diffraction grating 14 are formed thereon by the 2P method. By repeating this, the spacer layer 15, the output diffraction grating 16, the spacer layer 17, and the output diffraction grating 18 may be sequentially formed. In the case of a Blu-ray Disc or DVD multi-layer ROM medium, after forming each information holding layer (corresponding to the output diffraction grating of this embodiment) on which a diffraction grating pattern is formed, a dielectric reflection film, a thin metal thin film, or the like is formed. It is formed to control the reflectance 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 reflection film or a thin metal thin film on the output diffraction grating. At this time, since the diffraction efficiency of the output diffraction grating is substantially proportional to the reflectance of the reflection film formed based on the wave theory, the thickness can be easily designed. The present embodiment is an extension of the concept of the thickness of the light guide plate. By forming two or more output diffraction gratings formed on a transparent plastic substrate, the output diffraction grating is maintained while maintaining the mechanical strength. By reducing the interval in the thickness direction of the grid, the quality of the image recognized by the user is improved. The upper limit of the number of output diffraction grating layers that can be formed by this embodiment is approximately eight layers.

  FIGS. 24A and 24B are simulation results showing an incident diffraction grating suitable for the present example. Here, the calculation was performed on the assumption 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, high diffraction efficiency is required for the incident diffraction grating.

  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 of making the incident diffraction grating of the embodiment shown in FIGS. 16A and 16B. Here, a blazed diffraction grating having a normal triangular shape that does not overhang and that can be formed by an injection molding method, a 2P method, or the like was studied.

  FIG. 24A is a simulation result of the same transmission type diffraction grating as in “Patent Document 2”. The image light ray 2401 is 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 by the blaze surface and the diffraction by the periodic structure are phase-tuned. As shown in the figure, to achieve this, the height of the uneven pattern must be large, the pattern angle must be 70 to 80 degrees, and the aspect ratio obtained by dividing the pattern height by the period must be 10 or more. is there. In general, in the transfer of the concavo-convex pattern by the 2P method, when the aspect ratio exceeds 2, a problem such as releasability occurs, and the yield in 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 the reflection type diffraction grating. Similarly, the image light ray 2401 is 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 that the phase is tuned by the reflection by the blaze surface and the diffraction by the periodic structure. As shown in the figure, it can be seen that this condition is satisfied by the concave / convex pattern having a lower aspect ratio than that of the transmission type. At this time, the height of the concavo-convex pattern is about 250 nm, and the aspect ratio is about 0.57. In the above-described prototype device, a triangular uneven pattern having a pattern height of 374 nm can be transferred favorably. It can be said that it is preferable to use a reflection type diffraction grating for the light guide plate of this embodiment.

  The results shown in FIGS. 14A and 14B show that when the diffraction efficiency of the output diffraction grating is constant, the luminance of the projected image and the luminance unevenness have a trade-off relationship. Here, a configuration of an output diffraction grating suitable for the light guide plate of the present embodiment having high luminance and small luminance unevenness will be described. Hereinafter, the calculation was performed on the assumption 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.

  FIG. 25A and FIG. 25B are simulation results of visualizing the intensity distribution of 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 a 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 a region where the light beam reaching the pupil is finally diffracted on the emission diffraction grating. A high-luminance area on a straight line from the incident diffraction grating in the y direction indicates a main ray group (hereinafter, principal ray group) diffracted by the incident diffraction grating and propagated inside the light guide plate. The propagation direction of the chief ray group is indicated by an arrow 251.

  As can be seen from FIG. 25A, there is a characteristic that the intensity is gradually attenuated by the propagation of the principal ray group. The low-luminance light group that spreads around the principal light group is a light group that is diffracted by the output diffraction grating and whose traveling direction is deflected in the xy plane. Under this condition, it can be seen that the exit circle O and the pupil coincide in the xy plane. Therefore, what reaches the pupil and is recognized as an image is a part of the principal ray group having a high intensity.

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

  As can be seen, the chief ray group travels from the incident diffraction grating to the upper right. Also, it can be seen that the exit circle O and the pupil P do not coincide in the xy plane, and the light beam that reaches the pupil and is recognized as an image is deflected in the upper left direction from the principal 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 reason why luminance unevenness occurs when an image is projected using the light guide plate. In addition, the unevenness of brightness due to the diameter of the incident image beam, crosstalk between the RGB light guide plates, -1st-order diffraction of the incident diffraction grating, and the unevenness of the pitch of the diffraction grating and the height of the concavo-convex pattern are caused. It is a big cause. In the present embodiment, a method for improving the uneven brightness due to the mismatch between the principal ray group and the emission circle shown in the figure is provided below.

  FIG. 26A and FIG. 26B are schematic diagrams illustrating a method of improving the uneven brightness of the projected image according to the present embodiment. In the figure, the solution is schematically shown in superposition on the simulation results of FIGS. 25A and 25B. In the figure, 100 is a light guide plate, 210 and 220 are diffraction efficiency enhancement regions provided in a part of the output diffraction grating. As shown in the figure, the traveling direction of the principal ray group propagating in the light guide plate differs depending on the position of the image to be viewed.

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

  As shown in FIG. 26A, it is preferable that the diffraction efficiency enhancement region is formed in a region that does not coincide with the principal ray group directed to the center of the image. This can be realized by controlling the duty ratio and the height of the unevenness forming the grating when the master is created by the above-described EB drawing apparatus or the like.

  The light guide plate illustrated in FIG. 3 of “Patent Document 1” includes a waveguide and three linear diffraction gratings H0, H1, and H2. The incident light illuminates the diffraction grating H0, which is coupled into the waveguide. The two gratings H1, H2 formed in the waveguide overlap each other. Utilizing the knowledge of "Patent Document 1", the present embodiment can also be realized by forming only the diffraction grating H1 in the diffraction efficiency enhancement region 210 and forming only the diffraction grating H2 in the diffraction efficiency enhancement region 220. In the EB lithography system, it is easy to form a diffraction grating having two wavenumber vectors by patterning H1 and H2 in the same plane, and to provide a light guide plate at low cost by injection molding. It can be said that the light guide plate having the output diffraction grating formed on one surface is suitable for the embodiment in that the process of the optical disc is utilized.

  FIG. 27 is another schematic diagram showing a method for improving the uneven brightness 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 principal 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 luminance under the same conditions as above. As shown in the figure, when the outgoing 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 outgoing circle (the projected image In accordance with (upper), the luminance of the image is attenuated (characteristic 2701). On the other hand, when the attenuation is moderate, the unevenness in luminance of the projected image can be improved by correcting the diffraction efficiency of the output diffraction grating in a plane by linear approximation (characteristic 2702).

  FIG. 28 is a schematic diagram showing a light guide plate of the present embodiment for reducing uneven brightness of an image formed by the projector 281 as an image forming unit. In the figure, 300 denotes a light guide plate, 307 denotes an incident diffraction grating, and 303 denotes an output diffraction grating. An enlarged view of the concavo-convex pattern of the output diffraction grating is also shown. As shown in the figure, by changing the duty of the concavo-convex pattern of the output diffraction grating in the y-direction (light propagation direction), a distribution is given to the diffraction efficiency, so that the luminance unevenness shown in FIG. It becomes possible.

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

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

  FIG. 30 is a schematic diagram illustrating the configuration of the image display device according to the present embodiment. Light having image information emitted from the projector 281 in the figure is delivered to the user 400 by the action of the B, G, and R light guide plates 300B, 300G, and 300R, thereby realizing augmented reality. Each of the B, G, and R light guide plates 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 gratings depend on each color. Optimized.

  In FIG. 30, the image display device of this embodiment includes light guide plates 300B, 300G, and 300R, a projector 281 as an image forming unit, and a display image control unit (not shown). Here, as shown in FIG. 15, the light guide plate module 3001 integrates R, G, and B light guide plates corresponding to color display. Examples of the image forming means include an image forming apparatus including a reflective or transmissive spatial light modulator, a light source, and a lens; an image forming apparatus including an organic and inorganic EL (Electro Luminescence) element array and a lens; Widely known image forming apparatuses such as an image forming apparatus using a diode array and a lens, and an image forming apparatus combining a light source, a semiconductor MEMS mirror array, and a lens can be used. Also, an LED or laser light source and an optical fiber whose tip is caused to resonate by MEMS technology, PZT, or the like can be used. Among them, the most common one is an image forming apparatus including 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 such as LCOS (Liquid Crystal On Silicon) and a digital micromirror device (DMD). Or an LED or laser corresponding to each color. Furthermore, the reflection type spatial light modulator reflects a part of light from the liquid crystal display device and the light source to the liquid crystal display device, and passes a part of the light reflected by the liquid crystal display device. Then, a configuration including a polarizing beam splitter for guiding to a collimating optical system using a lens can be adopted.

  As a light emitting element constituting a light source, a red light emitting element, a green light emitting element, a blue light emitting element, and a white light emitting element can be given. The number of pixels may be determined based on the specifications required for the image display device. Specific values of the number of pixels include, in addition to 1280 × 720 shown above, 320 × 240, 432 × 240, 640 × 480 , 1024 x 768, 1920 x 1080.

  In the image display device of the present embodiment, the light beam including the image information emitted from the projector 281 is positioned so as to irradiate each of the incident diffraction gratings of the light guide plates 300B, 300G, and 300R, and is integrated with the light guide plate module 3001. Formed.

  A display image control unit (not shown) controls the operation of the projector 281 and performs a function of appropriately providing image information to the user 400.

  In the present embodiment, the case where the image information is provided to the user has been described. However, the image display device of the present embodiment additionally includes a touch sensor, a temperature sensor, an acceleration sensor, and the like for acquiring information on the user and the outside world. Various sensors and an eye tracking mechanism for measuring the movement of the user's eyes can be provided.

  According to the embodiment described above, a light guide plate (image display element) having an uneven diffraction grating described in “Patent Documents 1” to “Patent Document 3” advantageous for realizing a wide viewing angle, and using the same. The image display device can be provided to the user at low cost and light weight.

According to the embodiment described above, it is possible to provide a light guide plate which realizes low cost by utilizing the manufacturing technology of an optical disc and lightening by using plastic. The standard is about 1/10 to 1/100 for the cost and about 1/2 or less for the weight. In addition, due to the plasticization of the light guide plate, the plastic has the property of being less likely to scatter than glass when subjected to an unexpected external force when worn by the user, thus improving safety. The effect is also realized.
In addition, as shown in FIG. 1, a head-up display (HUD) for in-vehicle use described in Patent Document 6 has almost the same structure as a head-mounted display of the present invention, and provides appropriate information to a 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: output diffraction grating

Claims (15)

A first member made of a first resin,
A second member made of a second resin formed on at least a part of the first member;
With
The first resin and the second resin are different resins,
A first diffraction grating formed of a first pattern is formed on the first member;
A second diffraction grating having a second pattern is formed on the second member;
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 shape, aspect, height, duty, patterning 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. A 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;
A 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;
A first diffraction efficiency of the first diffraction grating is 0.1% to 7.5%;
The light guide plate according to claim 1. A first diffraction efficiency of the first diffraction grating is 0.5% to 5.0%;
The light guide plate according to claim 7. A diffraction efficiency enhancement region is provided in a part of the first diffraction grating,
The light guide plate according to claim 1. The first diffraction grating changes the duty of the first pattern along the propagation direction of light to give a distribution to diffraction efficiency.
The light guide plate according to claim 1. The first diffraction efficiency of the second diffraction grating is 30% to 70%;
The light guide plate according to claim 1. A plurality of light guide plates according to claim 1, wherein each of the plurality of light guide plates has a different first pattern corresponding to a predetermined wavelength, and is formed by stacking a plurality of the light guide plates.
Light guide plate module. The first patterns of the plurality of light guide plates are set at different pitches in a range of 300 nm to 550 nm.
The light guide plate module according to claim 12. The light guide plate according to claim 1,
With a projector that projects video,
Light emitted from the projector is arranged to irradiate the second pattern;
Image display device. A first step of preparing a substrate made of polycarbonate or acrylic resin and having a first diffraction grating pattern on its surface;
A second step of disposing a photocurable resin on the first diffraction grating pattern;
A third step of transferring a second diffraction grating pattern to the photocurable resin;
A fourth step of curing the photocurable resin,
A method for manufacturing a light guide plate, comprising:

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