Patent Application
20240061165
The disclosure herein generally relates to a light guide element, and a display apparatus using the same.
Augmented Reality (AR)/Mixed Reality (MR) headsets for personal use or professional use have been developed. Because the ARMR headset is required to have high resolution and a wide viewing angle, a heavy member is often used for a member for displaying an ARMR image. Thus, an entire headset is also heavy and wearing feeling is not good because the headset is fixed to the head. On the other hand, small and light-weight glasses-type displays that display simple information such as characters and symbols have also been developed.
An optical element for displaying ARMR images used in a personal display or an augmented reality display has been known having a configuration in which a projected image from a projector is displayed in front of the eyes using a diffracted light guide, in which an input coupling grating, an exit pupil expansion grating, and an output coupling grating are formed on a light guide plate (See, for example, Japanese translation of PCT international application publication No. 2020-521994 and Japanese translation of PCT international application publication No. 2017-528739). As optical glass used for the light guide plate, lead-free and arsenic-free optical glass with the refractive index nd to the d-line of greater than or equal to 1.91 and less than or equal to 2.05 is known (for example, see Japanese Patent No. 4970896).
If a light-weight display apparatus with the same function and the wide viewing angle as those of the ARMR headset is achieved, the application field and the user range would be further expanded. The present invention aims at providing a light guide element with light-weight and a wide viewing angle, and a display apparatus using the same.
According to an aspect of the present disclosure, a light guide element includes a light guide substrate that is a single-layer light guide substrate; and a diffraction layer formed on the light guide substrate. The diffraction layer includes a first diffraction grating configured to in-couple light into the light guide substrate, the light being incident on the light guide substrate; and a second diffraction grating configured to outcouple totally internally reflected light having propagated in the light guide substrate out of the light guide substrate. The first diffraction grating in-couples the incident light in a range of 60 degrees or more including a direction normal to the light guide substrate, for at least one wavelength of a first wavelength included in a 450 nm±20 nm band, a second wavelength included in a 530 nm±20 nm band, and a third wavelength included in a 630 nm±20 nm band. The second diffraction grating outcouples the totally internally reflected light in a range of 60 degrees or more including the direction normal to the light guide substrate, for at least one wavelength of the first wavelength, the second wavelength, and the third wavelength.
According to the invention of the present application, a light guide element with low weight and a wide viewing angle, and a display apparatus using the same can be provided.
Other objects and further features of the present disclosure will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:
In the embodiment, using the light guide substrate 11 that is a single-layer light guide substrate, light of a first wavelength included in the blue wavelength band, light of a second wavelength included in the green wavelength band, and light of a third wavelength included in the red wavelength band are taken into the light guide substrate 11 in a high FOV (field of view), and emitted from the light guide substrate 11 in a high FOV. The blue wavelength band is a range of, for example, 450 nm±20 nm. The green wavelength band is a range of, for example, 530 nm±20 nm. The red wavelength band is a range of, for example, 630 nm±20 nm.
Specifically, incident light is in-coupled in a range of 60 degrees or more, preferably 65 degrees or more, and more preferably 70 degrees or more, including the direction normal to the light guide substrate 11 for any one wavelength of the first wavelength (λ1), the second wavelength (λ2), and the third wavelength (λ3). The light is also outcoupled out of the light guide substrate 11 in a range of 60 degrees or more, preferably 65 degrees or more, and more preferably 70 degrees or more, including the direction normal to the light guide substrate 11 for any one wavelength of λ1, λ2, and λ3. The angle formed by the central direction of the angular range of the light to be in-coupled or the outcoupled light and the direction normal to the substrate is preferably 15 degrees or less, more preferably 10 degrees or less, even more preferably 5 degrees or less, and most preferably approximately 0 degrees (the central direction coincides with the direction normal to the substrate).
Furthermore, the in-coupling grating 121 in-couples incident light of any of the wavelengths of λ1, λ2, and λ3 in a common angular range of 55 degrees or more, including the direction normal to the light guide substrate 11. The outcoupling grating 123 outcouples light of any of the wavelengths of λ1, λ2, and λ3 in a common angular range of 55 degrees or more, including the direction normal to the light guide substrate 11. The angle formed by the central direction of the angular range of the light to be in-coupled or the outcoupled light and the direction normal to the substrate is preferably 15 degrees or less, more preferably 10 degrees or less, even more preferably 5 degrees or less, and most preferably approximately 0 degrees (the central direction coincides with the direction normal to the substrate).
In other words, the above descriptions are expressed as follows. The angular range of the light to be in-coupled or the outcoupled light preferably includes a range of at least −42.5 degrees to +12.5 degrees with respect to the direction normal to the substrate, or a range of −12.5 degrees to +42.5 degrees; more preferably a range of at least −37.5 degrees to +17.5 degrees, or a range of −17.5 degrees to +37.5 degrees; even more preferably a range of at least −32.5 degrees to +22.5 degrees or −22.5 degrees to +32.5 degrees; and most preferably a range of at least −27.5 degrees to 27.5 degrees. That is, preferably a fluctuation in the angular range is 15 degrees or less, more preferably 10 degrees or less, even more preferably 5 degrees or less, and most preferably the fluctuation in the angular range is approximately zero.
In the conventional configuration, monochromatic light guide plates respectively corresponding to the wavelengths are used in 3 layers. In this case, the FOV can be optimized for each wavelength, and a high FOV can be easily achieved. However, an entire optical element becomes thick and heavy. In the embodiment of the present application, because RGB light is guided by the light guide substrate 11 that is a single-layer light guide substrate, a compact and light-weight light guide element 10 can be achieved. When a light guide substrate 11 that is a single-layer light guide substrate is used, a FOV that each FOV of the wavelength of RGB overlaps is an entire FOV. In the present application, a high FOV is achieved by improving materials of the light guide substrate 11 and the diffraction layer 12, and designs of the in-coupling grating 121 and the outcoupling grating 123. For example, both the light guide substrate 11 and the diffraction layer 12 are formed of inorganic materials, and each of the substrate and the layer is designed to have a predetermined refractive index. In addition, the positive and negative one-dimensional diffractions are used. Bases of the configuration and results of analysis will be described later.
The light Lin is coupled into the light guide substrate 11, the light being incident on the light guide substrate, by the in-coupling grating 121 as a diffraction wavefront in a predetermined direction, e.g., the x-direction. The direction in which the light Lin is coupled into the light guide substrate 11, the light being incident on the light guide substrate, is not limited to the +x-direction. As described later, a configuration in which the light into the light guide substrate propagates in the +x-direction may be adopted, or a configuration in which the light into the light guide substrate propagates in the two-dimensional directions of the x and y-directions may be adopted. The in-coupling grating 121 couples light incident from the diffraction layer 12 side as well as light incident from the rear surface side of the light guide substrate 11 into the light guide substrate 11. Perpendicular incident light incident in the direction normal to the light guide substrate 11 is also coupled into the light guide substrate 11. In addition, the FOV of 55 degrees or more is achieved for all wavelengths of RGB.
The outcoupling grating 123 emits the light which is guided by total internal reflection inside the light guide substrate 11 in a predetermined angular range including the direction normal to the light guide substrate 11. Specifically, the outcoupling is achieved in the FOV of 60 degrees or more including the direction normal to the light guide substrate for any one of the wavelengths of RGB. The outcoupling is also achieved in the FOV of 55 degrees or more including the direction normal to the light guide substrate for all the wavelengths of RGB.
The light guide substrate 11 is, for example, a glass substrate. As the glass material, (1) Bi2O3—TeO2 based glass or (2) La2O3—B2O3 based glass may be used. In the specification of the present application, “composition” refers to a composition of elements or components designed to be 100% in total, in the unit of percent (mole %, weight %, or the like), excluding impurities that are inevitably mixed, or impurities and additives in parts per million (ppm) units that are intentionally added.
Examples of Bi2O3—TeO2 based glass include glass with a Bi2O3 content of 20% to 50% and a TeO2 content of 10% to 35%, when the total composition is 100% by mole % in terms of oxide.
Bismuth oxide, Bi2O3, is a preferred component to obtain high refractive index glass with high visible light transmittance. A lower limit of a content thereof is preferably 20% or more, and more preferably 25% or more, and even more preferably 30% or more. An upper limit of the content is preferably 45% or less, more preferably 40% or less, and even more preferably 35% or less.
Tellurium dioxide, TeO2, is a glass-forming component and may be contained in glass because TeO2 enables obtainment of a high refractive index glass with high visible light transmittance. A content of TeO2 is preferably 10% or more, more preferably 20% or more, and even more preferably 25% or more. However, the content of TeO2 is preferably 35% or less, and more preferably 30% or less, because excessive content thereof makes the glass unstable.
Boron oxide, B2O3, is a glass-forming component, and is preferably contained to stabilize glass. However, an excessive content thereof makes it difficult to achieve a high refractive index. A lower limit of the content is preferably 10% or more, and more preferably 12% or more. An upper limit of the content is preferably 40% or less, more preferably 35% or less, further more preferably 30% or less, and even more preferably 25% or less.
Diphosphorus pentoxide, P2O5, is an optional component. P2O5 is a glass-forming component, and is preferably contained to stabilize glass. However, an excessive content thereof makes it difficult to achieve a high refractive index. A lower limit of the content is preferably 0% or more, and an upper limit of the content is preferably 20% or less, and more preferably 15% or less.
A lower limit of a sum of the B2O3 content and the P2O5 content is preferably 10% or more, and more preferably 20% or more. An upper limit of the sum is preferably 45% or less, more preferably 40% or less, and more preferably 35% or less.
Niobium oxide, Nb2O5, titanium oxide, TiO2, tantalum oxide, Ta2O5, and tungsten oxide, WO3, are preferred components to be contained to enhance the refractive index of the glass. These components, including Bi2O3 and TeO2, are components that can be also used as the diffraction layer 12 described later. By increasing the content of these components, a configuration having a dispersion relation of the refractive index close to that of the diffraction layer 12 can be achieved. The content of Bi2O3—TeO2—Nb2O5—TiO2—Ta2O5—WO3 is preferably higher. The content is preferably 55% or more, and more preferably 60% or more.
Examples of La2O3—B2O3 based glass include glass with a La2O3 content of 10% to 40% and a B2O3 content of 10% to 35%, when the total composition is 100% by mole % in terms of oxide.
Lanthanum oxide, La2O3, has an excellent function of increasing the refractive index and decreasing the dispersion while maintaining the stability of the glass, is a component with a high visible light transmittance, and can provide high refractive index glass with high visible light transmittance. Therefore, La2O3 may be contained in the glass, but an excessive content makes the devitrification resistance decrease. A lower limit of the content is preferably 10% or more, and more preferably 20% or more. An upper limit of the content is preferably 40% or less, more preferably 30% or less, and even more preferably 25% or less.
Boron oxide, B2O3, is a glass-forming component, and is preferably contained to stabilize glass. However, an excessive content thereof makes it difficult to achieve a high refractive index. A lower limit of the content is preferably 10% or more, and more preferably 12% or more. An upper limit of the content is preferably 40% or less, more preferably 35% or less, further more preferably 30% or less, and even more preferably 25% or less.
Silicon dioxide, SiO2, is an optional component. SiO2 is a glass-forming component and may be included to stabilize the glass, but an excessive content makes it difficult to achieve a high refractive index. A lower limit of the content is preferably 0% or more, more preferably 5% or more, and even more preferably 10% or more. An upper limit of the content is preferably 30% or less, more preferably 20% or less, and even more preferably 15% or less.
Titanium oxide, TiO2, has an excellent function of increasing the refractive index while maintaining the stability of the glass, and may be contained in the glass. However, an excessive content makes the devitrification resistance decrease. A lower limit of the content is preferably 10% or more, more preferably 20% or more, and even more preferably 25% or more. An upper limit of the content is preferably 40% or less, and more preferably 35% or less.
Zirconium oxide, ZrO2, is an optional component. ZrO2 has an excellent function of increasing the refractive index while maintaining the stability of the glass, and may be contained in the glass. However, an excessive content makes the devitrification resistance decrease. A lower limit of the content is preferably 0% or more, and more preferably 5% or more. An upper limit of the content is preferably 15% or less, and more preferably 10% or less.
Gadolinium oxide, Gd2O3, niobium oxide, Nb2O5, tantalum oxide, Ta2O5, and tungsten oxide, WO3, are preferred components to be contained to enhance the refractive index of the glass. These components, including La2O3 and TiO2, are components which can also be used as the diffraction layer 12 described later. By increasing the content of these components, a configuration having a dispersion relation of the refractive index close to that of the diffraction layer 12 can be achieved. The content of La2O3—TiO2—Gd2O3—Nb2O5—Ta2O5—WO3 is preferably higher. The content is preferably 55% or more, and more preferably 60% or more. Furthermore, ZrO2 may be included, and the content of La2O3—TiO2—Gd2O3—Nb2O5—Ta2O5—WO3—ZrO2 is preferably 55% or more, and more preferably 60% or more.
Using the glass material having the above-described composition, the light guide substrate 11 with a refractive index to the d-line (λ2) exceeding 2.05 or the light guide substrate 11 with an internal transmittance per 10 mm thickness at 450 nm of 95% or more can be achieved.
The light guide substrate 11 may be a single crystal substrate. A single crystal refers to a crystal in which an orientation of an arrangement of atoms or molecules is the same in any part of the crystal. The light guide substrate 11 may be an isotropic single crystal substrate having optical characteristics independent of an orientation of the crystal, or may be a uniaxial single crystal substrate having a crystal axis oriented in a predetermined direction. In the case of the uniaxial substrate, an angle formed by the optical axis of the light guide substrate 11 and the direction normal to the light guide substrate 11 is preferably within ±4 degrees, and more preferably within ±0.4 degrees. This is because a center of a line of sight, when viewing an actual scene or characters that is not an ARMR image through the light guide substrate, roughly coincides with the direction normal to the substrate, and an image is doubled due to birefringence when the optical axis deviates from the propagating direction of light, that leads to a reduction in resolution.
In crystals, lattice defects called dislocation defects sometimes occur. For example, near a dislocation defect called micropipe having a size larger than 1 μm in diameter, a change in the refractive index often occurs. Even if the size of the defect does not affect the appearance as a light guide element, in the case where such defects occur within the range of a light guide, the change in the refractive index may lead to a reduction in the resolution of an image displayed from the light guide element. Thus, the above-described defects are preferably not present in the light guide element. The defect density of the micropipe is preferably 10/cm2 or less, more preferably 1/cm2 or less, and even more preferably 0.1/cm2 or less.
As the single-crystal light guide substrate 11, a substrate formed of TiO2, SrTiO3, KTaO3, LiNbO3, SiN, SiC, diamond, or the like may be used.
The thickness of the light guide substrate 11 is, for example, 1 mm or less. Each of the in-coupling grating 121 and the outcoupling grating 123 is, as an example, formed of a thin film of an inorganic material with a thickness of 100 to 1000 nm and has a diffraction grating pattern formed at a predetermined pitch. When the extended grating 122 is used in conjunction with the in-coupling grating 121 and the outcoupling grating 123, the extended grating 122 is also formed of the same thin film. The pitch of the diffraction grating is, for example, 300 to 500 nm. Instead of forming the diffraction layer 12 with a thin film of an inorganic material, the diffraction grating may be formed directly on the surface of the light guide substrate 11. In this case, the surface area where the diffraction grating is formed serves as the diffraction layer 12.
The in-coupling grating 121 achieves the FOV of more than 60 degrees, preferably more than 65 degrees, and more preferably more than 70 degrees for any one of the wavelengths of RGB. The in-coupling grating 121 achieves the FOV of more than 55 degrees for any of the wavelengths of RGB. The outcoupling grating 123 achieves the FOV of more than 60 degrees, preferably more than 65 degrees, and more preferably more than 70 degrees for any one of the wavelengths of RGB. The outcoupling grating 123 achieves the FOV of more than 55 degrees for any of the wavelengths of RGB.
The display apparatus 100 using the light guide element 10 may be configured to display color images in a range within the FOV of 55 degrees and to display simple information or monochromatic images in an area exceeding the FOV of 55 degrees. For example, simple information in letters and symbols, icons, toolbars, and the like may be displayed in an edge portion of the field of view. The display apparatus 100 may be provided with a cover covering a rear surface of the diffraction layer 12, a rear surface of the light guide substrate 11, or both. In that case, the cover may be a part of the light guide element 10 and held by the support 120 shown in
In the configuration shown in
The light guide element 10 may adopt any configuration of the grating shown in
The diffraction layer 12 including the in-coupling grating 121 and the outcoupling grating 123 is formed of, for example, ZrO2, HfO2, Ta2O5, Nb2O5, TeO2, MoO3, WO3, TiO2, SiN, SiON, SnO, ITO (indium tin oxide), Al2O3, Y2O3, AlN, MgO, or a mixture of two or more of the above-described compounds. Alternatively, the diffraction layer 12 may be formed of a glass material containing three or more inorganic elements. The diffraction layer 12 may be deposited on the surface of the light guide substrate 11 by vapor deposition, sputtering, or the like. The in-coupling grating 121, the extended grating 122, and the outcoupling grating 123 are formed by forming desired patterns on the diffraction layer 12 by etching, such as a line-and-space pattern, a rectangular grating pattern, or the like.
When the diffraction layer 12 is formed with a mixture of two or more of the above-described compounds, a film may be formed by co-sputtering to match the dispersion of the refractive index, that is, the wavelength dependence of the refractive index to that of the light guide substrate 11. The diffraction layer 12 may be formed by etching or by a lift-off method. As the diffraction layer 12, a diffraction grating may be carved directly into a glass substrate, a single crystal substrate, or the like. In this case, the light guide substrate 11 and the diffraction layer 12 can be formed integrally with a high-refractive-index material. When the light guide element 10 is applied to the display apparatus 100 shown in
Throughout the practical examples and comparative examples, wavelengths of λ1 of 450 nm, λ2 of 532 nm, and λ3 of 633 nm are commonly used. The aspect ratio of the input image is 16:9. The diffraction layer 12 is formed of an inorganic film having a higher refractive index than that of the light guide substrate 11 at λ1, λ2, and λ3. Alternatively, the grating is formed directly on the light guide substrate 11. In these cases, because an occurrence of total internal reflection of the guided light is determined by the refractive index of the light guide substrate 11, it is not necessary to consider the refractive index of the diffraction layer 12 in the discussion of the viewing angle.
A light guide element according to Example 1 includes the in-coupling grating 121 using a line-and-space pattern (denoted as “L&S” in
The refractive index of the light guide substrate 11 in Example 1 at λ3 is 2.08. The shorter the wavelength, the greater the refractive index affecting the wave, so the refractive index for the d-line (wavelength is 587.56 nm) is greater than 2.08. The diagonal FOV at λ2 in Example 1 is greater than 70 degrees, and a maximum guiding FOV at all of λ1, λ2, and λ3 is greater than 55 degrees.
In Example 2, a two-axis diffraction grating, in which the unit grating is a rectangular grating, is used for both the in-coupling grating 121 and the outcoupling grating 123, and diffractions in the positive and negative one-dimensional directions are used. In each of the in-coupling grating 121 and the outcoupling grating 123, the grating pitch in the x-direction is 310 nm and the pitch in the y-direction is 355 nm. For the light guide substrate 11, a Bi2O3—B2O3—TeO2—P2O5—Nb2O5—ZnO glass substrate of the same composition as in Example 1 is used. The refractive index of this light guide substrate 11 at λ3 is 2.08. The diagonal FOV at λ2 in Example 2 is greater than 70 degrees, and the maximum guiding FOV at all of λ1, λ2, and λ3 is greater than 55 degrees.
In Example 3, line-and-space patterns are used for both the in-coupling grating 121 and the outcoupling grating 123. The pitch of the in-coupling grating 121 in the x-direction is 270 nm, and the pitch of the outcoupling grating 123 in the y-direction is 300 nm. For the light guide substrate 11, a single-crystal substrate of SiC is used. The refractive index of this SiC substrate at λ3 is 2.63. The diagonal FOV at λ2 in Example 3 is greater than 100 degrees, and the maximum guiding FOV at all of λ1, λ2, and λ3 is greater than 65 degrees. By using the light guide substrate 11 with the large refractive index, a high FOV can be achieved while using a line-and-space diffraction grating.
In Example 4, a two-axis diffraction grating, in which the unit grating is a rectangular grating, is used for both the in-coupling grating 121 and the outcoupling grating 123. The in-coupling grating 121 has a pitch of 270 nm in the x-direction and a pitch of 310 nm in the y-direction. The outcoupling grating 123 has a pitch of 300 nm in the x-direction and a pitch of 310 nm in the y-direction. For the light guide substrate 11, a single-crystal substrate of SiC is used as in Example 3. The refractive index of the SiC substrate at λ3 is 2.63. The diagonal FOV at λ2 in Example 4 is greater than 110 degrees, and the maximum guiding FOV at all of λ1, λ2, and λ3 is greater than 85 degrees. In Example 4, a high FOV can be achieved by using the light guide substrate 11 having the high refractive index and the two-axis diffraction grating, in which the unit grating is a rectangular grating, for both the in-coupling grating 121 and the outcoupling grating 123.
In Comparative Example 1, the same light guide substrate 11 (refractive index is 2.08 at λ3) as in Examples 1 and 2 is used, but a two-axis diffraction grating, in which the unit grating is a square grating, is used for both the in-coupling grating 121 and the outcoupling grating 123. In each of the in-coupling grating 121 and the outcoupling grating 123, grating pitches in the x- and y-directions are 310 nm. Also in Comparative Example 1, diffractions in the positive and negative one-dimensional directions are used. In Comparative Example 1, the diagonal FOV at λ2 is less than 70 degrees, and the maximum guiding FOV at all of λ1, λ2, and λ3 is less than 55 degrees. Although the same light guide substrate 11 having the high refractive index as in Examples 1 and 2 is used, the diagonal FOV becomes smaller compared with Examples 1 to 3 by using the two-axis diffraction grating, in which the unit grating is a square grating, for an input image with an aspect ratio of 16:9.
In Comparative Example 2, lead-free and arsenic-free optical glass described in Japanese Patent No. 4970896 is used for the light guide substrate. The composition of this optical glass (mol %) is GeO3:Bi2O3:B2O3:ZnO:SiO2:Li2O:BaO:Sb2O3=30.9:25.0:15.9:10.0:8.0:5.0:5.0:0.1.
The refractive index of the optical glass substrate of Comparative Example 2 at λ3 is 1.99, and the refractive index nd for the d-line is in the range of 1.91≤nd≤2.05. A two-axis diffraction grating, in which the unit grating is a rectangular grating, is used for both the in-coupling grating 121 and the outcoupling grating 123, and diffractions in the positive and negative one-dimensional directions are utilized. The pitch of the in-coupling grating 121 in the x-direction is 310 nm and the pitch in the y-direction is 360 nm. The pitch of the outcoupling grating 123 in the x-direction is 310 nm and the pitch in the y-direction is 370 nm. In Comparative Example 2, the diagonal FOV at λ2 exceeds 70 degrees, but the maximum guiding FOV at all of λ1, λ2, and λ3 is less than 55 degrees. Although a two-axis diffraction grating, in which the unit grating is a rectangular grating, is used for the in-coupling grating 121 and the outcoupling grating 123 and diffractions in the positive and negative one-dimensional directions are used, a high FOV cannot be achieved for all colors of RGB, because the refractive index at λ3 of the light guide substrate 11 is 1.99.
In Comparative Example 3, the same optical glass as in Comparative Example 2 is used. The refractive index of the light guide substrate at λ3 is 1.99, and the refractive index nd for the d-line is 1.91−2.05 (1.91≤nd≤2.05). In Comparative Example 3, a line-and-space pattern is used for both the in-coupling grating 121 and the outcoupling grating 123. The pitch of the in-coupling grating 121 in the x-direction is 360 nm, and the pitch of the outcoupling grating 123 in the y-direction is 360 nm. In Comparative Example 3, the diagonal FOV at λ2 is less than 60 degrees, and the maximum guiding FOV at all of λ1, λ2, and λ3 is less than 35 degrees. When the light guide substrate 11 with the refractive index nd of 2.05 or less for the d-line is used, and a line-and-space pattern is used for both the in-coupling grating 121 and the outcoupling grating 123, the FOV value required for the display apparatus 100 cannot be achieved.
From the results shown in
Strontium titanate (SrTiO3), potassium tantalite (KTaO3), and diamond are optically isotropic. Titanium oxide (TiO2), lithium niobate (LiNbO3), and silicon carbide (SiC) are uniaxial and exhibit birefringence. However, the orientation of the optical axis with respect to the direction normal to the substrate is within ±4 degrees, and the reduction in resolution due to the generation of double images derived from the birefringence in viewing transmitted images of real images does not affect the FOV. In any of the crystals, an absorption edge resides in the ultraviolet region, and visible light is transmitted.
In Example 5, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is a Bi2O3—B2O3—TeO2—P2O5—Nb2O5—ZnO glass substrate used in Example 1 and Example 2. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.08 for the wavelength of 450 nm, 0.04 for the wavelength of 532 nm, and 0.02 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Example 6, ZrO2 is used for the diffraction layer 12. The light guide substrate 11 is a Bi2O3—B2O3—TeO2—P2O5—Nb2O5—ZnO glass substrate used in Example 1 and Example 2. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.05 for the wavelength of 450 nm, 0.06 for the wavelength of 532 nm, and 0.08 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Example 7, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is a Bi2O3—TiO2—Nb2O5—WO3—B2O3—P2O5—SiO2—BaO substrate. The specific composition of the light guide substrate 11 (mol %) is Bi2O3:TiO2:Nb2O5:P2O5:WO3:B2O3:BaO:SiO2=21.0:18.5:16.5:22.6:14.5:2.8:2.8:1.6. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.05 for the wavelength of 450 nm, 0.02 for the wavelength of 532 nm, and 0.01 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Example 8, ZrO2 is used for the diffraction layer 12. The light guide substrate 11 is a Bi2O3—TiO2—Nb2O5—WO3—B2O3—P2O5—SiO2—BaO substrate used in Example 7 and has the same composition as in Example 7. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.02 for the wavelength of 450 nm, 0.04 for the wavelength of 532 nm, and 0.06 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Example 9, TiO5 is used for the diffraction layer 12 and a single crystal substrate of LiNbO3 is used for the light guide substrate 11. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.07 for the wavelength of 450 nm, 0.03 for the wavelength of 532 nm, and 0.01 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Example 10, Nb2O5 is used for the diffraction layer 12. The light guide substrate 11 is a LiNbO3 single crystal substrate same as in Example 9. For each wavelength, the refractive index of the diffraction layer 12 is higher than the refractive index of the light guide substrate 11. The difference in refractive index is 0.07 for the wavelength of 450 nm, 0.05 for the wavelength of 532 nm, and 0.03 for the wavelength of 633 nm. Thus, any of the differences are less than 0.1.
In Comparative Example 4, Ta2O5 is used for the diffraction layer 12. The light guide substrate 11 is a lead-free and arsenic-free optical glass substrate disclosed in Japanese Patent No. 4970896 used in Comparative Example 2 and Comparative Example 3. With this combination, the refractive index of the diffraction layer 12 is high, but the refractive index of the light guide substrate 11 for each wavelength is 2.05 or less, and the difference in refractive index becomes large. The difference in refractive index is 0.18 for the wavelength of 450 nm, 0.14 for the wavelength of 532 nm, and 0.11 for the wavelength of 633 nm. Thus, any of the differences exceed 0.1.
In Comparative Example 5, ZrO2 is used for the diffraction layer 12. As in Comparative Example 4, the light guide substrate 11 is a lead-free and arsenic-free optical glass substrate in Japanese Patent No. 4970896. Even with this combination, the refractive index of the diffraction layer 12 is high, but the refractive index of the light guide substrate 11 for each wavelength is 2.05 or less, and the difference in refractive index becomes large. The difference in refractive index is 0.15 for the wavelength of 450 nm, 0.16 for the wavelength of 532 nm, and 0.16 for the wavelength of 633 nm. Thus, any of the differences exceed 0.1.
When the refractive index of the diffraction layer 12 is lower than the refractive index of the light guide substrate 11, light with the Numerical Aperture (NA=n×sin(θ)), which is determined by a refractive index between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11, cannot be emitted from the light guide substrate 11 to the diffraction layer 12 and does not diffract. Therefore, the refractive index of the diffraction layer 12 at λ3 is required to be greater than the refractive index of the light guide substrate 11. On the other hand, when the difference, Δn, between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 is large, reflection at an interface becomes large and the extraction efficiency at the outcoupling grating 123 decreases. Therefore, for the wavelengths of λ1, λ2, and λ3, the difference between the refractive index of the diffraction layer 12 and the refractive index of the light guide substrate 11 is preferably 0.1 or less (Δn≤0.1). In particular, for the wavelength of λ3, which performs total internal reflection guiding in the light guide substrate 11 at a large reflection angle, Δn is preferably smaller than 0.1, because influence of interface reflection becomes large. At λ3, Δn is more preferably smaller than 0.05, and even more preferably smaller than 0.03. When the differences in refractive indices at λ1, λ2, and λ3 are denoted by Δnλ1, Δnλ2, and Δnλ3, the influence from the interface reflection is easily reduced for all wavelengths in the case where Δnλ1?Δnλ2?Δnλ3. In many cases, for the wavelengths λ1 and λ2, it is possible to perform the total internal reflection light guiding in a desired FOV even when the refractive index of the diffraction layer 12 is lower than the refractive index of the light guide substrate 11. Therefore, a combination of materials may be selected such that the materials have a lower dispersion than the dispersion of the light guide substrate 11 and the refractive index of the light guide substrate 11 and the diffraction layer 12 match each other for a wavelength shorter than λ3.
The refractive index for each wavelength is obtained from the refractive index nd for the d-line and the Abbe's number νd for the d-line, in the same manner as in
In the outer circle, θprop is 90 degrees (θprop=90 degrees). The radius of the outer circle is determined by the refractive index n of the light guide substrate 11. Therefore, the higher the refractive index of the light guide substrate 11, the wider the outer circle and the wider the range of an angle in which the total internal reflection guiding can be performed.
In
With reference to
n
in×sin(θin)+mλ/Λ=nout×sin(θout).
With reference to
On the other hand, when light of the incident angle corresponding to the FOV on the positive side is in-coupled in the propagating direction, as shown in
Light corresponding to the positive side FOV of each of the colors of RGB is guided in the total internal reflection propagation region T on the left side of the NA diagram. Light corresponding to the negative side FOV of each of the colors of RGB is guided in the total internal reflection propagation region T on the right side of the NA diagram. Because there is a direction in which light corresponding to the FOV of each of the colors of RGB can be guided, an occurrence of vignetting is suppressed. Thus, the expansion effect of the FOV is obtained by using diffractions in the positive and negative one-dimensional directions. For example, by using a light guide substrate with the refractive index of 2.08 at λ3, a diagonal FOV of 55 degrees or more can be achieved for any of λ1, λ2, and λ3 included in RGB.
Also in the +y and −y-directions, light with each wavelength of RGB propagates within the total internal reflection propagation region T. Thus, an occurrence of vignetting is suppressed. Using a light guide substrate 11 that is a single-layer light guide substrate with the refractive index of 2.08 at λ3, the diagonal FOV of 55 degrees or more for each wavelength of RGB can be obtained.
According to the above description, it is possible to design an optical element having the FOV of 55 degrees or more for each wavelength of RGB and excellent visibility, in the case of projecting an image where an aspect ratio of a projected image is Ax:Ay, when the refractive index of the light guide substrate 11 at λ3 is n_λ3, at least the outcoupling grating is a two-axis diffraction grating, in which the unit grating is a rectangular grating, and a pitch in the x-direction, Λx, and a pitch in the y-direction, Λy, of the rectangular grating satisfy the following relations, where
1≤λ1/Λx−sin(FOVx−)
(λ3/Λx)2+sin(FOVy+)2≤(n_λ3)2
(λ3/Λx)2+sin(FOVy−)2≤(n_λ3)2 (1)
1≤λ1/Λx−sin(FOVx+)
(λ3/Λx)2+sin(FOVy+)2≤(n_λ3)2
(λ3/Λx)2+sin(FOVy−)2≤(n_λ3)2 (2)
1≤λ1/Λy−sin(FOVy+)
λ3/Λy+sin(FOVy−)≤(n_λ3)
(λ3/Λy+sin(FOVy−))2+sin(FOVx+)2≤(n_λ3)2
(λ3/Λy+sin(FOVy−))2+sin(FOVx−)2≤(n_λ3)2 (3)
tan(((FOVx+)+(FOVx−))/2)=Ax/(Ax2+Ay2)1/2·tan(FOVdiag/2) (4)
tan(((FOVy+)+(FOVy−))/2)=Ay/(Ax2+Ay2)1/2·tan(FOVdiag/2) (5)
FOVdiag≥55 degrees (6)
The middle figure in
In the diffraction pattern of the second grating, light with each wavelength of RGB diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. The grating pitch of the in-coupling grating 121 and the grating pitch of the outcoupling grating 123 are set so that the diffraction is performed in the NA diagrams shown in
In the G light guiding of
The middle figure in
In the diffraction pattern of the second grating, which is a two-axis diffraction grating, in which the unit grating is a rectangular grating, G light diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. It is possible to expand the FOV for G light using the light guide substrate 11 that is a single-layer light guide substrate with the above-described configuration of grating. However, even when R light or B light is incident normal to the second grating in the diagonal FOV of 70 degrees, total internal reflection light guiding is not necessarily achieved in both the ±x-directions and the ±y-directions.
<FOV characteristics of Example 2>
The middle figure in
In the diffraction pattern of the first grating, in the x-axis direction, light corresponding to the positive side FOV diffracts into the left side part of the total internal reflection propagation region, and light corresponding to the negative side FOV diffracts into the right side part of the total internal reflection propagation region. In the y-axis direction, light with each wavelength of RGB corresponding to all the FOVs diffracts into the total internal reflection propagation region. Also in the diffraction pattern of the second grating, light with each wavelength of RGB diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. The grating pitch of the in-coupling grating 121 and the grating pitch of the outcoupling grating 123 are set so that the diffraction is performed in the NA diagrams shown in
In the G light guiding of
The middle figure in
Also in the diffraction pattern of the second grating, in which the unit grating is a rectangular grating, G light diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. It is possible to expand the FOV for G light using the light guide substrate 11 that is a single-layer light guide substrate. However, even when R light or B light is incident normal to the second grating in the diagonal FOV of 70 degrees, total internal reflection light guiding is not necessarily achieved in both the ±x-directions and the ±y-directions.
The middle figure in
In the diffraction pattern of the first grating, light with each wavelength of RGB corresponding to the incident FOV diffracts into the total internal reflection propagation region in the +x-direction and the −x-direction. In the diffraction pattern of the second grating, light with each wavelength of RGB corresponding to the incident FOV diffracts into the total internal reflection propagation region in the +y-direction and the −y-direction. The grating pitch of the in-coupling grating 121 and the grating pitch of the outcoupling grating 123 are set so that the diffraction is performed in the NA diagrams shown in
In the G light guiding shown in
The middle figure in
The middle figure in
Also in the diffraction pattern of the second grating, light with each wavelength of RGB diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. The grating pitch of the in-coupling grating 121 and the grating pitch of the outcoupling grating 123 are set so that the diffraction is performed in the NA diagrams shown in
In the G light guiding shown in
The middle figure in
Also in the diffraction pattern of the second grating, which is a two-axis diffraction grating, in which the unit grating is a rectangular grating, G light diffracts into the total internal reflection propagation region in the ±x-directions and in the ±y-directions. It is possible to expand the diagonal FOV for at least G light to 110 degrees using the light guide substrate 11 that is a single-layer light guide substrate. However, even when R light or B light is incident normal to the first grating or the second grating in the diagonal FOV of 110 degrees, total internal reflection light guiding is not necessarily achieved in both the ±x-directions and the ±y-directions.
As described above, the present invention has been described based on the specific configuration examples. However, the present invention is not limited to the above-described examples. Various variations, modifications, substitutions, additions, deletions, and combinations are possible within the scope of claims. They also of course fall within the technical scope of the present disclosure. The display apparatus 100 using the light guide element 10 may be linked with a smartphone, a laptop personal computer (PC), or the like. The display screen of the smartphone or the laptop PC may be set to have a working field of view with the diagonal FOV of 55 degrees, and set to have an indirect field of view with the diagonal FOV of 70 degrees or more around the working field of view. In this case, a simple monochromatic image or information may be displayed in the indirect field of view within a range that does not interfere with daily activities.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-079180 | May 2021 | JP | national |
The present application is a continuation application of International Application No. PCT/JP2022/018697, filed Apr. 25, 2022, which claims priority to Japanese Patent Application No. 2021-079180 filed May 7, 2021. The contents of these applications are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP22/18697 | Apr 2022 | US |
| Child | 18384838 | US |
