Compact head-mounted display system displaying uniform image

Abstract

To provide an optical device that is as light and as compact as possible.SOLUTION: There is disclosed an optical device, including: a light-transmitting substrate having an input aperture, an output aperture, at least two major surfaces and edges; an optical element for coupling light waves into the light-transmitting substrate by total internal reflection; at least one partially reflecting surface located between the two major surfaces of the light-transmitting substrate for partially reflecting light waves out of the light-transmitting substrate; a first transparent plate, having at least two major surfaces, one of the major surfaces of the transparent plate being optically attached to a major surface of the light-transmitting substrate defining a flat interface; and a beam-splitting coating applied to the flat interface between the light-transmitting substrate and the transparent plate, where light waves coupled into the light-transmitting substrate are partially reflected from the flat interface and partially pass therethrough.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a substrate transmission type optical device, and more particularly to a device having a plurality of reflecting surfaces provided on a common optical transmission substrate, and also to a device also called a light-guide optical element (LOE).

[0002] The present invention can be advantageously realized for applications of a number of image technologies such as head-mounted displays, head-up displays, mobile phones, compact displays, 3-D displays, compact beam expanders, etc., and also for non-image devices such as flat panel display devices, compact lighting devices, and scanners, etc.

Background Art

[0003] As an important application of compact optical elements, there is a head-mounted display. Its optical module functions as both an imaging lens and a combiner, where a two-dimensional display image is imaged at infinity, and this is reflected and enters the observer's eye. The display image can be obtained directly or indirectly. As direct ones, there are spatial light modulators (SLMs), such as cathode ray tubes (CRTs), liquid crystal displays (LCDs), organic light-emitting diode arrays (OLEDs), scanning sources, or similar devices. As indirect ones, they are obtained from relay lenses or optical fiber bundles. The display image comprises an array of elements (pixels), and the array of elements (pixels) is imaged at infinity by a collimating lens and transmitted towards the observer's eye by a reflecting surface or a partially reflecting surface that acts as a non-see-through or see-through combiner respectively. Generally, conventional free-space optical modules are used for this purpose. However, as the desired field of view (FOV) of this system becomes wider, the conventional optical modules gradually become larger, heavier, and bulkier, and are not practical even for medium-performance types. This is a major drawback for all types of displays, especially head-mounted ones. Inevitably, this system is desired to be as lightweight and compact as possible.

Summary of the Invention

[0004] By pursuing compactness, various optical solutions can be obtained, but they become complex. None of the actual examples are sufficiently compact. On the other hand, major drawbacks remain from the perspective of manufacturing suitability, causing inconvenience. Furthermore, the optical field of view and the eye-motion-box (EMB) of the eye's movable range obtained based on these devices are quite small, for example, less than 8 mm. Therefore, the performance of the optical system is very sensitive even to a slight movement of the optical system with respect to the observer's eye, and it has not been sufficient to allow the pupil movement necessary to comfortably read text on such a display.

[0005] The disclosure content contained in all the published gazettes related to the present applicant, namely WO01 / 95027, WO03 / 081320, WO2005 / 024485, WO2005 / 024491, WO2005 / 024969, WO2005 / 124427, WO2006 / 013565, WO2006 / 085309, WO2006 / 085310, WO2006 / 087709, WO2007 / 054928, WO2007 / 093983, WO2008 / 023367, WO2008 / 129539, WO2008 / 149339, WO2013 / 175465, Israeli Patent Application No. 232197, Israeli Patent Application No. 235642, Israeli Patent Application No. 236490, and Israeli Patent Application No. 236491 are hereby incorporated by reference in this case.

[0006] In applications, the present invention facilitates the use of a very compact light guide optical element (LOE) for a head-mounted display. The present invention provides a wide field of view (FOV) with a relatively large EMB value. The optical system obtained here provides a large-screen, high-quality image and is adapted to large eye movements. The optical system according to the present invention is much more compact than the state-of-the-art implementations and can also be easily integrated into optical systems with special structures, thus having particularly excellent advantages.

[0007] A further major application of the present invention is to provide a compact display with a wide FOV for mobile handheld devices such as mobile phones. In today's market of wireless Internet access, a large bandwidth is available to suffice video transmission. The limiting factor is the quality of the display of the device held by the end user. Due to the requirements for mobile devices, the physical size of the display is limited, resulting in a low quality of the directly-viewed browsing image. The present invention enables the realization of a physically very compact display with a very large virtual image. This is a key feature in mobile communication, especially mobile Internet access, and provides a solution to one of the main limitations of the practical configuration. Therefore, the present invention enables browsing of digital contents of full-format Internet pages on small handheld devices such as mobile phones.

[0008] A major object of the present invention is thus to overcome the drawbacks of the compact optical display device according to the prior art and to provide other optical components and optical systems with improved performance based on specific requirements.

[0009] According to the present invention, there is provided an optical device comprising an optical transmission substrate having an incident aperture, an exit aperture, and at least two major surfaces and edges, an optical element for input-coupling light waves into the optical transmission substrate by internal total reflection, at least one partial reflection surface provided between the two major surfaces of the optical transmission substrate for partially reflecting light waves from the optical transmission substrate, a first transparent plate having at least two major surfaces, one of the major surfaces of the transparent plate being optically coupled and attached to the major surface of the optical transmission substrate defining a flat interface, and a beam splitter coating provided in a layer on the flat interface between the optical transmission substrate and the transparent plate, wherein the light waves input-coupled into the optical transmission substrate are partially reflected and partially transmitted at the flat interface. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] To better understand the present invention, the following preferred embodiments will be described with reference to the accompanying drawings.

[0011] Particularly in accordance with the content of the drawings, it is emphasized that the specific content shown is merely an example and is only intended to be described as an illustration of the preferred embodiments of the present invention. It is presented in a form that is considered to be the most beneficial and immediately understandable for explaining the principles and concepts of the invention. From this perspective, the invention is not intended to be shown structurally in more detail than is necessary for a fundamental understanding of the principles of the invention. The description made with reference to the drawings will provide suggestions to those skilled in the art regarding how various examples of the invention can be implemented.

[0012] In the drawings,

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13A

Figure 13B

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18A

Figure 18B

Embodiments for Carrying Out the Invention

[0013] Figure 1 is a cross-sectional view of a light guide optical element (LOE) according to the present invention. The first reflecting surface 16 is illuminated by the display light 18 of parallel light generated from a light source (not shown) disposed behind the optical element. The reflecting surface 16 reflects the incident light from the display light source so that the incident light is captured by total internal reflection within the planar substrate 20. After several reflections at the surfaces 26, 27 of the substrate, the captured light wave reaches the array of partial reflecting surfaces 22. This outputs the light from the substrate and directs it in the direction of the observer's eye 24 having a pupil 25. The input surface of the LOE is defined as the surface on which the input light wave is incident on the LOE, and the output surface of the LOE is defined as the surface from which the captured light wave exits the LOE. Also, the incident aperture of the LOE means the portion of the input surface through which the input light wave actually passes when entering the LOE, and the exit aperture of the LOE means the portion of the output surface through which the output light wave actually passes when entering the LOE. In the case of the LOE shown in Figure 1, both the input surface and the output surface coincide with the lower substrate surface 26, but other configurations are also envisioned where the input light wave and the image light wave are located on the opposite side of the substrate or at one edge of the LOE. Here, the central light wave of the display light source is output-coupled from the substrate 20 in the normal direction of the substrate surface 26, the partial reflecting surface 22 is flat, and the off-axis angle of the input-coupled light wave in the substrate 20 is α in If so, the angle α sur2 between the substrate plane (normal) and the reflecting surface is as follows.

[0014]

Equation

[0015] As can be seen from Figure 1, the captured light rays reach the reflecting surface in two different directions 28, 30. In this particular embodiment, the captured light rays reach the partial reflecting surface 22 in one of the plurality of directions 28 after an even number of reflections from the substrate surfaces 26, 27. Here, the incident angle β ref between the captured light ray and the normal of the reflecting surface is as follows.

[0016]

Equation

[0017] The captured light ray reaches the reflecting surface in the second direction 30 after being reflected an odd number of times from the substrate surfaces 26, 27. Here, the off-axis angle is α'. in = 180° - α in and the angle of incidence between the captured light ray and the normal of the reflecting surface is as follows.

[0018] [Number]

[0019] In the formula, the minus sign indicates that the captured light ray hits the opposite side of the partial reflecting surface 22.

[0020] As shown in FIG. 1, for each surface, the light ray reaches the substrate surface along the direction 30. Some of these light rays hit the substrate surface again along the direction 28. In order to prevent useless reflections and ghost images, it is important that the reflectivity is negligible for the light rays hitting the substrate surface located in the direction 28.

[0021] Here, the important issue to be considered is the actual effective range (area) of each reflecting surface. The non-uniformity that may occur in the obtained image may be due to different reflection sequences of different light rays reaching each selectively reflecting surface. Some light rays reach without prior interaction with the partial reflecting surface, and other light rays reach after one or more partial reflections. This situation is shown in FIG. 2A. For example, if α in = 50°, the light ray 80 intersects the partial reflecting surface 22 at the point 82. The angle of incidence of the light ray is 25°, and a part of the energy of the light ray is output-coupled from the substrate. Then the light ray intersects the same (selective) partial reflecting surface at the point 84 at an angle of incidence of 75° without being particularly strongly reflected, and intersects again at the point 86 at an angle of incidence of 25°. The remaining energy of the light ray is output-coupled from the substrate here. On the other hand, the light ray 88 shown in FIG. 2B makes only one reflection 90 from the same reflecting surface. Multiple reflections more than this occur at another partial reflecting surface.

[0022] Figure 3 shows the phenomenon of non-uniformity in a detailed cross-sectional view of the partial reflecting surface 22 that emits the light captured within the substrate toward the observer's eye 24. As shown, the light ray 80 is reflected by the upper surface 27 just before the line 100 which is the intersection line of the partial reflecting surface 22 and the surface 27. Since this light ray does not hit the partial reflecting surface 22, the brightness remains the same. The first incidence on the partial reflecting surface 22 occurs at point 102 after double reflections at both outer surfaces. At this point, the light wave is partially reflected and the light ray 104 is output-coupled from the substrate 20. For another light ray, for example, the light ray 88 located just below the light ray 80, the first incidence on the partial reflecting surface 22 occurs before reaching the surface 27 at point 106. At point 106, the light wave is partially reflected and the light ray 108 is output-coupled from the substrate. Therefore, after double reflections at the outer surfaces 26, 27 and hitting the partial reflecting surface 22 again at point 110, the brightness of the output-coupled light ray becomes smaller than that of the adjacent light ray 104. As a result, all the light rays having the same input-coupled angle as the light ray 80 reaching the partial reflecting surface 22 on the left side of point 102 have a smaller brightness. Therefore, the reflection from the partial reflecting surface 22 actually becomes "darker" on the left side of point 102 for this specific input-coupled angle.

[0023] In reality, the human eye has become accustomed to tolerating significant variations in brightness unconsciously, but it is difficult to completely correct such differences in the phenomenon of multiple crossings. In a near-to-eye display, the eye integrates the light emitted at a single viewing angle and forms an image of it at a point on the retina. Since the response curve of the eye is logarithmic, small variations in the brightness of the display unit may not be recognized. Even when the uniformity of light within the display unit is only at a medium level, the human eye can visually recognize high-quality images. This required medium level of uniformity can be properly achieved by using the optical element shown in FIG. 1. A system having a large FOV and requiring a large EMB requires a relatively large number of partial reflection surfaces to obtain the desired exit aperture. As a result, the non-uniformity caused by multiple crossings at a large number of partial reflection surfaces becomes more prominent, especially for a display unit located away from the eye, such as a head-up display. This non-uniformity is unacceptable. In such cases, a more systematic method is needed to solve the non-uniformity.

[0024] The "darker" part of the partial reflection surface 22 contributes less to the output coupling of the captured light wave from the substrate, so the influence on the optical performance of the LOE is only negative. That is, a dark part is generated in the exit aperture of the system, and a dark stripe is formed in the image. However, the transparency of each reflection surface is uniform with respect to the light wave from the external scene. Therefore, if the reflection surfaces are set to overlap with each other so as to correct the dark part within the exit aperture, the light rays from the external scene crossing the overlapping part will undergo double attenuation, and a dark stripe will be formed in the external scene. This phenomenon will not only greatly degrade the performance of a display unit located away from the eye, such as a head-up display, but also make the near-to-eye display unusable.

[0025] Figure 4 shows an embodiment for solving this problem. Only the "bright" portions of the partial reflecting surfaces 22a, 22b, 22c are embedded inside the substrate. That is, they do not intersect the lower major surface 26 and end in front of this surface. Since the ends of the reflecting surfaces are adjacent to each other beyond the length of the LOE, no gap occurs in the projected image. Since there is no overlap between the reflecting surfaces, there is no gap when viewed from the outside. There are several ways to fabricate the LOE, one of which is to attach a transparent plate 120 having a thickness T to the effective range of the substrate, preferably by optical cement bonding. In order to accurately utilize only the effective range of the partial reflecting surface 22, it is important to calculate the actual effective range of each partial reflecting surface and the required thickness T of the transparent plate 120.

[0026] In FIG. 5, as a function of the input coupling angle α in the size Dn of the effective aperture of the reflecting surface 22n formed in the plane of the surface 26 is as follows.

[0027]

Equation

[0028] The capture angle α in is variable as a function of the FOV, so it is important to know what angle to associate with each reflecting surface 22n in order to calculate the size of the effective aperture of the reflecting surface 22n.

[0029] FIG. 6 shows the effective aperture as a function of the field of view angle with respect to the system parameters. The system parameters are a substrate thickness d = 2 mm, a substrate refractive index ν = 1.51, the angle α of the partial sur reflecting surface = 64°. Considering the field of view angle, it can be seen that different portions of the resulting image are formed based on different portions of the partial reflecting surface.

[0030] Fig. 7, which is a cross-sectional view of the compact LOE display system based on this configuration, shows this effect. Here, a single plane light wave 112 forming a specific viewing angle 114 illuminates only a part of the entire array of partial reflection surfaces 22a, 22b, 22c. Thus, a nominal viewing angle is defined for each point on the partial reflection surface, and the required effective range of the reflection surface is calculated based on this viewing angle. The exact and detailed setting of the effective ranges of the various partial reflection surfaces is performed as follows. For each specific surface, a light ray is plotted (taking into account refraction based on Snell's law) from the left edge of the surface towards the center of a specific eye pupil 25. The calculated direction is taken as the nominal incident direction, and a specific effective range is calculated based on this direction.

[0031] As shown in Fig. 5, the exact value of the effective range (area) of the reflection surface can be used to determine each distance T between the left edge 102 of the bright part of each reflection surface 22n and the lower surface 26. To increase the effective range, it is required that the distance between the surfaces be small. This distance represents the thickness of the transparent plate 120 (Fig. 7) to be attached to the lower surface of the LOE. As shown in Fig. 5, the distance T as a function of the input coupling angle α in is as follows.

[0032]

Equation

[0033] Fig. 8 shows the required thickness T of the transparent plate 120 as a function of the viewing angle for the same parameters described above in relation to Fig. 6. Determining the thickness T as the maximum calculated value is suitable for ensuring that dark stripes do not appear on the image. If the transparent plate 120 is too thick, the opposite result will occur. That is, bright stripes will appear on the image.

[0034] As shown in FIG. 9, two light rays 122 and 124 are input-coupled into the planar substrate 20. These are partially reflected from the partial reflection surface 22a at points 126 and 128 respectively. However, only the light ray 122 hits the second partial reflection surface 22b at point 130 and is partially reflected here. The light ray 124 skips the partial reflection surface 22b without being reflected. As a result, the brightness of the light ray 124 incident on the partial reflection surface 22c at point 134 is greater than the brightness of the light ray 122 at point 132. Therefore, the brightness of the output-coupled light ray 138 from point 134 is greater than the light ray 136 output-coupled at point 132, and bright stripes appear in the image. Thus, the exact value of the thickness T should be selected so that neither dark stripes nor bright stripes appear in the image.

[0035] As shown in FIG. 10, a possible example of establishing a required structure in which the thickness T of the transparent plate 120 depends on the viewing angle is to use a wedge-shaped substrate 20' whose two main surfaces are not parallel. A complementary wedge-shaped transparent plate 120' is attached to the substrate, preferably by optical cement bonding, so that the resulting combined structure becomes a perfect rectangular parallelepiped, that is, the two main surfaces outside the final LOE are parallel to each other. However, this method has several drawbacks. First of all, the manufacturing method of the wedge-shaped LOE is more complex and time-consuming than that of the parallel LOE. Also, this solution is effective for a small EMB system in which the viewing angle and the horizontal position are properly matched on the substrate plane. However, in the case of a large EMB system, that is, a system in which the eyes can move significantly along the horizontal axis, it is not possible to make a good adjustment between the viewing angle and the actual thickness of the wedge-shaped transparent plate 120'. Therefore, light and dark stripes may appear in the image.

[0036] The occurrence of bright and dark stripes due to the structure of the partial reflecting surface within the LOE is not limited to the surface where this phenomenon occurs. As shown in FIG. 3, the brightness of the incident light ray 88 reflected twice by the partial reflecting surface 22a is smaller at point 110 than the brightness of the light ray 80 reflected once by the partial reflecting surface 22a at point 102. As a result, the brightness of the reflected light wave 112 is smaller than the brightness of the adjacent light ray 104. However, as shown in FIG. 11, not only is the brightness of the reflected light wave from the partial reflecting surface 22a different, but the brightness of the light rays 140 and 142 after transmission is also different. As a result, the brightnesses of the reflected light rays 144 and 146 from the partial reflecting surface 22b at points 148 and 150, respectively, also differ, and dark stripes are generated in the area of this image. Therefore, due to the differences between these light rays, the transmission within the LOE continues to the next adjacent partial reflecting surface. As a result, for an LOE having a large number of partial reflecting surfaces, based on the correct incident angle, each partial reflecting surface forms bright and dark stripes, and a large number of bright and dark stripes are concentrated at the far edge of the exit aperture of the LOE. For this reason, the image quality is significantly degraded.

[0037] Another source of image non-uniformity may be the non-uniformity of the image light wave coupled into the LOE. Usually, when the two edges of the light source have slightly different emission intensities, even if there is such a situation, it is hardly noticed by the observer. This is completely different for an image that is coupled into the substrate and gradually coupled out, such as in the LOE. As shown in FIG. 12, two light rays 152, 154 are located at the edges of a plane wave 156 generated from the same point within a display source (not shown). If the brightness of light ray 152 is less than that of light ray 154 as a result of an imperfect image system, since the light rays are separated from each other, even when directly viewing the plane wave 156, the difference between them is hardly recognized. However, after being coupled into the LOE 20, the conditions change. Light ray 154 illuminates the reflective surface 16 immediately to the right of the interface line 158 between the reflective surface 16 and the lower major surface 26. On the other hand, the right light ray 152 is reflected by the reflective surface 16, totally reflected by the upper major surface 27, and then hits the lower major surface 26 immediately to the left of the interface line 158. As a result, the two light rays 152, 154 are transmitted adjacent to each other within the LOE 20. The two emitted light rays 160, 162 generated from the light rays 152, 154 and reflected by the partial reflective surface 22a thus have different brightnesses. However, unlike the input plane wave 156, since the two different light rays are adjacent to each other, this difference is easily identified as a dark stripe on the image. These two light rays 164, 165 continue to be transmitted adjacent to each other within the LOE and cause a dark stripe at each position where they are coupled out together. Naturally, the best way to avoid this light non-uniformity is to ensure that all of the light waves coupled into the LOE have a uniform brightness across the entire incident aperture for all of the FOV. In a system having a large FOV with a wide incident aperture, it is very difficult to meet this requirement.

[0038] As shown in FIGS. 13A and 13B, the problem of light unevenness may also be solved by attaching a transparent plate to one of the main surfaces of the LOE already described with reference to FIG. 4. However, in the present embodiment, the beam splitter coating 166 is provided in a layer on the flat interface 167 between the planar substrate 20 and the transparent plate 120. As shown in FIG. 13A, two light rays 168 and 170 are input-coupled to the planar substrate 20. Only the light ray 168 hits the first partial reflection surface 22a at the point 172 and is partially reflected here. On the other hand, the light ray 170 skips the partial reflection surface 22a and is not reflected. As a result, if the two light rays have the same brightness when input-coupled to the LOE, the light ray 170 reflected upward from the lower main surface 26 will have a greater brightness than the light ray 168 reflected downward from the upper main surface 27. These two light rays intersect each other at the point 174 located on the flat interface 167. By the beam splitter coating provided here in a layer, the two intersecting light rays are each partially reflected and also partially transmit through the beam splitter coating. Therefore, the two light rays exchange energy, and the emitted light rays 176 and 178 from the intersection point 174 have the same brightness. This is approximately the average brightness of the two incident light rays 168 and 170. These light rays further exchange energy with two other light rays (not shown) at the intersection points 180 and 182. As a result of the energy exchange, the two reflected light rays 184 and 186 from the partial reflection surface 22b have approximately the same brightness, and the occurrence phenomenon of bright stripes is significantly improved.

[0039] Similarly, in FIG. 13B, light rays 188 and 190 are input-coupled into the planar substrate 20. However, only light ray 188 hits the first partial reflection surface 22a at point 192, where it is partially reflected and then reflected by the upper major surface 27. As a result, if the brightness when input-coupling into the LOE is equal for the two light rays, the light ray 190 reflected downward from the upper major surface 27 will have a greater brightness than light ray 188. However, the two light rays intersect with each other at point 194 located at the flat interface 167, where they exchange energy. In addition to this, the two light rays intersect with each other at points 196 and 198 located above the interface 167 for beam splitting. As a result, the light rays 200 and 202 reflected by surface 22a and the light rays 204 and 206 reflected by surface 22b will have substantially the same brightness, and accordingly, the phenomenon of dark stripes is significantly suppressed. Thus, the improved uniformity of the brightness effect can also be applied to the bright and dark stripes caused by the non-uniform illumination light entering the incident aperture of the LOE. Consequently, the brightness distribution of the light waves captured within the LOE is even more uniform within the range of the exit aperture than within the range of the incident aperture of the LOE.

[0040] As shown in FIG. 13A, the light rays 184 and 186 reflected by surface 22a intersect the interface 167 before output-coupling from the LOE. As a result, a simple reflective coating cannot be easily layered on the interface 167. This is because the interface 167 should be transparent to the light waves emitted from the substrate 20 and also transparent to the light waves from the external scene for use in a transmissive device. That is, the light waves should pass through the interface 167 at a small incident angle and be partially reflected at a larger incident angle. Usually, the incident angle for transmission is between 0° and 15°, and the incident angle for partial reflection is between 40° and 65°. In addition to this, since the light rays cross the interface 167 many times while being transmitted within the LOE, the absorption in the coating should be negligible. Therefore, a simple metal coating cannot be used, and a thin dielectric film coating with high transparency is required.

[0041] Figure 14 shows reflectivity curves as a function of the angle of incidence at three representative wavelengths, i.e., 470 nm, 550 nm, and 630 nm, in the bright-field region for s-polarized light. As shown, for example, for s-polarized waves, it is possible to perform the required operations of a reflectance of partial reflection (45 - 55%) at large angles of incidence in the range of 40 - 65° and a low reflectance (less than 5%) at small angles of incidence. For p-polarized waves, since it is close to the Brewster angle, it is impossible to obtain a large reflectance for angles of incidence in the range of 40 - 65°. Since the polarization commonly used in the LOE image system is s-polarization, the required beam splitter can be provided very easily. However, for light waves from an external scene that hit the interface at a small angle of incidence and are substantially unpolarized, the beam splitter coating should be substantially transparent, so the beam splitter coating should also have a low reflectance (less than 5%) for p-polarized waves at small angles of incidence.

[0042] The remaining difficulty lies in the fact that the LOE 20 is assembled from a plurality of different components. Usually, the manufacturing process includes cementing optical elements, and after the body of the LOE 20 is finished, the required angle-dependent reflective coating is applied to the light guide surface, so conventional high-temperature coating methods that may damage the cemented parts cannot be used. However, together with the ion-assisted coating method, new thin-film technologies can also be used for low-temperature processing. By eliminating the need to heat the parts, the cemented parts can be safely coated. As another method, by using a well-known high-temperature coating method and then performing cementing at the appropriate position, the required coating can be easily applied to the transparent plate 120 adjacent to the LOE 20. This additional example method can be clearly used only if the transparent plate 120 is not too thin and can be deformed during the coating process.

[0043] There are several issues to be considered when configuring the function of the beam splitter as described above.

[0044] a. The light rays captured within the LOE are not only totally reflected at the major surfaces 26, 27 but also at the flat interfaces 167 of internal partial reflections. Therefore, these three surfaces are parallel to each other, and it is important to set the initial input coupling direction within the LOE so that the light rays can be reliably maintained.

[0045] b. As shown in FIGS. 13A and 13B, the transparent plate 120 is thinner than the original LOE 20. Despite the fact that the thickness of the transparent plate 120 is important for optimizing uniformity as in the case of the non-coated plates in FIGS. 7 - 10, the thickness of the coated plate may be set for other reasons. That is, it is easy to fabricate a slightly thicker plate, apply a coating, and cement-join it. On the other hand, in the case of a slightly thinner plate, the effective volume of the LOE 20 that actually outputs and couples the light waves from the substrate becomes larger for a given substrate thickness. Also, the correct ratio of the thickness between the transparent plate 120 and the LOE 20 may affect the energy exchange process within the substrate.

[0046] c. Normally, for a beam splitter designed for a full-color image, the reflectivity curve should be as uniform as possible over the entire range of photopic vision in order to suppress the effects related to color. However, in the configuration illustrated in the present invention, many light rays cross each other many times before output-coupling from the LOE 20. Therefore, this requirement is not essential. Naturally, the beam splitter coating should consider the spectrum of the entire wavelength of the incident image, but the flatness regarding color with the partial reflection curve may be acceptable based on the system parameters.

[0047] d. The reflectivity / transmittance ratio of the beam splitter coating does not necessarily have to be 50% to 50%. Other ratios may be used to perform the necessary energy exchange between dark and bright light rays. Further, as shown in FIG. 15, a simpler beam splitter coating may be utilized. This reflectivity gradually increases from 35% at an incident angle of 40° and reaches 60% at an incident angle of 65°.

[0048] The number of beam splitter surfaces to be provided in the e.LOE is not limited to one. As shown in FIG. 16, another transparent plate 208 may be attached to the upper surface of the LOE by cement bonding. Further, a beam splitter coating may be provided on the flat interface 210 between the LOE 20 and the upper transparent plate 208 to obtain an optical device having two beam splitter surfaces. Two different light rays 212, 214 intersect each other at a point 215 on the coated flat interface 210. Further, light rays also intersect at points 216, 217 on the flat interface 210. This is done in addition to the intersection at the flat interface 167 for the lower beam split. As a result, it is expected that the uniformity of the reflected light rays 218, 220 will be even higher than that in the embodiments of FIGS. 13A and 13B. Of course, the method of creating an LOE having two flat interfaces for beam splitting is more difficult than that having only one flat interface. Therefore, it should only be considered for systems where there are significant problems with non-uniform surfaces. As described above, it is important that the four reflecting surfaces and the planes 26, 27, 167, 210 are all parallel to each other.

[0049] f. The transparent plate 120 does not necessarily have to be made of the same optical material as the LOE 20. Further, the LOE may be made of a silicate-based material, and for the purpose of eye safety, the transparent plate (layer) may be formed from a polymer-based material. Of course, it is necessary to pay attention to ensuring the optical performance of the outer surface and preventing deformation of the transparent plate.

[0050] g. So far, it has been assumed that all the transparent plates are blank. However, as shown in FIG. 17, partial reflecting surfaces 222a, 222b may be formed inside the transparent plate 120 to increase the available volume of the LOE. These surfaces need to be exactly parallel to the partial reflecting surfaces 22a, 22b and oriented in the correct same orientation.

[0051] All parameters of the above embodiments, such as the thickness of the transparent plate 120, the optical material, the original characteristics of the beam splitter coating, the number of beam splitter surfaces, and the positions of the partial reflection surfaces within the LOE, can take many different values. These are determined based on various parameters of the optical system and specific requirements for optical characteristics and manufacturing costs.

[0052] Hitherto, it has been assumed that light waves output-couple from the substrate at a partial reflection surface that is inclined with respect to the major surface and usually has a dielectric coating layer provided thereon. However, as shown in FIG. 18A, there is also a system in which light waves input-couple and output-couple with respect to a substrate using diffraction elements 230, 232. It can be said that the same uniformity problems as already described accompany this configuration. As shown in the figure, two light rays 234, 236 from the same point of the display source are positioned at intervals at two edges of the diffraction element 230 for input coupling and input-couple to the substrate 238. Adjacent light rays are output-coupled by the diffraction element 232 for output coupling, respectively. Therefore, if there are differences between the light rays, they can be easily seen in the output-coupled light waves. In addition to this, in order to enable a uniform output-coupled image, the diffraction efficiency of the diffraction element 232 for output coupling is gradually increased. As a result, different light rays from the same point light source may pass through different positions within the diffraction element 232 before output-coupling from the optical element, resulting in different brightnesses within the image. As another cause of light non-uniformity, while the light ray 236 hits the lower surface immediately to the left of the diffraction grating, the light ray 234 may be partially diffracted outside the substrate at the right edge 240 of the diffraction grating 232 and may not be diffracted there. As a result, for all positions of output coupling within the diffraction grating 232 for the two adjacent light rays 234, 236, the light ray 236 becomes brighter. This difference can be easily observed.

[0053] FIG. 18B shows a similar method for solving these problems. In the figure, a transparent plate 242 is cemented to the upper surface 244 of the substrate 238. A beam splitter coating similar to that described above is provided in layer at the interface 246.

Claims

1. An optical transmission substrate having an incident aperture, an exit aperture, and at least two major surfaces and edges, an optical element for input-coupling light waves to the optical transmission substrate by internal total reflection, at least one partial reflection surface provided between two major surfaces of the optical transmission substrate for partially reflecting light waves from the optical transmission substrate, a first transparent plate having at least two major surfaces, one of the major surfaces of the transparent plate being optically coupled and attached to the major surface of the optical transmission substrate defining a flat interface, a beam splitter coating provided on the flat interface between the optical transmission substrate and the transparent plate, and an optical device in which light waves input-coupled to the optical transmission substrate are partially reflected at the flat interface and partially transmitted therethrough.

2. The optical device according to claim 1, wherein light waves output-coupled from the optical transmission substrate by the partial reflection surface substantially pass through the flat interface without significant reflection.

3. The optical device according to claim 1, wherein the two major surfaces of the optical transmission substrate are parallel to the two major surfaces of the first transparent plate.

4. The optical device according to claim 1, wherein the beam splitter coating has a high reflectance at a large incident angle and a low reflectance at a small incident angle.

5. The optical device according to claim 4, wherein the beam splitter coating has a low reflectance at an incident angle between 0° and 15° and a high reflectance at an incident angle exceeding 40°.

6. The optical device according to claim 4, wherein the beam splitter coating has a reflectance exceeding 35% at an incident angle exceeding 40° and a reflectance less than 10% at an incident angle less than 15°.

7. The optical device according to claim 1, wherein the beam splitter coating is provided on one major surface of the optical transmission substrate.

8. The optical device according to claim 7, wherein the beam splitter coating is formed using a coating process at a low temperature.

9. The optical device according to claim 1, wherein the beam splitter coating is provided on one of the major surfaces of the first transparent plate.

10. The optical device according to claim 4, wherein the reflectance of the beam splitter coating is substantially constant at an incident angle greater than 40° and less than 60°.

11. The optical device according to claim 4, wherein the reflectance of the beam splitter coating is not constant when the incident angle exceeds 40°.

12. The optical device according to claim 11, wherein the reflectance of the beam splitter coating increases as a function of the incident angle when the incident angle exceeds 40°.

13. The optical device according to claim 4, wherein the reflectance of the beam splitter coating in the case of a large incident angle is uniform throughout the bright viewing region.

14. The optical device according to claim 1, wherein the transparent plate is thinner than the optical transmission substrate.

15. The optical device according to claim 1, further comprising a second transparent plate optically coupled and attached to the other major surface of the optical transmission substrate so as to form a second flat interface.

16. The optical device according to claim 15, wherein a beam splitter coating is provided on the second transparent plate.

17. The optical device according to claim 16, wherein the beam splitter coating has a large reflectance in the case of a large incident angle and a small reflectance in the case of a small incident angle.

18. The optical device according to claim 1, wherein the optical transmission substrate and the transparent plate are formed of the same optical material.

19. The optical device according to claim 1, wherein the optical transmission substrate and the transparent plate are formed of two different optical materials.

20. The optical device according to claim 1, wherein the transparent plate is formed of a polymer-based material.

21. The optical device according to claim 1, wherein the optical element for input-coupling light waves into the optical transmission substrate is a diffraction element.

22. The optical device according to claim 1, wherein the at least one partially reflective surface provided between the two major surfaces of the optical transmission substrate is a diffraction element.

23. The optical device according to claim 1, wherein the at least one partially reflective surface provided between the two major surfaces of the optical transmission substrate is oriented at an inclination angle with respect to the major surface of the optical transmission substrate.

24. The optical device according to claim 23, wherein a dielectric coating is provided on the at least one partially reflective surface.

25. The optical device according to claim 1, comprising a plurality of partially reflective surfaces provided between the two major surfaces of the optical transmission substrate, and the plurality of partially reflective surfaces are parallel to each other.

26. The optical device according to claim 1, wherein the brightness distribution of the light wave input-coupled to the optical transmission substrate is more uniform in the range of the output aperture than in the range of the input aperture of the optical transmission substrate.

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