A compact head-mounted display system that displays uniform images

Abstract

To disclose an optical device.SOLUTION: An optical device comprises: a light-transmitting substrate having an input aperture, an output aperture, and at least two major surfaces and edges; an optical element for input-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 from 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 coupled to the major surface of the light-transmitting substrate defining a flat interface plane, and a beam-splitter coating applied at the interface plane between the light-transmitting substrate and the transparent plate, wherein light waves input-coupled to the light-transmitting substrate are partially reflected on the flat interface plane and partially pass therethrough.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to substrate-transmission type optical devices, and in particular to devices having multiple reflective surfaces provided on a common optical transmission substrate, also known as light-guide optical elements (LOEs).

[0002] The invention can be implemented to advantage in many imaging applications, including head-mounted displays, head-up displays, mobile phones, compact displays, 3-D displays, compact beam expanders, and also in non-imaging devices, such as flat panel displays, compact lighting devices, and scanners. [Background technology]

[0003] An important application of compact optical elements is head-mounted displays, where the optical module acts as both an imaging lens and a combiner, and a two-dimensional display image is focused at infinity and reflected to the observer's eye. The display image can be obtained directly or indirectly, either from a spatial light modulator (SLM), e.g. a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light-emitting diode array (OLED), a scanning source, or similar, or indirectly from a relay lens or an optical fiber bundle. The display image comprises an array of elements (pixels), which are imaged at infinity by a collimating lens and transmitted to the observer's eye by a reflective or partially reflective surface acting as a non-see-through or see-through combiner, respectively. Conventional free-space optical modules are generally used for this purpose. However, as the desired field of view (FOV) of the system becomes wider, conventional optical modules become increasingly large, heavy and bulky, making them impractical for even moderate performance types. This is a major drawback for all types of displays, especially head-mounted ones. Naturally, it is desired that the system be as light and compact as possible. Summary of the Invention

[0004] The pursuit of compactness has led to various optical solutions, but at the expense of complexity. None of them are sufficiently compact for practical applications. On the other hand, they have significant drawbacks in terms of manufacturability, which cause inconveniences. Furthermore, the eye-motion-box (EMB) of the optical viewing angle obtained based on these ideas is rather small, e.g. less than 8 mm. The performance of the optical system is therefore very sensitive to even slight movements of the optical system relative to the observer's eye, and does not allow sufficient pupil movement to comfortably read text on such displays.

[0005] All of the published patent applications of 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, The disclosures contained in WO2007 / 093983, WO2008 / 023367, WO2008 / 129539, WO2008 / 149339, WO2013 / 175465, Israel Patent Application No. 232197, Israel Patent Application No. 235642, Israel Patent Application No. 236490, and Israel Patent Application No. 236491 are hereby incorporated by reference.

[0006] The present invention facilitates the use of very compact light guide optics (LOEs) for head mounted displays, among other applications. The present invention provides a wide field of view (FOV) with a relatively large EMB value. The resulting optical system provides large, high quality images and is compatible with large eye movements. The optical system according to the present invention is particularly advantageous because it is significantly more compact than state-of-the-art implementations and can be easily integrated into specialized optical systems.

[0007] A further 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 wireless Internet access market, a large amount of bandwidth is available to allow sufficient video transmission. The limiting factor is the display quality of the end user's equipment. The mobile requirements limit the physical size of the display, resulting in a low quality of the directly displayed viewed image. The present invention allows the realization of a physically very compact display with a very large virtual image. This is a key feature in mobile communication, especially in mobile Internet access, and provides a solution to one of the main limitations of practical implementations. Thus, the present invention allows viewing full-format digital content of Internet pages on small handheld devices such as mobile phones.

[0008] SUMMARY OF THE PRESENT EMBODIMENT It is therefore an object of the present invention to overcome the shortcomings of prior art compact optical displays and to provide other optical components and systems with improved performance based on specific requirements.

[0009] According to the present invention, there is provided an optical device comprising: an optical transmission board having an input opening, an output opening, and at least two major surfaces and edges; an optical element that couples light waves into the optical transmission board by total internal reflection; at least one partially reflective surface disposed between the two major surfaces of the optical transmission board and that partially reflects light waves from the optical transmission board; 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 optical transmission board defining a flat interface; and a beam splitter coating layered on the flat interface between the optical transmission board and the transparent plate, wherein the light waves coupled into the optical transmission board are partially reflected at the flat interface and partially transmitted therethrough. [Brief description of the drawings]

[0010] In order that the invention may be better understood, a preferred embodiment will now be described with reference to the following drawings, in which:

[0011] In particular, with respect to the contents of the drawings, it is emphasized that the specific contents shown are merely examples and are intended to be described merely as examples of preferred embodiments of the present invention. The explanation of the principles and concepts of the invention is presented in a manner believed to be most useful and immediately understandable. In this respect, no attempt is made to structurally depict the invention in more detail than is necessary for a fundamental understanding of the invention. The description made with reference to the drawings will provide suggestions to those skilled in the art as to how various examples of the invention can be implemented.

[0012] In the drawings: [Figure 1] FIG. 1 is a side view showing an example of a light guide optical element according to the prior art; [Figure 2A] FIG. 2 is a detailed cross-sectional view illustrating an example array of partially reflective surfaces. [Figure 2B] FIG. 2 is a detailed cross-sectional view illustrating an example array of partially reflective surfaces. [Diagram 3] FIG. 2 is a schematic cross-sectional view showing a reflective surface with two different incident light beams according to the present invention. [Figure 4] 1 is a cross-sectional view showing an example of an array of partially reflective surfaces with a transparent plate attached to the edge of a substrate. [Diagram 5] 1 is a schematic cross-sectional view of a reflecting surface according to the present invention showing the actual effective aperture of the reflecting surface; [Figure 6] 13 shows the effective aperture size of the reflective surface as a function of field of view angle for an example LOE. [Figure 7] FIG. 13 is a detailed cross-sectional view of the reflection from an example array of partially reflective surfaces for three different viewing angles. [Figure 8] FIG. 13 shows the required distance between two adjacent reflective surfaces as a function of viewing angle for an example LOE. [Figure 9] FIG. 11 is another schematic cross-sectional view showing a reflective surface with two different incident light beams according to the present invention. [Figure 10]FIG. 1 is a schematic diagram showing an example of an array of partially reflective surfaces with wedge-shaped transparent plates attached to the edge of a substrate. [Figure 11] FIG. 2 is another schematic cross-sectional view showing a reflective surface with two different incident light rays according to the present invention, where two light rays are reflected by two partially reflective surfaces. [Figure 12] FIG. 13 is yet another schematic cross-sectional view showing a reflective surface with two different incident light beams according to the present invention, where the two light beams are in-coupled into spaced LOEs and out-coupled from adjacent LOEs. [Figure 13A] FIG. 2 is a schematic cross-sectional view showing a beam splitter surface embedded within a light guide optical element. [Figure 13B] FIG. 2 is a schematic cross-sectional view showing a beam splitter surface embedded within a light guide optical element. [Figure 14] 1 is a graph showing curves of reflectivity of a beamsplitter surface as a function of incidence angle for an example angle-dependent coating for s-polarized light. [Figure 15] 11 is yet another graph showing curves of reflectivity of a beamsplitter surface as a function of incidence angle for an example angle dependent coating for s-polarized light. [Figure 16] 1 is a schematic cross-sectional view of two different beam splitter surfaces embedded in a light guide optical element. [Figure 17] FIG. 11 is another schematic cross-sectional view showing a beam splitter surface embedded within a light guide optic having a partially reflective surface formed within a transparent mounting plate. [Figure 18A] FIG. 13 is yet another schematic cross-sectional view showing an embodiment of a beam splitter surface embedded within a light guide optic in which the input and output coupling elements are diffractive optical elements. [Figure 18B] FIG. 13 is yet another schematic cross-sectional view showing an embodiment of a beam splitter surface embedded within a light guide optic in which the input and output coupling elements are diffractive optical elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] FIG. 1 is a cross-sectional view of a light guide optical element (LOE) according to the present invention. A first reflective surface 16 is illuminated by collimated display light 18 generated from a light source (not shown) located behind the optical element. The reflective surface 16 reflects the incident light from the display light source such that the incident light is captured by total internal reflection within a planar substrate 20. After several reflections off the substrate surfaces 26, 27, the captured light waves reach an array of partially reflective surfaces 22, which couple the light out of the substrate and direct it towards an observer's eye 24 with pupil 25. The input face of the LOE is defined as the face where the input light waves enter the LOE, and the output face of the LOE is defined as the face where the captured light waves exit the LOE. Also, the input aperture of the LOE refers to the portion of the input face through which the input light waves actually pass when entering the LOE, and the output aperture of the LOE refers to the portion of the output face through which the output light waves actually pass when entering the LOE. 1, both the input and output faces are coincident with the underlying substrate surface 26, although other configurations are envisioned in which the input and image light waves are located on opposite sides of the substrate, or at one edge of the LOE. Here, a central light wave from a display light source is outcoupled from substrate 20 in a direction normal to substrate surface 26, partially reflective surface 22 is flat, and the off-axis angle of the incoupled light wave within substrate 20 is α in Then, the angle α between the substrate plane (normal to the substrate plane) and the reflecting surface is sur2 is as follows:

[0014]

number

[0015] 1, the captured light ray reaches the reflective surface in two different directions 28, 30. In this particular embodiment, the captured light ray reaches the partially reflective surface 22 in one of multiple directions 28 after an even number of reflections from the substrate surfaces 26, 27. Here, the angle of incidence β between the captured light ray and the normal to the reflective surface is ref becomes:

[0016]

number

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

[0018]

number

[0019] where the minus sign indicates that the captured ray strikes the opposite side of the partially reflective surface 22.

[0020] 1, for each facet, a ray of light reaches the substrate surface along direction 30. Some of this ray re-interprets the substrate surface along direction 28. To prevent unwanted reflections and ghost images, it is important that the reflectivity be negligible for the ray that strikes the substrate surface in direction 28.

[0021] An important issue to consider here is the actual area of ​​each selectively reflecting surface. Possible non-uniformities in the resulting image can be attributed to different reflection sequences of different rays arriving at each selectively reflecting surface. Some rays arrive without prior interaction with the surface, others after one or more partial reflections. This situation is illustrated in Figure 2A. For example, α in = 50°, ray 80 intersects partially reflective surface 22 at point 82. The ray's angle of incidence is 25°, and some of the ray's energy is coupled out of the substrate. The ray then intersects the same (optionally) partially reflective surface at point 84 at an angle of incidence of 75° without significant reflection, and again at point 86 at an angle of incidence of 25°. The remaining energy of the ray is now coupled out of the substrate. On the other hand, ray 88 shown in FIG. 2B makes only one reflection 90 from the same surface. Further reflections occur off other partially reflective surfaces.

[0022] 3 illustrates the phenomenon of non-uniformity in a detailed cross-sectional view of partially reflective surface 22, which directs light trapped within the substrate toward viewer's eye 24. As shown, light ray 80 is reflected by upper surface 27 just before line 100, which is the intersection of partially reflective surface 22 and surface 27. Because this light ray does not strike partially reflective surface 22, its brightness remains the same. Its initial incidence on partially reflective surface 22 occurs at point 102, after double reflections at both outer surfaces. At this point, the light wave is partially reflected, and light ray 104 is coupled out of substrate 20. For other light rays, such as light ray 88, which is located just below light ray 80, its initial incidence on partially reflective surface 22 occurs before reaching surface 27 at point 106, where the light wave is partially reflected, and light ray 108 is coupled out of the substrate. Thus, after a double reflection at exterior surfaces 26, 27, when it strikes partially reflective surface 22 again at point 110, the coupled-out ray will be less bright than adjacent ray 104. As a result, all rays with the same coupled-in angle as ray 80 that arrive at partially reflective surface 22 to the left of point 102 will have a smaller brightness. Thus, the reflection from partially reflective surface 22 will actually be "darker" to the left of point 102 for this particular coupled-in angle.

[0023] In reality, the human eye is subconsciously adapted to tolerate significant variations in brightness, but it is difficult to completely correct these 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 focuses it at a single point on the retina. Small variations in the brightness of the display may not be noticed due to the logarithmic response curve of the eye. The human eye perceives high-quality images even when there is only a moderate level of light uniformity within the display. This required moderate uniformity is reasonably achieved with the optical element shown in Figure 1. Systems with large FOVs and requiring large EMBs require a relatively large number of partially reflective surfaces to obtain the desired exit aperture. As a result, the non-uniformity due to multiple crossings of multiple partially reflective surfaces becomes more pronounced, especially for displays located farther from the eye, such as head-up displays. This non-uniformity is unacceptable. In such cases, a more systematic method is required to resolve the non-uniformity.

[0024] The "darker" parts of the partially reflective surface 22 contribute less to the out-coupling of captured light waves from the substrate, so the only effect on the optical performance of the LOE is negative: they will cause dark areas in the exit aperture of the system, resulting in dark stripes in the image. However, the transparency of each reflective surface is uniform with respect to the light waves from the external scene. Therefore, if an overlap between each of the reflective surfaces is set to compensate for the dark areas in the exit aperture, the light rays from the external scene that cross the overlap will be doubly attenuated, resulting in dark stripes in the external scene. This phenomenon significantly degrades the performance of displays located away from the eye, such as head-up displays, as well as near-eye displays, making them unusable.

[0025] FIG. 4 shows an embodiment that solves this problem. Only the "bright" parts of the partially reflective surfaces 22a, 22b, 22c are embedded inside the substrate; that is, they do not intersect with the lower major surface 26, but terminate short of this surface. Because the reflective surfaces are end-to-end over the length of the LOE, there are no gaps in the projected image. Because there is no overlap between the reflective surfaces, there are no gaps 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 active area of ​​the substrate, preferably by optical cementing. To accurately utilize only the active areas of the partially reflective surfaces 22, it is important to calculate the actual active area of ​​each partially reflective surface and the required thickness T of the transparent plate 120.

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

[0027]

number

[0028] Acquisition angle α in Since θ varies as a function of the FOV, it is important to know what angle should correspond to each reflecting surface 22n in order to calculate the size of the effective aperture of the reflecting surface 22n.

[0029] Figure 6 shows the effective aperture as a function of field angle for the system parameters: substrate thickness d = 2 mm, substrate refractive index ν = 1.51, partial Reflection surface angle α sur = 64°. Taking the viewing angle into consideration, it can be seen that different parts of the resulting image are formed based on different parts of the partially reflective surface.

[0030] This effect is illustrated in Figure 7, which shows a cross-sectional view of a compact LOE display system based on this configuration, where a single plane light wave 112 with a particular viewing angle 114 illuminates only a portion of the entire array of partially reflective surfaces 22a, 22b, 22c. A nominal viewing angle is then defined for each point on the partially reflective surface, and the required effective range of the reflective surface is calculated based on this viewing angle. The precise and detailed determination of the effective ranges of the various partially reflective surfaces is performed as follows: For each particular surface, a ray is plotted (taking into account refraction according to Snell's law) from the left edge of the surface towards the center of the pupil 25 of the particular eye. The calculated direction is taken as the nominal direction of incidence, and the particular effective range is calculated based on this direction.

[0031] As shown in FIG. 5, the exact value of the area of ​​the reflective surfaces can be used to determine the respective distance T between the left edge 102 of the bright area of ​​each reflective surface 22n and the lower surface 26. A large area requires a small distance between the surfaces. This distance represents the thickness of the transparent plate 120 (FIG. 7) that must be attached to the lower surface of the LOE. As shown in FIG. 5, the in-coupling angle α in The distance T as a function of is:

[0032]

number

[0033] Figure 8 shows the required thickness T of the transparency 120 as a function of viewing angle for the same parameters as described above in relation to Figure 6. Determining the thickness T as a local maximum calculation is preferable to ensure that dark stripes on the image are avoided. Making the transparency 120 too thick will have the opposite effect, i.e. making light stripes appear on the image.

[0034] As shown in FIG. 9, two light rays 122 and 124 are in-coupled into the planar substrate 20. They are partially reflected from the partially reflective surface 22a at points 126 and 128, respectively. However, only the light ray 122 hits the second partially reflective surface 22b at point 130, where it is partially reflected. The light ray 124 skips the partially reflective surface 22b without being reflected. As a result, the brightness of the light ray 124 incident on the partially reflective surface 22c at point 134 is greater than the brightness of the light ray 122 at point 132. Thus, the brightness of the out-coupled light ray 138 from point 134 is greater than the out-coupled light ray 136 at point 132, resulting in a bright stripe in the image. Therefore, the exact value of the thickness T should be chosen to avoid dark and bright stripes in the image.

[0035] As shown in FIG. 10, a possible example of achieving the 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' with two non-parallel major surfaces. A complementary wedge-shaped transparent plate 120' is attached to the substrate, preferably by optical cementing, so that the resulting combined structure is a perfect rectangular prism, i.e., the two outer major surfaces of the final LOE are parallel to each other. However, this method has several drawbacks. First of all, the fabrication method of the wedge-shaped LOE is more complicated and laborious than that of the parallel LOE. Also, this solution is valid for systems with small EMBs, where there is a good match between the viewing angle and the lateral position on the substrate plane. However, for systems with large EMBs, i.e., systems where the eye can move significantly along the lateral axis, there is no good match between the viewing angle and the actual thickness of the wedge-shaped transparent plate 120'. Thus, light and dark stripes may be visible in the image.

[0036] The occurrence of light and dark stripes due to the structure of partially reflecting surfaces in the LOE is not limited to the surfaces where this phenomenon occurs. As shown in FIG. 3, the brightness of the incident light ray 88 reflected twice from partially reflecting surface 22a is less at point 110 than the brightness of light ray 80 reflected once from partially reflecting surface 22a at point 102. As a result, the reflected light wave 112 is less bright than the adjacent light ray 104. However, as shown in FIG. 11, not only are the reflected light waves from partially reflecting surface 22a different in brightness, but the transmitted light rays 140 and 142 are also different in brightness. As a result, the reflected light rays 144 and 146 from partially reflecting surface 22b at points 148 and 150, respectively, are also different in brightness, resulting in a dark stripe in this area of ​​the image. The difference between these rays therefore leads to continued transmission in the LOE to the next adjacent partially reflecting surface. As a result, for a LOE with multiple partially reflective surfaces, based on the correct angle of incidence, each partially reflective surface will produce a stripe of light and dark, resulting in multiple stripes of light and dark concentrated at the far edge of the LOE's output aperture, significantly reducing image quality.

[0037] Another source of image irregularity can be non-uniformity of the image light waves that are coupled into the LOE. Normally, when the two edges of a light source have slightly different emission intensities, they are barely noticeable to the viewer, if at all. This is quite different for an image that is coupled into and then out-coupled into a substrate, as in the LOE. As shown in FIG. 12, two light rays 152, 154 are located at the edges of a plane wave 156 that originates from the same point in a display source (not shown). If the brightness of light ray 152 is less than the brightness of light ray 154, as a result of an imperfect imaging system, then little difference between them would be discerned if one were to look directly at the plane wave 156, since the light rays are separated from each other. However, after coupling into the LOE 20, the conditions change. Light ray 154 illuminates the reflective surface 16 just to the right of the interface line 158 between the reflective surface 16 and the lower major surface 26. Meanwhile, the right ray 152 is reflected by the reflective surface 16, is totally internally reflected by the top major surface 27, and then strikes the bottom major surface 26 just to the left of the interface line 158. As a result, the two rays 152, 154 are transmitted side by side within the LOE 20. The two output rays 160, 162 resulting from rays 152, 154 and reflected by the partially reflective surface 22a thus have different brightness. However, unlike the input plane wave 156, the two different rays are side by side, and this difference is easily discerned as a dark stripe on the image. The two rays 164, 165 continue to be transmitted side by side within the LOE, causing a dark stripe at each location where they are outcoupled together. Of course, the best way to avoid this unevenness is to ensure that all of the light waves that are incoupled into the LOE are of uniform brightness across the input aperture for all of the FOV. In a system having a large FOV with a wide entrance aperture, this requirement is very difficult to meet.

[0038] As shown in Figures 13A and 13B, the problem of uneven light may be solved by attaching a transparent plate to one of the major surfaces of the LOE, as already mentioned with reference to Figure 4. However, in this embodiment, a beam splitter coating 166 is applied to the flat interface 167 between the planar substrate 20 and the transparent plate 120. As shown in Figure 13A, two light rays 168, 170 are coupled into the planar substrate 20. Only the light ray 168 hits the first partially reflective surface 22a at point 172, where it is partially reflected. Meanwhile, the light ray 170 skips the partially reflective surface 22a and is not reflected. As a result, the light ray 170 reflected upward from the lower major surface 26 is brighter than the light ray 168 reflected downward from the upper major surface 27, assuming that the two light rays have the same brightness when coupled into the LOE. The two light rays cross each other at point 174, which is located at the flat interface 167. The two intersecting beams are partially reflected and partially transmitted by the beam splitter coating applied thereto. Thus, the two beams exchange energy, and the output beams 176, 178 from the intersection point 174 have the same brightness, which is approximately the average brightness of the two input beams 168, 170. This beam further exchanges energy with two other beams (not shown) at the intersection points 180, 182. As a result of the energy exchange, the two reflected beams 184, 186 from the partially reflecting surface 22b have approximately the same brightness, and the bright stripe phenomenon is significantly improved.

[0039] Similarly, in FIG. 13B, rays 188 and 190 couple into the planar substrate 20. However, only ray 188 hits the first partially reflecting surface 22a at point 192, where it is partially reflected and reflected by the top major surface 27. As a result, ray 190 reflected downward from the top major surface 27 is brighter than ray 188, assuming that the two rays have equal brightness when coupled into the LOE. However, the two rays cross each other at point 194, which is located on the flat interface 167, where they exchange energy. In addition, the two rays cross each other at points 196 and 198, which are located above the beam splitting interface 167. As a result, rays 200 and 202 reflected from surface 22a and rays 204 and 206 reflected from surface 22b have approximately the same brightness, and the dark stripe phenomenon is correspondingly significantly suppressed. This improved uniformity of brightness effect also applies to light and dark stripes caused by non-uniform illumination entering the LOE's input aperture. As a result, the brightness distribution of light waves trapped within the LOE is significantly more uniform within the LOE's output aperture than within the LOE's input aperture.

[0040] As shown in FIG. 13A, the light rays 184 and 186 reflected from the surface 22a intersect with the interface 167 before being out-coupled from the LOE. As a result, a simple reflective coating cannot be easily layered on the interface 167 because the interface 167 should be transparent to the light waves emerging from the substrate 20 and transparent to the light waves from the external scene for transmission-type device applications. That is, the light waves should be transmitted through the interface 167 at small angles of incidence and partially reflected at larger angles of incidence. Typically, the angles of incidence for transmission are between 0° and 15°, and the angles of incidence for partial reflection are between 40° and 65°. In addition, since the light rays cross the interface 167 many times during transmission within the LOE, the absorption in the coating should be negligible. Therefore, a simple metal coating cannot be used, but a thin dielectric coating with high transparency must be used.

[0041] FIG. 14 shows the reflectance curves as a function of the angle of incidence for three representative wavelengths in the photopic region, namely 470 nm, 550 nm, and 630 nm, for s-polarized light. As shown, it is possible to achieve the required behavior of partial reflectance (45-55%) at large angles of incidence in the range of 40-65° and low reflectance (less than 5%) at small angles of incidence for s-polarized light, for example. For p-polarized light, it is not possible to achieve a large reflectance for angles of incidence in the range of 40-65°, since this is close to the Brewster angle. Since the polarization normally used in LOE imaging systems is s-polarized light, the required beam splitter can be provided very easily. However, the beam splitter coating should be substantially transparent for light waves from the external scene that strike the interface at small angles of incidence and are nearly unpolarized, so the beam splitter coating should have a small reflectance (less than 5%) for small angles of incidence even for p-polarized light.

[0042] The remaining difficulty is that the LOE 20 is assembled from several different parts. Since the fabrication process usually involves cementing together optical elements, and the necessary angle-dependent reflective coatings are applied to the light guide surfaces after the body of the LOE 20 is finished, traditional high-temperature coating methods that may damage the cemented parts cannot be used. However, new thin-film techniques, as well as ion-assisted coating methods, can also be used for low-temperature processing. By eliminating the need to heat the parts, cemented parts can be safely coated. Alternatively, the required coatings can be easily applied to the transparent plate 120 adjacent to the LOE 20 by using well-known high-temperature coating methods and then cementing them in place. Obviously, this additional approach can be used only if the transparent plate 120 is not too thin and can be deformed during the coating process.

[0043] In configuring the beam splitter function as described above, there are several issues that must be taken into consideration.

[0044] a. Because light rays trapped within the LOE are not only totally internally reflected from the major surfaces 26, 27, but also from the internal partially reflective flat interface 167, it is important that these three surfaces are parallel to each other to ensure that the light rays maintain their original in-coupling direction within the LOE.

[0045] b. As shown in Figures 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 shown in Figures 7-10 for the uncoated plate, the thickness of the coated plate may be set for other reasons: it is easier to fabricate, coat, and cement a slightly thicker plate. On the other hand, for a slightly thinner plate, the effective volume of the LOE 20 that actually couples light waves out of the substrate is larger for a given substrate thickness. Also, the correct ratio of thickness between the transparent plate 120 and the LOE 20 may affect the energy exchange process in the substrate.

[0046] c. Typically, for a beamsplitter designed for full color imaging, the reflectance curve should be as uniform as possible across the entire photopic range to avoid chromatic effects. However, in the configuration illustrated in this invention, many rays cross each other many times before being coupled out of the LOE 20. Therefore, this requirement is not necessary. Of course, the beamsplitter coating should take into account the full wavelength spectrum of the incident image, but chromatic flatness in the partial reflectance curve may be tolerated based on the system parameters.

[0047] d. The reflectance / transmittance ratio of the beamsplitter coating does not necessarily have to be 50% to 50%. Other ratios may be used to provide the necessary energy exchange between the dark and bright beams. Additionally, a simpler beamsplitter coating may be utilized as shown in FIG. 15, which has a reflectance that gradually increases from 35% at a 40° angle of incidence to 60% at a 65° angle of incidence.

[0048] e. The number of beam splitter surfaces to be provided on the LOE is not limited to one. As shown in FIG. 16, another transparent plate 208 may be cemented to the top surface of the LOE. A beam splitter coating may also be applied to the flat interface 210 between the LOE 20 and the top transparent plate 208 to obtain an optical device with two beam splitter surfaces. Two different beams 212, 214 intersect with each other at point 215 on the coated flat interface 210. The flat interface 210 also has further beams intersecting at points 216, 217. This is in addition to the intersection at the lower beam splitter flat interface 167. As a result, the uniformity of the reflected beams 218, 220 is expected to be even higher than that of the embodiment of FIGS. 13A, 13B. Of course, the process of making an LOE with two beam splitter flat interfaces is more difficult than that with only one flat interface. Therefore, it need only be considered in systems where uneven surfaces are a significant problem.As stated above, it is important that all four reflecting surfaces and planes 26, 27, 167, 210 are parallel to one another.

[0049] f. The transparent layer 120 does not necessarily have to be made from the same optical material as the LOE 20. Furthermore, the LOE may be made from a silicate-based material, and for eye safety purposes, the transparent layer may be formed from a polymer-based material. Of course, care must be taken to ensure the optical performance of the outer surface and to prevent deformation of the transparent layer.

[0050] g. Up to now, we have assumed that the entire transparent plate is blank. However, as shown in Figure 17, partially reflective surfaces 222a and 222b may be formed inside the transparent plate 120 to increase the available volume of the LOE. These surfaces must be strictly parallel to the partially reflective surfaces 22a and 22b and oriented in exactly the same manner.

[0051] All of the parameters of the above embodiments, such as the thickness and optical material of the transparent plate 120, the inherent properties of the beamsplitter coating, the number of beamsplitter facets, the location of the partially reflective surfaces within the LOE, etc., can take many different values, which are determined based on the various parameters of the optical system and the particular requirements of the optical performance and manufacturing costs.

[0052] So far, we have assumed that the light waves are coupled out of the substrate at a partially reflective surface that is oriented at an oblique angle to the major surface and is usually coated with a dielectric coating. However, as shown in FIG. 18A, there are also systems in which the light waves are coupled in and out of the substrate using diffractive elements 230, 232. The same uniformity problems already mentioned may be present in this configuration. As shown, two light rays 234, 236 from the same point of the display source are coupled into the substrate 238 at two spaced edges of the in-coupling diffractive element 230. Adjacent light rays are respectively out-coupled in the out-coupling diffractive element 232. Thus, any differences between the light rays are easily visible in the out-coupled light waves. In addition, to effect a uniform out-coupled image, the diffraction efficiency of the out-coupling diffractive element 232 is gradually increased. As a result, different light rays from the same point source may pass through different positions in the diffractive element 232 before being out-coupled from the optical element, resulting in different brightness in the image. Another cause of light unevenness is that ray 234 is partially diffracted out of the substrate at the right edge 240 of the grating 232, while ray 236 hits the underside of the grating immediately to the left, and is not diffracted there. This results in ray 236 being brighter for all positions of out-coupling in the grating 232 for the two adjacent rays 234, 236. This difference is easily observable.

[0053] 18B shows a similar approach to solving these problems, in which a transparent plate 242 is cemented to the top surface 244 of the substrate 238. At the interface 246 is deposited a beam splitter coating similar to that described above.

Claims

1. 1. An optical system comprising: a light guide optical element having two parallel major surfaces and edges, an input aperture through which light waves enter the light guide optical element, and an output aperture through which the light waves exit the light guide optical element; at least one beam splitter surface embedded within the light guide optical element between, separated from, and parallel to the two major surfaces of the light guide optical element, the at least one beam splitter surface being implemented as a thin film coating of a dielectric material; partially reflecting, with a reflectivity of less than 60%, and partially transmitting, light waves incident at an angle greater than 45° and less than 60° relative to the normal to the beam splitter surface; a reflectivity of less than 5% for the light waves incident at an angle of less than 10° relative to the normal to the beam splitter surface; transparent to unpolarized light from an external scene impinging on the beam splitter surface; the light waves include wavelengths of 470, 550, and 630 nm with S polarization; the light waves propagating by total internal reflection at the major surfaces undergo multiple total internal reflections at the two major surfaces and impinge multiple times on the at least one beam splitter surface between the input aperture and the output aperture; Optical system.

2. 10. The optical device of claim 1, wherein the reflectivity of the dielectric thin film coating increases gradually as a function of angle of incidence above 45 degrees and below 60 degrees relative to the normal.

3. The optical device of claim 1 , further comprising an optical element for coupling the light wave into the light guide optical element.

4. 10. The optical device of claim 1, further comprising at least one partially reflective surface disposed between the two major surfaces for output coupling the light waves, the at least one partially reflective surface including a first partially reflective surface on one side of the beam splitter surface and a second partially reflective surface on a second side of the beam splitter surface.

5. The optical device of claim 1 , wherein the at least one beam splitter surface at least partially overlaps the output aperture.

6. The optical device of claim 1 , wherein the light guide optical element is made of two different optical materials.

7. The optical device of claim 6 , wherein the two different optical materials include a silicate-based material and a polymer-based material.

8. 2. The optical device of claim 1, wherein the at least one beam splitter surface is implemented as at least two beam splitter surfaces, each of the at least two beam splitter surfaces embedded inside the light guide optical element between the two major surfaces of the light guide optical element and parallel to the two major surfaces of the light guide optical element.

9. An optical device as described in claim 4, wherein the at least one partially reflective surface is inserted between two of the beam splitter surfaces.

10. The dielectric thin film coating is 10. The optical device of claim 1, configured to be partially reflective and partially transmissive to light waves having S polarization.

11. An optical device as described in claim 1, wherein the reflectivity of the dielectric thin film coating at incident angles greater than 70° is uniform across the entire photopic vision range.

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