HK40064350B - Highly efficient compact head-mounted display system having small input aperture - Google Patents

Description

Highly efficient and compact head-mounted display system with a small input port

Technical Field

The present invention relates to optical devices for guiding light waves based on a substrate, and particularly to devices comprising a reflective surface supported by a light-transmitting substrate and an array of partially reflective surfaces attached to the substrate.

This invention can be advantageously implemented in a wide range of imaging applications, such as head-mounted displays, head-up displays, cellular phones, compact displays, and 3D displays.

Background Technology

One important application of compact optics is in head-mounted displays (HMDs), where the optical module acts as both an imaging lens and a combiner, in which a two-dimensional display is imaged to infinity and reflected into the observer's eye. The display can be obtained directly from spatial light modulators (SLMs) such as cathode ray tubes (CRTs), liquid crystal displays (LCDs), organic light-emitting diode arrays (OLEDs), scanning sources, and similar devices, or indirectly via relay lenses or fiber bundles. The display comprises an array of elements (pixels) imaged to infinity by collimating lenses and transmitted into the observer's eye via reflective or partially reflective surfaces, which serve as combiners for non-perspective and perspective applications, respectively. Typically, conventional free-space optical modules are used for these purposes. As the desired field of view (FOV) of the system increases, such conventional optical modules become larger, heavier, and bulkier, and therefore impractical even for medium-performance devices. This is a major drawback for all types of displays, but especially for HMDs, where the system should be as light and compact as possible.

The need for compactness has led to several different complex optical solutions, all of which are still not compact enough for most practical applications and suffer from significant shortcomings in terms of manufacturability, price, and performance.

The teachings contained in Publications WO2017/141239, WO2017/141240, WO2017/141242 and PCT/IL2018/051105 are incorporated herein by reference.

Summary of the Invention

In addition to other applications, this invention also facilitates the provision of compact substrates for HMDs. The invention allows for a relatively wide field of view (FOV) and a relatively large eye movement frame (EMB) value. The resulting optical system provides a large, high-quality image, which also accommodates significant eye movements. The optical system according to the invention is particularly advantageous because it is more compact than prior art implementations, and it can even be easily incorporated into optical systems with specialized configurations.

Therefore, the broad objective of this invention is to mitigate the drawbacks of prior art compact optical display devices and to provide other optical components and systems with improved performance according to specific requirements.

According to the present invention, an optical device comprising an optical element is provided, the optical element comprising a first light-transmitting substrate having at least two parallel main surfaces and two opposing edges; an input aperture; an output aperture positioned near one of the main surfaces of the substrate; an eye movement frame having the aperture; a first intermediate element having at least two surfaces positioned outside the substrate for coupling light waves having a field of view into the substrate through the input aperture; a first flat reflective surface having an effective area positioned between the two main surfaces of the light-transmitting substrate for reflecting the coupled input light waves to achieve total internal reflection from the main surfaces of the substrate; a second flat reflective surface parallel to the first flat reflective surface, having an effective area and positioned between the two main surfaces of the light-transmitting substrate for coupling light waves out of the substrate; and a redirecting optical element having at least two surfaces positioned outside the substrate for redirecting light waves coupled out of the substrate through the output aperture into the eye movement frame, wherein the input aperture is significantly smaller than the output aperture, the effective area of the first reflective surface is similar to the effective area of the second reflective surface, and each coupled light wave covers the entire aperture of the eye movement frame.

Attached Figure Description

The invention is described in conjunction with certain preferred embodiments and with reference to the following illustrative figures, so that the invention may be more fully understood.

Referring specifically to the detailed figures, it should be emphasized that the details shown are by way of example and are only for the purpose of illustrative discussion of preferred embodiments of the invention, and are presented to provide what is considered the most useful and readily understood description of the principles and concepts of the invention. In this respect, no attempt is made to show the structural details of the invention in more detail than necessary for a basic understanding of the invention. The description taken in conjunction with the accompanying drawings will serve as guidance to those skilled in the art on how the invention can be embodied in several forms in practice.

In the attached diagram:

Figure 1 is a side view of an exemplary transparent substrate of the prior art;

Figure 2 is a side view of another prior art exemplary light-transmitting substrate;

Figures 3A and 3B illustrate the desired reflectivity and transmissivity characteristics of a selectively reflective surface used in a prior art exemplary light-transmitting substrate for two incident angle ranges.

Figure 4 illustrates the reflectivity curve of an exemplary dielectric coating as a function of the incident angle;

Figure 5 is a schematic cross-sectional view of a prior art transparent substrate, wherein the coupling input and coupling output elements are diffractive optical elements;

Figures 6A, 6B and 6C illustrate cross-sectional views of a prior art transparent substrate having a coupling input and a coupling output surface and a partial reflection redirection element;

Figure 7 schematically illustrates the effective portion of the output surface coupled with the EMB according to the system's viewpoint;

Figures 8A, 8B, 8C and 8D schematically illustrate the effective portion of the coupled input surface of the system according to the system's viewpoint and EMB;

Figures 9A, 9B, 9C and 9D are schematic cross-sectional views of a substrate guiding embodiment according to the present invention, having a single coupled output element, an intermediate prism and an input aperture significantly smaller than the output aperture;

Figure 10 is a graph illustrating the reflection of incident light waves on the interface plane as a function of the incident angle for three different wavelengths according to the present invention.

Figure 11 is a graph illustrating the incident angle and critical angle of the interface plane of two different light waves as a function of wavelength according to the present invention.

Figures 12A, 12B and 12C are graphs illustrating the reflection of incident light waves on the interface plane as a function of the angle of incidence for three different wavelengths according to the present invention, and the angle of incidence for two specific light waves.

Figures 13A, 13B, 13C and 13D are schematic cross-sectional views of other substrate guiding embodiments according to the present invention, having a single coupled output element, an intermediate prism and an input aperture significantly smaller than the output aperture;

Figures 14A, 14B, 14C and 14D are schematic cross-sectional views of other substrate guiding embodiments according to the present invention, having a single coupled output element, an intermediate prism and an input aperture significantly smaller than the output aperture.

Figure 15 is a graph illustrating the reflection of incident light waves as a function of the angle of incidence for three different wavelengths according to the present invention on the coupled input surface.

Figure 16 is a graph illustrating the incident angle and critical angle of the coupling input surface for two different light waves as a function of wavelength according to the present invention.

Figures 17A, 17B, and 17C are graphs illustrating the reflection of incident light waves of three different wavelengths as a function of the angle of incidence on the coupled input plane according to the present invention, as well as the angle of incidence curves of two specific light waves.

Figures 18A, 18B, 18C and 18D are schematic cross-sectional views of a substrate guiding embodiment according to the present invention, having a single coupled output element, two intermediate prisms and an input aperture significantly smaller than the output aperture;

Figures 19A, 19B, 19C and 19D are further schematic cross-sectional views of a substrate guiding embodiment according to the present invention, having a single coupled output element, two intermediate prisms and an input aperture significantly smaller than the output aperture;

Figures 20A, 20B, 20C and 20D are further schematic cross-sectional views of a substrate guiding embodiment according to the present invention having a single coupled output element, two intermediate prisms and an input aperture significantly smaller than the output aperture;

Figure 21 is a schematic cross-sectional view of a substrate guiding embodiment with two adjacent substrates according to the present invention, the two adjacent substrates having different coupling input surface tilt angles;

Figures 22A, 22B, 22C and 22D are schematic cross-sectional views of a single light wave coupled within a substrate guiding embodiment having two adjacent substrates according to the present invention.

Figures 23A, 23B, 23C and 23D are schematic cross-sectional views of another light wave coupled inside a substrate guiding embodiment having two adjacent substrates according to the present invention;

Figures 24A, 24B and 24C are schematic cross-sectional views of another light wave coupled internally in a substrate guiding embodiment having two adjacent substrates according to the present invention.

Figure 25 is a schematic cross-sectional view of three different light waves coupled inside a substrate guiding embodiment according to the present invention having two adjacent substrates, an intermediate prism, and an input aperture significantly smaller than the output aperture.

Figures 26A and 26B are schematic cross-sectional views of a substrate guiding embodiment, in which unwanted light waves reach the system's EMB;

Figure 27 is a schematic cross-sectional view of a substrate guiding embodiment according to the present invention, which has an array of absorbing surfaces for eliminating total internal reflection from the outer surface;

Figures 28A, 28B, 28C, 28D, 28E and 28F are illustrations of a method for manufacturing a plate having an array of absorbing surfaces according to the present invention;

Figures 29A and 29B are schematic cross-sectional views of a substrate guiding embodiment according to the present invention, wherein unwanted stray light is absorbed inside the thin plate;

Figure 30 is a diagram illustrating a method according to the present invention of expanding an output aperture along two axes using a dual-substrate configuration, and

Figures 31A and 31B are further schematic cross-sectional views of a substrate guiding embodiment using a reflective lens as a collimating element for a polarized and non-polarized display source.

Detailed Implementation

Figure 1 illustrates a cross-sectional view of a prior art light-transmitting substrate, wherein a first reflective surface 16 is irradiated by a collimated light wave 12 emitted from a display source 4 and collimated by a lens 6 positioned between the source 4 and the substrate 20 of the device. The reflective surface 16 reflects the incident light from the source 4, such that the light wave is trapped within the planar substrate 20 by total internal reflection. After several reflections from the main surfaces 26, 27 of the substrate 20, the trapped light wave reaches a partial reflective element 22, which couples the light from the substrate to the viewer's eye 24 with a pupil 25. Herein, an input aperture 17 of the substrate 20 is defined as the aperture through which the input light wave enters the substrate, and an output aperture 18 of the substrate is defined as the aperture through which the trapped light wave exits the substrate. In the case of the substrate illustrated in Figure 1, both the input and output apertures coincide with the lower surface 26. However, other configurations are contemplated, in which the input and image light waves from the displacement source 4 are positioned on opposite sides of the substrate, or on one of the edges of the substrate. As shown in the figure, the effective areas of the input orifice and the output orifice are similar to each other, and they are approximately the projections of the coupled input element 16 and the coupled output element 22 on the main surface 26, respectively.

In HMD systems, the entire area of the EMB must be illuminated by all light waves emanating from the display source so that the viewer's eye can simultaneously see the entire field of view (FOV) of the projected image. Consequently, the system's output aperture must be correspondingly expanded. On the other hand, the optical module must be lightweight and compact. Because the lateral range of the collimating lens 6 is determined by the lateral dimension of the substrate's input aperture, it is desirable that the input aperture be as small as possible. In systems such as those illustrated in Figure 1, where the lateral dimension of the input aperture is similar to that of the output aperture, there is an inherent contradiction between these two requirements. Most systems based on this optical architecture suffer from the effects of a small EMB, a small achievable FOV, and a large and bulky imaging module.

Figure 2 illustrates an embodiment that at least partially solves this problem, wherein the element coupling the light waves output from the substrate is an array of partially reflective surfaces 22a, 22b, etc. The output aperture of this configuration can be expanded by increasing the number of partially reflective surfaces embedded within the substrate 20. Therefore, it is possible to design and construct an optical module with a small input aperture and a large output aperture. It can be seen that the captured light arrives at the reflective surfaces from two different directions 28, 30. In this particular embodiment, after an even number of reflections from the main surfaces 26 and 27 of the substrate, the captured light arrives at the partially reflective surface 22a from one of these directions 28, where the angle of incidence between the captured light and the normal to the reflective surface is θ.

After an odd number of reflections from substrate surfaces 26 and 27, the captured light reaches the partially reflective surface 22b from the second direction 30, wherein the angle of incidence between the captured light and the normal of the reflective surface is .

As further illustrated in Figure 2, for each reflective surface, each ray first arrives at the surface from direction 30, and some of these rays then strike the surface again from direction 28. To prevent undesirable reflections and ghosting, it is important that the reflectivity is negligible for the rays striking the surface with the second direction 28.

A solution for this requirement, utilizing the angular sensitivity of a thin-film coating, was previously proposed in the aforementioned publications. If one angle is significantly smaller than the other, the desired distinction between the two incident directions can be achieved. It is possible to provide a coating with very low reflectivity at high incident angles and high reflectivity at low incident angles. This property can be used to prevent undesirable reflections and ghosting by eliminating reflectivity in one of the two directions.

Referring now specifically to Figures 3A and 3B, these figures illustrate the desired reflective behavior of the partially reflective surface 34. When a ray 32 (Figure 3A) with an off-axis angle is partially reflected and coupled out from the substrate 20, a ray 36 (Figure 3B) arriving at the reflective surface 34 with an off-axis angle is transmitted through the reflective surface 34 without any apparent reflection.

Figure 4 illustrates the reflectivity curve of a typical partially reflective surface of this particular system as a function of the incident angle of S-polarized light with a wavelength λ = 550 nm. For full-color displays, similar reflectivity curves should be achieved for other wavelengths in the relevant visible spectrum, which is typically between 430 nm and 660 nm for most display sources. Two important regions exist in this curve: between 65° and 85°, where reflectivity is very low; and between 10° and 40°, where reflectivity increases monotonically with increasing incident angle. As can be seen in Figures 3 and 4, the reflectivity behavior required for the partially reflective surface 22 of the embodiment illustrated in Figure 2 is not conventional. Furthermore, to maintain low reflectivity in the higher angle regions, reflectivity in the lower angle regions cannot exceed 20%–30%. Furthermore, to achieve uniform brightness across the entire FOV, the reflectivity of the partially reflective surface needs to gradually increase towards the substrate edge, and therefore, the maximum achievable efficiency is relatively low and typically cannot exceed 10%.

Another method for coupling light waves into and out of a light-guiding optical element is by using diffraction elements. As illustrated in Figure 5, rays 34 and 36 are coupled into the transparent substrate 20 via diffraction element 48, and after several total internal reflections from the outer surface of the substrate, the rays are coupled out of the substrate via a second diffraction element 50. As illustrated, ray 34 is coupled out at least twice at two different points 52 and 54 on element 54. Therefore, in order to achieve a uniform output light wave, the diffraction efficiency of element 50 should gradually increase along the axis. As a result, the overall efficiency of the optical system is even lower than that of the system illustrated in Figure 2, and typically no more than a few percent. That is, in the embodiments illustrated in Figures 2 and 5, the output aperture is expanded to be much larger than the input aperture at the cost of significantly reducing the brightness efficiency of the optical module and complicating the substrate manufacturing process.

Figures 6A and 6B illustrate embodiments for overcoming the aforementioned problems. Instead of using a single element (22 in Figure 2, or 50 in Figure 5) that performs the dual function of coupling light waves from substrate 20 and directing them into the user's eye 24, the requested function is divided into two distinct elements; namely, an element embedded within the substrate couples light waves from the substrate, while a second, conventionally positioned reflective element outside the substrate redirects the light waves into the viewer's eye. As illustrated in Figure 6A, two rays 63 (dashed lines) from a planar light wave emitted from a display source and collimated by a lens (not shown) enter the light-transmitting substrate 64, which has two parallel main surfaces 70 and 72, through an input aperture 86 at an angle of incidence relative to the main surfaces 70 and 72 of the substrate. The rays strike a reflective surface 65, which is angled relative to the main surfaces of the substrate. The reflective surface 65 reflects the incident rays, such that the rays are trapped within the planar substrate 64 by total internal reflection from the main surfaces. To distinguish between the various "propagation orders" of the captured light wave, the superscript ( i ) will indicate order i . The input light wave striking the substrate with zero order is indicated by the superscript (0). After each reflection from the coupled input reflective surface, the order of the captured ray increases from ( i ) to ( i+1 ). The off-axis angle between the first-order captured ray and the normals of the main surfaces 70 and 72 is...

.

After being reflected several times from the substrate surface, the captured light reaches the second flat reflective surface 67, which couples the light out of the substrate. Assuming surface 67 is inclined to the main surface at the same angle as the first surface 65 (i.e., surfaces 65 and 67 are parallel), then the angle between the coupled-out light and the normal to the substrate plane is...

.

Therefore, the coupled output light beam is tilted towards the substrate at the same angle as the incident light beam. So far, the coupled input light wave behaves similarly to the light wave illustrated in Figure 1. However, Figure 6A illustrates different behavior, where two light beams 68 (dashed lines) with the same angle of incidence as light beam 63 strike the right side of the reflective surface 65. After being reflected twice from surface 65, the light wave is coupled into the interior of the substrate 64 via total internal reflection, and the off-axis angle of the captured light beam inside the substrate is now...

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After being reflected several times from the main surface of the substrate, the captured light reaches the second reflective surface 67. Light 68 is reflected twice from the coupling output surface 67 and coupled out of the substrate at the same off-axis angle as the other two rays 63, which are reflected only once from surfaces 65 and 67. This is also the same angle of incidence for all four rays on the main plane of the substrate. Although all four rays strike and are coupled out of the substrate at the same off-axis angle, there are significant differences between them: the two rays 68 incident on the right side of the reflective surface 65 are closer to the right edge 66 of the substrate 64, are reflected twice from surfaces 65 and 67, and are coupled out of the substrate at the left side of surface 67, which is closer to the opposite left edge 69 of the substrate. On the other hand, the two rays 63 incident on the left side of the reflective surface 65 are closer to the center of the substrate 64, are reflected once from surfaces 65 and 67, and are coupled out of the substrate at the right side of surface 67, which is closer to the center of the substrate.

As further illustrated in Figures 6A and 6B, the tilt angle of the image can be adjusted by adding a partially reflective surface 79, which is angled to the surface 72 of the substrate. As shown, the image is reflected and rotated such that it again passes through the substrate substantially perpendicular to the main surface of the substrate and reaches the viewer's eye 24 through the output aperture 89 of the substrate. To minimize distortion and chromatic aberration, it is preferable to embed the surface 79 in the redirection prism 80 and to use a second prism 82 to complete the shape of the substrate 64, both made of the same material, which should not necessarily be similar to the material of the prism 80. To minimize the thickness of the system, as illustrated in Figure 6B, it is possible to replace a single reflective surface 79 with an array of parallel partially reflective surfaces 79a, 79b, etc., where the number of partially reflective surfaces can be determined according to system requirements. Another way to redirect the coupled output light waves to the viewer's eye is to use a flat metasurface structured by subwavelength scale modes.

In the embodiments illustrated herein, it is assumed that light waves with only first and second order axis-to-axis angles propagate within the substrate. However, there are systems with considerably small tilt angles between the coupled input and coupled output surfaces, where even third and fourth order angles can be utilized.

As illustrated in Figure 6C, the input light ray 71 strikes the substrate 64 at an off-axis angle. After being reflected three times from surface 65 at points 75a, 75b, and 75c, the light ray couples inside the substrate and propagates within the substrate, having a third-order off-axis angle. After being reflected several times from the main surface of substrate 64, light ray 71 strikes surface 67. After being reflected three times from the surface at points 77a, 77b, and 77c, it is coupled out of substrate 64 at an off-axis angle. Light ray 71 is then reflected substantially perpendicularly to the main surface of the substrate by surface 79a into the viewer's eye 24. Generally, for a system with several coupling input orders, lower orders will couple into and out of the substrate at the reflective surface portion closer to the substrate edge, higher orders will couple at the reflective surface portion closer to the substrate center, and intermediate orders will couple from the center portion of the coupling input and coupling output surfaces.

There are two conflicting requirements arising from the coupling output surface 67. On the one hand, the first three orders of image should be reflected from this plane, while on the other hand, the zero-order image from the substrate 64 should pass substantially through the surface 79 after being reflected, without significant reflection. Furthermore, for a perspective system, the optical system should have the highest possible transparency to substantially perpendicular incident light rays 83 from the external scene. One way to achieve this is by using an air gap in the surface 67. However, to achieve a rigid system, it is preferable to apply an optical adhesive to the surface 67 to bond the substrate 64 to the prism 82 using an optical adhesive with a refractive index significantly lower than that of the substrate. However, there are cases where the required refractive index of the optical adhesive to produce the necessary total internal reflection effect for the entire coupling FOV is very low, approximately on the order of 1.31-1.35. Commercially available optical adhesives with the required refractive index exist. Nevertheless, their adhesive strength is generally insufficient, and their resistance to extreme environmental conditions is inadequate for military and professional applications. An alternative solution is to apply a dielectric film to the surface 67 using a spin-coating process. The applied coating material has a refractive index significantly lower than that of the substrate and should have an appropriate value to produce the required total internal reflection from surface 67 for the entire field of view. The substrate 64 can now be bonded to the prism 82 using an optical adhesive that has the required adhesive strength and resistance to environmental conditions, while its precise refractive index can have any reasonable value.

In any proposed scheme to minimize Fresnel reflection of transmitted light waves from the coupled output surface 67, it is preferable to apply a suitable anti-reflective (AR) coating to the surface. In this case, the overall efficiency of light waves passing through the substrate can be very high; that is, when light waves are coupled out from the substrate, the reflectivity of the surface 67 is 100% due to total internal reflection from the surface, and the transmission of reflected light waves from the surface 79 and light from the external scene is also close to 100% due to the AR coating. Similarly, it is preferable to use an optical adhesive with a refractive index significantly lower than that of the substrate to bond the prism 80 to the lower surface 72 of the substrate 64, thereby defining an interface plane 81 to which a suitable AR coating is applied. Here, too, total internal reflection from the surface 72 can be achieved by spin-coating a suitable material onto the surface 72 and bonding the prism 80 to the surface 72 using a conventional optical adhesive. Therefore, the brightness of the light waves coupled out from the substrate via surface 67 is similar to the brightness of the input light waves before they are coupled into the substrate via surface 65, and their brightness is only attenuated by partial reflection from surface 79. As a result, the brightness efficiency of the embodiments illustrated in Figures 6A-6C can be an order of magnitude higher than that of the configurations illustrated in Figures 2 and 5.

As explained above with respect to Figure 6A, in a perspective system such as an HMD for augmented reality (AR) applications, where the viewer should see the external scene through a substrate, the partially reflective surface 79 should be at least partially transparent so that external light rays 63 and 68 can pass through the substrate and reach the viewer's eye 24. Since surface 79 is only partially reflective, only a portion of the coupled light waves 63 and 68 is reflected by surface 79 and reaches the viewer's eye, while another portion of light wave 84 passes through surface 79, is coupled out from prism 80, and does not reach the viewer's eye. Similarly, since surface 79 is only partially transmissive, only a portion of external light ray 83 passes through surface 79 and reaches the viewer's eye, while another portion of light ray 85 is reflected by surface 79, coupled out from prism 80, and also does not reach the viewer's eye. Naturally, the efficiency of projecting the image can be increased due to the external scene, and conversely, the brightness of the coupled light rays 63 and 68 increases by increasing the reflectivity of the partial surface 79. However, as a result, the transmittance of surface 79 decreases, and consequently, the brightness of the external image 83 decreases accordingly.

In contrast to the embodiments illustrated in Figures 1-5, the surface 79, which reflects the coupled output light from the substrate to the viewer's eye while simultaneously transmitting external light, is a conventional partial reflector without any special or complex characteristics, as shown by surfaces 22 and 50 in the embodiments illustrated in Figures 2 and 5, respectively. As a result, it is possible to dynamically control the reflectivity (and thus the transmittance) of the partial reflective surface 79 according to external lighting conditions and the specific image projected onto the viewer's eye. One way to control the reflectivity of surface 79 is by using an electrically switchable transmissive mirror, a solid-state thin-film device made of a special liquid crystal material, which can rapidly switch between pure reflection, partial reflection, and full transparency. Another method to realize the switchable element 79 is by forming it as a dynamic metasurface. The desired state of the switchable mirror can be manually set by the user or automatically set using a photometer that controls the mirror's reflectivity based on external brightness. This feature is useful in situations where the projected image is properly combined with an external image, but the brightness of the external scene is relatively high, and therefore, it is important to prevent it from glaring the viewer and interfering with the projected image. On the other hand, the efficiency of the projected image should be high enough to achieve reasonable contrast. Therefore, the dynamic surface 79 can be switched to a primary reflection state, i.e., the reflection of the switchable mirror is much higher than its transmission. As a result, the light rays 63 and 68 coupled out from the substrate are primarily reflected from surface 79 to the viewer's eye, and the overall efficiency of the optical system can be greater than 90%, while still allowing a properly visible bright external scene. Therefore, the potential luminance efficiency of the embodiments illustrated in Figures 6A-6C can be more than an order of magnitude higher than the efficiency of the configurations illustrated in Figures 2 and 5.

As seen in Figures 6A-6C, the aperture of the coupled input surface 65 is similar to the aperture of the coupled output surface 67. Subsequently, the effective area of the input aperture 86 is similar to the effective area of the output aperture 89. As a result, although the embodiment illustrated in Figures 6A-6C may have very high potential luminance efficiency, it still suffers from similar problems with both the input and output apertures. Therefore, a suitable method should be found to reduce the input aperture given a given output aperture, or alternatively, to increase the output aperture given a given input aperture. To achieve this, the fact that light waves coupled from the substrate do not necessarily illuminate the entire effective area of the coupled output surface is utilized.

Figure 7 illustrates the light rays that should strike the output aperture of surface 89 to illuminate the EMB 100, including two marginal and central light waves of the image, which are coupled out from the substrate and redirected to the viewer's eye 24. As shown, light waves 107R, 107M, and 107L, having zero-order off-axis angles 107R, 107M, and 107L—which are the minimum, center, and maximum angles in the FOV, respectively—illuminate only portions 67R, 67M, and 67L of the coupled output reflective surface 67 and are reflected by surface 89 into the EMB 100. Therefore, a method can be determined in which the input aperture of the substrate is significantly reduced such that the coupled input light waves illuminate only the desired corresponding portions of surface 67, and thus, the original brightness is preserved.

Figures 8A-8D illustrate the backtracking of three light waves from the EMB toward the input aperture 86 of the substrate 64. As shown, light wave 107L (dashed line in Figure 8A) strikes the right-side portion of surface 65, is trapped inside the substrate after three reflections from surface 65—this has an off-axis angle—and is coupled out of the substrate after three reflections from surface 67, with the third reflection coupling the light wave out of the substrate landing on the left-side portion of surface 67. Light wave 107M (dashed line in Figure 8B) strikes the center portion of surface 65, is trapped inside the substrate after two reflections from surface 65—this has an off-axis angle—and is coupled out of the substrate after two reflections from surface 67, with the second reflection coupling the light wave out of the substrate landing on the center portion of surface 67. Light wave 107R (dashed line in Figure 8C) strikes the left-side portion of surface 65, is trapped inside the substrate after one reflection from surface 65—this has an off-axis angle—and is coupled out of the substrate after one reflection from the right-side portion of surface 67. As illustrated in Figure 8D, the lateral area of the incoming light wave covering the entire FOV of the input aperture 86 is similar to the lateral area of the output aperture 89, and therefore, in this embodiment, the goal of reducing the size of the input aperture 86 is not achieved.

However, it should be noted that although the incoming waves cover a fairly large input aperture, they strike the input aperture in an orientation opposite to that of a conventional optical system. That is, when the light waves are traced back from the input aperture 86, they do not diverge but converge closer together. As a result, an intermediate prism can be added to the optical system, which will allow the backtracking light waves to converge into a pupil significantly smaller than the pupil of the input aperture 86.

Figures 9A-9D illustrate the embodiment shown in Figures 8A-8D, wherein the intermediate prism 108 is attached to the substrate 64 at the input aperture 86. The surface 110 of the prism 108 may be optically attached to the upper surface 70 of the substrate 64, thereby defining an interface plane 111. To minimize dispersion, the optical material of the prism 108 should be similar to the optical material of the redirecting prism 80. Furthermore, the light wave input surface 112 of the prism 108 should be oriented such that the incoming waves 107R, 107M, and 107L will strike the surface 112 at the same angle as their coupling output from the substrate 64 through the upper surface 70 toward the viewer's eye 24. Additionally, the surface 112 should be positioned in the plane in which the retrograde light waves converge to the minimum aperture. As shown in Figure 9D, all incoming light waves strike the surface 112 inside the new input aperture 86'. Compared with the original input aperture 86 and output aperture 89, the new input aperture 86' is significantly smaller, less than half the size of the original input aperture 86 and output aperture 89.

There are two conflicting requirements for the interface plane 111 between the intermediate prism 108 and the substrate 64. On the one hand, the first three orders of light, and should be reflected from this plane, while the zero-order image entering the substrate 64 through the intermediate prism 111 should pass through it substantially without significant reflection. Similarly, the interface plane 81 between the substrate 64 and the redirecting prism 80 should be transparent to coupled output light waves with an input angle after the last reflection from surface 67, and simultaneously highly reflective to coupled light waves with higher-order input angles, and . Furthermore, for a perspective system, the optical system should have the highest possible transparency to substantially perpendicular incident light passing through the interface plane 81. A preferred way to achieve this is to apply an optical adhesive with a refractive index significantly lower than that of the substrate to these interface planes, or alternatively, to apply a thin film with the desired refractive index to the interface plane 81 using a spin-coating process. Furthermore, to minimize Fresnel reflection of transmitted light waves from interface planes 81 and 111, it is preferable to apply a suitable AR coating to these planes. In this case, the overall efficiency of the light waves interacting with these planes can be very high. In other words, when light waves are coupled into the substrate, the reflectivity of plane 111 is 100% due to total internal reflection from the surface, and the transmittance of the surface to the incoming light waves is also close to 100% due to the AR coating. Similarly, due to total internal reflection from surface 81, the reflectivity of light waves coupled into the substrate 64 from this surface is 100%, and the transmittance of this surface to light waves coupled out of the substrate 64 to the redirection prism 80, as well as to incoming light waves from the external scene, is also close to 100% due to the AR coating.

For most relevant display systems, these two requirements should be met across the entire relevant visible spectrum. Therefore, it is a reasonable assumption that the Abbe numbers of the optical adhesive (or alternatively, a thin film applied by spin coating) and the optical material of the substrate adjacent to the interface surface should be similar to avoid undesirable color effects in the image. To achieve the desired total internal reflection, the refractive indices of the substrate and the adhesive (or thin film) should be significantly different. As a result, this requirement will be difficult to meet, and typically the Abbe numbers of the adhesive (or thin film) and the optical material will differ significantly. However, the dispersion due to the variation in Abbe numbers can be compensated by selecting an optical material with an Abbe number different from that of the substrate 64 for the coupling input and redirection of prisms 108 and 80. With appropriate selection, the difference in Abbe numbers can induce dispersion with the same amplitude and relative direction. As a result, the two induced dispersions will compensate for each other.

Another issue to consider is the maximum achievable field of view (FOV) of the image projected into the viewer's eye. In most substrate-guided HMD technologies, whether by reflection or diffraction, light waves are coupled out from the guiding substrate essentially perpendicular to its main surface. Therefore, due to Snell refraction from the substrate, the external FOV of the image is:

The FOV inside the substrate is given by [formula missing], and the refractive index of the substrate is [index missing]. Furthermore, the orders of the light waves coupled inside the substrate must be strictly separated, i.e., [details missing].

.

Furthermore, in order to ensure the transmission of the entire zeroth order through interface planes 81 and 111, and the reflection of the entire first order from these planes, the following constraints must be satisfied:

Where is the critical angle of the interface plane. Therefore, the internal FOV is limited by the following constraints:

Typically, a margin of approximately one degree should be maintained between and to confirm the separation between the two orders. The constraint of equation (4) results in a system in which the refractive indices of the substrate, the coupling input, and the coupling output elements are equal.

The fact that optical waves enter the substrate 64 from the intermediate prism 108 at a highly tilted angle can be used to mitigate the aforementioned limitations. As illustrated in Figures 9A-9D, the intermediate prism 108 and the redirecting prism 80 are made of the same optical material with the following optical properties and refractive index.

Where v_{_p} is the refractive index of prisms 108 and 80. In addition, the Abbe numbers Ap _{_p} and As_{_s} of the prism and substrate are chosen respectively to compensate for the dispersion caused by the dissimilarity between the Abbe numbers of the substrate and the optical adhesive (or film), as explained above.

Due to the dissimilarity between the optical materials of substrate 64 and those of intermediate prism 108 and redirecting prism 80, and the high inclination of rays 107R, 107M, and 107L incident on interface surfaces 111 and 81, light waves undergo significant refraction as they pass through the interface surfaces. Since prisms 108 and 80 have identical optical properties, for each passing light wave, the refraction at surfaces 111 and 81 will have the same amplitude and relative direction, and therefore, they will compensate for each other. The angular deviation between the two different rays inside the prism, as a function of the substrate's internal deviation, can be calculated using the following approximate equation.

Where and are the off-axis angles inside the substrate and prism, respectively. Similarly, the angular deviation between light rays in air is .

.

Therefore, the ratio between the angular deviation in the air and the angular deviation inside the substrate 64 is:

or

In other words, by modifying the optical materials of prisms 108 and 80, the FOV of the system in air can be increased by a factor of 100.

The implementation of the embodiments shown in Figures 9A-9D is illustrated herein using an optical system having the following nominal parameters:

The light wave is unpolarized. The optical material of substrate 64 is Ohara S-LAH88 with a refractive index v _{_d} = 1.917 and an Abbe number _Ad = 31.6. The optical materials of prisms 81 and 111 are Schott N-BK7 with a refractive index v _{_d} = 1.516 and an Abbe number _Ad = 65.5. The optical adhesive of adjacent surfaces 81 and 111 in Figures 9A-9D is NOA 148 with a refractive index v _{_d} = 1.48 and an Abbe number _Ad = 48. As shown, the field of view (FOV) is 40° in air, 26° inside prisms 111 and 81, and 13° inside substrate 64. That is, even though the refractive index of the substrate is less than 2, the FOV in air is more than three times larger than the FOV inside the substrate. The maximum angle in the third order is greater than 90°, and therefore, it has an "illegal" propagation direction. However, as shown in Figures 9A-9C, the third propagation order is only effective for light waves in the lower region of the FOV. Light waves in the upper region of the FOV are coupled into the substrate after a single reflection from the coupling input surface 65, and therefore, they propagate only in the first propagation order, thus avoiding the contradiction.

Figure 10 illustrates the reflection curves of the AR coatings applied to interface surfaces 81 and 111. As a result of dispersion due to the variation between the Abbe numbers of substrate 64 and prisms 81 and 111, the critical angle largely depends on the wavelength. Therefore, it should be assumed that the following condition satisfies the condition of equation (5) throughout the relevant visible spectrum.

In other words, the FOV inside the substrate should be reduced to 12°, and therefore, the FOV in the air will be reduced to 36°.

The high dispersion of light waves entering the substrate 64 through the intermediate prism 111 causes each incoming white light wave to be spatially separated into components of different wavelengths. For example, for zero-order light waves with wavelengths of 450 nm, 550 nm, and 650 nm, respectively, the marginal light wave 107R with an off-axis angle of -20° across the entire visible spectrum is split into propagation directions of 36.2°, 36.6°, and 36.8°. The precise values of the parameters given in equation (13) for the three different wavelengths are

The subscripts sb , sg , and sr indicate the parameters of light waves with wavelengths of 450 nm, 550 nm, and 650 nm inside the substrate 64, respectively, and the subscripts surb , surg , and surr indicate the parameters of incoming light waves with the same wavelength that strike the coupling input surface 65.

Figure 11 illustrates the propagation direction and critical angle as a function of the wavelength of the entire relevant visible spectrum. As shown, the requirements given in equations (5)-(6) are satisfied for the entire spectrum without being subject to the limitations of equation (14), and a field of view (FOV) of at least 13° in the substrate and 40° in air is maintained.

Figures 12A-12C illustrate the reflection curves of the AR coating applied at interface surfaces 81 and 111 for wavelengths of 450 nm, 550 nm, and 650 nm, respectively. For each relevant wavelength, the two vertical lines on the graphs indicate the propagation direction. As shown, for all wavelengths, due to total internal reflection from the interface plane, the reflection at angle θ is 100%, while the transmission at angle θ is negligible as needed.

Figures 9A-9D illustrate one embodiment of an optical system that, even using only a single coupled output element 67, has a wide field of view (FOV) of 40° along the propagation direction of the light wave within the substrate 64. However, the input light wave has a highly skewed angle in its direction of propagation. In many applications, it is required that the input light wave strikes the substrate substantially perpendicularly to the main surfaces 70 and 72. Figures 13A-13C illustrate a configuration in which the light waves at the left edge 107L, center 107M, and right edge 107R strike the substrate substantially perpendicularly to the lower surface 72. As shown, the light wave enters the substrate and passes through the coupled input surface 65. Due to the significantly small angle of incidence of the input light wave and the AR coating applied at surface 65, the reflectivity of the light wave from this surface is negligible. Light waves leaving substrate 64 enter intermediate prism 114 via the lower surface 116 attached to the upper surface 70 of substrate 114, are reflected by reflective surface 118, and re-enter substrate 64 via the upper surface 70 of substrate 64 at an input angle. Light waves now striking the coupling input surface 65 have an incident angle higher than the critical angle and are coupled inside the substrate in a manner similar to that illustrated above with respect to Figures 9A-9D. As illustrated in Figure 13D, light waves throughout the FOV are incident on surface 72 inside the new input aperture 86', which is at least significantly smaller than one-third of the original input aperture 86 and output aperture 89. Here, the input aperture 86' is not adjacent to intermediate prism 114 but closer to the lower main surface 72 of the substrate. Generally, the optical system should be designed such that the input aperture will be positioned in a location convenient for placing the collimation module on the outer surface.

In the embodiment illustrated in Figures 13A-13D, light waves strike the substrate at surface 72 and exit the substrate through the opposing surface 70 into the viewer's eye, i.e., the viewer's eye and the display source are positioned on opposite sides of the substrate. This configuration is preferred for a top-down arrangement; however, other arrangements exist, such as eyeglass structures, where the viewer's eye and the display source are required to be positioned on the same side of the substrate.

Figures 14A-14C illustrate a configuration in which light waves from the left edge 107L, center 107M, and right edge 107R strike the substrate substantially perpendicularly to the upper surface 70 at the same side of the viewer's eye. A lens 6 is added to the figures to illustrate the collimation of the light waves from the display source 4. As shown, the light waves enter the substrate and pass through the coupling input surface 65 without significant reflection. The light waves leave the substrate 64, enter the intermediate prism 120 through the upper surface 124 attached to the lower surface 72 of the substrate, are reflected by the reflecting surface 122, and re-enter the substrate 64 through the lower surface 72. The light waves strike the coupling input surface 65 again with an angle of incidence below the critical angle, pass through the surface 65, and are totally internally reflected from the upper surface 70 of the substrate. The light waves strike the coupling input surface 65 again, now with an angle of incidence above the critical angle, and are coupled inside the substrate in a manner similar to that illustrated above with respect to Figures 13A-13C. As illustrated in Figure 14D, the light waves incident on the surface 70 inside the new input aperture 86' located near the outer surface of the collimating lens 6 are significantly smaller in the entire FOV than those in the original input aperture 86.

Unlike the other configurations illustrated above, in the embodiment described with respect to Figures 14A-14D, the light wave impacts the coupling input surface 65 three times. The requirement that the light wave passes through surface 65 without significant reflection during the first impact can be easily achieved by applying an AR coating to surface 65. However, for the other two impacts, there are two conflicting requirements from surface 65. On the one hand, the light wave incident on the surface at the third angle should be reflected from that surface. On the other hand, the light wave incident on the surface at the second angle should pass through it substantially without significant reflection. As described above with respect to interface planes 81 and 111, a preferred way to achieve this is to apply an optical adhesive (or a spin-coated film) to the coupling input surface 65, the optical adhesive (or spin-coated film) having a refractive index significantly lower than that of the substrate. Furthermore, to minimize Fresnel reflection of the light wave incident on surface 65 during the second impact, a suitable AR coating needs to be applied to these planes.

Figure 15 illustrates the reflection curves of an AR coating applied at the coupling input surface 65 to a substrate having the following parameters: the light wave is unpolarized; the optical material of the substrate 64 is Ohara S-LAF198 with a refractive index v_{_d} = 1.954 and an Abbe number Ad_{_d} = 32.32; and the optical adhesive adjacent to the surface 65 is NOA 1315 with a refractive index v _{_d} = 1.315 and an Abbe number Ad_{_d} = 56. As a result of dispersion due to variations in the Abbe numbers of the substrate 64 and the optical adhesive, the critical angle largely depends on the wavelength.

Figure 16 illustrates the propagation direction and critical angle as a function of wavelength across the entire relevant visible spectrum. As shown, there are differences in the angular spectra of the second and third impacts across the entire spectrum, and the spectra are positioned below and above the critical angle curve, respectively, as needed.

Figures 17A-17C illustrate the reflection curves of the AR coating applied at 65° of the coupling input surface for wavelengths of 450 nm, 550 nm, and 650 nm, respectively. For each relevant wavelength, the two vertical lines on the graphs indicate the propagation direction. As shown, for all wavelengths, the reflection of the third impact at the incident angle is 100% due to total internal reflection from the interface plane, while the transmission of the second impact at the incident angle is negligible as needed.

While Figures 13A-13D and 14A-14D illustrate embodiments in which the input light wave strikes the substrate substantially perpendicular to the main surface, there are configurations where the input light wave is required to be oriented at an angle to the substrate. Figures 18A-18D illustrate a modified version of the embodiment shown in Figures 13A-13D. A light wave illuminating the substrate at a predefined angle enters the substrate through the surface 128 of the first intermediate prism 126 attached to the lower surface 72 of the substrate 64, and passes through the coupling input surface 65 without significant reflection. The light wave then leaves the substrate 64, enters the second intermediate prism 132 through the lower surface 136 of the second intermediate prism 132 attached to the upper surface 70 of the substrate, is reflected by the reflecting surface 134, and re-enters the substrate 64 through the upper surface 70 of the substrate. The light wave is reflected by the coupling input surface 65 and is trapped inside the substrate in a manner similar to that illustrated above with respect to Figures 13A-13D.

Figures 19A-19D illustrate a modified version of the embodiment shown in Figures 14A-14D. Light waves illuminating substrate 64 at a predefined angle enter the substrate through surface 140 of the first intermediate prism 138 attached to the upper surface 70 of the substrate and pass through the coupling input surface 65 without significant reflection. Light waves leaving substrate 64 enter the second intermediate prism 144 through surface 148 of the second intermediate prism 144 attached to the lower surface 72 of the substrate, are reflected by reflective surface 146, and re-enter substrate 64 through the lower surface 72 of substrate 64. The light waves then strike the coupling input surface 65 again with an angle of incidence below the critical angle, pass through surface 65, and are totally internally reflected from the upper surface 70 of the substrate. The light waves strike the coupling input surface 65 again, now with an angle of incidence above the critical angle, and they are coupled within the substrate in a manner similar to that illustrated above with respect to Figures 14A-14C.

Figures 14A-14D and 19A-19D illustrate embodiments that can be used for eyeglass arrangements. However, particularly for consumer market applications, there is a situation where, for aesthetic reasons, the folding prism attached to the front surface of substrate 72 is required to be as small as possible. Figures 20A-20D illustrate a modified version of the embodiments shown in Figures 14A-14D and 19A-19D, wherein light waves illuminating substrate 64 at a predetermined angle enter the substrate through the surface 228 of the first intermediate prism 226 attached to the upper surface 70 and pass through the coupling input surface 65 without significant reflection. Light waves leaving substrate 64 enter the second intermediate prism 220 through the upper surface 224 attached to the lower surface 72 of the substrate, are reflected by the reflective surface 222, and re-enter substrate 64 through the lower surface 72 of substrate 64. However, here, the tilt angle of the reflective surface 222 is significantly smaller than the tilt angles of surfaces 122 and 146 in the configurations of Figures 14A-14D and 19A-19D, respectively, compared to the main surface 72. As a result, the light wave strikes the coupling input surface 65 again with an angle of incidence, which can be determined according to design considerations, but is typically greater than 5°. Now, even the maximum angle of incidence is much smaller than the critical angle, and therefore a simpler AR coating can be applied to surface 65. The light wave continues through surface 65 and re-enters the first intermediate prism 226 through the lower surface 230 attached to the upper surface 70 of the substrate. The wave is then totally reflected from the outer surface 228 and re-enters the substrate 64 through the lower surface 72 of the substrate 64. The tilt angle of surface 228 is set to compensate for the “lost” angle. Thus, the light wave, now with an angle of incidence greater than the critical angle, strikes the coupling input surface 65 again and couples into the substrate in a manner similar to that illustrated above with respect to Figures 14A-14C and 19A-19D.

In the embodiment described above, a high FOV of 40° along the propagation direction is achieved within the substrate using a single coupled output surface 67. For side-view configurations (such as eyeglasses), the diagonal FOV can be 47° or 50°, depending on the aspect ratio of the display source (9:16 or 3:4, respectively). For top-down configurations (such as helmet displays), the diagonal FOV can be extended to more than 80° for a 9:16 aspect ratio. To maximize luminance efficiency, assuming a single coupled output surface in the substrate is preferred, there are two conflicting requirements regarding the angular orientation of this surface. On the one hand, due to the limitations given in equation (7), it is preferable to increase the angle to expand the full FOV that can be coupled within the substrate. On the other hand, the range of the output aperture 89 of the substrate is proportional to d, where d is the thickness of the substrate, i.e., the output aperture, and therefore the EMB will be expanded by decreasing it. It is also possible to increase the output aperture by increasing the thickness of the substrate, but the input aperture will also increase accordingly. Furthermore, it is generally required that the substrate be as thin as possible.

Figure 21 illustrates a modified version of the embodiment shown in Figures 14A-14D. Instead of using a single substrate 64, the system 150 shown includes two adjacent substrates 64a and 64b. The upper surface 70b of substrate 64b is optically attached to the lower surface 72a of substrate 64a, thereby defining an interface surface 152. The orientation angles of the coupling input and coupling output surfaces 65b and 67b are set by the desired FOV according to the constraint of equation (7), while the orientation angles of the coupling input and coupling output surfaces 65b and 67b are set to the following lower values.

.

As a result, the entire FOV can be coupled inside the lower substrate 64b. However, to meet the requirements of equation (7), only a portion of the FOV can be coupled inside the upper substrate 64a. That is, the FOV coupled inside both substrates is...

.

Therefore, the lower part of the FOV is coupled only inside the lower substrate 64b, and to avoid crosstalk with the upper part of the FOV, it is not coupled inside the upper substrate 64a. Since light waves from the lower part of the FOV illuminate the viewer's eye from the left side of the output aperture, they should be coupled out from the left-side coupling output surface 67b; that is, they should only be transmitted to the eye through the lower substrate 64b. Therefore, the entire FOV can be preserved for the entire EMB. Furthermore, the output aperture is expanded to the following extent...

.

Alternatively, for a given output aperture, the thickness of the dual gratings can be thinner at the following ratio:

Where d _{_a} and d_{_b} are the thicknesses of substrates 64a and 64b, respectively, and d is the thickness of a single substrate, as illustrated in the embodiments shown in Figures 14A-14B. Therefore, the embodiment of Figure 21 has the advantage of a wider FOV determined by a larger angle and a larger output aperture determined by a smaller angle. Since each of the two substrates 64a and 64b functions independently, each individual substrate can have different parameters in addition to the tilt angle. Depending on the requirements of the optical system, the two substrates can in particular have different thicknesses, refractive indices, and Abbe numbers. Furthermore, the relative positions of the coupling input surfaces 65a and 65b and the relative positions of the coupling output surfaces 67a and 67b can be freely set to minimize the system's input aperture 86' (see Figure 25) and simultaneously maximize the system's output aperture 89 (see Figure 25).

As illustrated in Figures 22A, 22B, and 22C, three rays from the left-side interplanetary light wave 153 (153a, 153b, 153c) are coupled into the interior of the lower substrate 64b after being reflected three times from surface 65b. One ray 153d (Figure 22B) is coupled after being reflected twice, and two other rays 153e and 153f (Figure 22C) are coupled after being reflected once from surface 65b. As shown in Figure 22D, all rays are coupled out from substrate 64b through coupling output element 67b and redirected to illuminate the entire EMB 100.

Figures 23A, 23B, and 23C illustrate two rays (154a and 154b) from the central light wave 154, which are coupled inside the lower substrate 64b after a single reflection from surface 65b and output via surface 67b, respectively. Two rays (154c and 154d) are coupled inside the upper substrate 64a after three reflections from surface 65a and output via surface 67a. Two other rays (154e and 154f, Figure 23C) are coupled inside the upper substrate 64a after two reflections from surface 65a and output via surface 67a. As shown in Figure 23D, all rays are redirected by the redirecting prism 80 to illuminate the entire EMB 100.

Figure 24A illustrates two rays from the right-hand side light wave 155 (155a, 155b) coupled inside the upper substrate 64a after being reflected twice from surface 65a, and three other rays 155c, 155d, 155e coupled after being reflected once from surface 65a. As shown in Figure 24C, all rays are coupled out from substrate 64a through coupling output element 67a and redirected to illuminate the entire EMB 100. As illustrated in Figure 25, light waves in the entire FOV incident on surface 70 inside input aperture 86' illuminate the entire EMB, where input aperture 86' is significantly smaller than output aperture 89.

Another issue to consider is that ghosting can occur in the image due to undesirable reflections of stray light from the system's outer surfaces. As illustrated in Figure 26A, the input light 160 is coupled into the substrate 64 after a single reflection from surface 65, and then coupled out from the substrate after a single reflection from surface 67. The light is then partially reflected into the viewer's eye in the correct direction by surfaces 79i and 79j as output light rays 160a and 160b. However, a portion of light 160 passes through surface 79j, undergoes total internal reflection from the lower surface 162 of prism 80, then partially reflects from surface 79k, passes through the substrate 64, undergoes total internal reflection from the upper surface 70 of the substrate 64, passes through the substrate 64 again, and is then partially reflected into the viewer's eye as output light ray 160c from surface 79m in the "wrong" direction. That is, stray light ray 160c will appear as ghosting in the projected image. Figure 26A illustrates such ghosting originating from the coupled input image light waves. However, due to light waves from the external scene, other ghosting may occur. As illustrated in Figure 26B, external light ray 163 passes through the partially reflective surface 79n, through the prism 80 and the substrate 64, and reaches the viewer's eye in the original direction of light ray 163a. However, a portion of light ray 163 is partially reflected from surface 79n, totally reflected from the lower surface 162 of the prism 80, partially reflected from surface 79o, passes through the substrate 64, totally reflected from the upper surface 70 of the substrate 64, passes through the substrate 64 again, and is then partially reflected as output light ray 163b from surface 79p into the viewer's eye in an "incorrect" direction. Therefore, in the projected image, stray light ray 163b will also appear as ghosting.

As shown in Figures 26A and 26B, the primary cause of ghosting is unwanted reflection from surface 162. This phenomenon is typical not only for the embodiments illustrated in this application but also for other substrate-guided configurations. Unlike these other configurations, total internal reflection from surface 162 is not necessary for the propagation of light waves within the substrate and therefore can be completely eliminated. One possible way to eliminate unwanted reflection from surface 162 is to apply an absorbing layer to the surface. This simple method can be used in non-perspective systems where the outer surface 162 can be completely opaque. However, for perspective systems, since light from the external scene must pass through surface 162 to reach the viewer's eye 24, it is not permissible for surface 162 to be opaque.

Figure 27 illustrates a more efficient method to remove total internal reflection from surface 162 while maintaining the surface substantially transparent to light from an external scene. As shown, the upper surface 166 of a thin, flat, transparent plate 167 is optically attached to the lower surface 162 of a redirecting prism 80. An array of parallel absorbing surfaces ₁₆₈₁ , ₁₆₈₂ … oriented perpendicular to surface 166 is embedded within plate 167. To verify that all light striking surface 162 will be absorbed by these surfaces, the following relationship must be satisfied:

Where T is the thickness of plate 167, D is the distance between two consecutive surfaces _168i and 168i ₊₁ , and is the minimum off-axis angle of the light wave striking plate 167. As shown, light 171 is absorbed by surface _168i after total internal reflection from the lower surface 169 of plate 167, while light 172 is absorbed by direct impact on surface _168j . Since substrate 64 is thin and the absorbing surface is perpendicular to the main surface of the substrate and therefore perpendicular to the viewer's visual axis, plate 167 remains substantially transparent to light from the external scene.

Figures 28A to 28F illustrate the method for manufacturing plate 167. Multiple transparent plates 174i with a thickness T are manufactured (Figure 27). Since the main surfaces of these plates should be absorbent, they do not necessarily need to be polished, and their parallelism is not important. A thin absorbent layer 175 is applied to one of the main surfaces of each plate (Figure 28B). _This absorbent layer can be, in particular, black paint, a thin silicone coating, a metallic coating, or any other absorbent material that can be applied as a thin layer. Plates 176 are bonded together using a suitable optical adhesive to form a stack (Figure 28C). Multiple segments _167'i are then cut from the stacked form 176 in a direction perpendicular to the main surface of plate _174i (Fig. 28D), and are then processed by cutting, grinding, and polishing to create a plate 167” _i with a thickness T’ (Fig. 28E). One of the main surfaces of this plate is optically bonded to surface 162 (Fig. 28F). In many cases, it is required that plate 167 be very thin, approximately 0.1 mm. In such cases, it may be difficult to process a plate _167'i with the required thickness T. Therefore, a plate with the required thickness will be bonded to prism 80, and the lower surface 169' of the bonded plate 167” will be grounded and polished to achieve the required thickness T of the final plate 167.

Figures 29A and 29B illustrate an embodiment similar to the one shown in Figures 26A-26B, wherein plate 167 is optically attached to the lower surface 162 of prism 80. As shown, stray rays 160c and 163b are not totally internally reflected from surface 162 and continue to propagate in the system, but are absorbed in plate 167, and thus, ghosting originating from the projected image and the external scene is completely eliminated. This method of attenuating ghosting caused by undesirable total internal reflection can also be applied to other optical modules where stray rays are undesirably reflected from surfaces that should be transparent to normal incident light. Therefore, plate 167 can be optically attached to such surfaces to attenuate undesirable reflections while still maintaining the desired transmissivity of the surface.

The advantage of reducing the lateral size of the input aperture, as described above, is even more pronounced in cases where two-dimensional expansion of the coupled light wave is required. Figure 30 is a schematic diagram illustrating how a beam is expanded along two axes using a dual-substrate configuration. For simplicity, the intermediate prism and redirection element are omitted from the figures. The input image 256 is coupled to the first substrate 264a by the first reflective surface 265a through the input aperture 274 and then propagates along the η-axis. The first substrate 264a has a structure similar to one of the embodiments described above. The coupling output element 267a couples light out of the substrate 264a through the output aperture 276, and then the light is coupled to the second main substrate 264b through the input aperture by the coupling input element 265b, which coincides with the output aperture 276 of the first substrate 264a. The light wave then propagates along the η-axis and is coupled out by the coupling output element 267b through the output aperture 278. As shown, the original image 256 expands along two axes, where the total expansion is determined by the ratio between the lateral dimensions of apertures 274 and 278. As shown, each light wave (indicated by a single arrow in the figure) illuminates only a portion of the output aperture 278, but all light waves are coupled and output to the EMB 100 with the desired orientation.

In all the embodiments above, it has been assumed that the display source is unpolarized. However, there are microdisplay light sources, such as LCDs or LCOSs, where the light is linearly polarized, and this can be used to create more compact collimation systems. As illustrated in FIG31A, p-polarized input light waves 107L, 107M, and 107R from display light source 4 are coupled through surface 280 of light guide 279 into light guide 279, which is typically composed of light-transmitting material. The light waves pass through polarizing beam splitter 282 and are coupled out of light guide 279 through surface 283. The light waves then pass through quarter-wave delay plate 285, are collimated by lens 286 at reflective surface 289 of lens 286, return, pass through delay plate 285 again, and re-enter light guide 279 through surface 283. The now s-polarized light waves are reflected from polarizing beam splitter 282 and exit light guide through lower surface 290. The light wave is now coupled into the substrate 64 via intermediate prisms 226 and 220 in the same manner as illustrated above with respect to Figures 20A-20D. The reflective surface 289 may be materialized by a metal or dielectric coating.

As illustrated in Figure 31A, the use of the reflective collimating lens 286 has several significant advantages, such as achieving good performance through the use of a small number of optical components and with an additional compact collimating module. Therefore, it is advantageous to also use this embodiment for unpolarized light sources such as micro-LEDs and OLEDs. The main disadvantage in this case is that only a single polarization component of the display source can be used, and thus, the achievable brightness is reduced by more than 50%. Figure 31B illustrates an alternative method for utilizing two orthogonal polarization components of an unpolarized display source and thus avoiding brightness reduction. As shown, the s-polarization components of the input light waves 107L, 107M, and 107R from the display light source 4 are coupled into the light guide 279 via the right surface 280. After reflection from the polarization beam splitter 282, the light waves are coupled out from the substrate via the surface 291 of the light guide 279. The light wave then passes through the second quarter-wavelength delay plate 293, is collimated by the second lens 296 at the reflecting surface 297, returns and passes through the delay plate 293 again, and re-enters the light guide 279 through surface 291. The now p-polarized light wave passes through the polarizing beam splitter 282, exits the light guide through the lower surface 290, and is coupled into the substrate 64 through intermediate prisms 226 and 220 as before. As illustrated in Figure 31A, the p-polarized component of the light source is coupled into the substrate. The two collimating lenses should be identical and placed very precisely on the surface of the light guide 279 to avoid double images.

It will be apparent to those skilled in the art that the present invention is not limited to the details of the embodiments described above, and that the invention may be embodied in other specific forms without departing from the spirit or essential nature of the invention. Therefore, the present embodiments are to be considered illustrative rather than restrictive in all respects, and the scope of the invention is indicated by the appended claims rather than by the foregoing description; and thus, all changes within the meaning and scope of equivalents of the claims are intended to be included therein.

In particular, it should be noted that the features described with reference to one or more embodiments are described by way of example rather than by way of limitation on those embodiments. Therefore, unless otherwise stated or unless a particular combination is obviously unacceptable, it is assumed that optional features described with reference to only some embodiments also apply to all other embodiments.

Claims (20)

1. An optical device, comprising: The first light-transmitting substrate has at least two parallel main surfaces and two opposing edges; Input port; An output aperture located near one of the main surfaces of the substrate; An eye movement frame with perforations; The first intermediate element has at least two surfaces positioned outside the substrate for coupling an incoming light wave with a field of view into the substrate through an input aperture. A first flat reflective surface has an effective area positioned between two main surfaces of a light-transmitting substrate for reflecting incoming light waves from a first intermediate element to achieve total internal reflection from the main surfaces of the substrate. A second flat reflective surface, parallel to the first flat reflective surface, has an effective area and is positioned between the two main surfaces of the light-transmitting substrate, for coupling light waves out from the substrate; and A redirecting optical element having at least two surfaces positioned outside a substrate for redirecting light waves coupled out of the substrate through an output aperture to an eye motion frame. The input aperture is significantly smaller than the output aperture. All incoming light waves pass through the inside of the input aperture. All light waves coupled out from the substrate through the second reflective surface are coupled into the substrate through reflection from the first reflective surface. The effective area of the first reflective surface is similar to the effective area of the second reflective surface, and each coupled light wave covers the entire aperture of the eye motion frame. 2. The optical device according to claim 1, wherein the light waves coupled into and out of the substrate have brightness, and the brightness of the light waves coupled out of the substrate by the second flat reflective surface is substantially similar to the brightness of the light waves coupled into the substrate. 3. The optical device according to claim 1, wherein the first flat reflective surface and the second flat reflective surface couple light waves into the substrate and couple light waves from the substrate respectively through total internal reflection. 4. The optical device according to claim 1, wherein the light wave coupled inside the substrate is reflected the same number of times from the first flat reflective surface and the second flat reflective surface. 5. The optical device of claim 1, wherein the light wave passing through the input aperture and coupled into the substrate is incident only on a portion of the first flat reflective surface and the second flat reflective surface. 6. The optical device of claim 5, wherein light waves passing through the input aperture and the aperture of the eye movement frame are incident only on a portion of the first reflective surface closer to one of the substrate edges, and are coupled out from the substrate only by a portion of the second reflective surface closer to the other edge of the substrate. 7. The optical device of claim 5, wherein light waves passing through the input aperture and the aperture of the eye movement frame are incident only on the portion of the first reflective surface closer to the center of the substrate, and are coupled out from the substrate only by the portion of the second reflective surface closer to the center of the substrate. 8. The optical device according to claim 5, wherein the light wave passing through the input aperture and the aperture of the eye movement frame is incident only on the central portion of the first reflective surface, and is coupled out from the substrate only by the central portion of the second reflective surface. 9. The optical device of claim 1, further comprising a second intermediate element, wherein light waves pass through the first intermediate element and the second intermediate element before being coupled into the substrate by the first reflective surface. 10. The optical device of claim 1, wherein the coupled input light wave passes through the first flat reflective surface at least twice before being reflected by the surface to be coupled into the substrate. 11. The optical device of claim 1, wherein the first intermediate element and the redirecting optical element are optically bonded to the main surface of a substrate having a refractive index by a first optical adhesive, defining a first interface plane and a second interface plane, wherein the refractive index of the adhesive is significantly lower than that of the substrate, and an anti-reflective coating is applied to the interface plane. 12. The optical device of claim 11, wherein the interface plane is substantially transparent to light waves having an angle of incidence more than one degree lower than the critical angle of the interface plane. 13. The optical device of claim 1, wherein a second optical adhesive is applied to a first reflective surface and a second reflective surface, the refractive index of the adhesive being significantly lower than that of the substrate, an anti-reflective coating is applied to the reflective surface, and the surface is substantially transparent to light waves having an incident angle more than one degree lower than the critical angle of the surface. 14. The optical device of claim 12, wherein the first intermediate element and the redirecting optical element are made of the same optical material having a refractive index and Abbe number that are significantly different from the refractive index and Abbe number of the substrate, thereby creating a first dispersion of light waves coupled within the substrate. 15. The optical device of claim 14, wherein the Abbe number of the first adhesive is significantly different from the Abbe number of the substrate, thereby creating a second dispersion of light waves within the substrate, and the first dispersion and the second dispersion substantially compensate for each other. 16. The optical device of claim 1, further comprising a second light-transmitting substrate having at least two main surfaces, two opposing edges, and a third and a fourth flat reflective surface parallel to each other, each surface having an inclination angle, wherein the two substrates are optically attached, and the inclination angles of the third and fourth flat reflective surfaces toward the main surfaces of the second substrate are lower than the inclination angles of the first and second flat reflective surfaces toward the main surfaces of the first substrate. 17. The optical device of claim 1, further comprising a second light-transmitting substrate having at least two main surfaces, two opposing edges, and a third and a fourth flat reflective surface, an input aperture and an output aperture having lateral dimensions, wherein light waves coupled out from the first substrate are coupled into the second substrate, and the lateral dimension of the input aperture is significantly smaller than the lateral dimension of the output aperture along two different axes. 18. The optical device of claim 1, further comprising a plate optically attached to the surface of a redirecting optical element, wherein an array of flat absorbing surfaces substantially perpendicular to the surface of the redirecting optical element is embedded within the plate, the plate being substantially transparent to perpendicularly incident light waves, and light waves coupled out from the substrate and incident on the plate are absorbed by the absorbing surfaces. 19. The optical device of claim 1, wherein the first intermediate element and the redirecting element are optically bonded to the main surface of a substrate having a refractive index by an optical adhesive, thereby defining a first interface plane and a second interface plane, and an anti-reflective coating and a thin-film dielectric coating are applied to the interface plane, wherein the refractive index of the dielectric coating is significantly lower than that of the substrate. 20. The optical device of claim 1, wherein the substrate has a refractive index and a surface has a critical angle, the first reflective surface and the second reflective surface have a thin-film dielectric coating and a refractive index, the refractive index of the dielectric coating being lower than that of the substrate, and the reflective surface has an anti-reflective coating that is transparent to light waves having an incident angle lower than the surface critical angle.