Optical system including a light-directing optical element with a partially reflective internal surface - Patents.com

Abstract

To provide an optical system using a light guiding optical element (LOE) with a partially reflecting internal surface for use in a head-up display.SOLUTION: An optical system includes: (a) a light guiding optical element (LOE) having a pair of parallel main external surfaces; and (b) a plurality of mutually parallel reflective surfaces in the LOE, the reflective surfaces being obliquely angled with respect to the main external surfaces. At least one of the reflective surfaces is configured to have high reflectivity for an incident angle of greater than 60 degrees with respect to a reference plane and partial reflectivity for the incident angle of less than 35 degrees with respect to the reference plane. The high reflectivity is greater than 95%.SELECTED DRAWING: Figure 2C

Description

[Technical field]

[0001] The present invention relates to optical systems for use in head-up displays, and in particular to optical systems that use light-directing optical elements (LOEs) with partially reflective inner surfaces.

[0002] Various displays, particularly head-up displays (HUDs) and near-eye displays for augmented or virtual reality, use a light-directing optical element (LOE) that has a pair of parallel major exterior surfaces to deliver a collimated image that propagates into the LOE by internal reflection. The image is gradually outcoupled from the LOE, typically directly to the eye, or to another LOE, which delivers the image to the eye. In one example of such a device, outcoupling of the image from the LOE is accomplished by a set of mutually parallel partially reflective surfaces within the LOE that are positioned at an angle to the major exterior surface of the LOE. Gradually outcoupling onto a series of partially reflective surfaces achieves multiplication of the aperture of the optics coupled into the LOE.

[0003] Conventional LOEs impose stringent requirements on the reflectivity of partially reflective surfaces as a function of the angle of incidence. Typically, they require high transmission (near-perfect transmission) of image emission at a range of angles, and partial reflection at other angles relative to the plane of the facets. In practice, near-perfect transmission is difficult to achieve. One representative example is illustrated diagrammatically in Figures 1A and 1B, in which an LOE (10) with parallel major faces (12) and (14) contains a set of partially reflective faces (16) (also referred to interchangeably herein as "facets"). At an angle corresponding to a given pixel of an image generated from a given position in an input optical aperture (not shown), a typical light ray (18) propagates along the LOE by internal reflection at surfaces (12) and (14).

[0004] In a typical application, image illumination, exemplified by light ray (18), propagates to major faces (12) and (14) at angles steeper than the corners of the partially reflective surfaces (16). As a result, each illumination ray (18) may intersect a given facet (16) several times. For example, in Figures 1A and 1B, as light ray (18) propagates from left to right, it intersects the third facet three times, at locations labeled 1, 2, and 3, respectively. As a result, the light reflected and outcoupled from point 1 (labeled a in Figure 1B) is stronger than the light reflected and outcoupled from point 3 (labeled b), causing a non-uniformity in the output image.

[0005] Furthermore, it is typically required that the facet be transparent (no reflection) for rays (18) with an angle of incidence indicated at point 2, because any reflection there (dashed arrow) would further reduce the brightness of the propagating light reaching point 3 and would create "ghosting" due to illumination propagating in the wrong direction. This "ghosting" may result in portions of the image appearing misplaced in the final image. This requirement of perfect transmission (zero reflection) is difficult to achieve and becomes increasingly difficult to meet as the angle of incidence (AOI) becomes larger. Summary of the Invention

[0006] The present invention is an optical system that includes a light-directing optical element (LOE) with an internally reflective surface.

[0007] In accordance with the teachings of one embodiment of the present invention, an optical system is provided that includes: (a) a light-directing optical element (LOE) having a pair of parallel major exterior surfaces; and (b) a plurality of mutually parallel reflective surfaces within the LOE, the reflective surfaces being obliquely angled relative to the major exterior surfaces, at least one of the reflective surfaces being: Normal 60° to Exceed To have a high reflectance against the angle of incidence, Normal The reflector is configured to have partial reflectance for angles of incidence less than 35°.

[0008] According to a further feature of the embodiment of the present invention, the high reflectance is 60° Exceed It exceeds 95% for the angle of incidence.

[0009] According to further features of embodiments of the present invention, the partial reflectance is less than 50%.

[0010] According to a further feature of an embodiment of the invention, the LOE has an internal coupling region from which internally coupled image radiation propagates along the LOE and the partial reflectivity varies between a series of reflective surfaces to at least partially compensate for a reduction in intensity of the image radiation reaching the series of reflective surfaces.

[0011] According to a further feature of the present embodiment, the plurality of mutually parallel reflective surfaces within the LOE further comprises an incoupling reflective surface forming at least a part of an incoupling array, the incoupling reflective surface comprising: Normal 60° to Exceed It has a high reflectivity against the angle of incidence, Normal The reflectance is configured to be at least 66% for angles of incidence less than 35°.

[0012] According to further features of the embodiment of the present invention, the plurality of reflective surfaces including the incoupling reflective surface are part of a symmetrical arrangement of two sets of mutually parallel reflective surfaces including two incoupling reflective surfaces, the two incoupling reflective surfaces being arranged to form a chevron-shaped incoupling arrangement. Meet .

[0013] According to a further feature of the present invention, there is further provided an image projector for projecting a collimated image, wherein an internal coupling array optically couples the collimated image into the LOE as a first image illuminant that propagates within the LOE by internal reflection at the primary surface, the first image illuminant spanning a first angular field of view, the first angular field of view being at a steeper angle to the primary surface than the reflective surface.

[0014] According to a further feature of an embodiment of the invention, at least a portion of the first image illumination propagating along the LOE is transmitted and reflected by one of the reflective surfaces to generate a second image illumination that spans a second field of view angle at a shallower angle to the major surface than the reflective surface.

[0015] According to a further feature of the embodiment of the present invention, the second image illumination is reflected by a subsequent reflection at one of the reflective surfaces onto the first image illumination. It is deflected back.

[0016] According to a further feature of this embodiment of the invention, the reflective surface is inclined relative to the major exterior surface of the LOE at an angle of 20°-26°, and preferably at an angle of 23°-25°.

[0017] For purposes of defining the angle of incidence of a ray incident on a particular surface, the angle of incidence is determined by the relationship between the ray direction and the Normal , so that a ray normal to the surface has an angle of incidence referenced as 0°, while angles approaching 90° are critical incidence. Unless otherwise specified, the term "small angles of incidence" refers to angles between 0°-35°, while "large angles of incidence" refers to angles between 60°-90°.

[0018] The terms "steep" or "steeper" refer to light rays with a relatively small angle of incidence on a surface, or to light rays with a relatively large angle of incidence on a surface. Normal In contrast, the terms "shallow" or "shallower" refer to a surface that is inclined relative to a relatively critical incidence or Normal It is used to refer to a relatively large angle ray approaching a surface that is tilted at a relatively small angle to the [Brief description of the drawings]

[0019] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1A] FIG. 1A, discussed above, is a schematic side view showing the geometry of a light beam propagating along a LOE and a set of partially reflective surfaces placed at an angle within the LOE according to one conventional LOE design; [Figure 1B] FIG. 1B, discussed above, is a schematic side view showing the geometry of a light beam propagating along a LOE and a set of partially reflective surfaces placed at an angle within the LOE according to one conventional LOE design; [Figure 2A] FIG. 2A is a schematic side view of a LOE operative and constructed in accordance with the teachings of an embodiment of the present invention, illustrating various ray paths for image rays propagating along the LOE; [Figure 2B] FIG. 2B is an enlarged view of the area of ​​FIG. 2A in part II designated by the circle; [Figure 2C] FIG. 2C is a schematic side view of a LOE operative and constructed in accordance with the teachings of an embodiment of the present invention, illustrating various ray paths for image rays propagating along the LOE; [Figure 2D] FIG. 2D is a schematic side view of a LOE operative and constructed in accordance with the teachings of an embodiment of the present invention, illustrating various ray paths for image rays propagating along the LOE; [Figure 2E] FIG. 2E is a schematic side view of a LOE operative and constructed in accordance with the teachings of an embodiment of the present invention, illustrating various ray paths for image rays propagating along the LOE; [Diagram 3] FIG. 3 is a schematic diagram of an optical system that uses the LOE of FIGS. 2A-2E to provide a near-eye display. Description of the Preferred Embodiments

[0020] The present invention is an optical system that includes a light-directing optical element.

[0021] The principles and operation of an optical system according to the present invention may be better understood with reference to the drawings and the accompanying detailed description.

[0022] 2A-2E are schematic illustrations of a basic implementation of a portion of an optical system that includes a light-directing optical element (LOE) (100) having a pair of parallel major exterior surfaces (102) and (104). A plurality of mutually parallel reflective surfaces (106a), (106b), and (106c) are disposed within the LOE (100) and are at oblique angles relative to the major exterior surfaces (102) and (104).

[0023] At least one of the reflective surfaces (106b), (106c) is Normal Partial reflectance for angles of incidence less than 35°, 60° Exceed It is a particular feature of certain particularly preferred embodiments of the present invention that they are configured to have high reflectivity versus angle of incidence. "High reflectivity" in this context is 90%. Exceed , and more preferably greater than 95% reflectance. In some particularly preferred embodiments, the reflectance at 60° Exceed High reflectivity achieved due to the angle of incidence is 98% Exceed , and most preferably near 100%. Unlike the conventional approaches described above, this aspect of the invention does not require the reflective surface to have a reflectivity near zero for any range of angles of incidence. This greatly simplifies the implementation of a multi-layer dielectric coating or other reflective coating applied to the reflective surface.

[0024] The use of highly reflective surfaces at large angles creates unique ray paths that are different from conventional ones. Specifically, referring to the ray paths illustrated in Figures 2A and 2C-2E as well as the enlargement of Figure 2B, the collimated images delivered to the LOE (illustrated by the rays (108) incident at various different positions across the aperture and labeled A, B, C, D, and E) are combined into the LOE as the first image illumination. The collimated images delivered to the LOE are illustrated by image rays (110a) and their conjugate image rays (110b) as they propagate into the LOE (100) by internal reflections of the major faces (102) and (104). The illustrated rays AE are all parallel. This corresponds to illumination from one pixel of the image that is all incident in the collimated image, and the total field of view (FOV) of the combined images - referred to as the "first image illumination" - spans a first viewing angle. This first angular field is oriented at a steeper angle to the major faces than the reflective surfaces (106a), (106b), and (106c). As a result of this steeper angle of the first angular field, at least a portion of the first image illuminant propagating along the LOE undergoes reflection at a larger angle of incidence by one of the reflective surfaces, deflecting the light ray (110a) to generate a second image illuminant, as illustrated by light ray (112), which spreads into the second angular field at a shallower angle to the major faces (102) and (104) than the reflective surfaces (106a), (106b), and (106c). When the light ray (112) strikes the next reflective surface, the second image illuminant (112) is deflected back to the first image illuminant (110a) by reflection within one of the series of reflective surfaces. When ray (110b) strikes a reflective surface, which occurs at a small angle (less than 35°), it results in partial reflection to outcouple the image illumination as ray (114), as well as partial transmission of ray (110b) which carries the illumination forward for further outcoupling along the LOE.

[0025] In the non-limiting example illustrated herein, the incoupling of the image rays (108) is: Normal 60° to Exceed With high reflectivity at various angles of incidence, NormalThis is achieved using a reflecting surface (106a) implemented as an incoupling reflecting surface with a reflectivity of more than 50%, typically at least about 66%, at an angle of incidence of less than 35° to the surface of the facet (106a). The first reflection at the facet (106a) therefore incoupling the image illuminant into the first image illuminant (110b). Rays A and B illustrated in Figures 2A-2C enter the region of the input aperture and, at an angle leading them from the facet (106a) to a double reflection, give rise to a second image illuminant (112), which is converted back to the first image illuminant (110a) at the facet (106b). The first image illuminant then reflects from the major surface (104) to the facet (106a) where it traverses the facet (106b) while generating the outcoupling illuminant (114) by partial reflection. Rays A and B continue to propagate along the LOE, traversing further facets (106c) where further partial reflections occur, and then undergo additional large angle reflections at facets (106c) repeating the above process. Because of the high reflectivity of the reflective surfaces at large angles, conversion to and from the second image irradiance occurs without significant energy loss or the creation of ghost images. Furthermore, the use of relatively shallow angle reflective surfaces facilitates the implementation of relatively thin and lightweight LOEs. The preferred inclination of the reflective surfaces relative to the major faces of the LOE is between 20°-26°, and most preferably 23°-25°.

[0026] It will be noted that various rays undergo the above transformation between the first and second image illumination at various positions, and in some cases not at all. Thus, Figure 2D illustrates rays C and D undergoing a regular first image illumination propagation between facets (106a) and (106b) and then transformation to the second illumination by reflection off the back surface of the second reflective surface (106b). Figure 2E illustrates ray E, where the position and angle of the in-coupled rays are such that the ray remains as the first image illumination over the span of the three facets shown here.

[0027] These various different types of ray paths typically cooperate to provide outcoupling of image illumination from the LOE at a particular range of locations along the LOE, and generally produce a nearly continuous and overall image output over the desired output area. The partial reflectance of the reflective surface at small angles is preferably varied between surfaces to enhance the uniformity of the output image, according to the following principles: First, when the first facet (106a) is used as a coupling surface, the reflectance of the coupling reflective surface is preferably at least 50%, and most preferably approximately (1-1 / n), where n is the number of facets, unless the incoupling reflective surface is outside the area where outcoupling is required. In this case, a 100% reflector can be used.

[0028] The small angle partial reflectance of the remaining facets is preferably approximately 1 / n, where n for each facet is the number of remaining facets for which outcoupling is desired, including the current facet. Thus, for example, for the three-facet embodiment shown, the optimum reflectance values ​​for the facets at small and large angles are:

[0029] [Table 1] For the four facet embodiment:

[0030] [Table 2]

[0031] The above properties can be easily achieved using standard software tools for the design of multi-layer coatings, and in fact can be achieved more uniformly and require fewer coating layers than the conventional designs discussed above that have requirements for non-reflective properties over a certain angular range.

[0032] The above exemplary reflectance values ​​are suitable for implementations where the LOE is used for a first dimension of optical aperture expansion that serves as an input to another LOE on the opposite side of the eye, or for virtual reality applications. For applications where the LOE is placed against the eye for augmented reality applications, the inter-bonded facets are placed outside the field of view (or alternatively an inter-bonded configuration is used). Many facets with relatively low reflectance at small angles are preferred.

[0033] FIG. 3 illustrates generally an overall optical system (200) including an image projector (202) configured to project a collimated image. The image projector (202) is shown merely generally and may be any type of projector that projects a collimated image. In some embodiments, the image projector includes a light source, a spatial light modulator (such as a reflective liquid crystal on silicon, or "LCOS"), and a collimating optical system. All of these components may be advantageously arranged with the collimating optical system reflecting on each surface of a number of beam splitter prisms (e.g., a polarizing beam splitter (PBS) cube), as known in the art.

[0034] An incoupling array such as the first facet (106a) optically couples the collimated image into the LOE as a first image illumination, which then propagates into the LOE with an exchange between the first and second illuminations and progressive image outcoupling, all as shown above. In one particularly preferred, but non-limiting embodiment, as illustrated herein, a set of reflective surfaces, (106a), (106b), and (106c), are arranged to form a chevron-shaped incoupling array. Meet The two internally coupled reflective surfaces (106a) and (106a') are part of a symmetrical arrangement of two sets of mutually parallel reflective surfaces (106a), (106b), (106c), (106a'), (106b') or (106c').

[0035] The outcoupled image radiation from the LOE (100) is shown to be incoupled into the LOE (204) which conveys the image to the opposite side of the observer's eye and outcouples it to the observer's eye. The LOE (204) may be implemented with facets (206) implemented in accordance with the teachings of the present invention with high reflectivity at large angles, or may rather be implemented using conventional LOE techniques based on partially reflective facets and / or diffractive optical elements for outcoupling and incoupling, as known in the art.

[0036] While the coupling of the projected image into the LOE has been illustrated herein with reference to a coupling reflector surface, it is understood that other coupling in arrangements may also be advantageously used. Additional options include, but are not limited to, various forms of incoupling prisms attached to or integrated with one of the major surfaces and / or sides of the LOE, which provide the correct angled surface for direct injection of the projected image into the guided primary image illumination mode, and various incoupling arrangements based on diffractive optical elements.

[0037] Additional features may optionally be implemented in combination with the features described thus far to further enhance the uniformity of the outcoupled image intensity across the exit aperture. By way of one non-limiting example, one or both of the major faces of the LOE may be modified by the addition of a plate having parallel faces optically coupled to the LOE, and a partially reflective interface between the LOE and the plate may be created by application of a suitable coating to one or both surfaces of the interface, or by introduction of an interface layer of a suitable material. This partially reflective interface creates an overlap of multiple optical paths, serving as a "mixer," thereby enhancing the uniformity of the outcoupled image intensity across the exit aperture of the LOE.

[0038] It will be appreciated that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. 1. An optical system comprising: (a) a light-directing optical element (LOE) having a pair of parallel major exterior surfaces; (b) a plurality of mutually parallel reflective surfaces within the LOE, the reflective surfaces being obliquely angled with respect to the major exterior surface; at least one of the reflective surfaces is configured to have high reflectivity for angles of incidence greater than 60 degrees relative to a normal to the reflective surface and partial reflectivity for angles of incidence less than 35 degrees relative to the normal; The high reflectance is greater than 95% and the partial reflectance is less than or equal to 75%. The optical system, wherein the plurality of mutually parallel reflective surfaces within the LOE further includes an in-coupling reflective surface forming at least a portion of an in-coupling arrangement, the in-coupling reflective surface having a high reflectivity for angles of incidence greater than 60 degrees relative to the normal and a reflectivity of at least 66% for angles of incidence less than 35 degrees relative to the normal.

2. The optical system of claim 1 , wherein the partial reflectivity is at most 50%.

3. 2. The optical system of claim 1, wherein the plurality of reflective surfaces including the incoupling reflective surface are part of a symmetrical arrangement of two sets of mutually parallel reflective surfaces including two incoupling reflective surfaces, the two incoupling reflective surfaces intersecting to form a mountain-shaped incoupling arrangement.

4. 4. The optical system of claim 1, further comprising an image projector for projecting a collimated image, wherein an internal coupling arrangement optically couples the collimated image into the LOE as a first image illuminant that propagates within the LOE by internal reflection at the major external surface, the first image illuminant spanning a first angular field of view, the first angular field of view being at a steeper angle relative to the major external surface than the reflecting surface.

5. 5. The optical system of claim 4, wherein at least a portion of the first image illumination propagating along the LOE is transmitted and then reflected by one of the reflective surfaces to produce a second image illumination spanning a second angular field of view at a shallower angle relative to the major exterior surface than the reflective surface.

6. The optical system of claim 5 , wherein the second image illumination is deflected back to the first image illumination by subsequent reflection at one of the reflective surfaces.

7. The optical system of any one of claims 1 to 6, wherein the reflective surface is tilted with respect to the major external surface of the LOE at an angle of between 20° and 26°.

8. The optical system of any one of claims 1 to 7, wherein the reflective surface is tilted with respect to the major external surface of the LOE at an angle of between 23° and 25°.

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