Optical system comprising a light guide optical element having a partially reflective inner surface

Abstract

An optical system comprising: a light-guide optical element (LOE) (100) having a pair of parallel major external surfaces (102, 104); and a set of mutually parallel reflector surfaces (106a, 106b, 106c) at an oblique angle within the LOE. At least one of the reflector surfaces has high reflectivity for angles of incidence above 60 degrees from the normal and partial reflectivity for angles of incidence less than 35 degrees from the normal.

Description

[0001] Technical Field and Background Technology

[0002] The present invention relates to optical systems for head-up displays, and particularly to optical systems employing light-guiding optical elements (LOEs) having partially reflective inner surfaces.

[0003] Various displays, particularly head-up displays (HUDs) and near-eye displays used for augmented or virtual reality, employ light-guiding optical elements (LOEs) with a pair of parallel primary outer surfaces to transmit a collimated image propagating within the LOE via internal reflection. The image is gradually coupled out of the LOE, typically directly toward the eye or into another LOE that transmits the image to the eye. In one type of such device, image coupling from the LOE is achieved by a group of mutually parallel partially reflective surfaces within the LOE, wherein the group of mutually parallel partially reflective surfaces is arranged at an angle relative to the primary outer surface of the LOE. The gradual coupling out of a series of partially reflective surfaces increases the optical aperture coupled into the LOE.

[0004] Traditional LOEs impose stringent requirements on the reflectivity of the partially reflective surfaces as a function of the angle of incidence, typically requiring high transmission (near-complete transmission) of image illumination over certain angular ranges, and partial reflection at other angles relative to the plane of the facet. In practice, achieving near-complete transmission is difficult. A typical example is schematically illustrated in Figures 1A and 1B, where an LOE 10 with parallel main surfaces 12 and 14 includes a set of partially reflective surfaces 16 (which may also be interchangeably referred to herein as "facets"). Exemplary light ray 18 propagates along the LOE via internal reflection at surfaces 12 and 14 at an angle corresponding to a given pixel of an image generated from a given position in the input optical aperture (not shown).

[0005] In typical applications, the image illumination illustrated by ray 18 propagates at a steeper angle than the angle between the partially reflective surface 16 and the main surfaces 12 and 14. Therefore, each illumination ray 18 may intersect a given plane 16 several times. For example, in Figures 1A and 1B, as ray 18 propagates from left to right, it intersects the third plane three times at positions denoted as 1, 2, and 3, respectively. Consequently, the light reflected and coupled from point 1 (denoted as a in Figure 1B) will be stronger than the light reflected and coupled from point 3 (denoted as b), leading to non-uniformity in the output image.

[0006] Additionally, the small plane is typically required to be transparent (non-reflective) to ray 18 at the incident angle shown at position 2, because any reflection at position 2 (dashed arrow) will further reduce the brightness of the propagating light reaching point 3 and will generate "ghosting" due to illumination propagating in the wrong direction, which may cause a portion of the image to appear misaligned in the final image. This requirement for complete transparency (zero reflection) is difficult to achieve and becomes increasingly difficult as the incident angle (AOI) increases. Summary of the Invention

[0007] The present invention is an optical system comprising a light-guiding optical element (LOE) having an inner reflector surface.

[0008] According to the teachings of embodiments of the present invention, an optical system is provided, the optical system comprising: (a) a light-guide optical element (LOE) having a pair of parallel main outer surfaces; and (b) a plurality of mutually parallel reflector surfaces within the LOE, the reflector surfaces being obliquely angled relative to the main outer surfaces, wherein at least one of the reflector surfaces is configured to have high reflectivity for incident angles of 60 degrees or more with respect to the normal and partial reflectivity for incident angles of less than 35 degrees with respect to the normal.

[0009] Another feature of the embodiments of the present invention is that, for an incident angle of 60 degrees or more, the high reflectivity exceeds 95%.

[0010] Another feature of an embodiment of the invention is that the partial reflectivity is at most 50%.

[0011] According to another feature of an embodiment of the invention, the LOE has a coupling region from which image illumination propagates along the LOE, and wherein a portion of the reflectivity varies between successive reflector surfaces to at least partially compensate for the reduced intensity of the image illumination reaching the successive reflector surfaces.

[0012] According to another feature of an embodiment of the invention, the plurality of mutually parallel reflector surfaces within the LOE further include coupled reflector surfaces, which form at least a portion of the coupled arrangement. The coupled reflector surfaces have high reflectivity for incident angles of 60 degrees or more with respect to the normal and at least about 66% reflectivity for incident angles of less than 35 degrees with respect to the normal.

[0013] According to another feature of an embodiment of the invention, the plurality of reflector surfaces including the coupled reflector surfaces are part of a symmetrical arrangement of two sets of mutually parallel reflector surfaces including two coupled reflector surfaces, the two coupled reflector surfaces being connected to form a V-shaped coupled arrangement.

[0014] According to another feature of an embodiment of the invention, an image projector that projects a collimated image is also provided, wherein the coupling arrangement optically couples the collimated image into the LOE as a first-order image illumination to propagate within the LOE via internal reflection at the principal surface, the first-order image illumination spanning a first angular field of view, which forms a steeper angle with the principal surface compared to the reflector surface.

[0015] According to another feature of an embodiment of the invention, at least a portion of the first-order image illumination propagating along the LOE is transmitted and then reflected by one of the reflector surfaces to generate second-order image illumination spanning a second-angle field of view, which forms a shallower angle with the main surface compared to the reflector surface.

[0016] According to another feature of an embodiment of the invention, the second-order image illumination is deflected back to the first-order image illumination by reflection from a subsequent reflector surface in the reflector surface.

[0017] According to another feature of an embodiment of the invention, the reflector surface is inclined at an angle of 20° to 26° with respect to the main outer surface of the LOE, and preferably at an angle of 23° to 25°.

[0018] For the purpose of defining the angle of incidence of a ray incident on a plane, the angle of incidence is defined as the angle between the direction of the ray and the normal to the plane, such that a ray perpendicular to the surface has an angle of incidence called 0°, while an angle close to 90° is grazing incidence. Unless otherwise stated, the phrase "small angle of incidence" refers to an angle from 0° to 35°, while "large angle of incidence" refers to an angle from 60° to 90°.

[0019] The terms “steep” or “steeper” are used to refer to a ray with a relatively small angle of incidence to a plane, or a plane inclined at a relatively large angle to a reference plane. Conversely, “shallow” or “shallower” are used to refer to a ray that is closer to grazing incidence at a relatively large angle, or a plane inclined at a relatively small angle to a reference plane. Attached Figure Description

[0020] The invention has been described herein by way of example only with reference to the accompanying drawings, in which:

[0021] Figures 1A and 1B discussed above are schematic side views showing the geometry of light propagating along the LOE according to certain conventional LOE designs, as well as the partially reflective surface group with tilted orientation within the LOE.

[0022] Figure 2A , Figure 2C , Figure 2D and Figure 2EThis is a schematic side view of an LOE constructed and operated according to an embodiment of the present invention, which shows various ray paths of rays of an image propagating along the LOE;

[0023] Figure 2B yes Figure 2A An enlarged view of the area indicated by the circle marked II; and

[0024] Figure 3 Is adopted Figures 2A to 2E A schematic diagram of an LOE used to provide an optical system for near-eye displays. Detailed Implementation

[0025] The present invention is an optical system including a light-guiding optical element.

[0026] The principles and operation of the optical system according to the present invention can be better understood by referring to the accompanying drawings and description.

[0027] Now refer to the attached diagram, Figures 2A to 2E This is a schematic diagram of a basic implementation of an optical system including a light guide optical element (LOE) 100 having a pair of parallel main outer surfaces 102 and 104. Within the LOE 100 are a plurality of mutually parallel reflector surfaces 106a, 106b, and 106c, angled relative to the main outer surfaces 102 and 104.

[0028] A particular feature of certain preferred embodiments of the present invention is that at least one of the reflector surfaces 106b and 106c is configured to have high reflectivity for incident angles of 60 degrees or more with respect to the normal and partial reflectivity for incident angles of less than 35 degrees with respect to the normal. Hereinafter, "high reflectivity" generally means reflectivity of 90% or more, and more preferably, reflectivity exceeding 95%. In some particularly preferred embodiments, high reflectivity of 98% or more can be achieved for incident angles of 60 degrees or more, and most preferably close to 100%. Unlike the prior art methods described above, this aspect of the present invention does not require the reflector surface to have near-zero reflectivity in any incident angle range. This greatly simplifies the implementation of multilayer dielectric coatings or other reflective coatings applied to the reflector surface.

[0029] Using a reflector surface with high reflectivity at large angles can generate unique ray paths that differ from those of existing technologies. Specifically, refer to... Figure 2A and Figures 2C to 2E And enlarged image Figure 2BThe ray paths shown in the diagram, where the collimated image transmitted to the LOE (exemplified by injection rays 108 at various locations across the aperture, labeled A, B, C, D, and E) is coupled into the LOE as first-order image illumination to propagate within the LOE 100 via internal reflections at the main surfaces 102 and 104, are illustrated by image ray 110a and its conjugate image ray 110b. Rays A through E shown are all parallel, indicating in the collimated image that rays A through E each correspond to illumination from a single pixel of the injection image, where the total field of view (FOV) of the coupled image, referred to here as "first-order image illumination," spans a first angular field of view. This first angular field of view is oriented at an angle steeper than the angles formed by the reflector surfaces 106a, 106b, and 106c with the main surfaces. Due to the steeper angle of the first field of view, at least a portion of the first-order image illumination propagating along the LOE undergoes reflection at a large angle of incidence through one of the reflector surfaces, thereby deflecting ray 110a to generate second-order image illumination, exemplified by ray 112, spanning the second field of view. The second field of view forms a shallower angle with the main surfaces 102 and 104 compared to reflector surfaces 106a, 106b, and 106c. As ray 112 is projected onto the next reflector surface, the second-order image illumination 112 is deflected back to the first-order image illumination 110a by reflection at a subsequent reflector surface. This occurs at a small angle (less than 35 degrees) when ray 110b is projected onto the reflector surface, resulting in partial reflection for coupling the image illumination out as ray 114 and partial transmission of ray 110b, which carries forward a portion of the illumination intensity for further coupling out along the LOE.

[0030] In the non-limiting example shown here, the coupling of image ray 108 is achieved using reflector surface 106a, which is implemented as a coupling reflector surface having high reflectivity for incident angles greater than 60 degrees with respect to the normal and greater than 50% reflectivity for incident angles less than 35 degrees with respect to the normal, typically at least about 66%. Therefore, the first reflection from facet 106a couples the image illumination into first-order image illumination 110b. Figures 2A to 2CRays A and B, shown in the diagram, enter the region of the input aperture at an angle that causes them to be reflected a second time from facet 106a, thus generating second-order image illumination 112. This second-order image illumination 112 is transformed back to first-order image illumination 110a at facet 106b. This first-order image illumination is then reflected from the main surface 104 to become 106a, which passes through facet 106b while generating coupled ray 114 through partial reflection. Rays A and B continue to propagate along the LOE, also passing through facet 106c where further partial reflection occurs, and then undergoing another large-angle reflection at facet 106c to repeat the above process. Due to the high reflectivity of the reflector surface at large angles, the transformation to and from second-order image illumination occurs without significant energy loss or the generation of ghosting images. Furthermore, using a relatively shallow-angle reflector surface contributes to achieving a relatively thin and lightweight LOE. The preferred tilt angle of the reflector surface relative to the main surface of the LOE is between 20° and 26°, and most preferably between 23° and 25°.

[0031] Note that different rays undergo the transformation between the first-order and second-order image illumination at different locations, and in some cases, they do not undergo it at all. Therefore, Figure 2D Rays C and D are shown, which undergo regular first-order image illumination propagation between facets 106a and 106b, and then undergo a transformation to second-order illumination by reflection on the back of the second reflector surface 106b. Figure 2E Ray E is shown, for which the position and angle of the coupled ray are such that the ray remains as first-order image illumination across the span of the three small planes shown here.

[0032] These various types of optical paths provide image illumination from the LOE at certain locations along the LOE and generally cooperate to generate a generally continuous overall image output over the desired output area. Preferably, the partial reflectivity of the reflector surface at small angles is varied between surfaces according to the following principle to enhance the uniformity of the output image. First, when the first facet 106a is used as the coupling surface, the reflectivity of the coupling reflector surface is preferably at least 50%, and most preferably about (1-1 / n), where n is the number of facets, unless the coupling reflector surface is outside the area where coupling is required, in which case a 100% reflector can be used.

[0033] The reflectivity of the remaining facets at small angles is preferably approximately 1 / n, where n is the number of remaining facets required to couple out at each facet, including the current facet. Therefore, for example, in the case of the three facet implementation shown, the optimal reflectivity values ​​of the facets at small and large angles will be as follows:

[0034] 1 66％ >98％ 2 50％ >98％ 3 100％ >98％

[0035] The implementation for the four smaller planes will be as follows:

[0036] 1 75％ >98％ 2 33％ >98％ 3 50％ >98％ 4 100％ >98％

[0037] The aforementioned properties can be easily achieved using standard software tools for designing multilayer coatings, and in fact, compared to the aforementioned conventional designs that require non-reflectivity over certain angular ranges, these properties can be achieved more uniformly and require fewer coatings.

[0038] The aforementioned exemplary reflectance values ​​are suitable for implementations where an LOE is used as an input to another LOE opposite to the eye for first-dimensional optical aperture expansion, or for implementations where an LOE is used in virtual reality applications. For applications in which an LOE is deployed opposite to the eye for augmented reality applications, the coupling facets are deployed outside the field of view (or alternative coupling configurations are used), and preferably a greater number of facets with relatively low reflectance at small angles.

[0039] Figure 3 An overall optical system 200 is schematically shown, including an image projector 202 configured to project a collimated image. The image projector 202 is shown only schematically here, and the image projector can be any type of projector that projects a collimated image. In some embodiments, the image projector includes a light source, a spatial light modulator (e.g., a liquid crystal on silicon or "LCOS"), and collimating optics. These components can advantageously be arranged on the surfaces of several beam splitter prisms, for example, a polarizing beam splitter (PBS) cube with reflective collimating optics, all of which are known in the art.

[0040] The coupling arrangement, such as the first facet 106a, optically couples the collimated image into the LOE as first-order image illumination for propagation within the LOE, wherein there is an interchange between first-order and second-order image illumination and a gradual coupling out of the image, all as described above. In a particularly preferred but non-limiting implementation shown herein, the set of reflector surfaces 106a, 106b, and 106c is part of a symmetrical arrangement of two sets of mutually parallel reflector surfaces 106a, 106b, 106c, 106a', 106b', and 106c', comprising two coupled reflector surfaces 106a and 106a', wherein the two coupled reflector surfaces 106a and 106a' are abutted to form a V-shaped coupling arrangement.

[0041] The image illumination from LOE 100 is schematically shown here as coupled into another LOE 204, which transmits an image relative to the observer's eye and couples the image toward the observer's eye. LOE 204 can be implemented using a small facet 206 with high reflectivity at large angles, implemented according to the teachings of the invention, or can be implemented using other conventional LOE techniques based on partially reflective facets and / or diffractive optics for coupling in and out, as known in the art.

[0042] Although the coupling of a projected image to a LOE has been illustrated herein with reference to the coupling reflector surface, it should be understood that other coupling arrangements may also be beneficial. Other options include, but are not limited to: various forms of coupling prisms attached to or integrated with one of the main surfaces and / or the side surfaces of the LOE, providing surfaces at the correct angles for directly injecting the projected image into a guiding first-order image illumination mode; and various coupling arrangements based on diffractive optics.

[0043] Optionally, additional features can be implemented in conjunction with the features described so far to further enhance the uniformity of the coupled image intensity across the exit aperture. According to a non-limiting example, one or both of the main surfaces of the LOE are modified by additionally optically incorporating a plate facing parallel to the LOE and utilizing a partially reflective interface between the LOE and the plate. This partially reflective interface is generated by introducing an interface layer of suitable material or by applying a suitable coating to one or both surfaces at the interface. This partially reflective interface acts as a "mixer," generating an overlap of multiple optical paths, thereby enhancing the uniformity of the coupled image intensity across the exit aperture of the LOE.

[0044] It should be understood that the above description is intended to be illustrative only, and many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. An optical system (200), comprising: (a) A light-guiding optical element LOE (100) having a pair of parallel main outer surfaces (102, 104). as well as (b) A plurality of mutually parallel reflector surfaces (106a, 106b, 106c) within the LOE (100), the reflector surfaces (106a, 106b, 106c) being obliquely angled at an angle of 20° to 26° relative to the main outer surface (102, 104). The optical system is characterized in that the reflector surface is configured to have a reflectivity that is not close to zero for the entire range of incident angles, and has a coating that has high reflectivity for incident angles of 60 degrees or more with respect to the normal, and partial reflectivity for incident angles of less than 35 degrees with respect to the normal, wherein the high reflectivity exceeds 95% for incident angles of 60 degrees or more, and the partial reflectivity is at most 50%.

2. The optical system according to claim 1, wherein, The LOE has a coupling region from which image illumination propagates along the LOE, and wherein the partial reflectivity varies between successive reflector surfaces to at least partially compensate for the reduced intensity of the image illumination reaching the successive reflector surfaces.

3. The optical system according to claim 1 or claim 2, wherein, The plurality of parallel reflector surfaces within the LOE also include a coupled reflector surface, which forms at least a portion of the coupled arrangement. The coupled reflector surface has high reflectivity for incident angles of 60 degrees or more with respect to the normal and at least about 66% reflectivity for incident angles of less than 35 degrees with respect to the normal.

4. The optical system according to claim 3, wherein, The plurality of reflector surfaces, including the coupled reflector surface, are part of a symmetrical arrangement of two sets of mutually parallel reflector surfaces, the two coupled reflector surfaces being joined to form a V-shaped coupled arrangement.

5. The optical system according to claim 1 or claim 2, further comprising an image projector that projects a collimated image, wherein, The coupling arrangement optically couples the collimated image into the LOE as a first-order image illumination to propagate within the LOE via internal reflection at the main outer surface. The first-order image illumination spans a first-angle field of view, which forms a steeper angle with the main outer surface compared to the reflector surface.

6. The optical system according to claim 5, wherein, At least a portion of the first-order image illumination propagating along the LOE is transmitted and then reflected by one of the reflector surfaces to generate a second-order image illumination spanning a second field of view, which forms a shallower angle with the main outer surface compared to the reflector surface.

7. The optical system according to claim 6, wherein, The second-order image illumination is deflected back to the first-order image illumination by reflection from a subsequent reflector surface in the reflector surface.

8. The optical system according to claim 1 or claim 2, wherein, The reflector surface is inclined at an angle of 23° to 25° to the main outer surface of the LOE.

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