Transparent lightguide for viewing scene and near-eye display

Abstract

To meet the need that it is desirable that a coupled-out device is hardly visible or cannot be seen by an observer.SOLUTION: A light-guide optical element (LOE) for simultaneous viewing, of a real scene and of a projected image introduced into the LOE, having a transparent block along which light conveying a projected image propagates by internal reflection, and a plurality of internal partially reflecting surfaces obliquely oriented and configured so as to couple-out a part of said light, wherein the reflectance of each of the partially reflecting surfaces is such that the total power of the light that is coupled out is less than one third of the total power of the light that is introduced into the LOE. In some embodiments, the light of the projected image is polarized and the reflectance of the partially reflecting surfaces for light polarized in an orthogonal orientation is substantially reduced. In some embodiments, the reflectance of the partially reflecting surfaces for light not reaching the viewer is substantially reduced.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates generally to head-mounted display devices, and more particularly to light guide optics that are part of such devices. [Background technology]

[0002] Devices worn by a viewer for simultaneous viewing of a real scene and a projected image from a display device are well known and are commonly referred to as "head-mounted displays" (HMDs) or "near-eye displays" (NEDs). Such devices are typically configured as goggles or glasses, or a helmet or visor, worn on the viewer's head, and include one or two image projectors (each including an electro-optical display component) and optical components that transmit the projected image to the viewer's eyes. In some configurations of HMDs known in the art, one such optical component is a light guide positioned in front of each of the viewer's eyes.

[0003] Such light guides (alternatively referred to as "waveguides" or "substrates") serve to expand the field of view (i.e., the angular size of the display component's screen) and the observation window (i.e., the window within which a viewer's eye is positioned to view the entire display screen, also known as the "eye motion box"). Typically, such light guides are blocks (or slabs) of transparent material with two parallel major faces along which light carrying a collimated image projected from the display component propagates by total internal reflection. The block includes a structural coupling-out arrangement that serves to outcouple a portion of that light through one of the major faces toward the corresponding viewer's eye.

[0004] In some configurations of light guides known as diffractive light guides, the outcoupler includes a diffractive structure on one or both of the major surfaces. In other configurations known as reflective light guides and specifically "light guide optical elements" (LOEs), the outcoupler includes a set of obliquely angled, mutually parallel, partially reflective surfaces, alternatively known as facets, within a block.

[0005] In some cases, such as when the HMD is in the form of glasses, it may be desirable for the outcoupler to be difficult or invisible to an external observer. Summary of the Invention [Means for solving the problem]

[0006] The present invention seeks to provide improvements to light guides used in head-mounted displays (HMDs) that reduce the unwanted visual effects that an external observer would otherwise experience when viewing the face of a viewer wearing an HMD through such a light guide. Such effects occur when a portion of the light reflected from the viewer's eyes and face is coupled into the light guide, thereby attenuating the light reaching the external observer and causing the light guide to appear noticeably dark in the viewer's field of vision, masking the viewer's face and eyes. Another unwanted visual effect in prior art light guides occurs when light within the light guide is reflected off an end face and propagates backward, becoming outcoupled toward the external observer and perceived as glare. Accordingly, embodiments of the present invention provide various techniques for increasing the transmission of light through the light guide between the viewer's face and the external observer, while reducing the amount of other light emitted from the light guide toward the external observer.

[0007] By way of non-exclusive example, the improvements are described with respect to embodiments of light guide configurations that include partially reflective surfaces. Such reflective light guides, or conventional light guide optical elements (LOEs), are described, for example, in U.S. Patent No. 6,829,095, entitled "Substrate-Guided Optical Beam Expander," which is incorporated herein by reference. However, the improvements of the present invention are applicable, in whole or in part, to other embodiments and configurations of light guides for HMDs.

[0008] In particular, a light-guide optical element (LOE) for simultaneous observation by a viewer's eyes of a real scene and a projected image introduced into the LOE is disclosed, the LOE comprising: a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying the projected image introduced into the LOE propagates within the LOE by internal reflection at the first and second major surfaces; and a plurality of parallel partially reflective surfaces within the block oriented obliquely with respect to the first major surface, the partially reflective surfaces configured to outcouple a portion of the light via the second major surface. The reflectivity of each of the partially reflective surfaces is set so that the total power of the outcoupled light is less than 1 / 3 of the total power of the light carrying the projected image introduced into the light guide optical element.

[0009] In some embodiments, the reflectivity of each of the partially reflective surfaces is set so that the total power of the outcoupled light is less than 1 / 5, and in some embodiments less than 1 / 10, of the total power of the light carrying the projected image that was introduced into the light guide optical element.

[0010] In some embodiments, the reflectivity of each of the partially reflective surfaces is less than 13%, and in some embodiments, less than 5%.

[0011] Also disclosed is a light-guide optical element (LOE) for simultaneous observation by a viewer's eyes of an actual scene and a projected image carried by light polarized in a first orientation and introduced into the LOE, the LOE being a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying the projected image introduced into the LOE propagates within the LOE by internal reflection at the first and second major surfaces; and a plurality of mutually parallel partially reflecting surfaces within the block oriented obliquely with respect to the first major surface to outcouple a portion of the light towards the viewer's eyes. The reflectivity of each of the partially reflective surfaces in a direction perpendicular to the first and second major surfaces for light polarized in a second orientation perpendicular to the first orientation is less than one-third of its reflectivity in that direction for light polarized in the first orientation.

[0012] The first polarization orientation can be S-polarized light with respect to the partially reflective surface, hi some embodiments, the partially reflective surface substantially transmits P-polarized light over an angular range of at least about 30 degrees that includes a direction normal to the first major surface.

[0013] Also disclosed is a light-guide optical element (LOE) for simultaneous observation by a viewer's eyes of an actual scene and a projected image introduced into the LOE, the LOE comprising a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying the projected image introduced into the LOE propagates in a first direction within the LOE by internal reflection at the first and second major surfaces, and an eye motion box of a predetermined size is defined outside the first major surface in a plane parallel to the first major surface; and the block comprises a plurality of mutually parallel partially reflecting surfaces within the block, arranged in sequence along the first direction and oriented obliquely with respect to the first major surface to outcouple a portion of the light toward the eye motion box. The reflectivity of the last of the array of partially reflective surfaces for the portion of the light that is outcoupled therefrom toward any point within the eye motion box is at least twice its reflectivity for light propagating in a direction perpendicular to the first and second major surfaces.

[0014] In some embodiments, the reflectivity of the last of the array of partially reflective surfaces for the portion of the light that is coupled out from it to any point within the eye motion box is at least four times greater than its reflectivity for light propagating in a direction perpendicular to the first and second major surfaces.

[0015] In some embodiments, the block has an end face onto which light propagating within the light guide optical element after passing through the partially reflective surface is incident, and the end face is coated with a light absorbing layer configured to absorb light that is introduced into the light guide optical element and not outcoupled. The light absorbing layer may be implemented as black paint applied to the roughened end face.

[0016] An optical system for simultaneous viewing of a natural scene and an image on a near-eye image projector by a viewer is also disclosed, the optical system comprising: a light guide optical element according to any one of claims 1 to 10; and a support structure arranged to support the light guide optical element on the viewer's head in a position facing at least one eye of the viewer. [Brief explanation of the drawings]

[0017] The invention is herein described, by way of example only, with reference to the accompanying drawings in which: [Figure 1A] FIG. 1A is a partial top view of an exemplary light guide optics element (LOE) as used in a head-mounted display (HMD). [Figure 1B] FIG. 1B is a schematic front view of the LOE of FIG. 1A as perceived by an external observer. [Figure 2] FIG. 2 is a graph showing typical reflectance values ​​of a partially reflective surface along an LOE according to the present invention compared to that of a conventional LOE. [Figure 3A] FIG. 3A is a graph showing typical reflectance values ​​for two different orthogonal polarization orientations of a partially reflective surface in a conventional LOE. [Figure 3B] FIG. 3B is a graph showing typical reflectance values ​​for two different orthogonal polarization orientations of a partially reflective surface in an LOE according to the present invention. [Figure 4A] FIG. 4A is a partial top view of the LOE of FIG. 1A showing certain ray trajectories. [Figure 4B] FIG. 4B is an enlarged view of a detail in the LOE of FIG. 4A. [Figure 4C] FIG. 4C is a graph of reflectivity as a function of direction for the three partially reflective surfaces of the LOE of FIG. 1B. [Figure 5A] FIG. 5A is a partial top view of the LOE of FIG. 4A, showing the end face without the light absorbing layer. [Figure 5B] FIG. 5B is a partial top view of the LOE of FIG. 4A, showing the end face with the light absorbing layer. DETAILED DESCRIPTION OF THE INVENTION

[0018] 1A, by way of introduction, schematically illustrates a head-mounted display (HMD), in this example formed as glasses worn by a viewer, including a light guide optical element (LOE) 10 having a partially reflective surface that is positioned in front of the viewer's eyes 20 when the HMD is worn, an image projector 22 (which thus includes a spatial light modulator such as an electro-optic display device, or LCOS device) operative to generate an image in response to a signal supplied thereto, a collimating optical assembly (all not shown) configured to project light corresponding to the image in a collimated state, and coupling optics 24 that couple the projected image into the LOE 10.

[0019] Some configurations of HMDs have a single image projector associated with one eye, other configurations have two image projectors each associated with one of the viewer's eyes, and still other configurations have a single image projector configured to project images into two LOEs associated with corresponding eyes, or into a single long LOE that extends in front of both eyes.

[0020] A light guide optical element (LOE) 10 is illustrated in a horizontal cross section in FIG. 1A, tracing selected rays of a collimated image propagating within the LOE and emerging toward a viewer. LOE 10 is essentially an elongated block 11, constructed from a transparent material and having two parallel major surfaces, a front surface 12 and a rear surface 14. Near one end of block 11, a coupling-in arrangement associated with an entrance window is provided, through which the collimated image is introduced into the LOE. In this example, this is an angled reflective surface 18, and adjacent to it on rear surface 14, is an entrance window 19. In other LOEs, the entrance window may be, for example, an angled prism attached to one of the major surfaces, or the entrance window may be one end face of the block. Embedded within block 11 is an array of angled partially reflective surfaces 16, all parallel to one another and referred to as "facets." Adjacent to the entire set of partially reflective surfaces 16, and at a distance from and parallel to the rear surface 14, a field of view 17 is defined, also known as the eye motion box, which indicates the area where the viewer's eyes should be positioned to be able to see the entire image, allowing some freedom of eye movement and location of the HMD relative to the eyes 20.

[0021] It should be noted that although the entrance window is on the rear surface in the illustrated example, in other configurations of the HMD, the corresponding LOE may be configured to have an entrance window on the front surface or on an end surface of block 11. The present invention also addresses such configurations.

[0022] As represented by selected rays, image-carrying light from coupling optics 24 enters block 11 through entrance window 19 and, in this example, is deflected by angled reflective surface 18 and propagates along block 11, experiencing total internal reflection from major surfaces 12 and 14. In configurations where the entrance window is on an end face, light incident thereon travels in a straight line (i.e., is not deflected) and propagates along the block. During this propagation, the light is intercepted by partially reflective facets 16, which deflect or outcouple a portion of the light into observation window (eye motion box) 17.

[0023] One of the primary challenges in designing an LOE for any near-eye display device (such as a virtual reality or augmented reality device) is maximizing the optical throughput from the light source to the viewer's eye to reduce energy consumption and therefore extend battery life. Therefore, a common strategy in state-of-the-art reflective LOE designs is to maximize the overall reflectivity of the facets, maximizing the image intensity reaching the viewer's eye. At the same time, the reflectivity of the facets is typically varied between them to achieve a uniform intensity image across the field of view and observation window perceived by the viewer. More specifically, as light propagates along block 11 and some of it is outcoupled by the first facet, the intensity of the remaining light is reduced, and subsequent facets are required to have a correspondingly higher reflectivity, keeping the intensity of the outcoupled light constant; i.e., the last facet the light crosses should have the highest reflectivity.

[0024] Another challenge in LOE design, specific to parts of an HMD, is that the LOE must be sufficiently transparent so that the viewer can clearly see natural scenes. This requirement conflicts with the maximum facet reflectance requirement mentioned above, in that high reflectance effectively equates to low transmittance, thus attenuating the light from the natural scene that reaches the viewer's eyes. Thus, conventional LOE designs for HMDs compromise, reducing the facet reflectance proportionally to achieve a desired minimum transmittance value for light from natural scenes.

[0025] Also, in some conventional LOE designs (as mentioned in the Solution to Problem section above), the facets are substantially visible to an external observer because their relatively low transmittance reduces light reflected from the observer's face and eye 20 through the LOE's block 11 in a direction generally perpendicular to its major surfaces 12 and 14 toward the observer's eye (not shown). This effect is demonstrated diagrammatically in FIG. 1B, which shows a front view of the LOE 10 as seen by the external observer's eye. Here, the facets 16 appear as strips of varying darkness, obscuring the observer's face; the leftmost facet is the darkest because it is designed for maximum reflectance and therefore minimum transmittance. In these designs, the facets can also generate glia visible to the external observer because the remaining propagating light is reflected off the end surface 15 of the block 11 and outcoupled by the facet 16 through the front major surface 12 toward the observer's eye.

[0026] Below, exemplary embodiments of light guide optical elements are described that include novel features designed to reduce the aforementioned effects, such as the visibility of facets by an external observer. While these features are each described with respect to appropriate exemplary embodiments, some other embodiments may include two or more of these features simultaneously, as will be readily appreciated by those skilled in the art. Furthermore, some or all of these features may be included in various configurations of LOE embodiments, particularly reflective and diffractive LOEs.

[0027] The guiding principle of certain embodiments of the present invention is to reduce the reflectivity of the facets and increase their overall transmittance with respect to light transmitted across the LOE, making them appear transparent and invisible to an outside observer.

[0028] Furthermore, in a typical conventional optimized reflective LOE design, the reflectivity of the facets varies along the LOE from the first to the last incident facet over the range of incidence angles, polarization orientations, and wavelength bands of interest, typically ranging from 10% to 25%. The range of incidence angles of interest is determined by optical geometric considerations in the design of the LOE and the HMD of which the LOE is a part. The ranges of polarization orientations and wavelength bands of interest are largely determined by the characteristics of the image projector or by operational requirements. Optical designs seek to optimize the reflectivity of the facets within these ranges of interest, but reflectivity values ​​for any values ​​of incidence angles, polarization orientations, and wavelength bands outside these ranges are typically not constrained in the design. Therefore, a further guiding principle of some embodiments of the present invention is to reduce or, to the extent possible, minimize the reflectivity of one or more outer facets within these ranges, thereby increasing their transmittance for light across the LOE.

[0029] In a first exemplary embodiment of the present invention, or in accordance with a first aspect of the present invention, the overall reflectivity of each facet, including within angular ranges, polarization states and wavelength bands of interest, is substantially reduced by design compared to that of the prior art designs described above.

[0030] FIG. 2 is a comparative plot of the light intensity propagating along the LOE (decreasing lines) versus the facet reflectance values ​​(rising lines). The horizontal axis is relative distance along the LOE, and the vertical axis is percent of maximum value. The dotted lines represent optimal values ​​for an LOE designed for use in a virtual reality system (which is beyond the scope of this invention). Here, the optimal design would couple all of the light energy incident on the LOE out, maximizing the efficiency of viewing the display image while maintaining uniformity in the intensity of the outcoupled light. Thus, line 31, representing the propagated light intensity, descends linearly to near zero, while line 32, representing the reflectance of the array of facets, rises approximately exponentially with a corresponding increase in reflectance. As a result, the intensity of the light outcoupled toward the viewer is roughly uniform. Note that the lines represent optimal settings; in reality, the sloping lines resemble steps in parts corresponding to the facets.

[0031] The dashed lines represent typical values ​​for a conventional LOE designed for use in a head-mounted display (HMD), where the reflectivity is selected to provide a relatively clear view of natural scenes. These lines can be seen as similar to the dotted lines, but with a reduced slope. Thus, again, the linear descending line 34, starting at 100° (representing the maximum intensity of light incident on the LOE), reaches only approximately 50% at its end, meaning that only approximately 50% of the propagating light energy is coupled out (toward the observation window). Correspondingly, the ascending line 35 reaches only approximately 42°, meaning that the reflectivity of the last facet is only approximately 42%. This results in a transmittance of approximately 58% at the appropriate angle of incidence, which is expected to be equally high across the LOE, along which a natural scene is viewed—high enough that the scene appears satisfactorily clear.

[0032] As noted above, the transmittance of the latter is not high enough to avoid blurring of the HMD user's face and eyes and the attendant visibility of the facets to an external observer. To correct this, the reflectance of the facets in an exemplary embodiment of the present invention corresponding to the first aspect of the present invention is further reduced significantly, as represented by the solid line plot in FIG. 2 . Here, the ascending solid line 38 reaches only about 13, meaning that the total reflectance of the last (i.e., highest reflectance) facet preferably does not exceed about 13% (and in some particularly preferred embodiments, does not exceed about 5%), and the straight solid line 39 descends to only about 63%, meaning that only about 37% of the propagating light energy is outcoupled. As a result, the transmittance of the last facet rises to approximately 87%, which significantly reduces the facet's visibility to an external observer observing the user's face; the transmittance of the other facets is even greater. Also, as a result, in this preferred example, at least 63% of the image intensity coupled into the LOE continues to propagate along the LOE beyond the last facet and is therefore wasted. In other particularly preferred examples, the proportion of coupled-in illumination that propagates beyond the last facet is greater than 2 / 3, and in certain preferred cases even greater than 80% or 90%. Thus, in this embodiment, a counterintuitive design compromise is made: the optical efficiency of the LOE is significantly reduced in order to significantly reduce or eliminate the visibility of the facets as seen by an external observer. It should be noted that in the illustrated embodiment, the reduction in reflectivity of the last facet is a factor of 13% / 42%=0.31 relative to that value of the prior art design. More generally, in embodiments according to this first aspect of the invention, the reflectivity of the facet is reduced by a factor of between 0.5 and 0.1, preferably between 0.4 and 0.25, compared to conventional optical designs.

[0033] In a second exemplary embodiment of the present invention, corresponding to the second aspect of the present invention, image-carrying light incident on (or coupled into) the LOE is assumed to be S-polarized with respect to the facets. In some HMD configurations, this may be due to the image projector (e.g., a liquid crystal display) itself emitting inherently polarized light, or due to a polarizing filter inserted in the optical path between the image projector (or collimating assembly) and the LOE. According to a novel feature of this second aspect, the reflectivity of the facets for P-polarized light is minimized or significantly reduced compared to their reflectivity for S-polarized light. In some embodiments, the facets are substantially transparent to P-polarized light over an angular range of at least about 30 degrees, including the direction of light incidence normal to the first major surface. This maximizes the facet's transmittance for P-polarized light, allowing more light emanating from the viewer's face to reach an external observer, thus making the facet more transparent and less visible to that observer. Note that this feature may be applied in addition to optimizing reflectivity for S-polarized light, either conventionally or according to the first aspect of the present invention. The term "substantially transparent" is used in its ordinary sense. Quantitatively, it refers to a transmittance typically greater than 95%, and most preferably greater than 98%.

[0034] FIG. 3A shows, by way of example, the reflectivity of a typical facet in a conventional LOE for two polarization orientations, specifically P-polarized light and the orthogonal S-polarized light, as a function of incident light angle. FIG. 3B is a similar plot of reflectivity for an LOE in an exemplary embodiment corresponding to the second aspect of the present invention. In this embodiment, the reflectivity of incident S-polarized light within the range of incident angles of interest is optimized to strike a balance between efficient display image transmission and natural scene visibility, i.e., typically within the range of 10% to 25%, or a value representing the reduced reflectivity associated with the first aspect of the present invention discussed above. However, as can be clearly seen in FIG. 5B compared to FIG. 5A, the reflectivity of incident P-polarized light in a direction perpendicular to or close to the block major surface is significantly reduced. Preferably, this reduction is at least four-fold, more preferably at least eight-fold. Additionally or alternatively, the reflectivity value for P-polarized light in that direction is preferably no more than one-third, more preferably no more than one-fifth, of the facet's corresponding reflectivity value for S-polarized light.

[0035] In a third exemplary embodiment of the present invention, corresponding to a third aspect of the present invention that may be combined with one or both of the previous two aspects, the reflectivity of any of the facets is significantly reduced at angles of incidence different from the range of angles of incidence that propagate along the LOE and deflect incoming image-bearing light toward the wearer's eyes, and more generally toward the eye motion box. The range of angles of incidence within which reflectivity is reduced specifically includes those corresponding to the direction of light transmitted across the LOE, such as from the viewer's face and eyes toward an external observer. This equates to an increase in transmittance along that direction, making the facet difficult to see.

[0036] 4A, a partial view of an exemplary exemplary LOE 10 is shown, with display signal light rays (i.e., image-carrying light) propagating from the left edge therethrough and deflected (or outcoupled) by five facets 16. The illustrated light rays are central rays originally emanating from three points selected across the displayed image, briefly: the ray illustrated by solid line 42 emanates from the center point of the image, the ray illustrated by long dashed line 41 emanates from the far right of the image (as viewed by the viewer), and the ray illustrated by short dashed line 43 emanates from the far left of the image (as viewed by the viewer). As can be clearly seen, light rays from different image points reach the eye 20 via different facets. For example, a ray of light reaching the eye from the leftmost point (short dashed line) passes primarily through the first (leftmost) facet 16a, a ray of light reaching the eye from the central point (solid line) passes primarily through the third (center) facet 16b, and a ray of light reaching the eye from the rightmost point (long dashed line) passes primarily through the fifth (rightmost) facet 16c. For such a ray, there is a unique angle of incidence at the corresponding facet. More generally, for each facet, there is a range of angles of incidence (from the corresponding portion of the image) that direct the ray toward any point within the eye motion box (EMB) 17 where the ray enters the eye 20.

[0037] Focusing now on the final (right-most) facet 16c in the array of facets through which image-carrying light propagates, it has the highest reflectivity by design (as discussed above and shown, for example, at the right end of the plot in FIG. 2); therefore, it traditionally has the least transmittance for light passing across the LOE and is the most visible to an external observer (as demonstrated, for example, by the left-most band in FIG. 1B). This facet 16c and the light rays reflected thereby are shown enlarged in FIG. 4B and in the circled area labeled "Detail" in FIG. 4A. As marked on the diagram, three representative light rays can be observed arriving at unique angles of incidence. Thus, in this example, the ray from the left of the image (short dashed line) is incident at approximately 30 degrees, the ray from the center of the image (solid line) is incident at approximately 23 degrees, and the ray from the right of the image (long dashed line) is incident at approximately 16 degrees. Note that the same angles of incidence apply to all of the other facets.

[0038] In this case, only rays from the right side of the image (long dashed line) are of interest, and only these reach the EMB 17. More generally, rays emanating from near regions of the image in a range of angles of incidence near 16 degrees will be reflected into the EMB 17. This is the range in which reflectivity must be kept high by design (or potentially reduced by the first and / or second aspects of the invention). On the other hand, the reflectivity of facet 16c for optical signals arriving at angles significantly different from the design range described above is reduced relative to the design range reflectivity values ​​according to the third aspect of the invention.

[0039] Referring again to Figure 4A, light traversing the LOE upward in the direction indicated by vertical arrow 45, such as light reflected from the viewer's face toward an external observer, is seen to pass through facet 16c (as well as all other facets) at an angle of incidence significantly different from the ranges discussed above. As observed in Figure 4B, this angle is approximately 23 degrees. Thus, in this example, a range of incidence angles of approximately 23 degrees is one in which the reflectivity of facet 16c should be significantly reduced, increasing the transmittance in the transverse direction (arrow 45), and thus reducing the visibility of the facet.

[0040] More generally, in embodiments according to the third aspect of the invention, the reflectivity of the last facet of the array for the portion of light that is outcoupled from it towards any point within the eye motion box is preferably at least twice its reflectivity for light propagating in a direction normal to the major faces.

[0041] Figure 4C schematically illustrates the design goals for facet 16c of Figure 4A, namely, relatively high reflectivity for angles of incidence in the range of 16 to 21 degrees (for rays reaching the EMB), as illustrated by rectangles 51, and relatively low reflectivity for high angles of incidence, preferably in the range of 22 to 25 degrees, as illustrated by vertical lines 52. A plot 53 (solid line) of reflectivity versus angle of incidence for facet 16c of Figure 4A is also seen in Figure 4C to meet these requirements, and is compared to a similar plot 54 (dashed line) for a conventional facet.

[0042] Note that similar design considerations may apply to other facets in the LOE to further reduce their visibility to an outside observer.

[0043] Further aspects of the present invention, which may be useful alone or in combination with any one or more of the above-described aspects of the present invention, or which are applicable to all light guide configurations (including diffractive waveguides), are now disclosed with reference to FIGS. 5A and 5B. As illustrated in FIG. 5A, the remaining image-carrying light propagating along LOE 10 and not outcoupled toward the observation window continues to propagate to LOE end face 15#. At least a portion of this light is reflected back from that end face and propagates in the opposite direction 25 along the LOE, where an outcoupler typically couples some of the light outward 27 away from the user. This can result in unwanted glare emanating from the LOE and seen by an external observer. This effect is particularly pronounced with LOEs according to the first aspect of the present invention, because a relatively large portion of the injected image intensity propagates through all of the facets to reach end face 15.

[0044] To reduce this effect, in accordance with this aspect of the invention and as illustrated in FIG. 5B , a light-absorbing coating film or layer 35 is applied to end surface 15 of LOE 10. A light-absorbing coating may also be advantageously applied to any of the other three sides of the LOE. Light-absorbing coating 35 can conveniently be implemented as a layer of black paint. In some embodiments, the coating is configured to have a roughened surface, which can be achieved by roughening the edge of the LOE before applying the paint, or by employing a rough film or layer that is bonded to the relevant side of the LOE using an optical adhesive or the like. It should be noted that the orientation of the LOE illustrated in the drawings is considered a "side-injection" implementation, in which image illumination incident on the LOE enters near a side edge and propagates laterally. It should be noted that all of the illustrated features are equally applicable to a "top-down" implementation, in which the image is injected from the top surface of the LOE and propagates downward, which is also within the scope of the present invention. Other intermediate orientations may also be applicable in some cases and are included within the scope of the present invention unless expressly excluded. It is understood that the numerical examples provided above are merely examples and will vary during the design optimization process. It is understood that in various embodiments of the present invention, two or more aspects of the present invention may be combined in an optimized design. It will be understood 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.

[0045] To the extent that the appended claims are drafted without multiple dependencies, this is done solely to satisfy formality requirements in jurisdictions that do not permit such multiple dependencies. Note that all possible combinations of features implied by making the claims multiple dependent are expressly contemplated and should be considered part of the present invention.

Claims

1. A light-guide optical element (LOE) for simultaneous observation by a viewer's eye of a real scene and a projected image introduced into the LOE, comprising: a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying a projected image introduced into the light guide optical element propagates within the light guide optical element by internal reflection at the first and second major surfaces; a plurality of parallel partially reflective surfaces within the block and oriented obliquely relative to the first major surface, the partially reflective surfaces configured to outcouple a portion of the light through the second major surface; a reflectivity of each of the partially reflective surfaces is set so that the total power of the outcoupled light is less than one-third of the total power of the light carrying the projected image introduced into the light guide optical element.

2. 2. The light guide optical element of claim 1, wherein the reflectivity of each of the partially reflective surfaces is set so that the total power of the outcoupled light is less than 1 / 5 of the total power of the light carrying the projected image introduced into the light guide optical element.

3. 2. The light guide optical element of claim 1, wherein the reflectivity of each of the partially reflective surfaces is set so that the total power of the outcoupled light is less than 1 / 10 of the total power of the light carrying the projected image introduced into the light guide optical element.

4. 10. The light guide optical element of claim 1, wherein each of said partially reflective surfaces has a reflectivity of less than 13%.

5. 10. The light guide optical element of claim 1, wherein each of said partially reflective surfaces has a reflectivity of less than 5%.

6. A light-guide optical element (LOE) for simultaneous observation by an observer's eye of an actual scene and a projected image carried by light polarized in a first orientation and introduced into the LOE, comprising: a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying a projected image introduced into the light guide optical element propagates within the light guide optical element by internal reflection at the first and second major surfaces; a plurality of parallel partially reflective surfaces within the block and oriented obliquely relative to the first major surface to outcouple a portion of the light toward the viewer's eye; A light guide optical element, wherein the reflectivity of each of the partially reflective surfaces in a direction perpendicular to the first and second major surfaces for light polarized in a second orientation perpendicular to the first orientation is less than 1 / 3 of its reflectivity in the same direction for light polarized in the first orientation.

7. The light guide optical element of claim 6 , wherein the first polarization orientation is S-polarization with respect to the partially reflective surface.

8. 8. The light guide optical element of claim 7, wherein the partially reflective surface substantially transmits P-polarized light over an angular range of at least about 30 degrees that includes a direction perpendicular to the first major surface.

9. A light-guide optical element (LOE) for simultaneous observation by a viewer's eye of a real scene and a projected image introduced into the LOE, comprising: a block of transparent material having a first major surface and a second major surface parallel to the first major surface, wherein light carrying a projected image introduced into the light guide optical element propagates in a first direction along the light guide optical element by internal reflection at the first and second major surfaces, and wherein an eye motion box of a predetermined size is defined in a plane external to and parallel to the first major surface; a plurality of parallel partially reflective surfaces within the block, arranged in sequence along the first direction and oriented obliquely relative to the first major surface to outcouple a portion of the light toward the eye motion box; a reflectivity of a last one of said array of partially reflective surfaces for a portion of said light that is outcoupled therefrom towards any point within said eye motion box is at least twice its reflectivity for light propagating in a direction perpendicular to said first and second major surfaces.

10. 10. The light guide optical element of claim 9, wherein the reflectivity of the last of the array of partially reflective surfaces for the portion of the light that is outcoupled therefrom toward any point within the eye motion box is at least four times greater than its reflectivity for light propagating in a direction perpendicular to the first and second major surfaces.

11. 11. The light guide optical element according to claim 1, wherein the block has an end surface onto which light propagating within the light guide optical element after passing through the partially reflecting surface is incident, and the end surface is coated with a light absorbing layer configured to absorb light that is introduced into the light guide optical element and not out-coupled.

12. 12. The light guide optical element of claim 11, wherein the light absorbing layer is embodied as black paint applied to the roughened end face.

13. 1. An optical system for simultaneous observation by a viewer of a natural scene and an image on a near-eye image projector, the optical system comprising: (a) a light guide optical element according to any one of claims 1 to 10; and (b) an optical system comprising a support structure provided to support the light guide optical element on the head of the viewer in a position opposite at least one eye of the viewer.

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