Reflective sml image projector with intermediate image plane

Abstract

An image projector includes a spatial light modulator (SLM) (320) illuminated by light from an illumination source (316) to reflect light corresponding to an image. First optics (322) having positive optical power focus light reflected from the SLM at an image plane (324). Second optics (328) having positive optical power collimate light from the image plane (324) as a collimated output image.

Description

Technical Field

[0001] This invention relates to image projectors, and more particularly to image projectors based on spatial light modulators (SLMs). Background Technology

[0002] Many display systems (especially near-eye displays) employ image projectors that output collimated images to light-guide optical elements (LOEs), which then transmit the images in front of the user's eyes, coupling them towards the eyes for viewing.

[0003] Image projectors typically include an image generating element and collimating optics to generate a collimated image output. One desirable option for the image generator is a reflective spatial light modulator (SLM), typically implemented as a liquid crystal on silicon (LCOS) chip. This SLM uses polarized illumination to illuminate and reflects illumination with selectively rotated polarization corresponding to the intensity of the image pixels. The image is then selected by a polarizing beam splitter (PBS) before reaching the collimating optics. Summary of the Invention

[0004] This invention is an image projector based on reflective SLM and having an intermediate image plane.

[0005] According to the teachings of embodiments of the present invention, an image projector for outputting a collimated image is provided, the image projector comprising: (a) an illumination source; (b) a spatial light modulator illuminated by light from the illumination source to reflect light corresponding to the image; (c) a first optical device having positive optical power, arranged to focus light reflected from the spatial light modulator onto an image plane; and (d) a second optical device having positive optical power, arranged to collimate light from the image plane into a collimated output image.

[0006] According to another feature of an embodiment of the invention, at least the second optical device includes a reflecting lens associated with the surface of the prism containing the polarizing beam splitter.

[0007] According to another feature of an embodiment of the invention, the prism is optically integrated with a light-guiding optical element (LOE) having a pair of parallel primary outer surfaces for guiding light by internal reflection, wherein the portion of the prism between the polarizing beam splitter and the LOE has an outer surface that is coplanar with one of the primary outer surfaces of the LOE and forms an extension of that primary outer surface of the LOE.

[0008] According to another feature of an embodiment of the invention, the optical path from the spatial light modulator via the first optical device and the second optical device to the image output passes through two polarization beamsplitters, wherein the image plane is located between the two polarization beamsplitters.

[0009] Another feature of an embodiment of the invention is that the two polarization beam splitters are parallel.

[0010] According to another feature of an embodiment of the invention, two polarization beam splitters are included within a prism, and wherein the first optical device and the second optical device are implemented as reflective lenses associated with the surface of the prism.

[0011] Another feature of the embodiments of the present invention is that at least one optical element is spatially associated with an image plane.

[0012] According to another feature of an embodiment of the invention, at least one optical element includes a field lens.

[0013] Another feature of an embodiment of the invention is that at least one optical element includes a diffuser.

[0014] Another feature of an embodiment of the invention is that at least one optical element comprises a microlens array. Attached Figure Description

[0015] The invention is described herein by way of example only with reference to the accompanying drawings, in which:

[0016] Figure 1A This is a schematic side view of an image projector according to an embodiment of the present invention, illustrating its construction and operation.

[0017] Figure 1B Is with Figure 1A Similarly, a view is shown of a variant implementation of an image projector that includes optical elements at the image plane of the projector; and

[0018] Figure 2 This is a schematic side view of an image projector with a teaching structure and operation according to another embodiment of the present invention. Detailed Implementation

[0019] This invention is an image projector for outputting collimated images.

[0020] The principle and operation of the image projector according to the present invention can be better understood by referring to the accompanying drawings and description.

[0021] Now refer to the attached diagram, Figure 1A A non-limiting implementation of an image projector for outputting a collimated image, illustrating the teaching construction and operation according to an embodiment of the present invention, is shown. Generally, the image projector includes a spatial light modulator (SLM) 320, which is illuminated by light from an illumination source 316 (schematically indicated by an arrow) to reflect light corresponding to the image. A first optical device 322 having positive optical power is arranged to focus the light reflected from the SLM 320 onto an image plane 324. A second optical device 328 having positive optical power is arranged to collimate the light from the image plane 324 into a collimated output image.

[0022] Compared to the arrangement where the SLM itself must be located at the focal plane of the collimating optics, the focusing of light from the reflected SLM at the intermediate image plane 324 within the image projector assembly offers significant advantages. Specifically, to achieve a large field of view (FOV) of the projected image using a relatively small projector, the focal length of the collimating optics should be relatively short. However, the reflected SLM requires an illumination system for front illumination of the SLM. This limits the proximity of the SLM to the collimating optics, typically resulting in increased collimating optics size, reduced FOV, and / or other design trade-offs. By providing a first optical device 322 to generate an image at the image plane 324 within the projector, these design constraints are removed, leaving ample space for the illumination system while allowing the use of collimating optics with short focal lengths. This and other advantages of the invention will become more apparent from the following description and drawings.

[0023] In some particularly preferred implementations, at least the second optical element 328 is implemented as a reflecting lens associated with the surface of the prism 330, which includes the polarizing beam splitter 326. Advantageously, the image plane 324 can fall on a plane within the prism 330 without the need for any specific structure at that plane. Alternatively, in some implementations, it may be desirable to arrange one or more optical elements at or near the focal plane, as will be referred to below. Figure 1B exemplified.

[0024] In some applications, an image projector is integrated with a light-guide optics element (LOE) 204, which is typically a planar waveguide with two parallel primary outer surfaces within which image illumination propagates via total internal reflection (TIR). In some preferred implementations, the portion of prism 330 between polarization beamsplitter 326 and LOE 204 has an outer surface 332 that is coplanar with one of the primary outer surfaces of LOE 204 and forms an extension of one of the primary outer surfaces of LOE 204. This facilitates filling the optical aperture 334 into LOE 204 by generating a reflected (conjugate) image at the LOE entrance. LOE 204 can be any type of LOE known in the art, for example, an LOE employing a group of partially reflective inner surfaces to achieve aperture expansion in one or two dimensions and coupling image illumination toward the observer's eye. Alternatively, or additionally, the LOE may employ diffractive optical elements to achieve one- or two-dimensional optical aperture expansion and to couple image illumination toward the observer's eye, all of which are known in the art.

[0025] exist Figure 1A In the preferred but non-limiting example shown, the optical path from the SLM 320 to the image output 334 via the first optical device 322 and the second optical device 328 passes through two polarization beamsplitters, polarization beamsplitters 318 and 326, and the image plane 324 lies between the two polarization beamsplitters. Advantageously, the two polarization beamsplitters 318 and 326 are parallel. This helps to minimize noise in the output image because any original illumination (not fully filtered) leaking through the PBS 316 from the illumination source 316, for example due to the skewed angle of incidence, will encounter the exact same conditions at the second PBS 326, and therefore will also pass through the PBS 326 without becoming mixed with the image illumination.

[0026] The image projector of the present invention offers considerable design freedom regarding the position of the intermediate image plane, the focal power of the collimating optics, and the overall size of the optics. If the intermediate image plane 324 is designed to be closer to the collimating optics 328, the size of the optics can be reduced and / or the field of view (FOV) increased, relative to the SLM chip size, due to limitations imposed by optical aberrations and the required size of the intermediate image. The first optical device 322 is then designed based on the required position of the intermediate image and its dimensions relative to the SLM chip size.

[0027] In the implementation shown here, polarization beamsplitters 318 and 326 are included within prism 330, and both first optical device 322 and second optical device 328 are implemented as reflecting lenses associated with the surface of the prism. Each lens is integrated with a quarter-wave plate that rotates the polarization after two passes generated by reflection, thereby achieving transmission-followed-by-reflection or reflection-followed-by-transmission at the corresponding PBS surface, as known in the art.

[0028] The illumination source 316 can be any suitable illumination source known in the art, including but not limited to LEDs and laser diodes. The illumination source can include sources of different colors, which can be rapidly switched to illuminate the color separation image within a single frame period of the video to generate a color image. The illumination source can include various optical components for guiding and / or homogenizing the illumination, all of which are known in the art.

[0029] SLM 320 can be any suitable type of SLM, and in particular a front-emitting SLM. A particularly preferred example of an SLM suitable for implementing the present invention is an LCOS (liquid crystal on silicon) chip. The SLM is generally a rectangular array that also extends into the page in the side view shown herein, and all other optical components shown herein similarly extend into the page according to their relative size defined by the aspect ratio of the SLM, as will be clear to those skilled in the art.

[0030] It should be noted that the optical path shown here is for the light illuminating the center pixel of the SLM 320 by light reflected from the PBS 318 from the illumination source 316. The diverging light reflected from the SLM passes through the PBS 318 and is focused by the reflecting lens 322. The converging light is reflected again by the PBS 318 to generate an image at the image plane 324. The diverging light from this plane is reflected by a second PBS 326 onto the collimating reflecting lens 328. The reflected collimated light is coupled into the waveguide, while some of the reflected collimated light is reflected by the bottom plane 332.

[0031] Figure 1B It shows the usual with Figure 1AThis image projector is equivalent to an image projector, but in which at least one optical element 424 is arranged at or near the image plane 324 (i.e., spatially associated with the image plane 324). The optical element 424 may be a lens (typically a Fresnel lens) arranged to at least partially correct the field curvature of the optical device. Additionally or alternatively, the optical element 424 may include a diffuser or microlens array (MLA) for modulating the propagation of illumination to generate a more uniform output intensity across the optical aperture of the image projector output. In all other respects, Figure 1B The image projector is structurally and functionally equivalent to the one described above. Figure 1A Image projector.

[0032] Turn now Figure 2 It demonstrates functional similarity to Figure 1A and Figure 1B The second implementation of the present invention is similar in method but uses free-space optical devices to generate the internal image plane.

[0033] Specifically, in this configuration, the SLM 320, illuminated by the illumination source 316 via the PBS 318, is aligned with a first optical device 322', which is implemented as a free-space optical device typically employing one or more refractive lenses, arranged to focus the light reflected from the SLM 320 onto the image plane 324. A second optical device 328 with positive optical power is arranged to collimate the light from the image plane 324 into a collimated output image (shown here as entering the LOE 204 at the entrance aperture 334). Here, the second optical device 328 is implemented as a reflecting lens associated with the surface of the PBS prism 330, including the PBS 326, to guide the light from the image plane 324 toward the collimating optics. The reflected collimated light (as mentioned above, after passing through the quarter-wave plate twice) passes through the PBS 326 to reach the aperture 334—partially directly to the aperture 334 and partially after additional reflection at the prism surface 332.

[0034] A coupling prism 336 is provided to facilitate the coupling of image illumination after the first optical device 322'. An air space or buffer layer 338 of low refractive index material is provided to maintain the TIR conditions of collimated light propagating from the collimating optics 328 to the exit / entry aperture 334.

[0035] 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 by the appended claims.

Claims

1. An image projector for outputting collimated images, the image projector comprising: (a) Light source; (b) A spatial light modulator, illuminated by light from the illumination source to reflect light corresponding to the image, the illumination of the spatial light modulator being distributed around a vertical direction; (c) A first optical device having positive optical power is arranged to focus light reflected from the spatial light modulator onto the image plane; (d) A second optical device having positive optical power is arranged to collimate light from the image plane into a collimated output image; (e) A first polarization beam splitter (318) for integrating the illumination into the spatial light modulator; (f) A second polarization beam splitter (326) for integrating the diverging beam onto the second optical device; as well as (g) Output aperture (334), which is the entrance of waveguide (204) located after the second polarization beam splitter (326). The optical path from the spatial light modulator via the first and second optical devices to the image output passes through the first and second polarization beamsplitters, and the image plane is located between the first and second polarization beamsplitters. Wherein, at least the second optical device includes a reflective lens associated with the surface of a prism containing one of the first and second polarizing beamsplitters.

2. The image projector according to claim 1, wherein, The prism is optically integrated with a light-guiding optical element (LOE) having a pair of parallel primary outer surfaces for guiding light via internal reflection. The portion of the prism between the polarizing beam splitter and the light-guiding optical element has an outer surface that is coplanar with one of the primary outer surfaces of the light-guiding optical element and forms an extension of the primary outer surface of the light-guiding optical element.

3. The image projector according to claim 1, wherein, The first polarization beam splitter and the second polarization beam splitter are parallel.

4. The image projector according to claim 1, wherein, The first optical device (322) for focusing the image, located after the first polarization beam splitter (318), is reflective.

5. The image projector according to claim 1, wherein, Both the first polarizing beam splitter and the second polarizing beam splitter are included within the prism, and both the first optical device and the second optical device are implemented as reflective lenses associated with the surface of the prism.

6. The image projector of claim 1, further comprising at least one optical element spatially associated with the image plane.

7. The image projector according to claim 6, wherein, The at least one optical element includes a field lens.

8. The image projector according to claim 6, wherein, The at least one optical element includes a diffuser.

9. The image projector according to claim 6, wherein, The at least one optical element includes a microlens array.

Images