Optical projection system for aligning two images

Abstract: An optical guide (10, 40, 50, 60) for horizontally aligning two vertically stacked images generated by one or two SLMs. The optical guide has a channel separator (10a) that directs both images along two different paths. A pair of aligning reflectors (10b and 10c) on each path vertically shift the images with respect to each other so that at least part of the images on the first path are aligned side-by-side with at least part of the images on the second path. The channel separator (10a) then re-directs the images to the image plane 15. Along both paths, at least two of the reflecting surfaces of channel separator (10a) or aligning reflectors (10b and 10c) are optically powered so as to change the width or height of the images.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to optical devices, and more particularly to an optical projection system as defined in the precharacterizing portion of claim 1. Further, the invention relates to a method of aligning two images on an image plane. BACKGROUND OF THE INVENTION Spatial light modulators (SLMs) are commonly used for imaging applications, both for display and printing. In general, an SLM is an array of pixel elements, which are individually addressable, usually with electronic signals. Many SLMs are binary, having an addressing scheme that switches the pixels to either an "on" or an "off" state to form the image. Various modulation and integration schemes are used to provide greyscale images. For printing applications, the SLM is used to expose a photoreceptor drum and can be addressed so that its pixels selectively emit or reflect light to the drum in accordance with the desired image. One type of SLM is a digital mirror device (DMD), sometimes referred to as a deformable mirror device. The DMD has an array of hundreds or thousands of tiny tilting mirrors, which are the pixels. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts, and spaced by means of an air gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt. For printing applications, SLMs are typically used to generate long and narrow images, which expose a given number of rows on the photoreceptor drum. For example, a typical SLM might be 2.54 cm (an inch) long with rows of 900 pixels across its length. With appropriate magnification, an SLM of this size is suitable for exposing a 7.62 cm (3 inch) long strip of the image at 118 dots per cm (300 dots per inch (dpi)). However, it is often desired to have an image that is longer than 76.2 mm (3 inches) across. Although this could be accomplished by either adding more pixels so that the SLM is longer or by increasing the magnification, neither of these alternatives is desirable. A better technique for increasing the image size is described in Published European Patent document 0,493,831, assigned to Texas Instruments Incorporated. The patent describes using a DMD to simultaneously generate two images, one above the other. One image is the left half of the line to be printed, and one image is the right half of the line. An optical guide is used to project both images side-by-side in their correct alignment. SUMMARY OF THE INVENTION According to the present invention there is provided an optical projection system as defined in claim 1. The method of the invention comprises the steps defined in claim 16. Hereinafter, the optical projection system is also called optical guide. An advantage of the invention is that it provides a means for printing or displaying wider images with currently available narrow SLM arrays. Because of the anamorphic capability of the optical guide, these images can be made to have higher resolution without changing the size of the pixel elements of the SLM or the size of the array. Furthermore, for printing applications, the anamorphic capability permits the use of spatial variations for exposing each pixel so that greyscale levels can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further described, by way of example, with reference to the accompanying drawings in which; FIGURE 1 is a perspective view of an exposure system for a printer drum, with an anamorphic optical guide in accordance with the invention; FIGURE 2 is a plan view of the optical guide of FIGURE 1; FIGURE 3 is a front view of the optical guide of FIGURE 1; FIGURE 4 is a plan view of an alternative embodiment of the optical guide of FIGURE 1; FIGURE 5 is a perspective view of an alternative embodiment for an optical guide; and FIGUREs 6 - 8 are perspective, top, and front views respectively of an alternative embodiment for an optical guide. DETAILED DESCRIPTION OF THE INVENTION FIGURE 1 illustrates an anamorphic optical guide 10 in accordance with the invention. As explained below, optical guide 10 receives two "stacked" images. Optical guide 10 directs these images to an image plane 14, such that the two images are aligned side-by-side. It also adjusts the height or width of the images, such that the final image is anamorphic in accordance with a desired ratio of height to width. The following description is directed primarily to the anamorphic capability of optical guide 10. An optical guide without the anamorphic image capability of the present invention is described in Published European Patent document 0,493,831. In the example of this description, the source images are generated by an SLM 12 such as a DMD. As explained in the background, a DMD operates by reflecting light from tiny mirrors, which are selectively addressed. A typical DMD might have 1000 mirror elements per row, with each mirror element being individually addressable. A light source 11 illuminates SLM 12 via lenses 11a and 11b, and the image is reflected from SLM 12. However, if some other type of SLM is used, such as one whose pixels emit rather than reflect light, a light source 11 may not be necessary. As indicated by the dotted lines in FIGURE 1, SLM 12 generates a top image and a bottom image. These images are "stacked", one above the other. For this purpose, SLM 12 has at least two rows of mirror elements. For simplicity of description, it is assumed that SLM 12 generates the two stacked images with one row of pixels per image. However, it is possible that SLM 12 might use more than one row of pixels to generate each image. Also, SLM 12 might generate more than two images at a time. In the example of this description, a single SLM 12 generates both the top image and the bottom image. However, in other embodiments, instead of a single SLM 12 generating both images, two SLMs could be closely spaced, with one generating the top image and the other generating the bottom image. As generated by SLM 12, one image is above and one image is below an imaginary optical axis, which extends from the aperture of optical guide 10 to the image plane 14. Projection lens 13 receives the images generated by SLM 12 and projects them along the optical axis to optical guide 10. FIGURE 2 is a top plan view of optical guide 10. As indicated in both FIGUREs 1 and 2, optical guide 10 is comprised of a total of eight reflecting surfaces along two optical paths, a left path and a right path. Each optical path has four reflecting surfaces. In the embodiment of this description, a channel separator 10a provides two reflecting surfaces for each path, referred to herein as each path's "entry" surface and "exit" surface. Two aligning prisms 10b and 10c provide the other two reflecting surfaces for each path. The angles illustrated in FIGURE 2 are for purposes of example. Variations of these angles can be geometrically derived, based on known optical principles, such that the left optical path and the right optical path will eventually be positioned in the same plane as, and at least substantially parallel to, the optical axis. Channel separator 10a may be comprised of four mirrors, or, alternatively, of a prism with four silvered surfaces. Channel separator 10a receives the images from SLM 12 via lens 13, which provides a left image and a right image. Alternatively, two lens instead of a single lens 13, both viewing SLM 12, could provide the two images. Channel separator 10a directs both images along a left path and both along a right path. Thus, each optical path carries both the top image and the bottom image generated by SLM 12. In the example of this description, aligning reflectors 10b and 10c are right angle isosceles prisms. In FIGURE 1, each has its right angle milled off for packaging convenience. However, like channel separator 10a, aligning prisms 10b and 10c could each be two reflecting surfaces regardless of how attached to each other. FIGURE 3 is a front view of optical guide 10. As illustrated, aligning reflectors 10b and 10c are tilted with respect to the vertical. Because of their tilt, each aligning reflector 10b and 10c raises or lowers the image along one path with respect to the image along the other path, such that part of the left path's images are in the same horizontal plane as part of the right path's images. In the example of this description, reflector 10b is tilted inward toward channel separator 10a and reflector 10c is tilted outward from channel separator 10a. The tilt angles are chosen so that the top image from one optical path is aligned with the bottom image from the other optical path. However, it should be understood that if one image is directed along a path in the same horizontal plane as the optical axis, the alignment could be accomplished by raising or lowering only the other path's images. Also, the tilting could be reversed -- either the top or bottom image on one path could be aligned with the other image on the other path. The exit surfaces of channel separator 10a re-direct the images toward the image plane 15. An aperture 14 in front of image plane 15 transmits the two aligned images and blocks portions of the images that are not aligned. In the example of this description, the top image on one path is blocked, as is the bottom image on the other path. The final image is a long continuous image, comprised of the top image and the bottom image originally generated by SLM 12. This image illuminates the image plane 15, which for printing applications, is a photoreceptor drum. As stated above, the components of optical guide 10 are designed so as to adjust the size of the final images at image plane 14. In effect, each pixel becomes anamorphic. As explained below, the height or the width of the image can be adjusted by making two or more surfaces along each optical path optically powered. In the embodiment of FIGUREs 1 - 3, the two reflecting surfaces of each aligning reflector 10b and 10c are optically powered so as to decrease the vertical size of the image. For example, the vertical size might be adjusted so that it is 1/2 the horizontal size. At the pixel level, each pixel has become 1/2 as high as it is wide. This permits a two-fold increase in vertical resolution. In other words, where an image with non-anamorphic pixels might have a resolution of 236 dots per cm (600 dpi) in both the process (vertical) direction and the cross-process (horizontal) direction, these anamorphic pixels would provide an increase of resolution in the process direction to 472 dots per cm (1200 dpi). For printing applications, the anamorphic capability of optical guide 10 can also be used to provide greyscale images. For example, two rows of pixel elements of SLM 12 could be used to generate each row of the image on the image plane. Thus, two pixel elements of SLM 12 are used to generate one pixel of the image. For each pixel, a greyscale level of intensity can be accomplished according to whether neither, only one, or both pixel elements of SLM 12 are "on" for the corresponding pixel on the image plane. In FIGUREs 1 - 3, the first reflecting surface of each aligning reflector 10b and 10c is concave with respect to the images incident on it. The second reflecting surface of each aligning reflector 10b and 10c is convex with respect to the images. These two reflecting surfaces are "complementary" in the sense that the first surface converges the image and the second surface focuses the image at the image plane. In this case, the image is reduced in the vertical direction. Alternatively, the two reflecting surfaces could be complementary with their roles reversed, such that the first surface is convex with respect to the image and the second surface is concave. In this case, the final images would be larger in the vertical direction. For purposes of this description, "complementary" means that the reflecting surface first met by the images is optically powered so as to adjust the size of the images, and the second surface is optically powered so as to focus the images at the image plane. Also, three reflecting surfaces could be complementary, with two surfaces instead of one being used for adjusting size or collimating. Additional optical manipulation could be performed to correct astigmatism or other aberrations. Although in FIGUREs 1 - 3, it is the two surfaces of the aligning reflectors 10b and 10c that are optically powered, the same result could be accomplished with any two of the four reflecting surfaces along each path. For example, the four reflecting surfaces of channel separator 10a could be optically powered. In general, the invention is operative to provide anamorphic images when at least two surfaces along each optical path have complementary optical power. FIGURE 4 is a top view of an alternative embodiment of optical guide 10, which adjusts the size of the image in the horizontal direction. As with the optical guide 10 of FIGUREs 1 - 3, optical guide 40 has at least two reflective surfaces that are optically powered and that are complementary. However, in FIGURE 4, the optical power is in the horizontal instead of the vertical direction. Like optical guide 10, optical guide 40 has a channel separator 40a and two aligning reflectors 40b and 40c. These components serve the same function of separating the images onto two paths and of vertically shifting the images so that at least part of each pair of images is aligned at the image plane. In FIGURE 4, the optically powered surfaces are those of the aligning reflectors 40b and 40c. On each path, the first optically powered surface is concave with respect to the images incident on it. The images are converged to the second optically powered surface, which is convex with respect to the images and focusses the images to the image plane. The final images are reduced in the horizontal direction, making them less wide than tall. Pixels with these dimensions can be used to increase horizontal resolution. FIGURE 5 illustrates an alternative embodiment of the invention, in which optical guide 50 comprises a plane mirror 50a, a beam splitter 50b, and two aligning reflectors 50c and 50d. The two stacked images from SLM 12 are directed by plane mirror 50a to beam splitter 50b. Beam splitter 50b is partially reflective and partially transparent so that both images are incident on each reflector 50c and 50d. Thus, both images follow a first path that has a first aligning reflector 50c, and both images follow a second path that has a second aligning reflector 50d. The reflectors 50c and 50d are tilted with respect to the vertical so as to align the images. The images are then reflected or transmitted by beam splitter 50b to the image plane 15 via projection lens 13. Optical guide 50 has at least two reflecting surfaces along each path that are optically powered. In the example of FIGURE 5, plane mirror 50a and the aligning reflectors 50c and 50d are curved in the vertical direction. Plane mirror 50a is concave with respect to the images. Aligning reflectors 50c and 50d are convex with respect to the images on their respective paths. The result is an image that is reduced in size in the vertical direction. Like optical guide 40, the optical power could alternatively be in the horizontal direction. FIGUREs 6 - 8 illustrate another embodiment of the invention, in which optical guide 60 comprises two rhomboidal prisms 60a and 60b. Rhomboidal prisms 60a and 60b separate the reflected images onto two paths, vertically align the images, and direct them to image plane 15. FIGURE 6 is a perspective view and FIGURE 7 is a top view showing the optical paths. Optical guide 60 has two reflecting surfaces along each path that are optically powered. Along each path, a first reflective surface is concave with respect to the image and a second surface is convex. In the example of FIGUREs 6 - 8, the optically powered surfaces are the third and fourth reflective surfaces along each path, which are also the tilted aligning surfaces. The concave and convex curvatures are vertical so as to modifiy the size of the image in the vertical direction. Like the other optical guides discussed herein, the two optically powered surfaces are complementary so as to focus the images to the image plane 15. Also, the curvatures could alternatively be horizontal so as to modify the horizontal size of the image. As shown in FIGURE 8, which is a front view of optical guide 60, prisms 60a and 60b have reflecting surfaces that are tilted with respect to the vertical to provide the alignment. In the example of Figure 8, these tilted surfaces are also the optically powered surfaces. Figure 8 also illustrates a superimposed view of lens 13, which has a top portion for providing a top image and a bottom portion for providing a bottom image. Like the other embodiments described herein, the two images could alternatively be provided with two different lenses viewing the SLM 12. With respect to all of the above embodiments, it should be understood that the same concepts would apply if the invention were "turned on its side". Thus, two images could be generated that are parallel to each other in the vertical instead of horizontal direction. In this case, the optical guide could be re-positioned so as to align the images vertically so that one is above the other. For purposes of this description, this modification of the invention is an equivalent with the words "vertical" and "horizontal" being interchangeable.

Claims: An optical projection system (10; 40; 50; 60) for aligning a plurality of images comprising: a plurality of reflective surfaces for receiving light representing an image of an object plane, said plurality of reflective surfaces directing said light along a plurality of optical paths onto an image plane (15), said reflective surfaces being tilted such that images of said object plane formed at the image plane (15) by light travelling along each path are abutted in a first direction in said image plane (15); characterized in that at least two of said reflective surfaces along each optical path have optical power for adjusting the width or height of said images. The optical projection system of claim 1, said plurality of reflective surfaces comprising: a projection lens (13) for receiving light from said objects from a spatial light modulator (SLM) (12) and for directing said light along an axis of projection between said SLM (12) and said image plane (15); an entry pair of reflective surfaces for receiving said light from said projection lens (13) and for directing a portion of said light along a first path and a portion of said light along a second path, each path beginning with one of said entry pair of reflective surfaces; a first pair (10b; 40b) of reflective surfaces disposed along said first path for directing said light on said first path toward said axis; a second pair (10c; 40c) of reflective surfaces disposed along said second path for directing said light on said second path toward said axis; and an exit pair of reflective surfaces for directing said light to said image plane (15). The optical projection system in accordance with Claim 2, further comprising an aperture (14) for blocking at least part of said light of said first path and at least part of said light on said second path after said light is reflected from said exit reflective surfaces. An optical projection system according to any of Claims 2-3, wherein said optically powered reflective surfaces are said first pair of reflective surfaces and said second pair of reflective surfaces. An optical projection system according to any of Claims 2-4, wherein said optically powered reflective surfaces are said entry pair of reflective surfaces and said exit pair of reflective surfaces. An optical projection system according to any of Claims 2-5, where said optically powered reflective surfaces are said entry pair of reflective surfaces and one surface of said first pair of reflective surfaces and one surface of said second pair of reflective surfaces. An optical projection system according to any of Claims 2-6, wherein said optically powered surfaces are curved in one direction. An optical projection system according to any of Claims 2-7, wherein said optically powered reflective surfaces are complementary, one being concave and the other being convex. An optical projection system according to any of Claims 2-8, wherein one each of said entry pair and said first pair of reflective surfaces are.surfaces of a first rhomboidal prism (60a) along each path, and wherein one each of said second pair and said exit pair of reflective surfaces are surfaces of a second rhomboidal prism (60b) along each path. The optical projection system of claim 1 for aligning a plurality of images, said images generated by at least one spatial light modulator (SLM) (12) and aligned on said image plane (15), said plurality of reflective surfaces comprising: an entry reflective surface (50a) for receiving light from said representations from said SLM (12); a beam splitter (50b) for reflecting said light to a first path and for transmitting said light along a second path; a first reflective surface (50c) disposed along said first path for directing said light on said first path toward said image plane (15); and a second reflective surface (50d) disposed along said second path for directing said light on said second path toward said image plane (15); characterized in that said entry reflective surface (50a) and said first and said second aligning reflective surfaces (50c, 50d) are having optical power reflective surfaces for adjusting the width or height of said representations on said image plane (15). The optical projection system of Claim 10, wherein said optically powered surfaces are curved in one direction. An optical projection system according to any of Claims 10-11, wherein said optically powered surfaces are complementary, one being concave and the other being convex. The optical projection system of claim 1, for aligning and projecting two spatial light modulator (SLM)-generated images onto said image plane (15), further comprising: a first SLM region for generating a first image; a second SLM region for generating a second image; wherein said first SLM region and said second SLM region are generally in different horizontal planes; said plurality of reflective surfaces are arranged for guiding light from said first SLM region and light from said second SLM region to said image plane (15), said reflective surfaces tilted such that said images are horizontally aligned side-by-side on said image plane (15); and said at least two of said reflective surfaces are curved such that light from said regions is anamorphically imaged onto said image plane (15). The optical projection system of Claim 13, said optical projection system (10; 40; 60) comprising: a projection lens (13) for directing said light from said regions along an axis ofprojection between said SLM (12) and said image plane (15); an entry pair of reflective surfaces for directing light from both regions along both a first path and a second path that begins with said entry pair of reflective surfaces; a first pair (10b; 40b) of reflective surfaces disposed along said first path for directing said light on said first path; a second pair (10c; 40c) of reflective surfaces disposed along said second path for directing light on said second path; an exit pair of reflective surfaces, one along each said first and said second path, for directing said light on said paths to said image plane (15); wherein at least two of said reflective surfaces along each optical path are reflective surfaces having optical power. The optical projection system according to Claim 13, wherein said optical projection system (50) has an entry reflective surface (50a) for receiving light from said SLM regions; a beam splitter (50b) for reflecting said light to a first path and for transmitting said light along a second path; a first reflective surface (50c) disposed along said first path for directing said light on said first path; a second reflective surface (50d) disposed along said second path for directing light on said second path; wherein said entry reflective surface (50a) and said first and second aligning reflective surfaces (50c, 50d) have optical power. A method of aligning two images on an image plane (15), comprising the steps of: generating two representations of objects on one or more spatial light modulators (12), one representation above the other with respect to a vertical direction; directing light originating from both representations along a first light path and along a second light path to form an image of said representations on an image plane (15); wherein said first path and said second path each have at least two reflecting surfaces with optical power so as to change the height or width of said images; and adjusting the vertical angle of reflection so as to adjust the vertical position of an image directed along at least one of said paths, such that at least a portion of said image directed along said first path is in the same vertical plane as at least a portion of an image directed along said second path. The method of Claim 16, wherein said adjusting step is accomplished by elevating images on said first path and lowering said images on said second path. A method according to Claim 16, wherein one of said images is generated on an axis of projection, and wherein said adjusting step is accomplished by elevating or lowering the other of said images.

Inventors: ANDERSON CHARLES H, ANDERSON, CHARLES H.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: G02B27
IPC codes: H04N1, G02B27