Abstract:
Disclosed are a system and method for microprojection that uses multiple imagers to produce a high resolution output image. Each of a set of imagers produces a portion of the final image. Relay lenses then tile the individual image portions together into a combined image. Because the height of the individual imagers is smaller than the height of a monolithic imager, they can fit into a very thin device. The combined image has a resolution equal to the sum of the resolutions of the individual imagers. The individual images are tiled together within the microprojector itself rather than on a projection screen. This allows the tiling to be adjusted once at the factory and set forever. In some embodiments, the light created for use by the microprojector is split by a polarizing beamsplitter. Each resultant polarized beam is then sent to an imager. Another polarizing beamsplitter combines the individual images.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The subject matter of this patent application is related to that of U.S. patent application Ser. No. 11/687,884, filed on Mar. 19, 2007. 
     FIELD OF THE INVENTION 
     The present invention is related generally to projection of optical images, and, more particularly, to optical-image projectors subject to space limitations. 
     BACKGROUND OF THE INVENTION 
     A trend in personal portable devices (such as cell phones and personal digital assistants) is to add new features while keeping the devices small. Many of the new features, such as photograph sharing and video downloading, depend upon a high resolution, easy-to-read display screen. However, manufacturers cannot simply keep increasing the size of their display screens because that would eventually run counter to the desire to keep the devices small and portable. 
     Recently, “microprojectors,” a new category of display device, have been designed to address this conflict between greater display area and smaller device size. An image, either still or moving, is projected from the device onto a convenient surface (e.g., a projection screen or an office wall). The maximum size of the image is then effectively constrained by the amount of available wall space rather than by the size of the device itself. Using a microprojector-equipped device, several people can simultaneously view a photograph, for example, or review a full page of text, neither of which can be readily done with even the largest displays on current personal portable devices. 
     Promising as they are, microprojectors raise new headaches when engineers attempt to fit them into personal portable devices. While the overall size of the projected image may be effectively unlimited, expanding the image size is of little use if the resolution of the projected image is severely constrained. What customers want is a projected image that is both larger overall and has much greater resolution than a device&#39;s display screen. But, generally, the overall size of a microprojector grows with the amount of resolution it provides. This is especially true when a microprojector uses a microdisplay imager as its image source. The trend toward very thin personal portable devices renders it a challenge to fit in a microprojector that provides usefully high resolution. 
     Power use is another challenge. By its nature, a microprojector uses a significant amount of power to light a large display area. In addition, microprojectors usually use proven liquid-crystal displays which only work with linearly polarized light. Light created for use by the microprojector is first sent through a polarizer, a component that discards about half of the original light and thus discards about half of the power. Reducing the physical size of the microprojector exacerbates the power problem because the optics in microprojectors become less power-efficient as they become smaller. Designers of battery-based personal portable devices are already concerned about their power budgets and look askance at any new feature that threatens to reduce the utility of the device by reducing how long the device can operate between charges. 
     BRIEF SUMMARY OF THE INVENTION 
     The above considerations, and others, are addressed by the present invention, which can be understood by referring to the specification, drawings, and claims. According to aspects of the present invention, a microprojector uses multiple imagers to produce a high resolution output image and avoids the use of very small imager optics with their lowered efficiency. 
     Each of a set of imagers produces a portion of the final image. Relay lenses then tile the individual image portions together into a combined image. Because the height of the individual imagers is smaller than the height of a monolithic imager, they can fit into a very thin device. The combined image, on the other hand, has a resolution equal to the sum of the resolutions of the individual imagers. 
     In some embodiments, for example, two imagers are placed side by side. One imager produces the top half of the combined image, and the other imager produces the bottom half. When the two halves are combined, the combined image has a horizontal resolution equal to that of each imager and a vertical resolution equal to the sum of the vertical resolutions of the individual imagers. 
     The individual images are tiled together within the microprojector itself rather than on a projection screen. This allows the tiling to be adjusted once at the factory and set forever. Changing the distance between the projection lens and the display screen does not affect the tiling adjustment. This provides both higher tiling-image quality and much needed convenience for the consumer. Furthermore, it allows the use of a simple optical design for the projector. 
     In some embodiments, the light created for use by the microprojector is split by a polarizing beamsplitter. Each resultant polarized beam is then sent to an imager. Another polarizing beamsplitter combines the individual images. This technique uses essentially all of the original light, doubling the lighting efficiency of previous devices. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is an overview of a representative environment in which aspects of the present invention can be practiced; 
         FIG. 2  is a simplified isometric view of an exemplary microprojector with two imagers; 
         FIG. 3  is a flowchart of an exemplary embodiment of the present invention; 
         FIG. 4  is a schematic of an exemplary two-imager microprojector that efficiently uses polarized light; 
         FIGS. 5   a ,  5   b , and  5   c  are optical-path diagrams showing how relay lenses tile intermediate images together; and 
         FIGS. 6   a  and  6   b  are optical-path diagrams showing the effects of an optical diffuser. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable environment. The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein. 
     In  FIG. 1 , a user  100  is projecting an image  104  from her personal portable device  102 . The image  104  could be, for example, a photograph, a video, or a computerized display from a word processor or an Internet browser. The image  104  may be projected onto a screen or even onto a wall or ceiling. By projecting the large, high resolution image  104  rather than presenting it on a (necessarily small) display screen of her personal portable device  102 , the user  100  can invite others to share the image  104  with her. 
     The resolution of a digital image is defined as the product of its horizontal resolution and its vertical resolution. Resolution is measured in number of pixels. In  FIG. 1 , the image  104  has a horizontal resolution “Rx.” Rx measures the number of addressable pixels in the horizontal direction and is indicated by  106  in  FIG. 1 . The vertical resolution “Ry” counts addressable pixels in the vertical direction and is indicated by  108 . Note that here “horizontal” and “vertical” are merely convenient, and conventional, names for the two dimensions of a planar image, and are not confined to orientations taken with respect to the direction of gravity. 
     In a projector, an “imager” is a device that modulates light in order to imprint image information into a projected light beam. Generally, the resolution of a projected image is equal to the resolution of the imager that creates the image. Traditionally, including within the personal portable device  102  an imager that provides acceptable resolution for the projected image  104  makes the personal portable device  102  both thick and bulky. The present invention addresses this issue by allowing a small and thin personal portable device  102  to project a large, high resolution image  104 . 
       FIG. 2  shows how a microprojector made according to aspects of the present invention can achieve a high resolution in the projected image  104 .  FIG. 3  also illustrates embodiments of the present invention by following the light through the microprojector system. An illumination source  200  (discussed in greater detail below in reference to  FIG. 4 ) produces light (Step  300  of  FIG. 3 ). The light is directed toward multiple imagers  202   a  and  202   b  (second half of Step  302 ). (For clarity&#39;s sake,  FIG. 2  only shows an idealized light path for one of the imagers  202   a .) Each imager  202   a  and  202   b  generates a portion of the overall projected image  104  (Step  304 ). The light modulated by the imagers  202   a  and  202   b  is directed to relay lenses  204   a  and  204   b  (Step  306 ). The relay lenses  204   a  and  204   b  are so configured that they position the internal image portions  212   a  and  212   b  created by the imagers  202   a  and  202   b  so that when these internal image portions  212   a  and  212   b  are projected by the projection lens system  206  (Step  312 ), the image portions from the separate imagers are seamlessly combined into the overall image  104 . 
     The illustrative implementation of  FIG. 2  shows the imagers  202   a  and  202   b  as two physically separate entities. In some implementations, however, these logically separate imagers can be embodied in one physical entity, called here a “combined imager” to distinguish it from traditional monolithic imagers. A combined imager operating according to aspects of the present invention would operate in a manner similar to the manner discussed here for the separate imagers  202   a  and  202   b . For example, a combined imager could be “long and thin” as suggested by  FIG. 2  when compared to a traditional monolithic imager with the same overall resolution. Also for example, one section of the combined imager could create the internal image portion  212   a , while another section creates the internal image portion  212   b.    
     While the portions  208   a  and  208   b  of the final image  104  are stacked vertically in  FIG. 2 ,  FIG. 2  shows that the relay lenses  204   a  and  204   b  are so designed that the imagers  202   a  and  202   b  need not be stacked vertically but can reside side-by-side within the personal portable device  102 . This side-by-side layout of the imagers  202   a  and  202   b , each shorter in a vertical direction than a monolithic imager of the same overall resolution, permits the personal portable device  102  to remain small and thin. 
     The personal portable device  102  of  FIG. 2  also includes controller logic and image memory  210 . As directed by a user, the controller  210  retrieves image information and directs it to the imagers  202   a  and  202   b  for display. As the controller logic and image memory  210  are well known in the art, they are not further discussed here. 
     Because the final projected image  104  is produced by multiple imagers  202   a  and  202   b , there is no need to include in the personal portable device  102  room for a single monolithic imager that has the same resolution as the final image  104 . Instead, the system of  FIG. 2  is arranged in such a way that the resolution of the final image  104  is the sum of the resolutions of the individual imagers  202   a  and  202   b . If exactly two imagers are used in the system of  FIG. 2 , and if each imager  202   a  and  202   b  has a horizontal resolution equal to the horizontal resolution of the overall image  104 , and if each imager  202   a  and  202   b  has half the vertical resolution of the overall image  104 , then these two imagers  202   a  and  202   b  can, in combination, produce the total resolution of the overall image  104 . In this case, the thickness of the personal portable device  102  is constrained only by the vertical dimension of the “half-height” imagers  202   a  and  202   b  rather than by the vertical dimension of a “full-height” imager. This example, and the arrangement of  FIG. 2 , can be extended to include more than two imagers. The addition of more imagers allows further reductions in the thickness of the personal portable device  102  while potentially adding complexity to the light paths. 
     Note again that “vertical” and “horizontal” are used here merely for convenience&#39; sake and are used with respect to the figure under discussion. In most embodiments, the image  104  is expected to be projected from an end face of the personal portable device  102 . The shape of the end face of many personal portable devices  102  approximates a long, thin rectangle. In some embodiments of the present invention, the projected image  104  roughly follows this shape. Thus, to project an image in “landscape” mode (that is, with a greater horizontal than a vertical dimension), the user  100  holds her personal portable device  102  “flat” (with the long edge of the face of the device  102  parallel to the ground). To project an image  104  in the “portrait” mode as shown in  FIG. 1 , the user  100  turns her personal portable device  102  so that the long edge of its end face is vertical. Known technology can be used to tell the personal portable device  102  of its orientation so that it can project the image  104  appropriately. 
     The imagers  202   a  and  202   b  shown in  FIG. 2  are called “transmissive” imagers because they modulate light as it passes through them. “Reflective” imagers are also known and can be used in embodiments of the present invention. These imagers modulate light as it reflects off of them. 
     For simplicity&#39;s sake, the projection lens system  206  is drawn as a single lens in  FIG. 2  (and in  FIGS. 4 ,  5   c ,  6   a , and  6   b ). As is well known in the art, a projection lens system  206  can include numerous lenses of different curvatures and materials. Different projection lens systems  206  are chosen based on physical constraints and on anticipated use. 
       FIG. 4  shows an embodiment of the present invention that doubles the lighting efficiency of many previous devices. The illumination system here includes three light sources  400   a ,  400   b , and  400   c . Each source  400   a ,  400   b , and  400   c  produces unpolarized light of one color, while the combination of sources usefully covers the visible spectrum (e.g., one source produces red light, one green, and one blue). In some embodiments, each source  400   a ,  400   b , and  400   c  is a light-emitting diode. Light from the three monochromatic sources  400   a ,  400   b , and  400   c  is directed, possibly via light tunnels such as  402 , and combined together. In the example of  FIG. 4 , two colors are first combined in a dichroic beamsplitter  404 , and then the combination of the two colors is combined with the third color in a first polarizing beamsplitter  408 . Two polarized light beams leave the first beamsplitter  408  (Step  302  of  FIG. 3 ), one directed toward a first imager  202   a , and the other directed toward a second imager  202   b . Each polarized beam leaving the first beamsplitter  408  includes light of all three colors. 
     Typical imagers  202   a  and  202   b  are liquid-crystal devices that require polarized light. Because illumination systems typically produce unpolarized light, many previous systems filter their generated light through a polarizer before directing it to an imager. This technique, however, throws away all of the incipient light whose polarization is not aligned with that of the polarizer. This results in a loss of about 50% of the original light. In the system of  FIG. 4 , on the other hand, the first polarizing beamsplitter  408  uses all of the light directed to it, either in the beam directed toward the first imager  202   a  or in the other beam directed toward the other imager  202   b.    
     As discussed above in relation to  FIG. 2 , the imagers  202   a  and  202   b  modulate the light directed through them to imprint the light with portions of the final image  104 . Depending upon the physical layout of the components, the modulated light may be directed by total-internal-reflective prisms  406   a  and  406   b  as it travels to the relay lenses  204   a  and  204   b . Upon leaving the relay lenses  204   a  and  204   b , the modulated light is still polarized. Therefore, a second polarizing beamsplitter  410  can be used to direct all of the relayed light to the projection lens system  206 . 
     By using essentially all of the light produced by the sources  400   a ,  400   b , and  400   c , the embodiment of  FIG. 4  is approximately twice as light- and power-efficient as previous systems. In response to specific packaging and other constraints, known optical techniques can be used to rearrange the components of  FIG. 4  without departing from the teachings of the present invention. For example, reflective imagers can replace the transmissive imagers  202   a  and  202   b , and the two beamsplitters  408  and  410  can sometimes be combined into one component. 
       FIGS. 5   a ,  5   b , and  5   c  illustrate how, in some embodiments, the relay lenses  204   a  and  204   b  combine the image portions made by the individual imagers  202   a  and  202   b .  FIG. 5   a  shows the light modulated by the imager  202   a  directed toward the relay lens  204   a . The imager  202   a  is approximately centered above and below the optical center line  502  of the projection lens system  206  (not shown in  FIG. 5   a ). However, the relay lens  204   a  directs the image portion  212   a  created by this imager  202   a  so that it lies entirely above the optical center line  502 . Thus, the relay lens  204   a  both positions the image portion  212   a  and “shrinks” the vertical dimension of the image portion  212   a  so that it fits entirely above the optical center line  502 . In the example of  FIG. 5   a , the image portion  212   a  is approximately half the height of the imager  202   a . (In  FIGS. 5   a ,  5   b , and  5   c , “vertical” and “height” are used in reference only to the figures themselves.) 
       FIG. 5   b  is similar to  FIG. 5   a  but shows the other imager  202   b  and its associated relay lens  204   b . Like the first imager  202   a , the imager  202   b  is also approximately centered above and below the optical center line  502 . Its relay lens  204   b  shrinks the image portion  212   b  and directs it to lie entirely below the optical center line  502 . 
       FIG. 5   c  combines the results of  FIGS. 5   a  and  5   b  to show how the two image portions  212   a  and  212   b  are “tiled” together about the optical center line  502 . Because the relay lenses  204   a  and  204   b  “shrink” the image portions  212   a  and  212   b , these image portions  212   a  and  212   b  can be tiled together in a height that approximates the height of each imager  202   a  and  202   b . The projection lens system  206  then projects the tiled image out of the personal portable device  102 . The internal image portions  212   a  and  212   b  are projected to become the portions  208   a  and  208   b  that are seamlessly combined in the projected image  104 . 
       FIGS. 5   a ,  5   b , and  5   c  show how the use of relay lenses  204   a  and  204   b  allows the imagers  202   a  and  202   b  to each have a vertical dimension approximately equal to that of the projection lens system  206 . As shown in  FIGS. 2 and 4 , the individual imagers  202   a  and  202   b  are not stacked on top of one another, but can be placed in some kind of side-by-side arrangement. Thus, the relay lenses  204   a  and  204   b  enable the increased resolution provided by the multiple imagers  202   a  and  202   b  without increasing the thickness of the personal portable device  102 . 
     Another aspect of the present invention is illustrated in  FIG. 5   c . In many previous multiple-imager projection systems, the partial images are only combined outside the device housing the projection system, that is, they are combined on the projection screen (or wall or whatever). The alignment of the partial images into one image is then dependent upon the distance between the projection lens and the screen. In contrast to these previous systems, embodiments of the present invention tile the partial images  212   a  and  212   b  together within the personal portable device  102 , specifically, they are tiled together before they reach the projection lens system  206 . This arrangement allows the tiling to be carefully aligned and permanently set during manufacture, yielding a much more robust system whose alignment is independent of the environment outside of the personal portable device  102 . This arrangement also allows the use of simple optical design to reduce complexity for seamlessly tiling the two images on the final projection screen. 
       FIGS. 6   a  and  6   b  show a feature of some embodiments of the present invention.  FIG. 6   a  shows that some of the light from the imager  202   b , after passing through the relay lens  204   b  and creating the image portion  212   b , may miss the projection lens system  206  entirely and be wasted (thus reducing the power efficiency of the microprojector). Also, light from different areas of the imager  202   b  enters the projection lens system  206  at different angles, possibly leading to non-uniform lighting in the final projected image  104 . In  FIG. 6   b , an optical diffuser  600  is added at the location of the image portion  212   b  (Step  308  of  FIG. 3 ). The optical diffuser  600  redirects incident light so that it travels in a cone directly toward the projection lens system  206 . As a result, more light from the relay lens  204   b  reaches the projection lens system  206 . The projected image  104  is also more evenly lit because light leaves the optical diffuser  600  in a consistent cone regardless of where on the imager  202   b  the light comes from. In some embodiments, the optical diffuser  600  is designed to blend light from the different imagers  202   a  and  202   b  so as to effectively eliminate any seam between the image portions  212   a  and  212   b.    
     In view of the many possible embodiments to which the principles of the present invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of the invention. For example, the light paths in the figures are only meant to illustrate the functions of the various components and are not meant to be definitive. Other arrangements of the optical components shown in the figures and the addition of other known optical components are possible and may be called for in various environments. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.