Patent Publication Number: US-2009231495-A1

Title: Image projector with multiple imagers

Description:
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. 
     Light created for use by the microprojector is split by a polarization-sensitive element into a number of beams. Each polarized beam is then sent to an imager. Each imager modulates the light beam to produce a portion of the final image. Another polarization-sensitive element directs the individual image portions through a projection lens system so that when they are projected, the individual image portions tile together into a seamless projected image. This technique uses essentially all of the original light, doubling the lighting efficiency of previous devices. 
     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. 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. 
    
    
     
       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 schematic view of an exemplary microprojector with two transmissive imagers; 
         FIG. 3  is a flowchart of an exemplary embodiment of the present invention; 
         FIG. 4  is a schematic of an arrangement for directing polarized light toward two transmissive imagers; and 
         FIG. 5  is a simplified schematic view of an exemplary microprojector with two reflective imagers 
     
    
    
     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  gives an example of 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 ). A first polarization-sensitive element  202  splits the into multiple paths  204   a  and  204   b  (Step  302 ), one path  204   a  directed toward a first imager  206   a , and the other path  204   b  directed toward a second imager  206   b.    
     Typical imagers  206   a  and  206   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. 2 , on the other hand, the first polarization-sensitive element  202  uses all of the light directed to it, either in the beam  204   a  directed toward the first imager  206   a  or in the other beam  204   b  directed toward the other imager  206   b.    
     By modulating the polarized light transmitted to it, each imager  206   a  and  206   b  generates a portion of what will be the overall projected image  104  (Step  304  of  FIG. 3 ). The light modulated by the imagers  206   a  and  206   b  is directed by a second polarization-sensitive element  208  toward a projection lens system  210  (Step  306 ). Because the light modulated by the first imager  206   a  has a polarization different from that of the light modulated by the other imager  206   b , a third polarization-sensitive element  212  can separate and position the image portions created by the imagers  206   a  and  206   b  (Step  308 ). When these image portions are projected by the projection lens system  210  (Step  310 ), they become the projected image portions  214   a  and  214   b  which seamlessly combine into the overall image  104 . 
     For simplicity&#39;s sake, the projection lens system  210  is drawn as a single lens in  FIG. 2  (and in  FIGS. 4 and 5 ). As is well known in the art, a projection lens system  210  can include numerous lenses of different curvatures and materials. In some embodiments, the third polarization-sensitive element  212  is placed at the aperture stop of the projection lens system  210 . While  FIG. 2  shows the third polarization-sensitive element  212  located after the projection lens system  210 , in other embodiments the third polarization-sensitive element  212  can be placed either before or within the projection lens system  210 . While different projection lens systems  210  and different placements of the third polarization-sensitive element  212  are chosen based on physical constraints and on anticipated use, it is preferred that the choice of these two elements  210  and  212  be made together. In order to make the image portions  214   a  and  214   b  correctly tile to form the final image  104  without seam or overlap, it is recommended that the deflecting angle of the third polarization-sensitive element  212  be matched to the shooting angle of the projection lens system  210 . 
     In different embodiments, different technologies can be used for the polarization-sensitive elements  202 ,  208 , and  210 . For a few examples, they can be polarizing beamsplitters, calcite-crystals, liquid-crystal-type cells, thin-film polarization-sensitive elements, Wollaston prisms, or some combination of these. 
     The illustrative implementation of  FIG. 2  shows the imagers  206   a  and  206   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  206   a  and  206   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, a left section of the combined imager works with light of one polarization state and creates the image portion  214   a , while a right section of the combined imager works with light of another polarization state and creates the image portion  214   b.    
     While the portions  214   a  and  214   b  of the final image  104  are stacked vertically in  FIG. 2 ,  FIG. 2  shows that the imagers  206   a  and  206   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  206   a  and  206   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  216 . As directed by a user, the controller  216  retrieves image information and directs it to the imagers  206   a  and  206   b  for display. As the controller logic and image memory  216  are well known in the art, they are not further discussed here. 
     Because the final projected image  104  is produced by multiple imagers  206   a  and  206   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  206   a  and  206   b . If exactly two imagers are used in the system of  FIG. 2 , and if each imager  206   a  and  206   b  has a horizontal resolution equal to the horizontal resolution of the overall image  104 , and if each imager  206   a  and  206   b  has half the vertical resolution of the overall image  104 , then these two imagers  206   a  and  206   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  206   a  and  206   b  rather than by the vertical dimension of a “full-height” monolithic imager. 
     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. 
       FIG. 4  shows how embodiments of the present invention double 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 the first polarization-sensitive element  202 . Two polarized light beams leave the first polarization-sensitive element  202 , one directed toward the first imager  206   a , and the other directed toward another imager  206   b . Each polarized beam leaving the first polarization-sensitive element  202  includes light of all three colors. 
     As discussed above in relation to  FIG. 2 , the imagers  206   a  and  206   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 second polarization-sensitive element  208 . The second polarization-sensitive element  208  combines the paths of the polarized light and directs all of the light to the projection lens system  210  and third polarization-sensitive element  212 . 
     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. 
     The imagers  206   a  and  206   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.  FIG. 5  presents an embodiment of the present invention that uses reflective imagers. Light from the illumination source  200  is directed to a first polarization-sensitive element  500  which splits the light into multiple polarized paths  204   a  and  204   b . The polarized light is directed to the two reflective imagers  502   a  and  502   b . These imagers  502   a  and  502   b  impart image information into the light as it reflects off of them directly back to the first polarization-sensitive element  500 . That element  500  directs the light from both polarized light paths  204   a  and  204   b  toward the projection lens system  210  and the third polarization-sensitive element  212 , as in  FIG. 2 . In effect, this polarization-sensitive element  500  does the work of both the first and the second polarization-sensitive elements  202  and  208  of  FIG. 2 . The choice to use reflective or transmissive imagers is based on packaging and other considerations. 
       FIG. 2 ,  4 , and  5  show how the use of polarized light allows the imagers  206   a  and  206   b  (or  502   a  and  502   b ) to each have a vertical dimension approximately equal to that of the projection lens system  210 . As shown in  FIGS. 2 ,  4 , and  5 , the individual half-height imagers are not stacked on top of one another, but can be placed in some kind of side-by-side arrangement. Thus, the use of polarized light enables the increased resolution provided by the multiple imagers without increasing the thickness of the personal portable device  102 . 
     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.