Abstract:
A projection system includes multiple imaging heads for projecting a plurality of images, which may or may not be partially overlapped and edge blended to form a projected composite image. The projection system, however, includes only a single light source. The light from the light source is beam split into light portions, and the light portions are applied to the imaging heads via corresponding light guides. Use of the single light source to project all the images may solve colorimetry problems associated with generating projected composite image from multiple projected images, each of which uses a different light source. For projecting the images on a curved surface, the geometry of the images may be distorted using electronic image warping and/or lens-based optical distortion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority of the U.S. Provisional Application No. 60/238,199 filed Oct. 4, 2000 entitled “Improved Projection System for Arrayed or Tiled Displays,” the contents of which are fully incorporated by reference herein. The present application includes subject matter related to the subject matter disclosed in U.S. patent application Ser. No. 09/876,513 entitled “Method and Apparatus for Seamless Integration of Multiple Video Projectors” filed Jun. 6, 2001, the contents of which are fully incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to displaying a composite image using multiple projected images, and particularly to method and apparatus for projecting multiple images using a single light source. 
     BACKGROUND OF THE INVENTION 
     Seamless integration of images from multiple projectors has many applications in the simulation, visualization and virtual reality (VR) fields. The images are typically generated by computers, specialty cameras, or specially prepared media (e.g., multi-channel image generators). Recently, use of such technology has become widespread in various industries and government applications including, without limitation, training and simulation, oil/gas, automotive, aerospace, education, as well as command and control. 
     Pre-formatted output of a multi-channel image generator is typically sent to several video projectors that are carefully arranged and aligned to a geometry that precisely matches the output format of the generators. In order for this arrayed display to create an image that appears perfectly contiguous, a number of factors should generally be managed: 
     1) The geometry and alignment of the projectors should typically be precisely matched with the output geometry of the multi-channel image generator; 
     2) The color balance of the multiple projectors should typically be precisely matched so that the images from these projectors have matching color balances; and 
     3) Adjacent images should typically be edge blended to generate a seamless projected composite image. 
     A method typically used to generate a seamless composite image from multiple projectors is to overlap a portion of the images and then using smoothing, correction or ramping factors on each side of the overlap, to blend the brightness of the overlapping images together so that they appear uniform. The correction or smoothing factors may also be used to boost minimum black levels of non-overlapped regions to match the minimum black level of the overlapped region without affecting rest of the signal levels in the non-overlapped regions. 
     Examples of making a seamless composite image from multiple projectors are described in U.S. Pat. No. 4,974,073 entitled “Seamless Video Display,” U.S. Pat. No. 5,136,390 entitled “Adjustable Multiple Image Display Smoothing Method and Apparatus,” U.S. Pat. No. 6,115,022 entitled “Method and Apparatus for Adjusting Multiple Projected Raster Images and U.S. patent application Ser. No. 09/876,513 entitled “Method and Apparatus for Seamless Integration of Multiple Video Projectors” filed Jun. 6, 2001, the contents of all of which are incorporated by reference herein. 
     The projector technologies used for these arrayed or tiled displays have changed in the past few years from principally cathode ray tube (CRT) projectors to various formats of display engines that include Liquid Crystal Display (LCD), Image Light Amplification (ILA—Hughes/JVC), Digital Light Projection (DLP—Texas Instruments) L-Cos (Liquid Crystal on Silicon) and others, which may be referred to as digital projectors. 
     Digital projectors typically use some form of variable reflectivity or translucence to form images and project the images by applying a constant light source to the images having variable reflectivity or translucence. This is in contrast to CRT, which uses variable luminance of the cathode ray tube as opposed to variable reflectivity or translucence. 
     Maintaining all segments of a projected composite image to have the same precise color balance to create a contiguous arrayed display has been a challenge with CRT in that each red, blue and green phosphor tube typically should be carefully adjusted to behave the same on each projector. Additionally, with usage and component aging, the color balances often drift and change, thus requiring constant maintenance. 
     CRT projectors allow warping of images by changing the scanning path of the electron beam that is exciting the phosphors to glow. This allows adjustment of the display geometry to curves, off-axis presentation and fine tuning separate red, blue and green phosphor tubes to another, both for single full color images and for multiple images, which may include overlapping segments. However, since the CRT projectors are typically large and have three large heavy lenses for each of red, green and blue image channels, setting up and maintaining geometric alignment of multiple CRT projectors have been a daunting task. Therefore, an additional challenge with CRT arrays has been the setup and maintenance of geometric alignment of the images relative to one another. 
     Therefore, it is desirable to provide a digital projector that overcomes one or more challenges associated with color balancing and geometric alignment of CRT projectors, while allowing adjustment to match projection geometries. 
     SUMMARY 
     In an embodiment according to the present invention, a projection system is provided. The projection system comprises an image source, a light source and a beam splitter. The image source is used for generating a plurality of images to be projected. The light source is used for providing light to be used for projecting the images. The beam splitter is used for splitting the light from the light source to apply a corresponding portion of the light to each of the images. 
     In another embodiment according to the present invention, a method of projecting a plurality of images is provided. The plurality of images and light for applying to the images are generated. The light is split into light portions, and the images are projected by applying the light portions to the images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention can be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings, which are briefly described below. 
     FIG. 1 is a plain view diagram of an arrayed projection system in an embodiment according to the present invention; 
     FIG. 2 illustrates a side view of an imaging head similar to the one used in the arrayed projection system of FIG. 1; 
     FIG. 3 illustrates a bottom view of an imaging head similar to the one used in the arrayed projection system of FIG. 1; 
     FIG. 4 illustrates a sectional view of a digital projector mounted on a ceiling; 
     FIG. 5 illustrates 2×2 array configuration of imaging heads; 
     FIG. 6 illustrates a narrow angle array configuration of imaging heads; 
     FIG. 7 illustrates a same image plane array configuration of imaging heads; 
     FIG. 8 illustrates a wide angle array configuration of imaging heads; 
     FIG. 9 illustrates a plain view of an arrayed projection system in an embodiment according to the present invention; 
     FIG. 10 illustrates an electronic image warping in an embodiment according to the present invention; and 
     FIG. 11 illustrates an optical image warping using a special lens in an embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In an embodiment according to the present invention, an arrayed projection system suitable for projecting multiple images in arrayed or tiled configuration is provided. The arrayed projection system in this embodiment of the present invention preferably provides simpler setup, lower maintenance, greater flexibility, and other enhanced features. 
     FIG. 1 is a plain view diagram of an arrayed projection system  100  in an embodiment according to the present invention. The arrayed projection system  100  may also be referred to as a digital projection system or a digital projector. The arrayed projection system  100  includes a lamp housing  102 . The lamp housing  102  preferably includes a single light source (e.g., lamp), which preferably has high brightness. The lamp housing  102  may also include a power supply and color temperature control. 
     The arrayed projection system  100  preferably also includes a cooling system  104  for the lamp housing. The cooling system  104  preferably receives cool air, cools the lamp housing  102 , and outputs heated air through a vent. The arrayed projection system  100  preferably also includes a beam splitter  106  for splitting the light from the light source in the lamp housing  102  into two or more parts, each of which may be referred to as a light channel or a light portion. For splitting this light, any commercially available beam splitter or any other suitable method and apparatus known to those skilled in the art may be used. 
     The arrayed projection system  100  preferably also includes two or more light guides  108   a ,  108   b ,  108   c , which may include any commercially available light tubes, fiber optic cables or any other suitable apparatus and methods for guiding the light channels known to those skilled in the art. The light guides  108   a ,  108   b  and  108   c  preferably are used to send the light channels, respectively, to corresponding projection image heads  110   a ,  110   b  and  110   c , which may also be referred to as imaging heads. 
     Each imaging head may comprise a digital projector (without light source) such as, for example, liquid crystal display (LCD), image light amplification (ILA), digital light projection (DLP) or liquid crystal on silicon (L-Cos). It should be noted that, even though the imaging heads  108   a ,  108   b  and  108   c  may not include power supplies or light sources typical to conventional projectors, they may still be referred to as projectors, digital projectors or as any other term used by those skilled in the art to refer to a projection device. 
     The arrayed projection system  100  may be coupled to an image source (not shown) for generating the images to be projected. The image source may comprise a computer, a specialty camera and/or specially prepared media, and may generate multiple partially overlapped images for creation of a projected composite image. In other embodiments, each imaging head  110   a ,  110   b  or  110   c  may receive input from a separate image source. In still other embodiments, one or more imaging heads  110   a ,  110   b  and  110   c  may include an internal image source. 
     The imaging heads  110   a ,  110   b  and  110   c  preferably include projection lenses  112   a ,  112   b  and  112   c , respectively. Each projection lens, for example, preferably functions as an optical system for the corresponding imaging head to meet the display geometry requirements. 
     The arrayed projection system  100  as shown in FIG. 1 only has three light guides, three imaging heads and three projection lenses for illustrative purposes only. Those skilled in the art, of course, would appreciate that the present invention may actually include different quantity of each of these components as well as other components not shown in FIG.  1 . 
     The imaging heads preferably have reduced size and weight compared to a conventional digital projector having an internal light source, and preferably are arrayed in an overlap or adjacent geometry. For example in FIG. 1, projected images  114   a ,  114   b  and  114   c  from the imaging heads  110   a ,  110   b  and  110   c , respectively, are overlapped with one another. Further, the imaging heads may not include their own power supplies for internal electronics, with power being supplied by a common power supply located at the lamp housing. For example, each imaging head may only include a small electronics package, including the imaging chips, light valves or LCDs and lens protruding into the work environment. 
     Due to smaller size and weight of the imaging heads, it may be easier to design a mechanical alignment system to quickly and easily adjust the projection geometry of each array element. Once aligned, only severe mechanical interference preferably would alter the display geometry. For example, for suitable alignment of the imaging heads for various different applications, each imaging head preferably includes a mechanical alignment system, an example of which is illustrated in FIGS. 2 and 3. FIG. 2 illustrates a side view  120  of an imaging head, which may be similar to the imaging head  110   a ,  110   b  or  110   c  of FIG.  1 . As can been seen in FIG. 2, the imaging head includes a pivotal bolt  122  for rotatably adjusting the imaging head and a lock bolt for fixing the configuration at a particular rotation. 
     FIG. 3 illustrates a bottom view  130  of an imaging head, which may be similar to the imaging head  110   a ,  110   b  or  110   c  of FIG.  1 . As can be seen in the bottom view  130 , the imaging head includes a roll adjust knob  132  for adjusting roll of the imaging head, a side-to-side adjust knob  134  for sideways translational adjustment of the imaging head, a yaw adjust knob  136  for adjusting yaw of the imaging head, and a front-to-back adjust knob  138  for front-and-back translational adjustment of the imaging head. Of course, other adjustment knobs and bolts may be available for imaging head alignment in other embodiments. 
     Returning now to FIG. 1, by using a single light source (e.g., a high brightness lamp) in the common lamp housing  102  and sending portions of the light to the imaging heads  110   a ,  110   b  and  110   c , respectively, the challenge associated with color balance maintenance (or colorimetry) may be reduced or eliminated. Once the color reproduction characteristics of each imaging head in an array has been matched during the setup of the system, the effects of aging (e.g., changes in color temperature) and changes in brightness from use of the display preferably is evenly propagated to each of the imaging heads  110   a ,  110   b  and  110   c.    
     Although the accuracy of the color temperature relative to accurate reproduction of the source images preferably is controlled at the lamp housing using the color temperature control, the relative color of each imaging head preferably is constant and preferably does not require additional maintenance as the lamp ages or when replaced with a new one. Thus, using this method, the relative color of each imaging head preferably tracks with the overall color temperature changes of the single light source in the lamp housing  102 . 
     Use of a single lamp housing (including the light source) in an arrayed display device preferably also provides an ability to remotely control that lamp housing. In other words, for example, the lamp housing (including power supplies) may be located above a drop ceiling of a standard office facility, thus creating less intrusion into the actual workspace, reducing the fan noise of the projection device, and allowing for direct venting of substantial heat generated by the light source (e.g., lamp). 
     For example, FIG. 4 illustrates a sectional view of a digital projector  200  mounted on a ceiling. The digital projector  200  comprises a lamp housing  202 , a cooling system  204 , a beam splitter  206 , a light guide  208 , an imaging head  210  and a projection lens  212 . The lamp housing  202  preferably includes a high brightness light source, and may also include a power supply and color temperature control. 
     The digital projector  200  as shown only includes a single light guide  208  and a single imaging head  210 . However, the digital projector  200  preferably also includes additional light guides and imaging heads (not shown). For example, the digital projector  200  may be similar to the arrayed projection system  100  of FIG. 1 except that bulk of the components are installed above a ceiling line  201 . For example, the lamp housing  202 , the cooling system  204 , the beam splitter  206 , and a major portion of the light guide  208  are disposed above the ceiling line  201  in FIG. 4, while the imaging head  210  and the projection lens  212  are disposed below the ceiling line  201 . 
     When the digital projector  200  includes multiple light guides and imaging heads, the beam splitter  206  coupled to the lamp housing  202  preferably sends multiple light channels or light portions to the imaging heads (including the imaging head  210 ), e.g., via the light guides (including the light guide  208 ). The requisite technologies to accomplish distribution of light are well known to those skilled in the art, and a number of commercially available beam splitters, light tubes and/or fiber optic cables may be used for this light distribution. 
     Small size and weight of the imaging heads may afford increased flexibility in organizing groups of imaging heads into useful geometries as illustrated in FIGS. 5,  6 ,  7  and  8 . FIG. 5 illustrates a front view  222  and a top view  224  of four imaging heads arrayed in a 2×2 configuration. FIG. 6 illustrates a front view  232  and a top view  234  of four imaging heads arrayed in a single row, narrow angle configuration. FIG. 7 illustrates a front view  242  and a top view  244  of three imaging heads arrayed in a single row, same image plane configuration. FIG. 8 illustrates a front view  252  and a top view  254  of three imaging heads arrayed in a single row, wide angle configuration. The imaging heads having narrow and wide angle configurations may be particularly useful when projecting images onto a curved screen. 
     The imaging heads in various different embodiments of the present invention may be compatible with a number of specialty lenses and optical systems that should allow the arraying of images to various geometries such as flat displays, cylindrical displays and compound curve displays, whether front projected, rear projected or using folded light paths. An example of this would be a lens for a cylindrical display as illustrated in FIG.  9 . 
     FIG. 9 illustrates an arrayed projection system  300  in an embodiment according to the present invention. The arrayed projection system  300  may also be referred to as a digital projector or as a digital projection system. The arrayed projection system  300  includes a lamp housing  302 . The lamp housing  302  preferably includes a single high brightness light source. The lamp housing  302  may also include a power supply and color temperature control. 
     The arrayed projection system  300  preferably also includes a cooling system  304  for the lamp housing. The cooling system  304  preferably receives cool air, cools the lamp housing  302 , and outputs heated air through a vent. The arrayed projection system  300  preferably also includes a beam splitter  306  for splitting the light from the light source in the lamp housing  302  into two or more parts, each of which may be referred to as a light channel or a light portion. For splitting this light, any commercially available beam splitter or any other suitable method and apparatus known to those skilled in the art may be used. 
     The arrayed projection system  300  preferably also includes two or more light guides  308   a ,  308   b ,  308   c , which may include any commercially available light tubes, fiber optic cables or any other suitable apparatus and methods for guiding the light channels known to those skilled in the art. The light guides  308   a ,  308   b  and  308   c  preferably are used to send the light channels, respectively, to corresponding projection image heads  310   a ,  310   b  and  310   c , which may also be referred to as an imaging head. 
     Each imaging head preferably includes a projection lens  312   a ,  312   b  or  312   c . Each projection lens, for example, functions as an optical system for the corresponding imaging head to match the requirements of the display geometry, in this case, a cylindrical geometry. The arrayed projection system  300  shown in FIG. 9 includes three light guides, three imaging heads and three projection lenses for illustrative purposes only. Those skilled in the art, of course, would appreciate that the present invention may actually include different quantity of each of these components. 
     Unlike CRT projectors where the raster of the image can be warped to match the desired projection geometries, even for cylinders and compound curves, most current digital imagining projection technologies are based on fixed grid geometries. FIG. 10 illustrates one method that may be used to overcome this limitation for digital projectors, in which an electronic image warping or image mapping is used. In this application, the fixed geometry projection  330  preferably is distorted by an image processor  332  based on the projection geometry. The image processor  332  preferably then re-maps the image on the raster to create the desired geometrical result  334 . The mapped image  334  is then projected by a digital projector  336 , for example, on a curved screen  338 . 
     The electronic image warping, for example, may be used with the arrayed projection system  300  of FIG. 9, instead of or in addition to the special lenses  312   a ,  312   b  and  312   c , to match the cylindrical projection geometry. When this approach is used however, during the re-mapping, some available pixels of information may be discarded or thrown away because they do not fall into the desired geometry. 
     The image distortion may also be achieved using an optical lensing solution that is designed specifically for the desired geometry, such as in the arrayed projection system  300  of FIG.  9 . For example, in FIG. 11, the projection image  330  preferably is projected by a digital projector having corrective lens optics  352  to generate an image  354  with optical distortion. Then the distorted image may be projected onto a curved screen  356 . All pixels can be used to render the image in this case. 
     Accordingly, the present invention provides an improved projection system for arrayed or tiled display. Although this invention has been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that this invention may be practiced otherwise than as specifically described. Thus, the present embodiments of the invention should be considered in all respects as illustrative and not restrictive, the scope of the invention to be determined by the appended claims and their equivalents rather than the foregoing description.