Abstract:
An image projection system includes an image generator, a screen and a focus and aim component. The focus and aim component receives modulated light from the image generator and focuses and deflects the light as a function of a position on the screen. The component consists of an array of lenslets with wedge. The wedge or prism refracts the light and the lens focuses the light. As a result, the optics are selectively tailored to provide a clear image over the entire field of view, even in systems compact in depth that would otherwise rely on a short focal length lens. Acute incidence angles at the periphery of the screen are handled by TIR Fresnel lens elements or by diffractive elements. The focus and aim capability is alternatively provided by holographic elements in a holographic array.

Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application No. 60/143,058, entitled “Compact Rear Projection System Based upon a Curved Turning Mirror and Anamorphic Projection” filed Jul. 9, 1999, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to rear projection systems, and more particularly to compact rear projection systems that use separate optics for various areas of a viewing screen. 
     BACKGROUND OF THE INVENTION 
     Rear projection imaging systems typically include an image generation source, optics to enlarge and direct the image and a transmission screen for displaying the enlarged image. The image source can be of many different types, including cathode-ray tubes and liquid crystal displays (LCDs). In simple systems, the optics generally include a lens, such as a combined convex glass lens element and a methacrylic resin lens element, and a turning mirror for directing the image toward the screen. The transmission screens of typical systems generally include diffusing material, lenticular lens sheets and Fresnel lens sheets, which are intended to project a wide image with uniform brightness. 
     In operation, the image source is positioned behind the transmission screen and provides a small, bright image to the projecting lens. The projecting lens enlarges the image and directs it to the reflective surface of the turning mirror. The turning mirror reflects the image to the transmission screen. The lens sheets in the transmission screen further enlarge the image and collimate the projected light. The audience views the projected image from the transmission screen. 
     The depth dimension of known rear projection systems is constrained by the angle of incidence on and within the transmission screen&#39;s Fresnel lens. To make a compact rear projection package, a short focal length lens is required. A decreasing focal length increases the field of view as measured at the screen. As the field of view increases, the angles of incidence in air and within the Fresnel lens eventually approach the critical angle, causing transmission to drop to zero. Even before the angle of incidence reaches the critical angle, the angle of incidence will exceed the Brewster angle. Exceeding the Brewster angle can cause the S (perpendicular) and P (parallel) polarization transmission coefficients to diverge, resulting in image distortions, such as non-uniformity in brightness across the screen. 
     SUMMARY OF THE INVENTION 
     A projection system according to the principles of the invention tailors optics to various regions of a viewing screen. In one aspect of the invention, an image projection system includes an image generator, a viewing screen and a focus and aim component (FAC). The FAC is disposed to receive light corresponding to portions of the image from the image generator. The FAC deflects and focuses the light as a function of a position on the screen. An exemplary FAC includes a light-focusing device, such as a lens, and a light-deflecting device, such as a prism. The devices are disposed to receive a light beam in tandem representative of an image portion, such as a pixel. The light-deflecting device aims, and the light-focusing device focuses, the beam as a function of spot position on a screen. Another example of a light valve is a digital micro mirror device (DMD) or a digital light processor (DLP). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 provides perspective views of a focus and aim component. 
     FIG. 2 shows a side view of a focus and aim component. 
     FIGS. 3 ( a )-( b ) show, respectively, a side view of a projection system and a magnified screen detail. 
     FIGS. 4 ( a )-( b ) each illustrate respective embodiments of the invention. 
     FIGS. 5 ( a )-( b ) show a first scanning embodiment of the invention. 
     FIGS. 6 ( a )-( b ) show a holographic embodiment of the invention. 
     FIG. 7 shows a second scanning embodiment of the invention. 
     FIGS. 8 ( a )-( b ): show an embodiment of the invention using a color screen. 
     FIGS.  9 (a)-(b) show a second embodiment of the invention using a color screen. 
     FIG. 10 shows an embodiment of the invention in which holographic elements reflect light. 
     FIG. 11 shows a screen design in an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     A projection system according to the principles of the invention provides for decreased cabinet depth without sacrificing image quality due to variable focal lengths over the viewing screen. An exemplary image projection system  301  is shown in FIG.  3 ( a ). In this system  301 , a two-dimensional image generator  100  generates an image for viewing on a viewing screen  106 . The two-dimensional image generator  100  can be, for example, a scanner, a light valve, or some other image projection device. A light valve is an optical switch, such a liquid crystal display (LCD), which switches optical signals to corresponding pixels on a display screen. Another example of a light valve is a digital light modulator (DMD). 
     An optical component, hereafter referred to as a focus and aim component  102  (FAC), receives light from the image generator and focuses it on the screen. As will be explained more fully, the FAC can include multiple tandems of light focusing and light aiming devices. These tandem devices are optically responsive to selected portions of the image generated by the image generator  100 , and focus the portions on corresponding spot locations on the screen  106 . 
     In FIG.  3 ( a ), three exemplary beams  202 ,  204  and  206  output from the FAC  102  are shown. These beams strike the screen  106  at three different locations. A first beam  202  strikes the viewing screen  106  at an upper portion and at a relatively acute incident angle  103 . A second beam  206  strikes the screen  106  in a lower portion and at a less acute incident angle  105 . The third illustrated beam  204  strikes the screen  106  in a middle portion and at an intermediate incident angle  107 . The optical path lengths for these beams  202 ,  204  and  206  from the FAC  102  to the screen  106  differ. The FAC  102  includes focusing devices, such as lenses, to accommodate the differing optical path lengths and resulting different focal lengths. 
     The screen  106  in this system  301  includes a total-internal-reflection (TIR) Fresnel lens. 
     A screen detail  210  shown in FIG.  3 ( b ) shows a ray trace for one beam  202  as incident to the screen  106 . The screen  106  has an incident surface  212 . Fixed to the surface  212  are the TIR Fresnel lens elements  214  and  216 . The Fresnel lens elements receive from the FAC  102  projected light as illustrated by the incident rays  222  and  223 , respectively. One ray  222  is incident at the surface  212  at an angle that exceeds the critical angle, causing the ray  222  to reflect totally from the surface  212 , as shown by the reflected ray  224 . The reflected ray  224  then reflects off a reflective facet  220 , which collimates the light  226  and directs the light to the screen  106 . The collimated ray  226  is incident at the surface  212  at an angle less than the critical angle, and thus passes through the screen  106 . 
     Similarly, the other illustrative TIR Fresnel lens element  216  receives light from the FAC, as shown by a second ray  223 , and outputs a collimated ray  227  that is parallel to the collimated ray  226  output by the other TIR element  218  in the detail  210 . A plurality of collimated rays corresponding to the beam  202  form a portion of an image, or a “spot,” on the screen  106  at a spot position  213 . The spot at the spot position  213  can be a portion of an image that corresponds to a pixel. The image generator  100  modulates the plurality of light rays corresponding to the pixel by an image value that is characteristic of that portion. The image value may describe, for example, light intensity or color. 
     A perspective view of the image generator  100 , FAC  102  and screen  106  is shown in FIG.  1 . The screen  106  has multiple spot positions for multiple incoming beams  108 . The multiple incoming beams  108  striking their corresponding spots form a composite image  109  on the screen  106 . Each of the multiple incoming beams  108  is aimed and focused by the FAC  102  to a corresponding spot on the screen  106 . In this manner, the image maintains uniform focus across the screen  106 . As in the system of FIG.  3 ( a ), a TIR Fresnel lens (not shown) accommodates incident angle exceeding the critical angle. 
     FIG. 2 shows a side view of an FAC  102  according to the principles of the invention, such as the FAC  102  shown in FIGS.  1  and  3 ( a ). The FAC  102  is responsive to light from a group of pixels  300 . Light represented by rays  307 ,  315  and  317  corresponding to respective pixels  302 ,  304  and  306  passes to prisms  308 ,  310  and  312  on the FAC  102 . The prisms are light-deflecting devices that direct or aim the beams  307 ,  315  and  317  to corresponding spot locations on a screen (not shown). The beams pass respectively through lenses  314 ,  316  and  318 , which are light-focusing devices. The lenses  309 ,  311  and  313  have focal lengths tailored to the particular location on the screen (not shown). The FAC  102  has a plurality of FAC elements that form an array. The array elements each correspond both to a pixel of an image generator and to a screen location. 
     In one implementation of an image projection system, the image generator includes an LCD. The display screen of the LCD is placed in close proximity to the FAC so that each pixel in the display screen sends light to a corresponding FAC element, which aims and focuses the light to form a corresponding spot on a viewing screen. FIG.  4 ( a ) illustrates an image projection system  617  having an image generator  618  that consists of a laser  612 , a beam expander  614  and a light valve  616 . This embodiment uses a laser, but can also implemented instead with another light source, such as a metal halide lamp. The laser  612  emits a beam  619 , which the laser has modulated so that the beam  619  carries image information. The beam expander  614  expands the laser beam  619  and forwards it to the light valve  616 . The light valve  616  has a light valve pixel array  615 , which the light valve  616  operates based upon the received expanded beam  619 . Each pixel in the light valve pixel array  615  sends light to a corresponding FAC element of an FAC  510 . The FAC elements each focus and aim the received light onto a viewing screen  406 . 
     In FIG.  4 ( b ), an image projection system  618  operates according to the same principles as the embodiment in FIG.  4 ( a ), except that here the image generator and FAC are rotated by  90  degrees. An image generator  611  includes a laser  612 , a beam expander  614  and a light valve  616 . As in the previous embodiment, the light valve  616  illuminates an FAC  510 , which in turn illuminates a viewing screen  406 . The FIG.  4 ( b ) embodiment affords a cabinet design of slimmer depth than does the embodiment in FIG.  4 ( a ). 
     A scanning projection system  320  according to the principles of the invention is shown in FIG.  5 ( a ). This scanning projection system  320  is another implementation of the image projection system  301  discussed above. In this implementation, the image generator  321  is a scanner that consists of a laser  324  and a rotating prism  326 . The laser  324  modulates and projects a laser beam  330  that reflects off a facet  328  of the rotating prism  326  and arrives at the FAC  322 . A focus and aim device  342  on the FAC  322  focuses and deflects the beam  330  to a spot  332  on the viewing screen  338 . The laser  324  translates normal to the page in FIG.  5 ( a ), or, alternatively, rotates in a direction normal to the page. This scans the beam  330  across a row  340  of FAC elements, as shown in FIG.  5 ( b ). A corresponding row of spots (not shown) is traversed on the screen  338  during this scan. A next row of FAC elements is similarly scanned as the rotating prism rotates so that the beam  330  is directed to that next row. In this way, the entire FAC  322  is traversed during scanning, delivering an entire corresponding array of spots to the screen  338 . The laser  324  pulses the beam  330  in correspondence with the discrete locations of FAC elements in the FAC  322 . Each pulse corresponds to a direction at which the beam  330  leaves the laser  324 . Those directions are known here as addressable light directions of the scanner  321 . The addressable light directions in this embodiment serve the same purpose as do the LCD pixels of the previous embodiment. There, each pixel corresponds with an FAC element. Here, each addressable light direction corresponds with an FAC element. 
     A projection system  430  according to the principles of the invention can also be implemented with a holographic element array (HEA) operating as a FAC. In FIG.  6 ( a ), a scanning projection system  430  using a HEA  510  as the FAC is shown. The system  430  includes a laser  504 , a vertical scanner  506 , a horizontal scanner  508 , and the HEA or holographic “memory”  510 . The array  510  is arranged in a holographic array configuration  510  composed of holographic array elements  502 , as shown in FIG.  6 ( b ). The elements  502  are arranged in locations corresponding to horizontal lines and vertical rows. The elements  502  have a diffractive holographic grating with light-focusing means and light-deflecting means to focus an incoming beam on a corresponding spot location of a viewing screen. 
     The laser  504  modulates and outputs a light beam  503  to the vertical scanner  506 . The vertical scanner  506  directs the beam  503  in accordance with a desired vertical location on the HEA  510 . The horizontal scanner  508  also acts on the beam  503  to direct it to a desired horizontal location on the array  510 . The beam is then directed to the corresponding holographic element  502  which focuses the beam  503  to a spot  512  on the viewing screen  406 . The viewing screen  406  has a plurality of spot locations  514 , each corresponding to an element  502  that acts to form the image spot  512  on the screen  406 . As the system  430  scans the HEA, an image is projected to the screen  406 . 
     FIG. 7 shows another scanning projection system  601  including a laser line array  600 , a vertical scanner  604  and a HEA  510 . The laser line array  600  further includes a line of lasers  602 , each laser of the line  602  corresponding to a pixel of information. The lasers  602  modulate and project laser light to the vertical scanner  604 , which vertically deflects the light to a line of holographic elements of the holographic array element  510 . The line of holographic elements projects a beam line array  606  to form a spot line array  608  on the viewing screen  406 . 
     An HEA may be used to implement an FAC in the scanning projection system  320 , shown in FIG.  5 . Moreover, an HEA can also replace an FAC in the non-scanning image projection systems  617  and  618  in FIGS.  4 ( a )-( b ). In that case, the FIG.  4 ( b ) configuration can offer an additional advantage, beyond slimming cabinet depth. As the viewing screen  406  is brought closer to the FAC  510  in FIG.  4 ( a ), the angle at which light leaves the top of the HEA (since the FAC is implemented as an HEA) becomes more acute. The angle may become acute enough to interfere with diffraction from HEA elements at the top of the HEA. Rotating the configuration, as in FIG.  4 ( b ) lessens the acuteness of the angle, so that diffraction operates correctly even with slim cabinet depth. As an alternative, rotation of an image generator may be lessened or eliminated by interposing a folding mirror between the generator and the HEA. The folding mirror deflects light from the generator to the HEA, in lieu of rotating the generator. As a further alternative, if the pixel size of a light valve is too small, a projection lens can be interposed between the light valve and the HEA. The beam  1044  arrives at a holographic array  1042  and continues on to a viewing screen (not shown). 
     An first embodiment of the invention using a color screen is shown in FIG.  8 ( a ). A color projection system  1400  includes a red light valve  1402 , a green light valve  1404 , a blue light valve  1404  and a viewing screen  1408 . The red light valve  1402  consists of an LCD  1460 , a phase screen  1414  and an HEA  1424 . A phase screen is a screen having areas of variable thickness so that the index of refraction is a function of position. Here, three phase screens are used to converge correspondingly differently colored light projected for a single pixel. The phase screen  1414  serves as a screen for the LCD  1460 . The phase screen  1414  has a LCD array  1470  that defines each pixel  1412  of the LCD  1460 . The HEA  1424  is “printed” on the phase screen  1414 , as by lamination or by other techniques. As a result, each holographic element of the HEA  1424  is incorporated in a corresponding pixel  1412  of the LCD  1460 . Similarly, for the green light valve  1404 , an HEA  1424  is combined with a phase screen  1420  on an LCD  1462 . Also, for the blue light valve  1406 , an HEA  1426  is combined with a phase screen  1422  on an LCD  1464 . 
     Operationally, for the red light valve  1402 , the phase screen  1414  acts in tandem with the HEA  1424  to direct the light beam  1440  from the pixel  1412  to a spot location  1410  on the viewing screen  1408 . Similarly, for the green light valve  1404 , the phase screen  1420  acts in tandem with the HEA  1424  to direct the light beam  1444  from the pixel  1416  to the same spot location  1410  on the viewing screen  1408 . Also, for the blue light valve  1406 , the phase screen  1422  acts in tandem with the HEA  1426  to direct the light beam  1448  from the pixel  1418  to the same spot location  1410  on the viewing screen  1408 . Accordingly, each of the phase screens acts in tandem with its corresponding HEA to converge the light beam for a given pixel. An analogous architecture holds for the spot location  1430  on the viewing screen  1408 . The red light valve  1402  projects a light beam  1442  to the spot location  1430 , the green light valve projects the light beam  1446  to the same spot location  1430  and the blue light valve projects the light beam  1450  to the same spot location  1430 . In similar fashion, for a plurality of other screen locations, light from corresponding pixels  1412 ,  1416  and  1418  converge to a corresponding spot location of the viewing screen  1408 . 
     FIG.  8 ( b ) illustrates a second embodiment of the invention using a color screen. A color projection system  1300  includes a red light valve  1302 , green light valve  1304  and blue light valve  1306 , a volume hologram or volume HEA  1308 , and a viewing screen  1310 . The red light valve  1302  consists of an LCD  1340  and a phase screen  1342 . Each element of the volume HEA has a volumetric grating that superposes incident light beams of different colors to focus and aim the superposed beam to a desired screen location. As in the previous embodiment, the phase screen  1342  serves as a screen for the LCD  1340 . The phase screen  1342  has an LCD array  1350  that defines each pixel  1350  of the LCD  1340 . Similarly, the green light valve  1304  consists of an LCD  1346  and a phase screen  1348 , which serves as screen for the LCD  1346 . Also, the blue light valve  1306  consists of an LCD  1352  and a phase screen  1354 , which serves as a screen for the LCD  1352 . As with the red light valve  1302 , both the green light valve  1304  and the blue light valve  1306  have their respective LCD arrays that define corresponding pixels  1359  and  1361 . 
     Operationally, for the red light valve  1302 , a light beam  1320  from a pixel  1350  is projected from the phase screen  1342 . The light beam  1320  arrives at a volume HEA  1312  of the volume hologram  1308 . The green light valve  1304  and blue light valve  1306  similarly project their corresponding light beams  1322  and  1324  from corresponding pixels  1359  and  1361  to the volume HEA  1312 . Element  1312  superposes the three differently colored light beams  1320 ,  1322  and  1324  to form a light beam  1330  that arrives at the viewing screen  1310  at a spot location  1316 . The phase screens  1342   1348  and  1354  are each configured differently to converge the three beam  1320 ,  1322  and  1324  to a common volume HEA  1312 . In an analogous way, light beams  1326 ,  1328  and  1331  are received from light valves  1302 ,  1304  and  1306  by a volume HEA  1314  of the volume hologram  1308 . The three light beams  1326 ,  1328  and  1331  are superposed to form a light beam  1332  that arrives at a spot location  1318  of the viewing screen  1310 . A plurality of other red, green and blue light beams, in similar fashion, are superposed by the volume hologram  1308  and directed to the viewing screen  1310 . 
     FIG.  9 ( a ) illustrates a third embodiment of the invention using a color screen illuminated by an infrared (IR) laser. A color projection system  1200  includes a scanning IR laser  1202 , an HEA  1204  and a viewing screen  1210 . The viewing screen  1210  includes a plurality of triads  1212 . As shown in FIG.  9 ( b ), each triad  1212  consists of a red triad section  1214 , a green triad section  1216  and a blue triad section  1218 , which are correspondingly coated with up converting phosphors  1220 ,  1222  and  1224 . The laser  1202  modulates and projects a laser beam  1206  that arrives at a holographic element  1208  of the HEA  1204 . The holographic element  1208  has been pre-configured to focus and aim the laser beam  1206  onto a specific one the sections  1214 ,  1216  or  1218  of the triad  1212 . All elements of the HEA  1204  are similarly configured to focus and aim on a corresponding triad section. As the laser  1202  scans the HEA  1204 , a color image is created on the viewing screen  1210 . 
     FIG. 10 shows an embodiment of the invention in which an HEA reflects incident light. A scanner  900  comprises an image generator  904 , a first rotating drum  906  and a second rotating drum  908 . The image generator  904  projects a modulated laser beam  910  that the first rotating drum  906  receives. The second rotating drum  908  has a plurality of tracks, that each correspond with a screen line. The track  912  has an HEA  914 . The modulated laser beam  910  is received by element  914 , which deflects and focuses the beam to form a spot  916  on screen  406  (the screen is not shown). Rotation of drum  908  scans spot  916  to trace a screen line  902 . The next screen line is traced by reflection of a beam from the next track of drum  908 , and so on. 
     In the above embodiments of the invention, the screen was disclosed to have Fresnel lens elements to collimate incoming beams at acute incidence angles. A diffractive element, as shown in FIG. 11, serves the function of the Fresnel lens elements  214  or  216  in FIG.  3 ( b ). An incident beam  345  arrives at a diffractive element  347  of a screen  346  and is diffracted to yield a collimated beam  348 . An advantage of the diffractive screen is that the holographic pattern may be very well embossed upon a thin plastic sheet. An extremely low-cost screen is realized by binding this sheet to a sturdier piece and using a diffuser or filter. 
     The examples given herein are presented to enable those skilled in the art to more clearly understand and practice the invention. The examples should not be considered as limitations upon the scope of the invention, but as merely illustrative. Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description.