Abstract:
The invention relates to a light valve system that enhances the contrast ratio for light and dark video images and reduces contouring artifacts. The light valve system comprises a color selection device configured to temporally attenuate component color bands of light to correspond with a video input signal. A first polarizing beam splitter configured to polarize the component color bands into oppositely polarized components, and a microdisplay configured to receive at least one of the oppositely polarized components for forming a projected light matrix.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit, under 35 U.S.C. § 365 of International Application PCT/US03/37768, filed Nov. 26, 2003, which was published in accordance with PCT Article 21(2) on Jun. 17, 2004 in English and which claims the benefit of U.S. provisional patent application No. 60/430,818, filed Dec. 4, 2002. 

   FIELD OF THE INVENTION 
   The invention relates to a light valve system and, more particularly, to a light valve system with a microdisplay for use in a rear projection television. 
   BACKGROUND OF THE INVENTION 
   In a conventional light valve system for, example in displays such as, rear projection televisions (RPTVs), digital cinema, etc., white light output from a lamp is directed to a microdisplay such as a liquid crystal display (LCD), liquid crystal on silicon (LCOS), or digital light processing (DLP) system, through a series of integrating and collimating optics. In the LCD or LCOS systems, white light is separated into its component red, green, and blue (RGB) bands of light, polarized by a polarizing beam splitter (PBS) in the case of LCOS, and directed onto the microdisplay. The microdisplay has a matrix of pixels. The microdisplay operates to modulate each of the pixels of the component RGB bands of incident light by a gray-scale factor control output from a controller based on a video input signal to form a light matrix of discrete modulated light signals or pixels. The light matrix is reflected or output from the microdisplay and directed to a system of projection lenses that projects the modulated light onto a display screen, combining the pixels of light to form a video image. 
   In the DLP system, the white light is separated into its component RGB bands of light, and reflected onto a DLP microdisplay. The microdisplay is a semiconductor device containing an array of hinge-mounted microscopic mirrors. Each of the mirrors corresponds to one pixel in a video image input to the microdisplay. When the semiconductor is driven by the video input signal, the mirrors are tilted or switched on and off to reflect all or some of the incident light. The array of pixels reflected from the mirrors form a light matrix corresponding to the video-input signal. The light matrix is reflected or output from the microdisplay and directed to a projection lens system that projects the modulated light onto a display screen to form a video image. 
   A disadvantage of these display systems is that the video images projected in a dark state scene are inferior in quality to the video images that are projected in a bright state scene. In the LCD or LCOS systems, the difference in quality occurs because the amount of light directed onto the microdisplay remains constant regardless of the brightness of the video image input to the microdisplay. Gray-scale variation from pixel to pixel is thereby limited by the number of bits used to process the video-input signal. Because the video input signal is a fixed number of bits, which corresponds to the full scale of light, there tend to be very few bits available for subtle differences in darker areas of the video image. For example, if the microdisplay is capable of reproducing 1024 gray shades (10-bit output digital to analog converter (DAC)) when the program contains only 0 to 64 gray shades, the net effect is that contrast appears poor and the video image appears to have a severe level of noise and contouring due to quantization effects and truncation effects. The DLP system suffers from more severe contouring effects than the LCOS or LCD systems due to the intrinsically linear response of the semiconductor. 
   To alleviate the differences in quality occurring between the light and dark video images, it is known to increase the contrast of the microdisplay itself. Increasing the contrast of the microdisplay, however, leads to very high data rates, very high resolution DAC&#39;s, and very critical optical and liquid crystal performance requirements. It is, therefore, desirable to develop a light valve system that enhances the contrast ratio for the video images, particularly in dark video images, and reduces contouring artifacts. 
   SUMMARY OF THE INVENTION 
   The invention relates to a light valve system that comprises a color selection device configured to temporally attenuate component color bands of light to correspond with a video input signal. A first polarizing beam splitter configured to polarize the component color bands into oppositely polarized components, and a microdisplay configured to receive at least one of the oppositely polarized components for forming a projected light matrix. 
   The invention further relates to a light valve system that comprises a color selection device configured to temporally separate light into its component color bands to correspond with a video input signal. A first polarizing beam splitter configured to polarize the component color bands into a first set of oppositely polarized components. First and second liquid crystal displays. Each of the first and second liquid crystal displays configured to receive one of the first set of oppositely polarized components for forming first and second light matrices, respectively. A second polarizing beam splitter configured to receive the first and second light matrices for separating the first and second light matrices into a second set of oppositely polarized components, and a microdisplay configured to receive at least one of the second set of oppositely polarized components for forming a projected light matrix. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the following figures, wherein: 
       FIG. 1  is block diagram of a light valve system according to a first embodiment of the invention; 
       FIG. 2  is block diagram of a light valve system according to a second embodiment of the invention; and 
       FIG. 3  is a schematic diagram of a polarizing beam splitter arrangement for use in the system of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a light valve system  1  according to a first embodiment of the invention. The system  1  includes a lamp  10 . The lamp  10  generates white light  4  and projects the white light  4  toward a set of illumination optics  11 . The illumination optics  11  may include, for example, a polarizer and/or an integrator. In this embodiment a polarizer is included to rotate incident light to an s-polarization. The illumination optics  11  directs a telecentric beam of the white s-polarized light  4  toward a color selection device  5 . In the illustrated embodiment, the color selection device  5  is a color switching device, which is an optical device having several layers of liquid crystal displays stacked together. Examples of such a color selection device  5  include the COLORSWITCH® made by ColorLink, Inc. of Boulder, Colo. and the Application Specific Integrated Lens (ASIL) made by DigiLens, Inc. of Sunnyvale, Calif. The white light  4  enters the color selection device  5 , and the color selection device  5  temporally filters the white light  4  incident thereon into sequential component red, green, and blue (RGB) bands of light  12 . A selected band of light is transmitted or reflected depending on a digital control signal voltage applied to the color selection device by a display controller  3 . The color selection device  5  also has an on/off state voltage input for receiving a control signal from the display controller  3 . When the voltage level from the display controller  3  is high, it drives the color selection device  5  to an off state and when the voltage level from the display controller  3  is low, it drives the color selection device  5  to an on state in which light is transmitted therethrough. The display controller  3 , by virtue of its processing of the video-input signal to the microdisplay  7 , performs analysis on the video signal to determine its content. In this analysis, the display controller  3  analyzes the video-input signal on a pixel-by-pixel basis for the frame to be displayed. If none of the pixel input values exceed half of full scale, then the voltage level controlling attenuation in the color selection device  5  is set at 50% of full scale. If on the other hand the input pixel values are all zero thus indicating a full black screen, the voltage level controlling a color selection device is reduced to the full on state voltage. This attenuation control enhances contrast especially in frames containing mostly dark content. Since ultimate contrast is the product of contrast achieved through the optical components in the system, if for example the color selection device  5  has a contrast of 50:1 and the microdisplay  7  has a contrast of 600:1 then the measured sequential contrast is 30000:1 allowing for improved contrast levels especially in the dark state. 
   The display controller  3  is programmed with the transfer function of the microdisplay  7 . To program the display controller  3  the microdisplay  7  may be calibrated at a factory level or auto-calibrated by photosensors in a cabinet or a projection light path, e.g., behind a folding mirror. Because the calibration may be performed in binary steps, the calibration would take no more than a few seconds and may be performed during normal operation after the video-input signal is known. As a result, the dynamic contrast of the system  1  is improved without the cost of any additional hardware, and a customer has the option of reducing the peak brightness of the video image as she chooses without producing undesired contouring effects. 
   The sequential component RGB bands of light  12  exit the color selection device  5  and are directed toward a polarizing beam splitter  8  (PBS). Incident s-polarized components  19  of the incident light  12  are reflected from the polarizing surface  17  to a third surface  15 . A microdisplay  7  is disposed beyond the third surface  15  of the PBS  8 , and the s-polarized component  19  of the light  12  is incident thereon. 
   In the illustrated embodiment, the microdisplay  7  is a liquid crystal on silicon (LCOS) imager. Alternatively, a liquid crystal display (LCD) may be used and the optical system adjusted accordingly. The LCOS microdisplay  7  serves to modulate incident light with video signal coming from the display controller  3 . Each of the pixels of the projected light matrix  18  has an intensity or luminance proportional to the individual gray scale value provided for that pixel in the microdisplay  7 . As a result of the modulation, the LCOS microdisplay  7  reflects a light matrix  18  comprising a matrix of pixels or discreet dots of p-polarized light back through the third surface  15  of the PBS  8 . The p-polarized components of the projected light matrix  18  pass through the polarizing surface  17  and out of the PBS  8  through a fourth surface  16 . The projected light matrix  18  is directed from the fourth surface  16  to a projection lens system  9 . The projection lens system  9  projects the light matrix  18  onto a display screen  6 , combining the pixels of light to form the video image corresponding to the video input signal  2 . 
     FIG. 2  shows a light valve system  20  according to a second embodiment of the invention. The system  20  includes a lamp  35 . The lamp  35  generates white light  23  and projects the white light  23  toward illumination optics  31 . The illumination optics  31  may include, for example, an integrator, such as, a sequential color recapture (SCR) integrator. The integrator  31  directs a telecentric beam of the white light  23  toward a color selection device  24 . In the illustrated embodiment, the color selection device  24  is a color wheel, which has a disc with fan-shaped sectors uniformly disposed along a circumference of the disk. The sectors filter the white light  23  incident thereon into its component RGB bands of light  25  in a timed sequence corresponding to color wheel rotation. The color selection device  24  is rotated by a motor (not shown) and is controlled by a display controller  22  to transmit corresponding component RGB bands of light  25  in synchronization with a video input signal  21  to transmit the respective component RGB bands of light  25  on a frame-by-frame basis. 
   The component RGB bands of light  25  are directed toward a PBS arrangement  50 . The PBS arrangement  50  includes first and second PBSs  46 ,  49 , first and second mirror prisms  47 ,  48 , and first and second LCDs  26 ,  28 . Alternatively, the first and second LCDs  26 ,  28  may be arranged before the integrator  31 . As shown in  FIG. 3 , the component RGB bands of light  25  enter a first face  42  of the first PBS  46  and are polarized by a first polarizing surface  43  to have an s-polarized component  27  and a p-polarized component  45 . The path of the s-polarized component  27  of the RGB bands of light  25  through the PBS arrangement  50  will first be described in greater detail, and then, the path of the p-polarized  45  component will be described in greater detail. 
   The s-polarized component  27  is reflected through a second face  56  of the first PBS  46  and is received in the first mirror prism  47 . The s-polarized component  27  is reflected by a first mirror surface  59  out of the first mirror prism  47  and toward the first LCD  26 . The first LCD  26  is for example, a single cell panel containing a matrix of liquid cells coupled to an electrical signal from the display controller  22 . The electrical signal controls the LCD  26  to have it either rotate polarization of light passing therethrough or pass the light without rotation. 
   As a result the first LCD  26  transmits a first light matrix  38  comprising a matrix of pixels or discreet dots of light with s-polarized and p-polarized components. The first light matrix  38  enters a first face  44  of the second PBS  49  and is polarized by a second polarizing surface  53 . The s-polarized component (not shown) of the first light matrix  38  is reflected through a second face  57  of the second PBS  49  and is discarded while, the p-polarized component  60  of the first light matrix  38  passes through the second polarizing surface  53  and out of the second PBS  49  through a third face  52  toward illumination lens  33 . 
   The p-polarized component  45  of the component RGB band of light  25  passes through the first polarizing surface  43  and through a third face  51  of the first PBS  46  toward the second LCD  28 . The second LCD  28  is identical to the first LCD  26  in structure and function and, as such, further description thereof has been omitted. The second LCD  26  transmits a second light matrix  55  comprising a matrix of pixels or discreet dots of light with s-polarized and p-polarized components. The second light matrix  55  enters the second mirror prism  48  and is reflected by a second mirror surface  58  out of the second mirror prism  48  and toward the second PBS  49 . The second light matrix  55  enters a fourth face  54  of the second PBS  49  and is polarized by the second polarizing surface  53 . The p-polarized component (not shown) of the second light matrix  55  passes through the second polarizing surface  53  and is discolored through second face  57  of the second PBS  49 . The s-polarized component  61  of the second light matrix  55  is reflected out of the second PBS  49  through the third face  52  and is received in a light stop (not shown) in combination with the s-polarized component  45 , so that there is a fairly low loss of total brightness. 
   As shown in  FIG. 2 , the s-polarized component  61  of the second light matrix  55  and the p-polarized component  60  of the first light matrix  38  are simultaneously focused by illumination lenses  33  into a third mirror prism  34  for high-through-put efficiency. The third mirror prism  34  may be, for example, a total internal reflection (TIR) prism or off axis optics. The s-polarized component  61  of the second light matrix  55  and the p-polarized component  60  of the first light matrix  38  pass through a first surface  36  of the third mirror prism  34 . The s-polarized component  61  of the second light matrix  55  and the p-polarized component  60  of the first light matrix  38  are reflected at an angle away from a reflection surface  41  of the third mirror prism  34  and through a third surface  37  the third mirror prism  34 . A DLP microdisplay  30  is disposed beyond the third surface  37  of the mirror prism  37 , and the combined s-polarized and p-polarized components  60 ,  61  are incident thereon. 
   The DLP microdisplay  30  may be any suitable digital light processor (DLP), such as the DLP made by Texas Instruments Incorporated of Dallas, Tex. The microdisplay  30  has an optical semiconductor (not shown), such as the DIGITAL MICROMIRROR DEVICE made by Texas Instruments Incorporated of Dallas, Tex. The semiconductor contains an array of hinge-mounted microscopic mirrors. Each of the mirrors corresponds to one pixel in a video image (not shown) of the video-input signal  21 . When the semiconductor is driven by the controller  22  based on video input signal  21 , the mirrors are tilted or switched on or off to reflect all or some of the first and second light matrices  51 ,  49 . The array of pixels reflected from the switched mirrors forms a projected light matrix  40  corresponding to the video-input signal  21  from the display controller  22 . 
   Operation of the LED&#39;s  26 ,  28  serve as attenuation control whereby some p-polarized and some s-polarized light is discarded before recombination. For example, as described above in the first embodiment if none of the video input pixel values exceeds half of full-scale, then the first and second LCDs  26 ,  28  control fifty percent of incident light. In an instance where the video input signal  21  indicates a full black screen, the first and second LCDs  26 ,  28  are set by the display controller  22  to maximum, and the microdisplay  30  is driven with zeros to achieve very high sequential contrast. Thus, if the first and second LCD&#39;s  26 ,  28  have a peak attenuation of 50:1, and the microdisplay  30  has a sequential contrast of at least 600:1, then the measured sequential contrast is 30,000:1. 
   The projected light matrix  40  is reflected from the microdisplay  30  back through the third surface  37  of the TIR prism  34 . The projected light matrix  40  passes through the reflecting surface  41  of the TIR prism  34  and out of the TIR prism  34  through a fourth surface  39 . The projected light matrix  40  is directed from the fourth surface  39  to a system of projection lenses  32 . The projection lenses  32  project the projected light matrix  40  onto a display screen  29 , to form the video image corresponding to the video input signal  21 . 
   The system  20  has the benefit of allowing the microdisplay  30  to be illuminated with alternating polarizations of light, which allows for polarization-based stereographic imaging. 
   The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.