Abstract:
A method of increasing image dynamic range by breaking each pixel into two or more time displaced samples that are presented to the display in synchronization with appropriate illumination.

Description:
TECHNICAL FIELD 
   The present invention relates in general to a method of increasing the number of gray scale levels in an image and, more particularly, to a method of increasing the number of gray scale levels in an image without increasing the dynamic range of the digital to analog conversion. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   In the development of improved television systems, especially in the projection field, it has been found that reflective liquid crystal displays (LCD) or liquid crystal on silicon (LCOS) has been found to have attributes for high resolution in large screen digital displays. Because the display is fabricated on silicon, LCOS displays have the ability to create small pixel size and high-resolution. Such reflective displays have a large pixel aperture ratio which results in a smooth and continuous image especially suitable for display of natural images and video. The development of a single panel color projection video display enables the creation of a color image without the ⅔ light loss associated with a conventional color wheel system by keeping all colors on the display panel at all times. 
   The continuous scrolling action of differently colored stripes, usually red, green and blue, are such that as a stripe leaves the bottom of the display panel the stripe is split and discontinuously redirected to the top of the panel. Concurrently with this optical scan an electrical scan is provided which addresses the rows of pixels with a signal corresponding to the color of the light impinging on the pixel row. The sequence of light bands passing over the display panel occurs so quickly as to give the viewer an appearance of simultaneous full color. 
   In such video systems wherein digital video samples are converted to analog voltages, and then sampled into the pixel array, the number of gray scale levels is limited by the dynamic range of the video digital to analog conversion. The present invention is directed to overcoming one or more of the problems or disadvantages associated with the relevant technology. 
   In the method of this invention, the number of gray scale levels in an image is increased without increasing the dynamic range of the digital to analog conversion. In one embodiment of this method such an increase is effected by separating each pixel into two or more time-displaced samples that are presented to the display panel in synchronization with appropriate illumination. For example, the number of gray levels in a system utilizing 8-bit to 12-bit pixels can be increased from 256 steps to 4,096 steps by using an 8-bit digital to analog conversion. Such a technique would be very useful in cinema, medical and advanced CAD applications. The present invention addresses one or more of these concerns. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, like reference numerals indicate corresponding parts throughout, wherein: 
       FIG. 1  is an optical schematic of a suitable single panel color projection video display system for utilizing the present invention; 
       FIG. 2  is a generalized representation of the row and column drivers of a pixel array for use with this invention; 
       FIG. 3  is a diagrammatic representation of the signal processing for red, green and blue signals which are input to analog to digital converters. 
       FIG. 4  is an optical diagram of a two-stripe monochrome system for utilizing the present invention; and 
       FIG. 5  is an illustration of the manner in which the stripes of the optical system illustrated in  FIG. 4  will appear on a display panel. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , there is illustrated an optical schematic of a single panel scrolling color video display system  50  in which 3 prisms  58 ,  72  and  80 , one for each of the red, green and blue colors respectively, are non-coaxially mounted. It is to be understood that optical elements for shaping the light bands have been omitted from this illustration for purposes of clarity. Such scanning systems as illustrated are particularly useful in high illumination applications such as cinema, medical and advanced CAD applications. 
   The scanning system  50  includes a light source  52  which includes a reflector lamp which emits white light. The light beam  54  emitted by the lamp  52  first passes to a first dichroic splitting mirror  56  which passes red light and reflects light of other colors. A red light beam  57  which exits mirror  56  then impinges on a first rotating prism  58  which operates on red light beam  57  to scan the beam in a vertical direction. Light beam  57  then exits prism  58  and impinges upon a mirror  60  which reflects light beam  57  through a dichroic mirror  74  (which passes red light) to a dichroic re-combining mirror  62  which reflects red beam  57  to a display panel or light valve  64  which modulates the red light in accordance with the video information and passes the light in accordance therewith to a projection lense  66 . 
   The green and blue light reflected from dichroic mirror  56  impinges upon a dichroic mirror  68  which acts to reflect green light and passes light of other colors. Accordingly, a green beam  70  is reflected by mirror  68  and impinges upon a rotating prism  72  which serves to scan green beam  70  in the direction as illustrated. The green beam  70  then passes to dichroic mirror  74  which reflects the green beam  70  to re-combining mirror  62  which reflects green beam  70  onto the LCD display panel or light valve  64 . 
   The light beam which passes through dichroic mirror  68  forms a blue beam  76  as the red and green components have been subtracted from light beam  54  by the actions of dichroic mirrors  56  and  68 , respectively. Blue beam  76  is thereafter reflected by mirror  78  to a rotating prism  80  which scans the blue beam  76  and passes the blue beam  76  to re-combining mirror  62  which passes the blue beam  76  to the display panel or light valve  64 . By action of rotating prism  80  blue beam  76  is also scanned in a vertical direction. 
   The action of prisms  58 ,  72  and  80  results in a scanning of red, green and blue light bands across the display panel  64 . 
   As is known in this art, in such displays the rows are addressed sequentially with all of the pixels in one row being turned on simultaneously by one of the row drivers R 1 , R 2 , R 3  ( FIG. 2 ). The individual pixels in a row are driven by a series of column drivers  1 ,  2 ,  3 , and  4 . The column drivers, which are basically memory devices, sample the incoming video signal and store the sampled value in the respective memory cell. With single panel color projection video display system, the 3 color bands red, green, and blue are scanned vertically over the display panel or light valve  64 . During one video frame each row is illuminated by first passing red, then a green and finally a blue light band. The programming of a particular row is performed in a way that e.g. the green values are loaded before the green light band reaches this row, but after the red band has passed by. Since all 3 color bands are illuminating the panel at any one time, 3 rows have to be programmed during the time of one regular video line. This operation is performed sequentially and the programming tracks the color bands as they move over the panel  64 . 
   The signal processing for the red, green and blue signals is illustrated in  FIG. 3  in a diagrammatic manner. Each of the color signals is input to analog to digital (A/D) converters  22 ,  24  and  26  so that signal processing takes place in digital form. Thereafter the R signal is input to a first delay line  28  which will delay the red signal for a time t 1 . The green signal is input to a delay line  30  which will delay it for a time t 2  and the blue line is input to a delay line  32  to delay at a time t 3 . These times are selected according to the position and scan speed of the respective color band on the display panel  64 . 
   The color signals then pass to a switch  34  which selects each of the outputs of the delay circuits  28 ,  30  and  32  sequentially so that the output of switch  34  is a serial stream with the pixels of the video lines in the aforementioned sequence. Thereafter the signals are input to a switching mechanism for applying the serialized delayed stream to the display panel  64 . 
   The video stream passes to switch  36  which separates the video stream into first and second streams  38  and  40 . The switch  36  is operated to divide the video stream into halves corresponding to the first and second half of each line. Thereafter, the output from switch  36  is connected by a line  38  to switch  42  which is operable to separate the inputs to the odd and even pixels. Input to the odd pixels is directed to a buffer memory  44  and thereafter the output of the buffer memory  44  is passed to a digital to analog converter  46  whose output is coupled to column driver  1  shown in  FIG. 2 . The even pixel stream is directed to a buffer memory  48  and a digital to analog converter  90  and thereafter to column driver  2  in a similar manner. The second halves of the video lines are coupled by line  40  and similarly processed by odd even switch  92  alternating between the odd and even pixels and utilizing buffers  94 ,  98  and D/A converters  96 ,  99  which, respectively, are output to column drivers  3  and  4 . 
   To increase the number of gray levels in such a video system, or in a monochrome imaging system, e.g. from 256 steps to 4096 steps (an 8-bit to 12-bit pixels array using an 8-bit digital to analog (D/A) conversion), the upper 8-bits of each pixel is first converted to an analog voltage and applied to each pixel location. The addressed pixel location is then illuminated with full intensity light. The lower 4-bits of each pixel are then converted to (full scale) analog voltage which is applied to each pixel, sampled, and the array illuminated with 1/256 intensity light. An alternative method would be to convert the lower 4-bits of each pixel at a level less than full scale and increase the intensity of the illumination proportionally. This same technique can be applied to monochrome or color images. The frame rate should be sufficiently high to avoid flicker or other temporal artifacts. 
   To better explain the method of this invention, reference is made to the optical diagram of  FIG. 4  and the attendant scanning of the stripes onto a display panel or light valve  64   a  as illustrated in  FIG. 5 . Because the method of this invention can be used for both color or monochrome images, for convenience of illustration a two-stripe monochrome system will be utilized for reference. It is to be understood, however, that a system as described with reference to  FIGS. 4 and 5  could be used for each of the three colors Red, Green and Blue. 
   Similar to the color imaging system illustrated in  FIG. 1 , a light source  52   a , which includes a reflector lamp which emits white light, is passed to a beam splitter  53  whereby the beam splitter  53  transmits (m/256) L in a first light path  55  and reflects (n/256) L in a second light path  65  to a mirror  63 , wherein n+m=256, preferably n=1 and m=255. 
   The dim light (n/256) L is passed from the mirror  63  in the light path  65  through a slit  71  into a rotating prism  73 . The (m/256) L light is passed along light path  55  through a slit  51  and into a rotating prism  59 . Thereafter, the light beams will pass through or be redirected by beam splitter  74   a , to a polarizing beam splitter  69  and display panel  64   a.    
   The height of the respective stripes (n/256) L and (m/256) L is determined by the light passing through the slits  71  and  51 , respectively. Preferably the height will be one-half the height of the display panel. 
   Rotating prims  73  and  59  are phase shifted such that they cause the dim stripe (n/256) L to scroll down the display panel 180 degrees out of phase with the bright stripe (m/256) L. Accordingly, assuming no overscan (overscan=0%) the two stripes will scroll down the panel 180 degrees out of phase, and light will always be in the panel, as illustrated in  FIG. 5 . 
   While the size of each prism face will determine the amount of overscan onto the display panel, and for purposes of simplicity no overscan has been described above, a more realistic example would be to provide an approximately 5% overscan to allow for component tolerance for preventing stripe overlaps and to provide a black gap between stripes for writing data. As illustrated, video addressing for each stripe is addressed to the display panel just ahead of the leading edge of each stripe. 
   For an illustrative example, assume the bright stripe has a luminance of (255/256)k L and the dim stripe has a luminance of (1/256) k L, and the required dynamic range of each pixel is 12-bits. If it is desired to update a pixel in row X, column Z, with a 12-bit value of 47A hexadecimal, then when the front edge of the bright stripe reaches row X, column Z is updated with an analog value equivalent to the upper 8-bits of the pixel, or 47 hex. When the front edge of the dim stripe reaches row X (a half frame later), column Z is updated with an analog value equivalent to the lower 4-bits of the pixel, or A hex. Accordingly, when integrated by the human eye over a frame time, the pixel appears as a 12-bit value, 47A hex. 
   FUNCTIONAL DESCRIPTION 
   In operation, the present invention increases the number of gray scale levels in an image without increasing the dynamic range of the digital to analog conversion applied to the pixels of an image display panel. The increase in a video system, either color or monochromatic, from 256 steps to 4096 steps, an 8-bit to 12-bit pixel array using an 8-bit D/A conversion, first converts the upper 8-bits of each pixel to an analog voltage applied to each pixel location. The addressed pixel location is then illuminated with substantially full intensity light. The lower 4-bits of each pixel are subsequently converted to full scale analog voltage which is applied to each pixel sampled, and the array illuminated approximately 180 degrees out of phase with the initial scroll with 1/256 intensity light. When integrated by the human eye over a frame time, the pixel appears as a 12-bit value. 
   An alternative method of increasing the gray scale level would be to convert the lower 4-bits of each pixel at a level less than full scale, and increase the intensity of the illumination proportionally. 
   Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.