Abstract:
A light source used for projection displays that produces more light of a particular color relative to one or more other colors can be operated in a way to increase the performance of the overall optical system. A first color component that is produced in a lesser amount, may be separated out from the light and discarded while the other light components may be modulated. A separate light source may generate light of the first color that is modulated and combined with the previously modulated color components.

Description:
BACKGROUND 
     This invention relates generally to projection displays. 
     Many projection display systems are driven by ultra high pressure (UHP) mercury halide arc lamps. These arc lamps generate most of their light in the green portion of the spectrum and very little light in the red portion of the spectrum. 
     In order to produce display images that have color temperatures meeting various display standards, the projection systems discard a high percentage of the green light in order to achieve the right color balance between the green, blue and red components. This accommodation may decrease the brightness of the display, complicate projection system design and limit the size of the display that the arc lamp can power. 
     Thus, there is a need for better ways to utilize light sources that generate light with an imbalance between the color components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A light modulation engine  10  for a projection display includes a lamp  12  that produces more light in one portion of the spectrum than in another. For example, the lamp  12  may be an ultra high pressure mercury halide arc lamp that produces more light in the green portion of the spectrum and less light in the red portion of the spectrum. 
     The blue and green randomly polarized light (B r , G r ) light generated by the lamp  12  is reflected from a cold mirror  14 . The cold mirror  14  passes red (R r ), infrared (IR) and ultraviolet (UV) components that are collected by an absorber  16 . The blue and green randomly polarized light (G r , B r ) is reflected from the cold mirror  14  and is subjected to beam conditioning by the optics  18  to transform it into a uniform intensity beam with a shape matching the microdisplays  30 ,  32 ,  42 . 
     The light (G r , B r ) then interacts with a polarization converter  20  that reflects the s polarizations (G s , B s ) and passes the p polarizations (G p , B p ). 
     The blue and green (G p , B p ) components are then passed through a lens  22  and input to a cyan notch dichroic filter  24  that removes a portion of the green spectrum that would be presented to both the blue and green spatial light modulators  30 ,  32 . The blue and green (G p , B p ) components are then input to a green polarization filter  26  that rotates the green component (G p ) to s polarization (G s ) while leaving the polarization of the blue component (B p ) unchanged. The blue and green light components (G s , B p ) then transit a polarization beam splitter (PBS)  28  where, the green component (G s ) is reflected by the polarizer of the splitter  28  to image on the green spatial light modulator  30 , and the blue component (B p ) passes through the polarizer of the splitter  28  to image on the blue spatial light modulator  32 . 
     The green spatial light modulator  30  serves to rotate the polarization of the green light component (G s ) back to p polarization (G p ) for those pixels that are “on”. The “on” green light component (G p ) then transits the PBS  28  passing through its polarizer, also passing through a Blue Twist polarization filter  34  unchanged, and through the combining PBS  36  to contribute to the final image  48 . 
     The green spatial light modulator  30  also serves to leave the polarization of the green light component (G s ) that images to “off” pixels unchanged in the s polarization state (G s ). This “off” green light component (G s ) is reflected by the PBS  28  polarizer back toward the light source  12  and does not contribute to the final image. 
     The blue light component (B p ) that passes through the PBS  28  polarizer images on the blue spatial light modulator  32 . The blue spatial light modulator  32  serves to rotate the polarization of the blue light component (B p ) to the s polarization (B s ) for those pixels that are “on”. The “on” blue light component (B s ) then reflects from the polarizer in the PBS  28  and passes through a Blue Twist polarization filter  34 , where its polarization is changed to p polarization (B p ). It then passes through the combining PBS  36  to contribute to the final image  48 . 
     The blue spatial light modulator  32  also serves to leave the polarization of the blue light component (B p ) that images to “off” pixels unchanged in the p polarization state (B p ). This “off” blue light component (B p ) passes through the PBS  28  polarizer back toward the light source  12  and does not contribute to the final image  48 . 
     A red light component (R s ) is introduced into a PBS  40  from a second light source  46  such as a red laser array or an arc lamp. The conditioning, despeckling and imaging optics  44  serve to put the red light component into the s polarization state if needed, and to prepare the light to match the f number of the imaging optics  22  used for the blue and green light components. 
     This red light component (R s ) reflects from the PBS  40  polarizer to image to the red spatial light modulator  42 . The red spatial light modulator  42  serves to rotate the polarization of the red light component (R s ) back to p polarization (R p ) for those pixels that are “on”. The “on” red light component (R p ) then transits the PBS  40  passing through its polarizer, also passing through a half wave achromatic phase retarder, which changes it back to the s polarization (R s ). It then enters the combining PBS  36  where it reflects from the PBS  36  polarizer combining with the “on” green and blue spatial light components to form the final image  48 . 
     The red spatial light modulator  42  also serves to leave the polarization of the red light component (R s ) that images to “off” pixels unchanged in the s polarization state (R s ) . This “off” red light component (R s ) is reflected by the PBS  28  polarizer back toward the light source  46  and does not contribute to the final image. 
     The combined light output  48  may then be displayed on a projection screen (not shown). 
     In embodiments of the present invention, the design of light modulation engine  10  may be simplified. In addition, the brightness standard display color temperatures may be increased in some embodiments. Much larger display screens may be driven by arc lamps used in projection systems in some cases. In addition, arc lamps may be re-optimized for luminance efficiency to a condition where they readily emit green and blue components, thus increasing their light output and lifetime. By making the light source  46  emit a red light with an étendue that matches that of the lamp  12  and light modulation engine  10 , light from the two light sources may be effectively combined. 
     The light modulation engine  10  may dump red light from the lamp  12  in an early optical stage and may re-introduce red light from a second, high intensity source in a later optical stage. The red light (R) from the second, high intensity source  46  is imaged on the red spatial light modulator  42  and combined with modulated blue and green beams (GB). 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.