Abstract:
A luminosity channel may be provided in an electro-optic projection display by beam splitting the incident light source into first and second polarization beams. One polarization beam may be used to illuminate the three modulators for the three primary color channels. The other beam may be utilized to illuminate a white light modulator which provides a luminosity channel. The two beams may be recombined to produce the final image. As a result, in some embodiments, higher resolution, or wider color gamut, and better image quality may be achieved without unduly effecting the light efficiency of the display.

Description:
BACKGROUND 
     This invention relates generally to electro-optic projection displays such as liquid crystal projection displays. 
     It has been appreciated for some time that the addition of an luminosity channel to a projection display, including an electro-optic display, would be advantageous. The luminosity channel contains the lightness and darkness information and, viewed by itself, looks like a gray scale image. A substantial portion of the image detail exists in the luminosity channel. The luminosity channel uses white light while the conventional electro-optic display uses a set of three spatial light modulators for each of the primary color planes. 
     It is believed that luminosity channels have not been widely used heretofore, despite the understanding that the resolution of the projection display would be substantially enhanced. This may be because of the problem with light efficiency which arises from extracting a significant portion of the light for a four channel system. Thus, while those skilled in the art may appreciate the benefits of luminosity channel, it has not been widely adopted in practice. 
     One known approach to providing luminosity channel is utilized in the Texas Instruments digital mirror based projection engines. However, the luminosity channel is provided by field sequential addressing mode using a color wheel. The color wheel has slots for the primary color planes and two white light or luminosity channels. While time sequential approaches have some advantages, they also suffer from some problems. Firstly, their optical efficiency is low. Secondly, they suffer from color breakup. It is not believed that the addition of a luminosity channel in this fashion will sufficiently improve the overall characteristics of the display to make the addition of a luminosity channel cost effective. 
     Thus, there is a continuing need for a way to provide a luminosity channel without adversely effecting the characteristics of the overall display, including its light efficiency. 
     SUMMARY 
     In accordance with one aspect, a method of operating an electro-optic projection display includes generating a light source. The light from the source may be split into two separately polarized light beams. One polarized beam is used to form three color channels. The other polarized light beam is utilized to form a luminosity channel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of one embodiment of the present invention; and 
     FIG. 2 is a depiction of an array of electro-optic spatial light modulators that may be utilized in connection with the embodiment shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a projection display  10  may include a light source  12 . The light source may be conventional in all respects. It may include a point source of light, an element for focusing the light and any desired optical elements. A polarizing beam splitter  14  separates the light from the light source  12  into two polarized beams  13  and  15 . 
     The beam  15  which is polarized in a first direction (such as the {right arrow over (p)} direction) may be converted to the form of polarization ({right arrow over (s)}, for example) of the beam  15  using a half wave plate  16 . The beam  15  (either in its original polarization without the half wave plate  16  or as converted to the opposite polarization) is then passed to a beam splitter  18 . The beam  15  then illuminates a spatial light modulator  20 . 
     Since the beam  15  is a white light beam, the white light spatial light modulator  20  provides a luminosity channel. The luminosity channel is a channel containing white light which may be used to improve the resolution of the display  10 . The suitably modulated luminosity channel is then reflected by the beam splitter  18  along the path  21  to a beam combiner  22 . 
     Meanwhile, the other polarization beam  13  is passed to a beam splitter  26  and is directed to an electro-optic spatial light modulating array  24 . The array  24  creates modulated color planes. For example, in one embodiment of the present invention, a spatial light modulator may be utilized for each of the primary color planes. In other words, one polarization of the two possible polarizations is utilized to create the three color plane signals while the other polarization is utilized to produce the luminosity channel. The color plane information from the array  24  is combined with the luminosity channel information in the beam combiner  22 . This signal is processed by a projection lens  30  and projected on a display  32 . 
     The projection system  10  may be any of a variety of electro-optic devices including a liquid crystal display or a spatial light modulator that uses liquid crystal elements. The system may based on reflection or transmission. 
     The mix of light along each of the paths  13  and  15  may be controlled by a variable ratio beam splitter in one embodiment. Thus, depending on the importance of the luminosity channel, any proportion of the overall light produced by the light source  12  can be dedicated to producing the luminosity channel. For example, half of white light may be split to each path  13  and  15  in one embodiment. In any case, the luminosity channel is provided through the use of a portion of the light source which would otherwise have been wasted. Thus, the provision of the luminosity channel does not significantly adversely effect the light efficiency of the display  10 . As a result, in some embodiments, the resulting image may have greater brightness and better color gain in addition to better resolution. The light modulator  20  may be tuned to accept a wider band of wavelengths instead of the single wavelength that modulators are normally tuned to receive. 
     The efficiency of the system  10  may be improved by using good beam splitters which have high optical throughput. Most white light includes ultraviolet and infrared wavelengths that are normally discolored by optics. This restricts the available color gamut. Embodiments of this invention allow for wider color bandpass, extending the color gamut so as to take advantage of the true range of human color perception. For example, the display&#39;s light responsivity may be broadened to approximately 400 to 800 nanometers. 
     Some embodiments of the present invention may advantageously improve the light efficiency of the overall system due to polarization recovery. In addition, some embodiments may provide a way to improve the gray scale and color gamut of the images. Also, some embodiments may provide a way to increase the resolution of the images by using one modulator with higher resolution for the luminosity channel while lower resolution modulators may be used for the color planes. 
     Since the brightness of the resulting display may be increased, the lifetime of the lamp used as a light source may be increased as lower brightness settings may be utilized. Alternatively, lower wattage light bulbs may be used in embodiment of the present invention. 
     Referring to FIG. 2, an embodiment  24  of a color plane modulation array in accordance with the invention has electrical features to cause the convergence of modulated beams images (modulated red, green and blue images, for example) that collectively form a composite image on a screen  32 . In some embodiments, the array  24  may be a liquid crystal display (LCD) projection system, and the display panels  60  may be reflective LCD display panels. Other arrangements are possible. 
     In some embodiments, the array  24  may include prisms  52  (prisms  52   a ,  52   b ,  52   c  and  52   d , as examples) that direct an incoming beam of white light (formed from red, green and blue beams) from a light source  63  to the display panels  60 , as described below. In particular, the prism  52   a  receives the incoming white beam of light at a prism face  52   aa  that is normal to the incoming light and directs the beam to a prism face  52   ab  that is inclined toward the face  52   aa . The reflective face of a red dichroic mirror  54   a  may be mounted to the prism face  52   ab  or to the prism face  52   ca  by a transparent adhesive layer. 
     The red dichroic mirror  54   a  eseparates the red beam from the incoming white beam by reflecting the red beam so that the red beam exits another prism face  52   ac  of the prism  52   a  and enters a prism face  52   ba  of the prism  52   b . The prism faces  52   ac  and  52   ba  may be mounted together via a transparent adhesive layer. The prism  52   b , in turn, directs the red beam to the incident face of the display panel  60   a  that is mounted to another prism face  52   bb  of the prism  52   b  that is inclined toward the prism face  52   ba . The display panel  60   a  modulates the incident red beam, and the modulated red beam follows a similar path to the path followed by the incident red beam. 
     The remaining blue and green beams (from the original incoming white beam) pass through the red dichroic mirror  54   a . The opposite face of the mirror  54   a  is attached to a prism face  52   ca  of the prism  52   c , an arrangement that causes the blue and green beams to pass through the red dichroic mirror  54   a , pass through the prism face  52   ca  of the prism  52   c , travel through the prism  52   c  and pass through a prism face  52   cb  (of the prism  52   c ) that forms an acute angle with the prism face  52   ca . The reflective face of a blue dichroic mirror  54   b  is mounted to the prism face  52   cb . As a result, the blue dichroic mirror  54   b  reflects the blue beam back into the prism  52   c  to cause the blue beam to exit another prism face  52   cc  of the prism  52   c . The incident face of the display panel  60   b  is mounted to the face  52   cc  and modulates the incident blue beam. The modulated blue beam, in turn, follows a path similar to the path followed by the incident blue beam. 
     The green beam passes through the blue dichroic mirror  54   b  and enters the prism  52   d  through a prism face  52   da  that may be mounted to the other face of the blue dichroic mirror  54   b  via a transparent adhesive layer. The green incident beam exits another prism face  52   db  of the prism  52   d  to strike the incident face of the display panel  60   c  that is mounted to the prism face  52   db . The display panel  60   c  modulates the incident green beam before reflecting the modulated green beam along a path similar to the path followed by the incident green beam. The beam splitter  26  directs the modulated green beam through the projection lens  30 . The three modulated beam images form a color composite image on the screen  32 . 
     The system  24  depicted in FIG. 2 is an example of one of many possible embodiments of the invention. Other modulation systems, prism arrangements and optical systems are possible. 
     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.