Abstract:
An optical projection system having optical components immersed in a liquid. An enclosure holds the liquid, a spherical lens, a polarizer, a polarizing beam splitter, a reflective light valve having a retarder foil, and an exit window. The liquid stabilizes the thermal environment, reduces the need for antireflective coatings, enables a reduction in the number of optical components, and enables high optical flux.

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     This invention relates to light projectors. More particularly it relates to light projectors having optical components, such as retarder foils, polarizers, beam splitters, and light valves. 
     2) Description of the Related Art 
     Optical projectors are common devices that are used to produce images on a screen. Such projectors are finding wide spread use as television displays, computer screen displays, and theatre displays. Optical projectors, particularly liquid crystal optical projectors, are beneficial because they produce large images using relatively small, inexpensive devices. 
     Typical liquid crystal optical projectors include a light source that illuminates an optical projection system. Such optical projection systems usually include an input optical subsystem having a first window, an input lens that collimates and focuses the illuminating light, and a polarizing prism. The polarizing prism selectively reflects the portion of the focused light that has the correct polarization direction experienced by off axis light rays, while the reflective liquid crystal light valve selectively changes the polarization of the light that is reflected back through the second window and into the prism. The prism then passes the portion of the light that has changed polarization through a third window onto the display screen. 
     While generally successful, typical optical projection systems have problems. First, the light from the light source passes through a large number of optical elements, each of which increases cost and each of which can produce undesirable birefringence. Furthermore, the various windows often require relatively expensive antireflective coatings on one or more optical surfaces. Also, the prism tends to be relatively expensive and tends to have imperfections that are detrimental to overall operation. Another problem relates to the alignment of the optical components. Thermal differences across the system can produce significant optical path length changes that can have detrimental effects on overall system performance. 
     Therefore, a new optical projection system would be beneficial. Even more beneficial would be a new optical projection system that reduces the number of optical components. Even more beneficial would be an optical projection system that reduces thermal differences, and that reduces the need for antireflective coatings. 
     SUMMARY OF THE INVENTION 
     The principles of the present invention enable optical projection systems having a reduced number of optical components. The principles of the present invention also enable optical projection systems having reduced thermal differences. The principles of the present invention further enable optical projection systems that do not require antireflective coatings. 
     An optical projection system in accord with the principles of the present invention includes a polarizer, a polarizing beam splitter, and a reflective light valve immersed in a liquid, such as water, contained in a housing. Beneficially, the housing also includes an input lens, such as a spherical lens, an exit window, and a light valve. Also beneficially, the light valve includes a retarder foil. 
    
    
     Additional features and advantages of the present invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and which are incorporated in and constitute a part of this specification, illustrate an embodiment of the present invention and, with the description, serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 illustrates an enclosure of an optical projection system that is in accord with the principles of the present invention; and 
     FIG. 2 simplified, schematic, sectional view of the optical projection system of FIG. 1 taken along line  2 — 2 . 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to an illustrated embodiment of the present invention, the example of which is shown in the accompanying drawings. 
     FIG. 1 illustrates an enclosure  10  of an optical projection system that is in accord with the principles of the present invention. FIG. 1 shows the enclosure  10  with a front wall  12 , an angled side wall  14 , and a side attachment  16 . That side attachment  16  includes a mounting bracket  18 . The enclosure  10  is a sealed, leak proof unit that also includes top, bottom, and back walls (the back wall is shown in FIG.  2 ). The front wall  12  includes an opening for a spherical lens  20 , while the angled side wall  14  includes an opening for an exit window (shown in FIG.  2 ). 
     Turning now to FIG. 2, which is a simplified, schematic, sectional view of the optical projection system of FIG. 1 taken along line  2 — 2 , the enclosure  10  is filled with a liquid  22 . The liquid  22  is preferably an organic compound. The liquid  22  is selected to have a desired index of refraction, preferably an index matching an index of refraction of optical elements in the enclosure  10 . The liquid may also be selected to have desirable boiling point and freezing point properties. One particularly suitable liquid is Ethylene Glycol. It is also possible to use water. 
     As shown in FIG. 2, a polarizer  24  is disposed behind the spherical lens  20 . Alternatively, the polarizer  24  may be located in front of the spherical lens  20 , outside of the enclosure  10 . 
     Behind the polarizer  24  is a polarizing beam splitter  26 . On the side attachment  16  is a reflective liquid crystal light valve  28 . In front of the liquid crystal light valve is a retarder foil  30 . Mounted at the opening of the angled side wall  14  is an exit window  32 . The exit window  32  may be a lens, and may comprise a first optical element of a projection lens. Also shown in FIG. 2 are openings  34  in the mounting bracket  18 , the back wall  36 , a second bracket  38  on the back wall, and a slotted opening  40  in the second bracket. The mounting brackets  18  and  38  are used to mount the enclosure  10  in the overall projection system and will not be described further. 
     Still referring to FIG. 2, in operation light enters the enclosure  10  through the spherical lens  20 . That lens  20  collimates the input light and directs the collimated light to the polarizer  24 . The polarizer  24  substantially passes the component of the collimated light that has a predetermined polarization, and substantially prevents the component that has a polarization perpendicular to the predetermined polarization from passing through it. The passed light component is reflected by the polarizing beam splitter  26  toward the liquid crystal light valve  28 . The retarder foil  30  corrects the “off-axis” component of the light from the polarizing beam splitter  26 . 
     The liquid crystal light valve  28  selectively reflects the light from the polarizing beam splitter  26  in accordance with video information (which is not shown, but which are well known in the art of liquid crystal projectors). Significantly, the liquid crystal light valve  28  changes the polarization of its reflected light. Light reflected by the liquid crystal light valve  28  then passes though the polarizing beam splitter  26  and exits the enclosure  10  through the exit window  32 . The exit window may be a first lens of a projection lens arrangement for projecting the light onto a display screen. 
     In the illustrated optical projection system, all optical paths between the optical components are through the liquid  22 . By properly matching the refractive indexes of the liquid  22 , the exit window  32 , and the retarder foil  30 , antireflective coatings can be eliminated. Furthermore, when all of the optical components are in the same liquid environment, thermal variations are significantly reduced or eliminated. This reduces design difficulty and improves overall performance. Furthermore, by using the spherical lens as an optical input, the need for an input window is eliminated. Additionally, stresses found in prior art optical projection systems that can cause undesirable birefringence, such as in retarder foils having antireflective coatings, are reduced. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.