Abstract:
A class of novel structures which make novel use of polymer based reflective polarizing films in an improved polarization conversion system which are useful in liquid crystal projection systems that are easily manufactured, of lower cost, and permit the versatility of higher numerical aperture polarization conversions. Another aspect of the present invention are polarization modulating liquid crystal projection display systems utilizing the polarization conversion systems of the present invention.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to projection type liquid crystal systems such as projection computer displays, projection monitors, projection video or television systems, and more particularly to a low cost polarization conversion system and the optical arrangement for a projection type liquid crystal system based on polarization modulating liquid crystal displays. 
     2. Description of the Related Art 
     Polarization conversion systems for use in projection type liquid crystal displays (LCDs) are well known in the prior art. In a projector, light from a source such as a metal halide arc lamp is collected by a reflector and relayed onto an object (e.g., a LCD light valve), with the help of condenser and relay lenses. The illuminated object is then projected onto a display screen at a desired magnification. However, since a dominant type of liquid crystal displays form the image by discriminating between the polarization of light from bright and dark pixels, the light collected from the light source must be polarized by means such as a linear polarizer before being incident on the liquid crystal light valve. In the light valve, the bright and dark pixels are discriminated by the polarization of the light leaving them. This light is then analyzed by means of an analyzer. Typically, the act of polarization of unpolarized light by the conventional polarizers implies that the light of unwanted polarization be lost due to absorption in the polarizers. The alternative of using polarizing cube or plate beam splitters results in transmitting one polarization and reflecting the other. In such a situation, the unused light of unwanted polarization must be recaptured and its polarization altered to be that of the used polarization before it can be used to illuminate the light valve. These polarization conversion systems generally comprise a light reflector, a quarter wave plate and/or a half wave plate. A quarter wave plate shifts the polarization of incident light by 45 degrees. Similarly, a half wave plate shifts the polarization of incident light by 90 degrees. 
     In U.S. Pat. No. 5,200,843 to Karasawa et al., the polarization and polarization conversion takes place over the full aperture of the light source and therefore the outgoing light must go through a intensity homogenizer before it is incident upon the light valve. While illumination systems of this type have their benefits, they tend to be bulky and expensive. 
     Still other polarization conversion systems are known in the prior art, such as those disclosed in EP A1 0753780 and EP A1 0753971, shown in FIG. 1A, and referred to generally as reference numeral  100 . In these systems the full aperture of the light coming from a light source  101  and a reflector  102  which reflects the light from the light source in the direction indicated by arrows A, is sampled, subdivided, and focused with the help of an array of lenses  103 , resulting in an array of beam samples  114 . Near the focus of these beam samples is the polarization conversion system  120  which consists of a stack of plate polarizing beam splitters  115 , each plate of the stack seeing one row of beam samples  114 . Since the polarization conversion system  120  is in the vicinity of the focus of the beam samples, the beam samples are separated from each other and a copy of the beam sample can be placed at its side. For example, the beam sample  114  is separated into components of two polarizations with the P-polarization  117  being transmitted and the S-polarization  116  being reflected from the front side  115   a  of the polarizing beam splitter  115 . The S-polarization is further reflected by the rear side  115   b  of the plate polarizing beam splitter  115 . The reflected beam then goes through a half wave plate  130  where its polarization is shifted 90° to become a P-polarization  118  upon transmission from the plate. Thus the outgoing beam has suffered minimal amplitude loss and has acquired a single type of linear polarization in the form of beams  117  and  118 . This polarization conversion occurs for all the beam samples which are then redirected towards the light valve (not shown) by a second array of lenses  113 . The appropriately magnified and overlapping beam samples illuminate the light valve with polarized light in a very efficient way. The side of the polarization conversion system  120  of FIG. 1A facing the reflector has the alternating plates of the stack blocked in the form of light blocks  122  to maximize polarization purity. The system has many components and while the assembly is compact and efficient, it is relatively expensive to manufacture such polarization conversion systems  120 . 
     FIG. 1B shows an enlarged schematic view of the polarization conversion system  120  of FIG.  1 A. It consists of a stack of plane parallel plate polarizing beam splitters  115  with one side of the plate  115   a  having a polarizing beam splitting multi-layer dielectric coating and the other side  115   b  being blocked by the light blocking grid  122  so it does not receive any direct light. As shown, the light is incident on alternate plates only. The grid of half wave plates  130  is similarly applied to alternate plates so the half wave plate  130  only sees the reflected light  116   a  of S-polarization  116  and converts it to P-polarization  118 . While this arrangement has its advantages the assembly and alignment of all the components is expensive. Also, the multi-layer thin film polarizing films only work over a limited range of angles. 
     For the above reasons, there is a need in the art for a simple, low cost polarization conversion system which is capable of operation at high numerical apertures. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a polarization conversion system which produces linearly polarized light from unpolarized light without losing half the light intensity. 
     Another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which enables a brighter image without increasing the wattage of the light source. 
     Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light for use in projection displays. 
     Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which is of simpler construction and thus more economical. 
     Yet still another object of the present invention is to provide a polarization conversion system which produces linearly polarized light from unpolarized light which performs over a broad range of incident light angles (i.e., numerical apertures). 
     Yet another object of the present invention is to provide a polarization conversion system which produces linearly polarized light which performs over a broad range of wavelengths covering the three primary colors, red, green and blue. 
     Accordingly, a polarization conversion system for converting incident light is provided in which the incident light having at least a first and second polarization is converted to light of one of the first and second polarizations. The polarization conversion system comprises an input side at which the incident light enters and an output side at which light of one of the first and second polarizations exits. A polymer based reflective polarizing material disposed between the input and the output sides, in communication with the incident light, and angled with respect to the incident light for transmitting light of one of the first and second polarizations and reflecting the other is provided. A reflector for reflecting the reflected light from the polymer based reflective polarizing material and a light block disposed on the input side for blocking the incident light from communication with the reflective means are also provided. Lastly, a polarization convertor for shifting the polarization of the reflected light to that of the transmitted light is provided. The polymer based reflective polarizing material, the reflector, the polarization convertor, and the light block are arranged such that the transmitted light of one of the first and second polarizations exits the output side, and the reflected light is directed towards the reflector which directs it towards the polarization convertor which in turn shifts the polarization of the reflected light to that of the transmitted light before exiting the output side. 
     Another aspect of the present invention are polarization modulating liquid crystal projection display systems utilizing the polarization conversion systems of the present invention. 
     It can be realized that while the invention is described relative to a parabolic reflector that produces a largely collimated beam of light, it may also be readily employed with elliptical or other types of reflectors by incorporation of collimating condenser lens to produce a collimated beam as is known in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein: 
     FIG. 1A illustrates a LCD projector illumination system of the prior art; 
     FIG. 1B illustrates an enlarged view of the Polarization Conversion System of FIG. 1A; 
     FIG. 2A illustrates an isometric view of a Polarization Conversion System of the present invention; 
     FIG. 2B illustrates a sectional view of the Polarization Conversion System of FIG. 2A taken along line  2 B— 2 B; 
     FIG. 3A illustrates a sectional view of an alternative embodiment of the Polarization Conversion System of FIG. 2B; 
     FIG. 3B illustrates a sectional view of yet another alternative embodiment of the Polarization Conversion System of FIG. 2B; 
     FIG. 3C illustrates a sectional view of still yet another alternative embodiment of the Polarization Conversion System of FIG. 2B; 
     FIG. 4 illustrates a graph of the transmittance and reflectance of the P- and S-polarizations for the Polarization Conversion System of the present invention; 
     FIG. 5 illustrates a graph of the percent transmission of unpolarized light through Polarization Conversion System of the present invention; and 
     FIG. 6 illustrates a polarization and modulating liquid crystal display system utilizing the Polarization Conversion System of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 2A and 2B, there is illustrated a polarization conversion system of the present invention, referred to generally as reference numeral  220 , in which the expensive multi-layer thin film polarizing beam splitting coating on the plane parallel plates as used in the prior art are replaced with a polymer based reflective polarizing material  215 , such as DBEF available from 3M Corporation. The reflective polarizing material  215  is preferably suspended in air as shown, or alternatively sandwiched between two thin glass plates (not shown), or between glass wedges (not shown). The reflective polarizing material  215  is generally several hundred microns thick, sturdy enough to be suspended on its own, and does not necessarily need supporting substrates as is the case with the multi-layer thin film coatings of the prior art. This leads to a reduction in parts, weight, and manufacturing costs. 
     The reflective polarizing material  215  is angled at an angle (90-θ) with respect to the incident light  114  such that it transmits the P-polarized light  117  and reflects the S-polarized light  116  towards the S-reflecting mirror  225  (or alternatively reflective polymer material). A reflector, such as a S-reflecting mirror  225  is proximate to the reflective polarizing material and similarly angled with respect to the incident light. The S-reflecting mirror  225  reflects the S-polarized light  116  and is preferably the same reflective polarizing material that splits the S- and P-polarizations. Alternatively, the S-reflector  225  may be a non-polarizing reflector or a regular reflective mirror on a thin glass substrate (not shown). 
     As is done in the prior art, a light block  222  is provided such that the incident light  114  is prevented from communicating with the S-reflecting mirror  225 . Similarly, a polarization convertor, such as a half wave plate  230  is positioned such that the S-polarized light  116   a  reflected from the S-reflecting mirror  225  is directed towards the half wave plate  230  whereby the S-polarized light  116  is converted to P-polarized light  118 . The components as shown in FIG. 2B are preferably suspended in air with the help of holders  240 . The holders  240  have slots  245  for acceptance of the components and are preferably fabricated of metal or plastic. 
     The polarization conversion system  220  is preferably arranged as shown in FIG. 2A where the polymer based reflective polarizing material  215 , the S-reflecting mirrors  225 , the half wave plates  230 , and the light blocks  222  are configured as an array of slats, the slat arrays are referred to generally as reference numerals  215   a,    225   a,    230   a,  and  222   a,  respectively. The slats are preferably arranged in an array of polarization conversion systems which operate as a single unit. However, it is understood by someone skilled in the art that the polymer based reflective polarizing material  215 , the S-reflecting mirrors  225 , the half wave plates  230 , and the light blocks  222  can be configured and shaped in a number of ways without departing from the scope and spirit of the invention. It is also understood by someone skilled in the art that the polarization system  220  does not have to be an array of elements, it can consist of a single reflective polarizing material  215 , reflector  225 , light block  222 , and polarization convertor  230 . 
     The operation of the polarization conversion system  220  will now be discussed with reference to FIGS. 2B,  4 , and  5 . For purposes of this discussion, it is assumed that the polarization conversion system  220  is configured as an array of repeating elements, as shown in FIG. 2B, however, as discussed previously, its operation would be the same for a single group of elements. Unpolarized light from a light source and reflector is directed onto the polarization conversion system  220  by an array of lenses. The incident light  114  directed towards the polarization conversion system  220  consists of P-polarized  117  and S-polarized  116  light. Alternating areas of light blocks  222  block the incident light  114  from communicating with the S-reflecting mirrors  225 . In portions of the polarization conversion system where the incident light  114  is not blocked by the light blocks  222 , the light  114  communicates with the reflective polarizing material  215 , which is disposed between light blocks  222  and angled at an angle e such that the reflective polarizing material  215  transmits the P-polarized light  117  and reflects the S-polarized light  116  towards the S-reflecting mirrors  225  (or alternatively reflective polarizing material). The S-polarized light  116  is then reflected  116   a  towards a half wave plate  230  where it is converted to P-polarized light  118 . Thus, the incident light  114  containing both P- and S-polarized light is converted to light containing only P-polarized light  117 , 118 . 
     Whereas the multi-layer thin film polarizing films of the prior art work over a limited range of angles (90-θ) with respect to the incident light, the polymer reflective polarizers  215  perform over a broader range of angles. FIG. 4 illustrates a measurement of the S and P transmission ( 610 ,  620  respectively), and P and S reflectivity ( 630 ,  640  respectively), verses angle θ for a typical reflective polarizing film operating over a broad range of angles, much broader than any thin film polarizing beam splitter coating enabling the Polarization Conversion System  220  to operate over much higher numerical apertures. FIG. 5 shows the transmission of unpolarized light through a sample film as a function of wavelength. The substantially uniform transmission of light through these multi-layer reflective polymer films across the whole spectrum makes them very useful for a polarization conversion system. 
     Another embodiment of the polarization conversion system is shown in FIG.  3 A and referred to generally as reference numeral  320 . In this embodiment the reflective polarizing material  315  is inclined at an angle θ, where θ is preferably 30 degrees, allowing the use of the same film surface for transmitting the P-polarization  117  and for reflecting the S-polarization  116 . In this configuration, the P-polarized light  117  passes through the lower half  315   a  of the reflective polarizer  315  and the reflected S-polarized light  116  proceeds to the upper half  315   b  of the adjacent reflective polarizing segment reflecting again at this surface to be transmitted through the half wave plate  330  for polarization conversion  118 . With this configuration, dual use is made of each reflective polarizing segment  315  and the number of tilted surfaces is cut nearly in half further reducing the cost of the assembly. However, the metal light block  322  and the half wave plate  330  must be aligned to each other. 
     The components shown in FIG. 3A can be suspended with the help of holders as discussed with regard to the previous embodiment or they can be affixed to a transmissive wedge  340 , the wedge is preferably fabricated of a high quality optical glass. The transmissive wedge  340  further has an input side  340   a,  an output side  340   b  corresponding to the input and output sides of the polarization conversion system, a polarization side  340   c,  and a reflecting side  340   d.  The polymer based reflective polarizing material  315   a  is affixed to the polarization side  340   c,  the S-reflector mirror  315   b  is affixed to the reflecting side  340   d,  the light block  322  is affixed to the input side  340   a,  and the half wave plate  330  is affixed to the output side  340   b.  A plurality of wedges are preferably joined to form the polarization conversion system  320  as shown in FIG. 3A which operates as a single unit. 
     Still another embodiment of the present invention is shown in FIG. 3B, and referred to generally as reference numeral  420 . The embodiment of FIG. 3B is similar to that of the previous embodiment except that the reflective polarizing material  415  is now suspended in air with the help of holders  440  having slots  445  for acceptance of the components of FIG.  3 B. Alternatively, the reflective polarizing material  415  could be affixed to a thin glass plate (not shown) or sandwiched between thin glass plates (not shown), or affixed to glass wedges. In these cases, the lower part  415   a  of the surface at 30 degrees has the reflective polarizing material while the upper part  415   b  may be a simple thin film mirror coating. 
     FIG. 3C shows another embodiment of the polarizing conversion system, referred to generally as reference numeral  520 , utilizing a reflective polarizing polymer film  515  where the polarization conversion is accomplished with a quarter wave plate  550  in place of the half wave plate in the previous embodiments. Here the reflective polarizing film  515  is applied to the lower half of a thin glass plate  560  and inclined at an angle θ, preferably 30 degrees. The upper half of the glass plate  560  is coated with a mirror coating  570  on the side furthest from the S-polarized light  116  and a quarter wave plate coating  550  closest to the S-polarized light  116 . This serves as the polarization converter, the metal block and the reflector. The S-polarized light  116  is converted to P-polarized light  118  by passing through the two quarter wave plates  550  twice before being transmitted into a direction parallel to the transmitted P-polarized light  117 . The thin plates  560  as described are easily manufactured and placed simply into a metal or plastic holder having slots to assemble a very low cost Polarization Conversion System  520  for use in projection displays. 
     Referring now to FIG. 6, there is illustrated a polarization modulating liquid crystal projection display system, referred to generally as reference numeral  700  in which a polarization conversion system of the present invention is utilized. The polarization modulating liquid crystal projection display system  700  comprises a light source  710 , and a reflector  720  for reflecting and directing unpolarized light in the direction of arrow B. 
     A first array of lenses  730  is provided to sample, subdivide and focus the unpolarized light. The light from the first array of lenses  730  is focused near a polarization conversion system  740  of the present invention. Polarized light from the polarization conversion system  740  is then directed to at least one liquid crystal light valve by a second array of lenses  750 . Preferably three liquid crystal light valves are used, red, green, and blue,  760   a ,  760   b ,  760   c  respectively for a color display. Preferably the liquid crystal light valves  760   a , 760   b , and  760   c  are transmissive as shown in FIG. 6, however, it can be appreciated by someone skilled in the art that reflective light valves can also be used with the inventive polarization conversion system and not depart from the scope and spirit of the present invention. 
     If more than one color liquid crystal light valve is used, then a color separating and combining means  770   a ,  770   b , respectively, must be used to separate and combine the colors of the individual light valves to form a color image. The image from the separating and combining means  770  is then directed towards a projection lens  780  which projects the image onto a screen  790 . The polarization modulating liquid crystal projection display system  700  can be used for a projection computer display, a projection monitor, and a projection video or television system. 
     From the above description it is understood by someone in the art that the Polarization Conversion System of the present invention can be produced at a very low cost. The proposed layouts are simpler and contain fewer elements than the earlier systems and may be extended to higher numerical apertures due to the properties of the polymer based reflective polarizing material coatings as compared with the previous systems that utilize multi-layer thin dielectric films. 
     While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.