Abstract:
An optical system for a projection display comprising a light source; an illumination module for receiving light from the light source; a prism assembly including a first prism pair consisting of a first prism and a second prism, and a second prism pair consisting of a third prism and a fourth prism, wherein a first dichroic mirror is provided in between the first prism and the second prism, and a second dichroic mirror is provided in between the third prism and the fourth prism, a polarizing beam splitter is provided in between the first prism pair and the second prism pair; a projection lens; and first to third reflective light valves for changing the polarity of light and for reflecting light to the prism assembly.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a projection display, and more particularly to an optical system having reflective light valves for a projection display.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, the importance of information visualization has been widely perceived and the need for projection displays has been increased rapidly. High projection quality, low production costs, and lightweight designs are the primary R&amp;D objectives of the industry.  
           [0005]    U.S. Pat. No. 5,777,789 to Chiu discloses an optical system consisting of reflective light valves, a polarizing beam splitter, color image combining prisms, an illumination system, and a screen. The primary disadvantages of this prior art optical system lie in that due to angular dependence of the polarizing beam splitter, in other words, for light beams incident on the polarizing beam splitter at different angles, the transmitted light beams are polarized in different directions and this can result in a serious problem of non-uniform contrast and a problem of low contrast; the optical system involves a long back focal length and this causes a problem of a projected image of low brightness or of the need for a big size projection lens; and the optical system involves a serious problem of birefringence that can affect projection quality thereof.  
           [0006]    U.S. Pat. No. 5,826,959 to Atsuchi discloses an image projection apparatus consisting of three dichroic mirrors, three polarizing beam splitters, a plurality of reflective light valves, a cross dichroic prism device, an illumination system, and a screen. The primary disadvantages of this prior art image projection apparatus lie in that it includes too many elements that result in a complicated structure, high production costs, and a big size; and it involves a long back focal length and this causes a problem of a projected image of low brightness or of the need for a big size projection lens.  
         SUMMARY OF INVENTION  
         [0007]    An object of the invention is to provide an optical system for a projection display, wherein a prism assembly includes a polarizing beam splitter, e.g. a wire grid polarizer, involving no angular dependence instead of a conventional polarizing beam splitter involving angular dependence, thereby avoiding or mitigating the above-mentioned problem of non-uniform contrast and problem of low contrast.  
           [0008]    Another object of the invention is to provide an optical system for a projection display, which involves no any long back focal length. Therefore, the above-mentioned low brightness problem of a projected image or of the need for a big size projection lens can be avoided or mitigated.  
           [0009]    Still another object of the invention is to provide an optical system for a projection display, which has a unique geometrical structure that can help avoid or mitigate the above-mentioned problem of birefringence. Therefore, the projection quality can be improved.  
           [0010]    Yet another object of the invention is to provide an optical system for a projection display, which is simple in structure and thereby can help avoid or mitigate the above-mentioned problems of high production costs and of a big size.  
           [0011]    To achieve the above and other objects, the present invention provides an optical system for a projection display comprising a light source for providing light; an illumination module for receiving light from the light source and for outputting linear polarized white light; a color-selecting component for receiving said linear polarized white light and for selectively outputting first color light of a second polarity, second color light of a first polarity, and third color light of the second polarity; a prism assembly including a first prism pair consisting of a first prism and a second prism, and a second prism pair consisting of a third prism and a fourth prism, wherein a first dichroic mirror is provided in between the first prism and the second prism, and a second dichroic mirror is provided in between the third prism and the fourth prism, a polarizing beam splitter is provided in between the first prism pair and the second prism pair, the first dichroic mirror can separate the third color light apart from the first color light and the second color light, and the second dichroic mirror can recombine the third color light together with the first color light and the second color light, and the prism assembly receives the first color light of the second polarity, the second color light of the first polarity, and the third color light of the second polarity from the color-selecting component; a projection lens; and first to third reflective light valves for changing the polarity of the first color light, the second color light, and the third color light respectively and for reflecting the first color light, the second color light, and the third color light respectively to the prism assembly so that the first color light, the second color light, and the third color light can pass through the prism assembly and be irradiated into the projection lens. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    The above and other objects, advantages, and features of the present invention will be understood from the following detailed description of the invention when considered in connection with the accompanying drawings below.  
         [0013]    [0013]FIG. 1 is a schematic view showing an optical system for a projection display in accordance with a preferred embodiment of the invention.  
         [0014]    [0014]FIG. 2 is a schematic view showing the imaging system in the optical system.  
         [0015]    [0015]FIG. 3 is a schematic view showing other imaging system in the optical system.  
         [0016]    [0016]FIG. 4 is a schematic view showing another imaging system in the optical system 
     
    
     DETAILED DESCRIPTION  
       [0017]    Referring to FIGS. 1 and 2, an optical system for a projection display in accordance with a preferred embodiment of the invention comprises an illumination system  1  and an imaging system  2 . The illumination system  1  includes a light source  12  for providing light and an illumination module  14  for receiving light from the light source  12  and for outputting linear polarized white light  10 .  
         [0018]    The imaging system  2  comprises a color separating assembly  20  and a projection lens  30 . The color separating assembly  20  comprises a prism assembly  22 , a color-selecting component  24 , and three reflective light valves  25 R,  25 G, and  25 B. The reflective light valves  25 R,  25 G, and  25 B can reflect red light, green light, and blue light respectively and change their polarity, i.e. change S-polarized light into P-polarized light and vice versa. In the present invention, referring to FIG. 2, P-polarized light means the vector of the electrical field of the electromagnetic wave thereof is parallel to the paper plane and perpendicular to the light rays of FIG. 2; S-polarized light means the vector of the electrical field of the electro-magnetic wave thereof is orthogonal to the paper plane of FIG. 2.  
         [0019]    Referring to FIG. 2, the prism assembly  22  includes four prisms  221 ,  222 ,  223 , and  224  made of, e.g., glass. The prisms  221  and  223  have similar shape and size, and the cross-sections thereof are isosceles triangles. Although, in this embodiment, the base angles of isosceles triangles are 30° for purposes of illustration, the base angles are not limited and preferably within in the range of about 15° to about 35°. The cross-section of the prism  222  is a trapezoid with a right base angle. The length of the base side of the trapezoid is the same as that of the adjacent side of the prism  221 . The height of the prism  222  equals to a half of the length of the base side of the prism  223 . A dichroic mirror  27  is provided in between the base side of the trapezoidal prism  222  and one of the two equal sides of the prism  221 . The slant side of the trapezoidal prism  222  and the other of the two equal sides of the prism  221  are in the same line, as shown in FIG. 2, i.e. the side surface  221   a  of the prism  221  and the side surface  222   a  are in the same plane.  
         [0020]    The cross-section of the prism  224  is an equilateral triangle. The length of any side thereof is equal to that of the adjacent side of the prism  223 . A dichroic mirror  28  is provided in between the prism  224  and the prism  223 . A side surface  223   a  of the prism  223  and a side surface  224   a  of the prism  224  are in the same plane. A wire grid polarizer  29  is provided on the side surface  223   a  and the side surface  224   a . The wire grid polarizer  29  is spaced apart, by a small gap  220 , from the side surface  221   a  and the side surface  222   a , wherein the gap  220  is filled with air or the optical liquid, such as optical liquid, optical glue or gel, which has the refractive index close to the material of the prism assembly  22 .  
         [0021]    Both the dichroic mirror  27  and the dichroic mirror  28  transmit blue light and reflect red light and green light. The wire grid polarizer  29  transmits P-polarized light and reflects S-polarized light.  
         [0022]    The color-selecting component  24  and the three reflective light valves  25 R,  25 G, and  25 B are suitably positioned around the prism assembly  22 , as shown in FIG. 2. Those skilled in the art can understand that the light paths of red light, green light, and blue light reflected by the reflective light valves  25 R,  25 G, and  25 B, transmitted through the prism assembly  22 , and irradiated into the projection lens  30  are preferably equal in length. The linear polarized white light  10  enters the color-selecting component  24  and the latter selectively outputs S-polarized red light Rs, P-polarized green light Gp, and S-polarized blue light Bs. The polarized trichromatic light hits the dichroic mirror  27  that transmits S-polarized blue light Bs and reflects S-polarized red light Rs and P-polarized green light Gp. Thereafter, S-polarized blue light Bs passes the surface  222   a  of the prism  222  and the gap  220 , and hits the wire grid polarizer  29  that involves no angular dependence, is reflected and hits the reflective light valve  25 B. The reflective light valve  25 B reflects S-polarized blue light Bs and changes its polarity simultaneously so that S-polarized blue light Bs is changed into P-polarized blue light Bp that is then transmitted through the wire grid polarizer  29  and the dichroic mirror  28  and irradiated into the projection lens  30 .  
         [0023]    The above-mentioned S-polarized red light Rs reflected by the dichroic mirror  27  is totally reflected at the outer surface of the prism  221 , is reflected on the wire grid polarizer  29 , and then hits the reflective light valve  25 R. The reflective light valve  25 R reflects S-polarized red light Rs and changes its polarity simultaneously so that S-polarized red light Rs is reflected into P-polarized red light Rp that is then transmitted through the wire grid polarizer  29 , totally reflected on the outer surface of the prism  223 , reflected by the dichroic mirror  28 , and irradiated into the projection lens  30 .  
         [0024]    The above-mentioned P-polarized green light Gp reflected by the dichroic mirror  27  is totally reflected at the outer surface of the prism  221 , is transmitted through the wire grid polarizer  29 , and then hits the reflective light valve  25 G. The reflective light valve  25 G reflects P-polarized green light Gp and changes its polarity simultaneously so that P-polarized green light Gp is reflected into S-polarized green light Gs that is then reflected from the wire grid polarizer  29 , totally reflected on the outer surface of the prism  223 , reflected by the dichroic mirror  28 , and irradiated into the projection lens  30 .  
         [0025]    Due to that the wire grid polarizer  29  involves no angular dependence, all S-polarized light (orthogonal to the paper plane of FIG. 2) incident on the wire grid polarizer  29  at different angles involves no change in polarity when reflected therefrom, i.e. the vector of the electrical field of the electro-magnetic wave thereof is still orthogonal to the paper plane of FIG. 2; similarly, all P-polarized light (parallel to the paper plane and perpendicular to the light rays of FIG. 2) incident on the wire grid polarizer  29  at different angles involves no change in polarity when transmitted therethrough, i.e. the vector of the electrical field of the electro-magnetic wave thereof is still parallel to the paper plane and perpendicular to the light rays of FIG. 2. Thereby, the above-mentioned problem of non-uniform contrast and problem of low contrast can be avoided or mitigated.  
         [0026]    Referring to FIGS. 3 and 4, to achieve the flexibility of various alignments, the present invention also can replace the prism  224  by a prism  224 ′, and change the cross-section of the prism  224 ′ from an equilateral triangle to trapezoid with a right base angle. Therefore, the reflective light valves  25 R can be moved to the position of the reflective light valves  25 R′, or the white light  10  can be moved to the position of the white light  10 ′. Furthermore, a color-selecting component  31  and a polarizer  32  may be installed between the prism assembly  22  and the projection lens  30  to help purify color and polarized light.  
         [0027]    While a preferred and particular embodiment of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. For example, replacing the wire grid polarizer  29  by a conventional polarizing beam splitter, providing a color filter, a retardation film, or a color polarizer between each of the reflective light valves  25 R,  25 G, and  25 B and the prism assembly  22  to help purify polarized light, providing an additional retardation film or color polarizer on the dichroic mirror  27  and the dichroic mirror  28  to help purify polarized light, and replacing the wire grid polarizer  29  by a double brightness enhanced film (DBEF) with trademark Vikuiti available from 3M all obviously fall within the true spirit and scope of this invention. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.