Abstract:
The present invention provides a projecting device for displaying electrical images. The projecting device comprises an illumination device for emitting a polarized trichrome light of various polarizations, three modulating devices for modulating three polarized monochrome lights to display the same electrical image, a light separating device having two dichroic mirrors perpendicular to two polarization beam splitting mirrors. One dichroic mirror and the two polarization beam splitting mirrors are used for separating the polarized trichrome light into three monochrome lights and passing each monochrome light to a correspondent modulating device for optical modulation. The three modulated lights reflected from the three modulating devices are transmitted to another dichroic mirror and synthesized into an output light beam for displaying the electrical image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a projecting device, and more specifically, to a projecting device for displaying electrical images. 
     2. Description of the Prior Art 
     Please refer to FIG. 1. FIG. 1 is a perspective view of a prior art projecting device 10 of an LCD projector. The projecting device 10 comprises a light source 12 that produces a white light beam, a uniform illumination optical device 14 installed in front of the light source 12 for converging the white light emitted from the light source 12 into a uniformly distributed rectangular light beam, a light separating device 16 for separating the rectangular light beam into red, green and blue color input light beams, a dichroic prism 18 having three input sides and an output side for synthesizing the three input light beams into an output light beam, three modulating panels 20 each formed by a monochrome liquid crystal panel and separately installed in front of the three input sides of the dichroic prism 18 for modulating the three input light beams, three focusing lenses 17, 19 and 21 separately installed in front of the three modulating panels 20 for focusing the three input light beams from the light separating device 16 onto the three modulating panels 20, and a projecting lens 22 installed in front of the output side of the dichroic prism 18 for projecting the synthesized output light beam from the dichroic prism 18 onto a screen 24. Each of the modulating panels 20 is formed by a transparent monochrome liquid crystal panel for displaying a monochrome image. The dichroic prism 18 synthesizes the three monochrome images to form the output color image. 
     The light separating device 16 comprises a first dichroic mirror 26 for separating the red light from the rectangular light beam, a reflecting mirror 27 for reflecting the red light from the first dichroic mirror 26 onto the focusing lens 17, a second dichroic mirror 28 for separating light reflected from the first dichroic mirror 26 by reflecting blue light to the focusing lens 19, and two optical lenses 30 and two reflecting mirrors 32 for passing and reflecting green light to the focusing lens 21. FIG. 1 clearly shows that the distance traveled by the green light is much longer than that of the red and blue lights. Since traveling distances affect light intensities, the two optical lenses 30 installed in front of the two reflecting mirrors 32 are essential to converge the green light so as to compensate for the loss of its light intensity. 
     However, the installation of the two optical lenses 30 makes a structure of the light separating device 16 complicated and costly. It is therefore an important objective to provide a projecting device with a simple structure that can solve the problem of unequal traveling distances for the three color beams. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide a projecting device to solve the above mentioned problem. 
     Briefly, in a preferred embodiment, the present invention provides a projecting device comprising: 
     an illumination device for emitting a polarized trichrome light comprising three primary colors with various polarization; 
     a first, second and third modulating device, each modulating, reflecting, and changing the polarization of its own distinct polarized monochrome light; 
     a light separating device having two dichroic mirrors arranged along a first diagonal direction and two polarization beam splitting mirrors arranged along a second diagonal direction perpendicular to the first diagonal direction; 
     wherein the first dichroic mirror separates the polarized trichrome light into a first polarized monochrome light and a polarized dichrome light, and transmits these two lights separately to the first and second polarization beam splitting mirror, the first polarization beam splitting mirror transmits the first polarized monochrome light to the first modulating device and transmits the modulated light reflected from the first modulating device to the second dichroic mirror, the second polarization beam splitting mirror separates the polarized dichrome light into second and third polarized monochrome lights which are directly transmitted to the second and third modulating devices, and then transmits the modulated lights reflected from the second and third modulating devices to the second dichroic mirror, and the modulated lights transmitted from the first and second polarization beam splitting mirrors are synthesized by the second dichroic mirror to form an output light beam. 
     It is an advantage of the present invention that the projecting device has a very simple structure. 
     It is another advantage of the present invention that the traveling distances of the three polarized light beams are approximately equal and shorter than that of the prior art device. 
     These and other objects and the advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art projecting device of an LCD projector. 
     FIG. 2 is a perspective view of a projecting device of an LCD projector according to the present invention. 
     FIG. 3 is a schematic diagram of the dichroic-polarization beam splitter prism of the present invention. 
     FIG. 4 is a schematic diagram of the modulation apparatus of the present invention. 
     FIG. 5 is a schematic diagram of the illumination device of the projecting device of an LCD projector shown in FIG. 2. 
     FIGS. 6 and 7 are schematic diagrams of the alternative light separating device and projecting device of an LCD projector, respectively, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to FIGS. 2 to 4. FIG. 2 is a perspective view of a projecting device 40 of an LCD projector according to the present invention. FIG. 3 is a schematic diagram of the dichroic-polarization beam splitter prism 50 of the present invention. FIG. 4 is a schematic diagram of the modulation apparatus 44 of the present invention. The projecting device 40 comprises an illumination device 42, three modulating devices 44, 46 and 48, a dichroic-polarization beam splitter prism 50 and a projecting lens 52. The illumination device 42 emits a uniformly distributed approximately white light beam which comprises red, green and blue colors, each with distinct polarizations. For example, the illumination device 42 of the present invention emits a polarized light comprising red R, green G* and blue B polarized lights wherein the polarization of the green G* polarized light is different from the other polarized lights R and B. The three modulating devices 44, 46 and 48 are used for modulating and shifting the polarization of the three polarized monochrome lights by utilizing a method involving reflection of light. The dichroic-polarization beam splitter prism 50 receives and sends the three polarized lights R,G* and B to the three corresponding modulating devices 44, 46, and 48 where each polarized light undergoes processing, modulation, and change in polarization with synthesizing of the three polarized monochrome lights and formation of an output light beam. The projecting lens 52 is installed in front of the output side of the dichroic-polarization beam splitter prism 50 for projecting the output light beam synthesized by the three polarized monochrome lights and emitted from the output side of the dichroic-polarization beam splitter prism 50 to a screen 54. 
     The dichroic-polarization beam splitter prism 50 comprises four identical triangular prisms 60, each having a thin reflective coating on their interfaces. The dichroic-polarization beam splitter prism 50 further comprises a first dichroic mirror 62 and a second dichroic mirror 64 installed within the dichroic-polarization beam splitter prism 50 along the first diagonal line and the first polarization beam splitting mirror 66 and second polarization beam splitting mirror 68 are also installed within the dichroic-polarization beam splitter prism 50 along the second diagonal line perpendicular to the first and second dichroic mirrors. 
     Each of the three modulating devices 44, 46 and 48 comprises a mirror-type optical modulator 76 for modulating and reflecting an incident light to generate a modulated light, and a quarter-wave retarder 78 for retarding both the incident light and modulated light by a quarter of a wavelength so that the incident light of the modulating device and the modulated light generated by the modulating device have opposite polarizations. The mirror-type optical modulator 76 can be a digital micro-mirror device or a mirror-type liquid crystal display. 
     After the polarized trichrome light emitted from the illumination device 42 enters the dichroic-polarization beam splitter prism 50, it is separated by the first dichroic mirror 62 into a first blue polarized monochrome light B and a polarized dichrome light consisting of a red polarized light R and green polarized light G*. The blue polarized light B is transmitted to the first polarization beam splitting mirror 66 while the red polarized light R and green polarized light G* light is transmitted to the second polarization beam splitting mirror 68. The first polarization beam splitting mirror 66 has a special coating and can reflect a P-STATE polarized light and pass an S-STATE polarized light. Thus, the blue polarized light B is transmitted by the first polarization beam splitting mirror 66 to the first modulating device 48 which forms and reflects a modulated light B* which is transmitted by the first polarization beam splitting mirror 66 to the second dichroic mirror 64. The second polarization beam splitting mirror 68 passes P-STATE polarized light and reflects S-STATE polarized light so that the red polarized light R is reflected to the second modulating device 44 where it is modulated into red polarized light R* and the green polarized light G* is transmitted to the third modulating device 46 where it is modulated into green polarized light G. Both modulated lights are transmitted by the second polarization beam splitting mirror 68 to the second dichroic mirror 64. Finally, the second dichroic mirror 64 will reflect the blue polarized light B* and pass the red polarized light R* and green polarized light G into the projecting lens 52 to form a synthesized polarized trichrome light B*GR* which is projected onto the screen 54. The simple structure of the projecting device 40 of the present invention uses the dichroic-polarization beam splitter prism 50 to separate and synthesize the trichrome light beam where the traveling distances for each of the polarized light beams are approximately equal and shorter than the distances of the input light beams of the prior art projecting device 10. 
     The dichroic-polarization beam splitter prism 50 uses a set of two dichroic mirrors 62, 64 and two polarization beam splitting mirrors 66, 68 to separate and individually modulate the three polarized lights. Therefore, the illumination device 42 must provide the dichroic-polarization beam splitter prism 50 with a polarized trichrome light comprising a polarized monochrome light and a dichroic polarized light with a different polarization for final output of a perfectly synthesized trichrome light beam. 
     Please refer to FIG. 5. FIG. 5 is a schematic diagram of the illumination device 42 of the projecting device 40 of an LCD projector. The illumination device 42 comprises a light source 80 for generating a trichrome unpolarized light RR*GG*BB* which comprises red, green and blue lights, a light polarizing device 82 for transforming the trichrome unpolarized light RR*GG*BB* into a polarized trichrome light RGB, and a light separating device 84 for separating the polarized trichrome light RGB into a green polarized light G and a polarized dichrome light RB. The light separating device 84 further comprises two dichroic mirrors 86, 88 for separating the green polarized light G from the polarized trichrome light RGB and synthesizing the green polarized light G* into a polarized trichrome light RG*B, two reflecting mirrors 90, 92 for reflecting the green polarized light G and the polarized dichrome light RB, and a half-wave retarder 94 for retarding and modulating the green polarized light G into the green polarized light G*. As shown in FIG. 2, the illumination device 42 outputs a polarized trichrome light RG*B to the first dichroic mirror 62 of the dichroic-polarization beam splitter prism 50. 
     Please refer to FIG. 6 and FIG. 7. FIGS. 6 and 7 represent schematic diagrams of an alternative light separating device 100 and a projecting device 110 of an LCD projector, respectively, according to the present invention. The light separating device 100 has a far simpler structure than the previously described light separating device 84 and comprises a dichroic mirror 102, two reflecting mirrors 104 and 106, and a half-wave retarder 108. The output orientation of the green polarized light G* is perpendicular to that of the polarized dichrome light RB. Unlike the previously described dichroic mirror 62, the dichroic mirror 112 shown in FIG. 7 is coated to reflect both the green polarized light G* and the blue polarized light B while passing the red polarized light R. 
     From the foregoing, it is clear that the projecting devices 40 and 110 of the present invention are simple in structure and provide approximately equal light traveling distances that are shorter than that of the prior art device projecting device 10. Thus, special optical lenses to compensate for the loss of light intensity are not needed in projecting devices 40 and 110 thereby simplifying the structure, lowering manufacturing costs, and improving image quality. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the propeller may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.