Color image projecting device

0001The present invention relates to a color image projecting device in accordance with the pre-characterizing clause of claim, 1.

0002A prior art projecting device of this kind provides a light path of one of the monochrome lights in the device which is considerably longer than the light paths of the two other monochrome lights. Accordingly, since the travelling distances of the monochrome lights affect their intensities, two optical lenses have to be installed in the longer light path to converge the respective light so as to compensate for the loss in light intensity. However, the installation of the two optical lenses renders the structure of the light splitting device complicated and costly.

0003This in mind, the invention as claimed aims at simplifying the structure of any such projecting device.

0004This is achieved by the invention in that its light splitting device employs two polarization beam splitting mirrors in addition to two dichroic mirrors to separate three polarized monochrome lights and the three modulating devices modulate these polarized monochrome lights, at the same time changing their polarization. In this way the travelling distances of the three monochrome lights can be made equal.

0005In the following, the invention is illustrated by way of example on the background of the prior art with reference to the accompanying drawings, in which <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a schematic view of a prior art projecting device of an LCD projector,</li><li>Fig. 2 is a schematic view of a projecting device of an LCD projector according to the present invention,</li><li>Fig. 3 is a schematic diagram of a dichroic-polarization beam splitter prism of Fig. 2,</li><li>Fig. 4 is a schematic diagram of one of the modulating devices of Fig. 2,</li><li>Fig. 5 is a schematic diagram of the illumination device of the projecting device shown in Fig. 2 including a light separating device,</li><li>Fig. 6 is a schematic diagrams of an alternative light separating device and</li><li>Fig. 7 is a schematic view of an LCD projector similar to that one of Fig. 2 yet employing the light separating device of Fig. 6.</li></ul>

0006The prior art projecting device 10 of Fig. 1 comprises a light source 12 that produces a white light beam, a uniform illumination 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 splitting 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 transparent 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 focussing lenses 17, 19 and 21 separately installed in front of the three modulating panels 20 for focussing the three input light beams from the light splitting 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 adapted for displaying a monochrome image and the dichroic prism 18 synthesizes the three monochrome images to form the output color image.

0007The light separating device 16 comprises a first dichroic mirror 26 for separating the red light from the rectangular light beam from the uniform illumination device 14, a reflecting mirror 27 for reflecting the red light from the first dichroic mirror 26 onto the focussing lens 17, a second dichroic mirror 28 for separating light reflected from the first dichroic mirror 26 by reflecting blue light to the focussing lens 19, two optical lenses 30 and two reflecting mirrors 32 for passing and reflecting green light to the focussing lens 21. The figure clearly shows that the distance traveled by the green light is much longer than that of the red and blue lights. Since travelling 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, as mentioned before, the installation of the two optical lenses 30 renders the structure of the light splitting device 16 complicated and costly.

0008Fig. 2 shows a projecting device 40 of an LCD projector, this projecting device being constructed according to the present invention. Fig. 3 is a schematic diagram of a dichroic-polarization beam splitter prism 50 in Fig. 2 and Fig. 4 is a schematic diagram of a modulating device 44 in Fig. 2. 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.

0009The 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 a first polarization beam splitting mirror 66 and a second polarization beam splitting mirror 68 installed within the dichroic-polarization beam splitter prism 50 along the second diagonal line perpendicular to the first and second dichroic mirrors.

0010Each 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.

0011After 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.

0012The 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.

0013Please 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.

0014Please 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.

0015From 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.