Abstract:
An optical system of a liquid crystal projector that is capable of reducing a full length of the optical system including three reflective liquid crystal display panels. In the system, an illuminating unit allows a white light generated from a light source to have any one linear polarized light and a uniform light distribution. A color separator separates the white light from the illuminating unit in accordance with a wavelength band to obtain a first color light, and polarization-converts the remaining lights in accordance with a wavelength band and then separates them into second and third color lights depending on their polarization components. First to third liquid crystal display panels uses the first to third color lights from the color separator to implement first to third color pictures, respectively. A picture combining unit combines the first to third color lights having acquired picture information from the first to third liquid crystal display panels. A projection lens unit projects the combined picture from the picture combining unit onto a screen on an expanded scale. Accordingly, the optical system can reduce the number of optical elements.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a liquid crystal projector, and more particularly to an optical system of a liquid crystal projector that is capable of minimizing the size and the number of optical elements of an optical system using three reflective liquid crystal displays.  
           [0003]    2. Description of the Related Art  
           [0004]    Nowadays, there has been highlighted a flat panel display that is capable of replacing a cathode ray tube display having a limit in the size of screen and a large system size and realizing both a thin thickness and a large-scale screen. The flat panel display includes a projector that projects a small-field picture onto a large screen on an expanded scale.  
           [0005]    The projector employs a cathode ray tube or a liquid crystal display (LCD) as a display device for implementing a small-field picture. Recently, there has been mainly used the LCD to keep up with a trend toward a thin thickness of the projector. Generally, a liquid crystal projector adopts a transmissive or reflective LCD. The liquid crystal projector has been developed with intent to obtain small bulk, low weight and high brightness, whereas an LCD panel has been developed with intent to obtain high aperture ratio and high resolution. Thus, there has been used the reflective LCD panel so as to keep up with a recent trend toward a liquid crystal projector realizing high resolution, small bulk and low cost.  
           [0006]    The liquid crystal projector takes advantage of a light emitting from a light source to implement a picture on the LCD panel. The liquid crystal projector images a picture of the LCD panel onto a screen using a projective optical system to observe a picture imaged on the screen. When the projector is implemented by directly projecting a picture of the LCD panel onto a rear screen, a projection distance should be assured between the screen and the projective optical system. Accordingly, since the rear side of the screen requires a relatively large space to enlarge a thickness of the projector, it is difficult to obtain a thin thickness of the projector.  
           [0007]    In order to solve this problem, a reflecting mirror is introduced between the screen and the projective optical system to change a light path, thereby reducing a thickness of the projector. However, since an arrangement angle of the reflecting mirror has to be more than a critical angle thereof so as to project a picture onto the rear screen without any distortion, there is a limit in reducing a thickness of the system. Also, there is a limit in reducing a thickness of the system due to an inherent full length of an optical system consisting of an illumination system, the LCD and the projective lens system of the projector.  
           [0008]    Referring to FIG. 1, there is shown an optical system of a conventional liquid crystal projector. The liquid crystal projector includes first and second fly eye lenses (FEL&#39;s)  6  and  8 , a polarizing beam splitter (PBS) array  10  and a first condensing lens  12  arranged between a light source  4  and a full-reflecting mirror  14 , and a second condensing lens  16  arranged between the full-reflecting mirror  14  and a first Dichroic mirror  18 .  
           [0009]    A white color light emitting from a lamp of the light source  4  is reflected by a parabolic mirror to be progressed toward the first FEL  6 . The first FEL  6  divides an incident light into the cell units and allows them to be focused onto each lens cell of the second FEL  8 . The second FEL  8  converts an incident light into a parallel light for a specific portion and transmits the same to the PBS array  10 . The PBS array  10  separates an incident light into linear polarized lights having any one light axis, that is, a P-polarized light and a S-polarized light. Then, a half wavelength plate (not shown) partially attached to the rear side of the PBS array  10  converts the transmitted P-polarized light into a S-polarized light. Thus, all incident lights are converted into S-polarized lights by means of the PBS array  10 , thereby permitting most lights emitting from the light source  4  to be incident to picture implementing elements  26 R,  26 G and  26 B of the LCD panel.  
           [0010]    The first condensing lens  12  focuses an incident light from the PBS array  10  onto the full-reflecting mirror  14 . The full-reflecting mirror  14  makes a full reflection of an incident light from the first condensing lens  12  and allows the same to be progressed toward the second condensing lens  16 . The second condensing lens  16  focuses an incident light from the full-reflecting mirror  14  onto the first Dichroic mirror  18 . The first Dichroic mirror  18  transmits a light at a blue color area in the incident lights while reflecting lights at a green color area and at a red color area having a larger wavelength than a blue light.  
           [0011]    Further, the optical system of the liquid crystal projector shown in FIG. 1 includes a second Dichroic mirror  20 , a first polarizing film  22 R and a first polarizing beam splitter prism (PBSP)  24 R arranged between the first Dichroic mirror  18  and the red LCD panel  26 R, a second polarizing film  22 G and a second PBSP  24 G arranged between the second Dichroic mirror  20  and the green LCD panel  26 G, a first relay lens  27 , a second full-reflecting mirror  28 , a second relay lens  29 , a third polarizing film  22 B and a third PBSP  24 B arranged between the first Dichroic mirror  18  and the blue LCD panel  26 B, a Dichroic prism  30  arranged among the first to third PBSP&#39;s  24 R,  24 G and  24 B, and a projection lens  32  installed in opposition to an light output surface of the Dichroic prism  30 .  
           [0012]    The second Dichroic mirror  20  reflects a light at the blue area in lights reflected from the first Dichroic mirror  18  and being incident thereto to progress it toward the second polarizing film  22 G while transmitting a light a the red area to progress it toward the first polarizing film  22 R. The second full-reflecting mirror  28  reflects a light at the blue area transmitted from the first Dichroic mirror  18  and being incident thereto to progress it toward the third polarizing film  22 B.  
           [0013]    The first and second relay lenses  27  and  29  are field lenses, which relay an imaging point of a light at the blue area to re-image the blue light onto the blue LCD panel  26 B. Each of the first to third polarizing films  22 R,  22 G and  22 B transmits only a S-polarized light parallel to its optical axis in the incident lights and allows it to be progressed toward the first to third PBSP&#39;s  24 R,  24 G and  24 B, respectively.  
           [0014]    The first to third PBSP&#39;s  24 R,  24 G and  24 B reflects red, green and blue S-polarized lights transmitted from the first to third polarizing films  22 R,  22 G and  22 B and being incident thereto and allows them to be progressed into the red, green and blue LCD panels  26 R,  26 G and  26 B, respectively. Further, the first to third PBSP&#39;s  24 R,  24 G and  24 B obtain picture information from the red, green and blue LCD panel  26 R,  26 G and  26 B to transmit red, green and blue lights converted into P-polarized lights, respectively and allows them to be progressed toward the Dichroic prism  30 .  
           [0015]    Each of the red, green and blue LCD panels  26 R,  26 G and  26 B is a reflective LCD panel, which converts a S-polarized light reflected from each of the first to third PBSP  24 R,  24 G and  24 B and being incident thereto into a P-polarized light to thereby implement a picture.  
           [0016]    The Dichroic prism  30  obtains picture information from the red, green and blue LCD panels  26 R,  26 G and  26 B to combine the incident red, green and blue lights and output the combined light to the projection lens  32 . First and second polarization converting films (not shown) for converting P-polarized lights from the first and third PBSP&#39;s  24 R and  24 B into S-polarized lights are arranged between the first and third PBSP&#39;s  24 R and  24 B and the Dichroic prism  30 , respectively. Accordingly, the Dichroic prism  30  reflects red and blue lights having a S-polarization component being inputted via the first and second polarization converting films into the projection lens  32 . At the same time, the Dichroic prism  30  transmits a red light having a P-polarization component inputted via the second PBSP  24 G into the projection lens  32 , to thereby combine the red, green and blue lights. The projection lens  32  magnifies a picture being inputted from the Dichroic prism  30  and projects it onto the screen.  
           [0017]    The conventional liquid crystal projector having as described above requires a plurality of Dichroic mirrors to separate a white light from the light source into red, green and blue colors because of an adoption of three LCD panels. Also, since the conventional liquid crystal projector uses a reflective LCD panel, it requires a plurality of PBSP&#39;s differentiating paths of input and output lights on a basis of that reflective LCD panel. As an optical system of the conventional liquid crystal projector employing three reflective LCD panels requires a large number of optical elements, the size of the optical system is enlarged. Particularly, since the optical system has an inherent full length L, it has a limit in reducing a thickness thereof proportional to said inherent full length L. Thus, it becomes difficult to realize a thin thickness of the optical system.  
           [0018]    Accordingly, there has been suggested an optical system that has a two-layer structure in which a color separator is arranged at the upper layer of the color-combining part and the projecting lens system so as to reduce a full length L of the optical system. However, the two-layer structure optical system also employs a large number of optical elements to have a limit in reducing its size.  
         SUMMARY OF THE INVENTION  
         [0019]    Accordingly, it is an object of the present invention to provide an optical system of a liquid crystal projector that is capable of reducing the number of optical elements as well as the size of the optical system.  
           [0020]    In order to achieve these and other objects of the invention, an optical system of a liquid crystal projector according to an embodiment of the present invention includes an illuminating unit for allowing a white light generated from a light source to have any one linear polarized light and a uniform light distribution; a color separator for separating the white light from the illuminating unit in accordance with a wavelength band to obtain a first color light and for polarization-converting the remaining lights in accordance with a wavelength band and then separating them into second and third color lights depending on their polarization components; first to third liquid crystal display panels using the first to third color lights from the color separator to implement first to third color pictures, respectively; a picture combining unit for combining the first to third color lights having acquired picture information from the first to third liquid crystal display panels; and a projection lens unit for projecting the combined picture from the picture combining unit onto a screen on an expanded scale.  
           [0021]    In the optical system, the color separator includes a first Dichroic mirror for separating the white light in accordance with a wavelength band and allowing the separated first color light to be progressed toward the first liquid crystal display panel; and a first color selector for polarization-converting the remaining lights from the first Dichroic mirror in accordance with a wavelength band to differentiate polarization components of the second and third color lights. The optical system further includes a first polarized light separating prism for separating the second and third color lights from the first color selector in accordance with a polarization direction and allowing them to be progressed toward the second and third liquid crystal display panels, respectively.  
           [0022]    Particularly, each of said first to third liquid crystal display panels is a reflective liquid crystal display panel for converting and outputting a polarization component of an incident light depending on the picture information; and said first polarized light separating prism for allowing the second and third color lights polarization-converted by acquiring picture information from the second and third liquid crystal display panels to be progressed into the picture combining unit. The optical system further includes a second polarized light separating prism for allowing the first color light from the Dichroic mirror to be incident to the first liquid crystal display panel and allowing the first color light polarization-converted by acquiring picture information from the first liquid crystal display panel to be progressed toward the picture combining unit.  
           [0023]    In the optical system, the picture combining unit further includes any one of a third polarized light separating prism and a Dichroic prism for acquiring picture information from the first to third liquid crystal display panels to combine the first to third color lights being incident thereto via the first and second polarized light separating prisms and output the combined light.  
           [0024]    The picture combining unit further includes a second color selector for selectively converting polarization components of the second and third color lights outputted from the first polarizing prism in accordance with a wavelength band and allowing it to be progressed toward any one of the third polarized light separating prism and the Dichroic prism in the same linear polarized state.  
           [0025]    In the optical system, the first and second color selectors make a polarization conversion of the red light while transmitting the light at other wavelength band as it is.  
           [0026]    The optical system further includes a half wavelength plate for converting a linear polarized light of the first color light having acquired the picture information from the second polarizing prism and allowing it to be progressed toward any one of the third polarized light separating prism and the Dichroic prism.  
           [0027]    The optical system further includes a third color selector for polarization-converting the combined picture from the picture combining unit in accordance with a wavelength band and allowing it to be progressed toward the projection lens.  
           [0028]    Herein, if a P-polarization screen for transmitting only a P-polarized light is applied to said screen, then the third color selector allows all lights going between the picture combining unit and the projection lens unit to be converted into P-polarized lights.  
           [0029]    In example of another system, the picture combining unit includes a second Dichroic mirror for selectively reflecting and transmitting the first to third color lights from the first and second polarized light separating prisms in accordance with a wavelength band to combine a picture.  
           [0030]    The optical system further includes first and second color light filters arranged between the second and third liquid crystal display panels and the first polarized light separating prism, respectively to enhance color purity. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which:  
         [0032]    [0032]FIG. 1 is a plan view showing a configuration of an optical system of a conventional liquid crystal projector;  
         [0033]    [0033]FIG. 2 is a plan view showing a configuration of an optical system of a liquid crystal projector according to an embodiment of the present invention;  
         [0034]    [0034]FIG. 3 is a transmission characteristic diagram of the first color selector shown in FIG. 2; and  
         [0035]    [0035]FIG. 4 is a plan view showing a configuration of an optical system of a liquid crystal projector according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0036]    Referring to FIG. 2, there is shown an optical system of a liquid crystal projector according to an embodiment of the present invention.  
         [0037]    The optical system includes three reflective LCD panels  46 R,  46 G and  46 B for displaying a picture, an illuminating unit for allowing a white light from a light source  30  to be uniformly and efficiently irradiated onto the LCD panels  46 R,  46 G and  46 B, a color separator for spatially separating a white light from the illuminating unit into red, green and blue color lights and allowing the separated color lights to be the reflective LCD panels  46 R,  46 G and  46 B, a light combining unit for combining three color lights obtaining picture information from the reflective LCD panels  46 R,  46 G and  46 B, and a projection lens  50  for magnifying and projecting the combined picture.  
         [0038]    The illuminating unit consists of the light source  30 , first and second FEL&#39;s  32 A and  32 B and a PBS array  34 . The color separator includes a Dichroic mirror  38 , a first color-selective polarization-converting device  42 , hereinafter referred to as “color selector” for making a polarization conversion in accordance with a wavelength area, and a first PBSP  44 A. The light combining unit consists of said first PBSP  44 A, a second PBSP  44 B, a second color selector  42 B and a third PBSP  44 C.  
         [0039]    A white color light emitting from the light source  30  goes toward the first FEL  32 A. The first FEL  32 A divides an incident light into the cell units and allows them to be focused onto each lens cell of the second FEL  32 B. The second FEL  32 B converts an incident light into a parallel light for a specific portion. The PBS array  34  is integral to the second FEL  32 B to separate an incident light into a P-polarized light and a S-polarized light. Then, a half wavelength (λ/2) plate (not shown) partially attached to the rear side of the PBS array  34  converts the transmitted P-polarized light into a S-polarized light. The first condensing lens  36 A focuses an incident light. Such an illuminating unit converts all incident lights into a linear polarized light in any one direction, that is, a S-polarized light in such a manner to be uniformly incident to the LCD panels  46 R,  46 G and  46 B, thereby improving a light efficiency.  
         [0040]    The first Dichroic mirror  38  selectively reflects or transmits an incident light from the illuminating unit in a wavelength area. For instance, the first Dichroic mirror  38  transmits a light at a blue color area in the incident lights while reflecting lights at a green color area and at a red color area having a larger wavelength than said light at the blue color area.  
         [0041]    The first color selector  42 A make a selective polarization conversion of lights at the red and green areas transmitting the first Dichroic mirror  38  and being incident thereto in accordance with a wavelength area. For instance, the first color selector  42 A rotates a light at the blue area at 90° to convert a S-polarized light into a P-polarized light while transmitting a light at the red area with keeping a S-polarization state as it is. The first color selector  42 A is most preferably designed such that it makes a polarization conversion of a red light R having a good transmittance characteristic after polarization conversion as can be seen from a transmission characteristic diagram of the color selector shown in FIG. 3. Such a first color selector  42 A making a selective polarization conversion according to a wavelength area has been disclosed in U.S. Pat. No. 5,990, 996.  
         [0042]    A transmitted light output side and a reflected light output side of the Dichroic mirror  38  are further provided with condensing lenses  36 B and  36 C and S polarizers  40 A and  40 B, respectively. The condensing lenses  36 B and  36 C focuses a light and the S polarizers  40 A and  40 B, thereby enhancing a purity of the S-polarized light.  
         [0043]    The first PBSP  44 A transmits a P-polarized light at the red area polarization-converted by means of the first color selector  42 A and being incident thereto toward the red LCD panel  46 R while reflecting a S-polarized light at the green area being incident thereto as it is into the red LCD panel  46 G. Further, the first PBSP  44 A reflects a S-polarized light at the red area polarization-converted by acquiring picture information from the red LCD panel  46 R and being incident thereto into the third PBSP  44 C while transmitting a P-polarized light at the green area polarization-converted by acquiring picture information from the red LCD panel  46 G and being incident thereto toward the third PBSP  44 C.  
         [0044]    The second PBSP  44 B reflects a S-polarized light at the blue area reflected from the Dichroic mirror  38  and being incident thereto into the blue LCD panel  46 B while transmitting a blue light converted into a P-polarized light by acquiring picture information from the blue LCD panel  46 B toward the third PBSP  44 C.  
         [0045]    Each of the red, green and blue LCD panels  46 R,  46 G and  46 B is a reflective LCD panel, which polarization-converts and outputs a line-polarized light being incident thereto to implement the corresponding color picture.  
         [0046]    The transmitted light output side of the second PBSP  44 B is further provided with a P polarizer  40 C and a half wavelength (λ/2) plate  48 . The P polarizer  40 C transmits only a P-polarized light to enhance a purity of a P-polarized light acquiring green picture information. The half wavelength plate  48  converts an incident P-polarized light into a S-polarized light.  
         [0047]    The second color selector  42 B selectively polarization-converts an incident light from the first PBSP  44 A in accordance with a wavelength area in similarity to the fore-mentioned first color selector  42 A. More specifically, the second color selector  42 B transmits a green P-polarized light acquiring picture information as it is while rotating a red S-polarized light acquiring picture information at 90° to output it as a P-polarized light.  
         [0048]    The third PBSP  44 C combines incident red, green and blue lights acquiring picture information and output them to a projection lens unit  50 . In other words, the third PBSP  44 C reflects a blue light converted into a S-polarized light by means of the half wavelength plate  48  and being incident thereto into the projection lens  50  while transmitting a red light converted into a P-polarized light by means of the second color selector  42 B and being incident thereto and a green light being incident thereto without any polarization conversion toward the projection lens  50 . The third PBSP  44 C can be replaced by a Dichroic prism. The projection lens  50  projects the combined picture inputted from the third PBSP  44 C onto the screen on an expanded scale.  
         [0049]    As described above, the optical system of the liquid crystal projector according to the embodiment of the present invention employs the color selectors for making a selective polarization conversion according to a wavelength area, to thereby reduce the number of PBSP&#39;s differentiating paths of input and output lights of the Dichroic mirrors for a light separation and the LCD panels. Accordingly, it can reduce the number of optical elements and the size of optical system. Particularly, the present optical system is capable of more reducing a full length L′ of the optical system than a full length L of the conventional optical system shown in FIG. 1, to thereby reduce the thickness of the liquid crystal projector.  
         [0050]    The optical system of the liquid crystal projector according to the embodiment of the present invention further includes a third color selector  42 C between the third PBSP  44 C and the projection lens  50  so as to allow polarization components of all lights being incident to the projection lens unit  50  to be a P wave. The third color selector  42 C rotates only a blue S-polarized light reflected from the third PBSP  44 C and being incident thereto at 90° to convert it into a P wave while transmitting red and green P-polarized lights going through the third PBSP  44 C as it is.  
         [0051]    If a P-polarization screen transmitting only a P-polarized light is applied to the projective screen for allowing polarization components of all lights being incident to the projection lens  50  to be consistently P-polarized lights and imaging a picture enlarged by means of the projection lens  50 , it becomes possible to improve a contrast of picture. Because the P polarizer adopted for the projective screen can absorb an external light.  
         [0052]    Referring to FIG. 4, there is shown an optical system of a liquid crystal projector according to another embodiment of the present invention.  
         [0053]    When compared with the optical system shown in FIG. 2, the optical system of FIG. 4 uses a second Dichroic mirror  38 B, instead of the third PBSP  44 C, as a combining unit, and further includes green and red color filters  54 A and  54 B provided between the red and green LCD panels  46 R and  46 G and the second PBSP  44 B to enhance a color purity and a full-reflecting mirror  52  provided between the light source  30  and the first Dichroic mirror  38 A to change a light path. Also, the optical system shown in FIG. 4 excludes the polarizing filters  40 A,  40 B and  40 C, but may additionally include them. Herein, a detailed explanation as to elements of the optical system of FIG. 4 identical to the optical system shown in FIG. 2 will be omitted.  
         [0054]    First and second FEL&#39;s  32 A and  32 B, a PBS array  34  and first and second condensing lenses  36 A and  36 B allow a white light from the light source  30  to be uniformly and efficiently irradiated onto red, green and blue LCD panels  46 R,  46 G and  46 B. The full-reflecting mirror  52  full-reflects an incident light from the first condensing lens  36 A to progress it toward the second condensing lens  36 B.  
         [0055]    A first Dichroic mirror  38 A reflects a S-polarized light at the blue color area and allows it to be irradiated, via a third condensing lens  36 C and a first PBSP  44 A, onto the blue LCD panel  46 B. Further, the first Dichroic mirror  38 A transmits S-polarized lights at the red and green color area having a larger wavelength than a light at the blue color area and allows them to be incident to the red and green LCD panels  46 R and  46 G via a first color selector  42 A, a fourth condensing lens  36 D and a second PBSP  44 A.  
         [0056]    The first color selector  42 A rotates a S-polarized light at the blue area at 90° as mentioned above to convert it into a P-polarized light, thereby allowing it to be transmitted to the second PBSP  44 B and to be incident to the red LCD panel  46 R. Further, the first color selector  42 A transmits a light at the green area with keeping a S-polarized light as it is, thereby allowing it to be reflected from the second PBSP  44 B and to be incident to the green LCD panel  46 G.  
         [0057]    The red and green color filters  54 A and  54 B installed at the second PBSP  44 B in opposition to the red and green LCD panels  46 R and  46 G improve purities of red and green colors, respectively.  
         [0058]    The second Dichroic mirror  38 B transmits a red light converted into a S-polarized light by acquiring picture information from the red LCD panel  46 R and being incident thereto and a green light converted into a P-polarized light by acquiring picture information from the green LCD panel  46 G and being incident thereto toward the projection lens  50 . Further, the second Dichroic mirror  38 B reflects a blue light converted into a P-polarized light by acquiring picture information from the blue LCD panel  46 B and being incident thereto into the projection lens  50 . As described above, the second Dichroic mirror  38 B combines red, green and blue lights acquiring picture information and allows the combined light to be progressed toward the projection lens  50 .  
         [0059]    The projection lens unit  50  projects the combined picture inputted from the second Dichroic mirror  38 B onto the screen on an expanded scale.  
         [0060]    As described above, the optical system of the liquid crystal projector according to another embodiment of the present invention employs the color selectors for making a selective polarization conversion according to a wavelength area, to thereby reduce the number of Dichroic mirrors for a light separation and the number of PBSP&#39;s for a light combination. Accordingly, it can reduce the number of optical elements and the size of optical system. Particularly, the present optical system is capable of more reducing a full length L″ of the optical system than a full length L of the conventional optical system shown in FIG. 1, to thereby reduce the thickness of the liquid crystal projector.  
         [0061]    As described above, according to the present invention, the color selectors for making a polarization conversion according to a wavelength are used to the number of optical elements such as the Dichroic mirror and the PBSP, so that it becomes possible to simplify the optical system and thus reduce the size of the optical system. Particularly, a full length of the optical system of the liquid crystal projector can be reduced to make the projector having a thin thickness.  
         [0062]    In addition, according to the present invention, the P-polarization screen is adopted and the color selectors convert all light being incident to the projection lens unit into P-polarized lights, so that it becomes possible to improve a contrast of picture.  
         [0063]    Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.