Abstract:
Optical system of a liquid crystal projector with three reflective liquid crystal displays, wherein a first light path changing part among first to third light path changing parts is designed such that a first color path surrounds three faces of a first of liquid crystal panel except a side of a connector thereof, for avoiding interference of the color light path with the connector, thereby reducing a total length of the optical system.

Description:
This application claims the benefit of the Korean Application No. P2000-72707 filed on Dec. 02, 2000, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal projector, and more particularly, to an optical system of a liquid crystal projector with three reflective liquid crystal displays. 
     2. Background of the Related Art 
     Recently, as demands on large sized, and high resolution pictures become the stronger, projectors are paid attention as displays, for making enlargement projection of a picture by means of a projection lens system. The picture may be projected in front or rear direction. 
     The liquid crystal projector projects a light from a light source to a liquid crystal panel, and therefrom to a screen by means of a projection optical system. When the picture is projected to the screen in rear of the liquid crystal panel directly, a distance of projection distance between the screen and the projection optical system requires a large space in rear of the screen, to make the projector thick, that impedes fabrication of a thin projector. 
     As a solution for this problem, a total reflection mirror may be provided between the screen and the projection optical system, to fold an optical path, that can reduce the thickness of the projector. In this instance, though the thickness of the projector may be reduced further by reducing an angle of dispose of the total reflection mirror, there has been a limit in the angle of dispose between the mirror and the projection optical system for projection of the picture on the screen without distortion, and there has been a limitation in reduction of a system thickness coming from intrinsically required total length of an optical system consisting of a lighting system, the liquid crystal panel, and the projection lens system. 
     Accordingly, there have been various plans for reducing the total length of the optical system. FIG. 1 illustrates a related art optical system of a liquid crystal projector, employing a polarizing beam sprite prism which reflects, or transmits an incident light depending on a wavelength and a direction of polarization, for reducing a total length of the optical system. 
     In detail, the optical system in FIG. 1 is provided with a fly eye lens (FEL)  42  between a light source  40  and a first total reflection mirror  44 , and a second FEL  46 , a polarizing beam sprite array (PBS array)  47 , and a focusing lens  48  between the first total reflection mirror  44  and a second total reflection mirror  50 . 
     The optical system of a liquid crystal projector in FIG. 1 is further provided with a dichroic mirror  52  on an optical path of a light reflected at the second total reflection mirror  50 , a first polarizing beam sprite prism (PBSP)  54 A on an optical path between the first dichroic mirror  52  and a blue liquid crystal panel  56 A, a second PBSP  54 B on an optical path between the first dichroic mirror  52  and a green liquid crystal panel  56 B, a first relay lens  60 , a third total reflection mirror  62 , a second relay lens  64 , fourth, and fifth total reflection mirrors  66  and  68 , a third PBSP  54 C on an optical path between the second PBSP  54 B and a red liquid crystal panel  56 C, a dichroic prism  70  among the first, to third PBSPs  54 A,  54 B, and  54 C, and a projection lens  72  opposite to a light projecting surface of the dichroic prism  70 . 
     Since the second PBSP  54 B can separate colors, the foregoing optical system can reduce a number of dichroic mirrors, to reduce a total length of the optical system. In this instance, a red light passed through the second PBSP  54 B proceeds along an optical path that is turned in a form surrounding the third liquid crystal panel  56 C by the fourth to sixth total reflection mirrors  62 ,  66 , and  68 , and is directed to the third liquid crystal panel  56 C. 
     In the meantime, in general, a connector  10  is fitted at a side of the third liquid crystal panel,  56 C i.e., at a bottom thereof in FIG. 1 for connecting the third liquid crystal panel  56 C to external signal lines, causing an interference with the path of the red light. Accordingly, as the path of the red light is required to be spaced away from the connector  10  of the third liquid crystal panel  56 C as far as possible, there is a limitation in reducing the total length of the optical system. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an optical system of a liquid crystal projector that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an optical system of a liquid crystal projector, in which a total length of the optical system is reduced to reduce a thickness of an entire system. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the optical system of a liquid crystal projector for superimposing, enlarging, and projecting pictures displayed on first to third liquid crystal panels by using color lights received through first to third light path changing parts, wherein the first light path changing part includes a first sub-light path part for making a first time alteration of a linearly polarized light component of the color light received from one of the first to third light path changing parts, a second sub-light path part for reflecting the color light from the first sub-light path part, and making a second alteration of the linearly polarized component of a reflected color light, and forwarding to the first sub-light path part, and a third sub-light path part for directing the color light received through the second sub-light path part and the first sub-light path part to the first liquid crystal panel relevant to the color light, wherein the first to third sub-light path parts are arranged spaced away from the first liquid crystal panel, on sides of the first liquid crystal panel respectively except a side the connector is fitted thereto. 
     The first sub-light path part includes a first λ/2 plate for altering a linearly polarized component of a color light received from one of the second, and third light path changing parts, the second sub-light path part includes a first total reflection mirror for reflecting the color light from the second sub-light path part at a right angle, a second total reflection mirror for reflecting back the color light from the first total reflection mirror, a λ/4 plate for altering a first linearly polarized light in one direction from the first total reflection mirror to a circular polarized light and forwarding the circular polarized light, and altering a circular polarized light from the second total reflection mirror to a second linearly polarized light in the other direction, and forwarding the second linearly polarized light, and the third sub-light path part includes a first polarizing beam sprite prism (PBSP) for transmitting the first linearly polarized light from the first λ/2 plate toward the first total reflection mirror, and reflecting the second linearly polarized light from the first total reflection mirror at a right angle, a second PBSP for reflecting the second linearly polarized light from the first PBSP toward the first liquid crystal panel at a right angle, and a second λ/2 plate for altering the first linearly polarized light from the second PBSP to a second linearly polarized light. 
     The first, second, and third sub-light path parts further includes at least one refractive lens for preventing light scattering and relaying an image forming point, and the refractive lens in the first sub-light path part is arranged between the first λ/2 plate and the first PBSP, the refractive lens in the second sub-light path part is arranged between the λ/4 plate and the second total reflection mirror, and the refractive lens in the third sub-light path part is arranged between the first PBSP and the second PBSP. 
     The second sub-light path part further includes a color filter for enhancing a purity of the color light, and the color filter is arranged between the λ/4 plate and the second total reflection mirror. 
     The λ/4 plate is arranged between the first total reflection mirror and the second total reflection mirror, the first PBSP is arranged between the first λ/2 plate and the first total reflection mirror, and the second PBSP is arranged between the first liquid crystal panel and the second λ/2 plate. 
     In another aspect of the present invention, there is provided an optical system of a liquid crystal projector for superimposing, enlarging, and projecting pictures displayed on first to third liquid crystal panels including a light source, an illuminating part for altering a white light from the light source to a linearly polarized light, and forwarding the linearly polarized light, a color separating part for separating the white color from the illuminating part into first to third color lights, a third light path changing part for reflecting the three color lights to the third liquid crystal panel, a second light path changing part for reflecting two color lights received through the third light path changing part to the second liquid crystal panel, and a first light path changing part including a first sub-light path part for making a first time alteration of a linearly polarized light component of the first color light received from the second light path changing part, a second sub-light path part for reflecting the first color light from the first sub-light path part, and making a second alteration of the linearly polarized component of a reflected first color light, and forwarding to the first sub-light path part, and a third sub-light path part for directing the first color light received through the second sub-light path part and the first sub-light path part to the first liquid crystal panel, wherein the first to third sub-light path parts are arranged spaced away from the first liquid crystal panel, on sides of the first liquid crystal panel respectively except a side the connector is fitted thereto. 
     In the first light path changing part, the first sub-light path part includes a first λ/2 plate for altering a linearly polarized component of a color light received from one of the second, and third light path changing parts, the second sub-light path part includes a first total reflection mirror for reflecting the color light from the second sub-light path part at a right angle, a second total reflection mirror for reflecting back the color light from the first total reflection mirror, a λ/4 plate for altering a first linearly polarized light in one direction from the first total reflection mirror to a circular polarized light and forwarding the circular polarized light, and altering a circular polarized light from the second total reflection mirror to a second linearly polarized light in the other direction, and forwarding the second linearly polarized light, and the third sub-light path part includes a first polarizing beam sprite prism (PBSP) for transmitting the first linearly polarized light from the first λ/2 plate toward the first total reflection mirror, and reflecting the second linearly polarized light from the first total reflection mirror at a right angle, a second PBSP for reflecting the second linearly polarized light from the first PBSP toward the first liquid crystal panel at a right angle, and a second λ/2 plate for altering the first linearly polarized light from the second PBSP to a second linearly polarized light. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a related art optical system of a liquid crystal projector; 
     FIG. 2 illustrates an optical system of a liquid crystal projector in accordance with a preferred embodiment of the present invention; and, 
     FIG. 3 illustrates a detail of a red light path in an ‘R’ optical system in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 2 illustrates an optical system of a liquid crystal projector in accordance with a preferred embodiment of the present invention. 
     Referring to FIG. 2, the optical system of a liquid crystal projector in accordance with a preferred embodiment of the present invention has a red light path provided to avoid a direction a connector of the red light liquid crystal panel is fitted. To do this, the optical system of a liquid crystal projector in FIG. 2 includes first, and second FELs  6  and  8 , a PBS array  10 , and a first focusing lens  12  between a light source  2  and the first total reflection mirror  14 , and a second focusing lens  16  between a first focusing lens  12  and a first dichroic mirror  18 . 
     A visible light from the light source  2  proceeds toward the first FEL  6 . The FEL  6  divides the light in cell units, and focuses onto respective lens cells in the second FEL  8 . The second FEL  8  collimates the lights to be parallel to a particular part, and directs toward the PBS array  10  fitted on a rear surface thereof, for assuring uniformity of light distribution directed to the liquid crystal panels  26 R,  26 G, and  26 B by means of the first, and second FELs  6  and  8 . The PBS array  10  splits the light into linearly polarized lights each having either one of optical axes, i.e., a P polarized light, and an S polarized light, and a ½ wavelength plate (not shown) partly fitted to rear of the PBS array  10  alters the transmitted P polarized light to an S polarized light. Accordingly, as all input lights are altered into the S polarized light by the PBS array  10 , most of the light from the light source  2  is directed to the liquid crystal panels  26 R,  26 G, and  26 B to be explained later. The first total reflection mirror  14  reflects the input light fully toward the dichroic mirror  18 . The first, and second focusing lenses  12  and  16  focus the input light, for preventing light scattering. 
     The optical system of a liquid crystal projector in FIG. 2 also includes a first PBSP  24 B between the first dichroic mirror  18  and the blue liquid crystal panel  26 B, a second PBSP  24 G between the first dichroic mirror  18  and a green liquid crystal panel  26 G, a red light optical system R between the second PBSP  24 G and the red liquid crystal panel R so as not to interference a connector of the red liquid crystal panel  26 R, a dichroic prism  82  between a third PBSP  24 R and the first, and second PBSPs  24 B and  25 G, and a projection lens  84  on a light projection side of the dichroic prism  82 . 
     The dichroic mirror  82  reflects a blue light in an input light, and transmits a red light and a green light both having wavelengths longer than the blue light. The first PBSP  24 B has different reflective, or transmissive characteristic depending on the linearly polarized light. In detail, the first PBSP  24 B reflects a blue linearly polarized light (S polarized light) reflected at the first dichroic mirror  18  toward the blue liquid crystal panel  26 B, and transmits a blue linearly polarized light (P polarized light) from the blue liquid crystal panel  26 B to proceeds toward the dichroic prism  82 . There is a third focusing lens  20  between the first dichroic mirror  18  and the first PBSP  24 B for prevention of light scattering. The second PBSP  24 G has different reflective or transmissive characteristic depending on a linearly polarized light and a wavelength. In detail, the second PBSP  24 G reflects a green linearly polarized light (S polarized light) in lights transmitted through the first dichroic mirror  18  toward the green liquid crystal panel  26 G, and transmits a green linearly polarized light (P polarized light) from the green liquid crystal panel  26 G to proceeds toward the dichroic prism  82 . Also, the second PBSP  24 G transmits a red linearly polarized light (S polarized light) in the lights transmitted through the first dichroic mirror  18  to proceed toward the red liquid crystal panel  26 R. 
     The red optical system R forms a red light path not to interfere with the connector  25  of the red liquid crystal panel  26 R. To do this, referring to FIG. 3, the red optical system R includes a first λ/2 plate  86 , and a fourth PBSP  30  arranged in succession on a red light path, a second total reflection mirror  32 , a λ/4 plate  34 , a red light filter  90 , and a total reflection mirror  38  arranged on a path of a red light transmitted through the fourth PBSP  30  in succession, a third PBSP  24 R between a fourth PBSP  30  and a red liquid crystal panel  26 R, and a second λ/2 plate  88  between the third PBSP  24 R and the dichroic prism  82 . The red optical system R also includes third to fifth focusing lenses  28 ,  36 , and  80  for focusing a light on a red light path. 
     The first λ/2 plate  86  alters the red S polarized light transmitted through the second PBSP  24 G to a P polarized light, and forward the P polarized light. The fourth PBSP  30  transmits the red P polarized light from the first λ/2 plate  86 . The second total reflection mirror  32  totally reflects the red P polarized light from the fourth PBSP  30 . The λ/4 plate  34  alters the red P polarized light from the second total reflection mirror  32  to a circular polarized light rotating in one direction, and forwards the circular polarized light. The circular polarized light from the λ/4 plate  34  is totally reflected at the third total refection mirror  38 , and returns to the λ/4 plate  34 . The λ/4 plate  34  alters the circular polarized light thus returned at the third total reflection mirror  38  to an S polarized light in a direction different from the P polarized light from the second total reflection mirror  32 , and forwards the S polarized light. The red filter  90  between the λ/4 plate  34  and the third total reflection mirror  38  enhances a purity of a red light. The S polarized light from the λ/4 plate  34  is totally reflected at the second total reflection mirror  32 , and proceeds toward the fourth PBSP  30 . The fourth PBSP  30  reflects the S polarized light reflected at the second total reflection mirror toward the third PBSP  24 R. The third PBSP  24 R reflects the red S polarized light reflected at the fourth PBSP  30  toward the red liquid crystal panel  26 R, and transmits the red P polarized light from the red liquid crystal panel  24 B. The second λ/2 plate  88  alters the red P polarized light from the third PBSP  24 R to an S polarized light, and provides to the dichroic prism  82 . The fourth to sixth total reflection mirrors  62 ,  66 , and  68  turns a light path by totally reflecting an incident light. The focusing lenses, for an example, the third focusing lens  28  between the first λ/2 plate  86  and the fourth PBSP  30 , the fourth focusing lens  36  between the red filter  90  and the third total reflection mirror  38 , and the fifth focusing lens  80  between the fourth, and third PBSPs  30  and  24 R, focus the red light as well as relay an image forming point of the red light, to form the image on the red liquid crystal panel  26 R again, for preventing a light loss caused by elongation of a light path of the red light longer than other lights. The first to third liquid crystal panels  26 B,  26 G, and  26 R, reflective type liquid crystal panels, alter the S polarized lights respectively reflected at the first to third PBSPs  24 B,  24 G, and  24 R to P polarized lights according to a picture signal, and present the P polarized lights, to form an image. The dichroic prism  82  superimposes the red light, the green light, and the blue light incident thereto respectively after obtaining picture information at the first to third liquid crystal panel  26 B,  26 G, and  26 R, and forwards a superimposed light through a projection surface to the projection lens  84 . Then, the projection lens  84  enlarges, and projects the picture from the dichroic prism  82  to a screen. 
     Thus, the optical system of a liquid crystal projector in the embodiment of the present invention forms a red light path so as to avoid a connector  25  at one side of the red liquid crystal panel  26 R, to resolve the problem of interference between the red liquid crystal panel  26 R and the connector  25 , thereby permitting further reduction of a total length of the optical system. 
     As explained, the optical system of a liquid crystal projector of the present invention forms a path of a particular color light so as to avoid a connector at one side of a liquid crystal panel for the particular color light, to resolve the problem of interference between the liquid crystal panel for the particular color light and the connector, thereby permitting further reduction of a total length of the optical system, that in turn reduces a thickness of the liquid crystal projector system. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the optical system of a liquid crystal projector of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.