Abstract:
A projection type picture display apparatus, includes a light source; a polarizing beam splitter capable of reflecting or transmitting an incident light; a decomposing/synthesizing prism assembly capable of receiving a reflected light from the polarizing beam splitter to decompose the incident light into red, green and blue lights, also capable of receiving reflected red, green and blue lights to synthesize these lights so as to produce a synthesized light; a plurality of reflective type liquid crystal displays capable of receiving, modulating and reflecting red, green and blue lights; a projection optical system capable of receiving a picture light passing through the polarizing beam splitter to project an enlarged picture on a screen. In particular, the decomposing/synthesizing prism assembly has a first dichroic mirror reflecting a first color light but transmitting a second and third color lights, and has a second dichroic mirror reflecting the second color light but transmitting the first and third color lights, with the first and second dichroic mirror intersected with each other at a predetermined operable angle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a picture display apparatus, in particular to a projection type picture display apparatus using reflective type liquid crystal display means. 
     FIG. 4 is an explanatory view indicating a conventional system for use as a projection type picture display apparatus. As shown in FIG. 4, a white light from a light source  1  is incident on a YDM (Yellow Reflecting Dichroic Mirror)  2   a  to be divided into a blue light which is a rectilinear light rectilinearly propagated through the YDM  2   a , and a two-color light (including a green light and a red light) reflected from the YDM  2   a.    
     Referring again to FIG. 4, the blue light rectilinearly propagated through the YDM  2   a  is reflected by a reflecting mirror  3   a  and then incident on a light transmissible liquid crystal panel  4   a  (for use in processing a blue light). On the other hand, the two-color light (including a green light and a red light) is incident on a GDM (Green Reflecting Dichroic Mirror)  2   b  to be divided into a green light and a red light, respectively. Here, the green light is a reflected light from the GDM  2   b , whilst the red light is a rectilinear light rectilinearly propagated through the GDM  2   b . The green light reflected from the GDM  2   b  is incident on a light transmissible liquid crystal panel  4   b  (for use in processing a green light). On the other hand, the red light rectilinearly propagated through the GDM  2   b  is reflected by a reflecting mirror  3   b  and a further by a reflecting mirror  3   c  so as to be incident on a light transmissible liquid crystal panel  4   c  (for use in processing a red light). 
     The light transmissible liquid crystal panel  4   a  (for use in processing a blue light), the light transmissible liquid crystal panel  4   b  (for use in processing a green light), the light transmissible liquid crystal panel  4   c  (for use in processing a red light), each comprises a pair of transparent electrode substrates forming an internal space therebetween which is filled with a liquid crystal so as to form a plurality of liquid crystal cells. Further, each of the light transmissible liquid crystal panels  4   a ,  4   b ,  4   c  includes on both sides thereof a pair of polarizing plates which are provided to cause a blue, green or red light to be incident onto a light synthesizing cross-prism  5 . 
     The light synthesizing cross-prism  5  has a pair of dichroic mirrors  5   a ,  5   b  each consisting of a deposition film, which are arranged in a mutually crossed manner so that they are able to reflect or transmit various lights (blue, green and red lights). 
     In this way, a red light is allowed to pass through the dichroic mirror  5   b  and then reflected by the dichroic mirror  5   a . A blue light is allowed to be reflected by the dichroic mirror  5   b  and then pass through the dichroic mirror  5   a . Further, a green light is allowed to first pass through the dichroic mirror  5   b  and thens through the dichroic mirror  5   a . Therefore, various lights (blue, green and red lights) are synthesized in the light synthesizing cross-prism  5  to form a picture which is then enlarged through a projection lens  6  so as to be projected on a screen  7 . 
     However, with the above-described conventional system shown in FIG. 4, since an optical system for dividing an incident white light into various color lights is space-apart from an optical system for synthesizing together individual color lights, the whole system for use as a projection type picture display apparatus has to be large in size, making it difficult to manufacture a picture display apparatus at a low cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved system for use as a projection type picture display apparatus which is compact in size and may be manufactured at a low cost, so as to solve the above-mentioned problems peculiar to the above-mentioned prior arts. 
     According to the present invention, there is provided a projection type picture display apparatus, comprising: a light source; a polarizing beam splitter capable of reflecting or transmitting an incident light; a decomposing/synthesizing prism assembly capable of receiving a reflected light from the polarizing beam splitter to decompose the incident light into red, green and blue lights, also capable of receiving reflected red, green and blue lights to synthesize these lights so as to produce a synthesized light which is then incident on the polarizing beam splitter; a plurality of reflective type liquid crystal display means capable of receiving, modulating and reflecting red, green and blue lights; a projection optical system capable of receiving a picture light passing through the polarizing beam splitter to project an enlarged picture on a screen. In particular, the decomposing/synthesizing prism assembly has a first dichroic mirror reflecting a first color light but transmitting a second and third color lights, and has a second dichroic mirror reflecting the second color light but transmitting the first and third color lights, with the first and second dichroic mirrors intersected with each other at a predetermined operable angle. 
     In one aspect of the present invention, the decomposing/synthesizing prism assembly comprises four right-angle prisms bonded together by means of an adhesive on their right-angle surfaces, forming the first and second dichroic mirrors intersected with each other in a crossed manner. In detail, the decomposing/synthesizing prism assembly is formed in a manner such that, an angle between the main beam of an incident light and the normal line of each dichroic mirror is 45°. 
     In another aspect of the present invention, the decomposing/synthesizing prism assembly may comprise four cubical trapezoidal prisms bonded together by means of an adhesive, forming the first and second dichroic mirrors intersected with each other at an angle of about 60°. In detail, the decomposing/synthesizing prism assembly may be formed in a manner such that, an angle between the main beam of an incident light and the normal line of each dichroic mirror is 30°. 
     In a further aspect of the present invention, the decomposing/synthesizing prism assembly may be comprised by a plate forming a first dichroic mirror and another plate forming a second dichroic mirror, both of the plates being disposed in an optical case filled with an optical liquid. 
     The above objects and features of the present invention will become better understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an explanatory view schematically illustrating an optical system for use as a projection type picture display apparatus, according to a first embodiment of the present invention. 
     FIG. 2 is an explanatory view schematically illustrating an optical system for use as a projection type picture display apparatus, according to a second embodiment of the present invention. 
     FIG. 3 a  is a perspective view illustrating a cubical trapezoidal prism forming a part of a decomposing/synthesizing prism assembly used in the projection type picture display apparatus, according to the second embodiment. 
     FIG. 3 b  is a perspective view illustrating a decomposing/synthesizing prism assembly for use in the projection type picture display apparatus, according to the second embodiment. 
     FIG. 4 is an explanatory view schematically illustrating a conventional optical system for use as a projection type picture display apparatus, according to a prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 showing a first embodiment of the present invention, reference numeral  10  is a white light source including a halogen lamp and a xenon lamp. A white light emitted from the white light source  10  is reflected by a reflector  11  to form a number of generally collimated beams which are then incident on a polarizing sheet  12   a . The polarizing sheet  12   a  is adapted to transmit a first polarized light component of the light from the reflector  11 , but absorb a second polarized light component thereof. A light beam passed through the polarizing sheet  12   a  is then incident on a polarizing beam splitter  13  and reflected by a polarizing surface thereof. A reflected light from the polarizing beam splitter  13  is thus incident to a decomposing/synthesizing prism assembly  14 . 
     The decomposing/synthesizing prism assembly  14  includes four right-angle prisms bonded together by means of an adhesive, forming a first dichroic mirror  14   a  (for selecting and reflecting a first wavelength light) and a second dichroic mirror  14   b  (for selecting and reflecting a second wavelength light). The first and second dichroic mirrors  14   a ,  14   b  are arranged in a generally crossed manner such that an angle of about 45° will be formed between a main beam of an incident light and the normal line of each dichroic mirror. 
     Further, the first and second dichroic mirrors ( 14   a ,  14   b ), are each formed by several tens of dielectric layers such as TiO 2  and SiO 2  layers which are alternatively laminated together to be able to select and reflect a light having a predetermined wavelength. 
     The first dichroic mirror (RDM)  14   a  is adapted to reflect a red light (a first light), but transmit a green light (a second light) and a blue light (a third light). 
     The second dichroic mirror (BDM)  14   b  is adapted to reflect a blue light (a third), but transmit a red light (a first light) and a green light (a second light). 
     A red light reflected by the first dichroic mirror (RDM)  14   a  is passed through a phase differential plate  15   a  to be incident to a reflective type liquid crystal display means  16   a  (for use in processing a red light). A blue light reflected by the second dichroic mirror (BDM)  14   b  is passed through a phase differential plate  15   b  to be incident to a reflective type liquid crystal display means  16   b  (for use in processing a blue light). Further, a green light passed first through the first dichroic mirror (RDM)  14   a  then through the second dichroic mirror (BDM)  14   b  is passed through a phase differential plate  15   c  to be incident to a reflective type liquid crystal display means  16   c  (for use in processing a green light). 
     A light modulated in the reflective type liquid crystal display means  16   a  (for use in processing a red light) is again passed through the phase differential plate  15   a  to be incident to the decomposing/synthesizing prism assembly  14 , in which the red light is first reflected by the first dichroic mirror  14   a  (RDM) and is then passed through the second dichroic mirror  14   b  (BDM). A light modulated in the reflective type liquid crystal display means  16   b  (for use in processing a blue light) is again passed through the phase differential plate  15   b  to be incident to the decomposing/synthesizing prism assembly  14 , in which the blue light is first passed through the first dichroic mirror  14   a  (RDM) and is then reflected by the second dichroic mirror  14   b  (BDM). 
     A light modulated in the reflective type liquid crystal display means  16   c  (for use in processing a green light) is again passed through the phase differential plate  15   c  to be incident to the decomposing/synthesizing prism assembly  14 , in which the green light is first passed through the first dichroic mirror  14   a  (RDN) and is then passed through the second dichroic mirror  14   b  (BDM). 
     A composite light synthesized in the decomposing/synthesizing prism assembly  14  is passed through the polarizing beam splitter  13  and the polarizing sheet  12   b , then is enlarged and projected onto a screen  18  through a projection lens  17 . 
     It will be appreciated from the above description that one of the most important significances between the present invention and the above-discussed prior art is that a plurality of reflective type liquid crystal display means  16   a - 16   c  are used together with a decomposing/synthesizing prism assembly  14 , so that both light decomposition and light synthesizing may be effected within a single decomposing/synthesizing prism assembly  14  by the reciprocating of the individual lights. 
     FIG. 2 illustrates a second embodiment of the present invention. As shown in FIG. 2, a decomposing/synthesizing prism assembly  40  is comprised of four cubical trapezoidal prisms  30   a - 30   d  having shapes identical with one another. 
     Referring to FIG. 3 a , each cubical trapezoidal prism  30  has a rectangular top surface  21 , a rectangular bottom surface  22 , a side surface  24  orthogonal to both the top surface  21  and bottom surface  22 , an inclined surface  23  (hatched portion) intersected with the top surface  21  at an angle of 120°, and intersected with the bottom surface  22  at an angle of 60°. 
     FIG. 3 b  illustrates a decomposing/synthesizing optical system  40  formed by assembling the four cubical trapezoidal prisms  30   a - 30   d . As shown in FIG. 3 b , a first dichroic mirror (RDM)  25  for selecting and reflecting a first wavelength light is provided on the inclined surface  23  of a cubical trapezoidal prism  30   a , whilst a second dichroic mirror (BDM)  25  for selecting and reflecting a second wavelength light is provided on the bottom surface  23  thereof. Similarly, each of the first and second dichroic mirrors  25 ,  26  is formed by several tens of dielectric layers such as TiO 2  and SiO 2  layers which are alternatively laminated together to be able to select and reflect a light having a predetermined wavelength. 
     Referring again to FIG. 3 b , the inclined surface  23  of the cubical trapezoidal prism  30   a  (for use in processing a red light) and the inclined surface  23  of the cubical trapezoidal prism  30   c  (for use in processing a green light) are mutually bonded to each other by means of an adhesive. Namely, the two inclined surfaces  23 ,  23  are bonded together completely by making one side of the top surface  21  of the prism  30   a  coincident with a corresponding side of the top surface  21  of the prism  30   c , such that the side surface  24  of the prism  30   c  is arranged in a direction forming an angle of 30° with the inclined surface  23  of the prism  30   a.    
     Further, the side surface  24  of the prism  30   a  is arranged in a direction forming an angle of 30 with the inclined surface  23  of the prism  30   c.    
     Similarly, the bottom surface  22  of the cubical trapezoidal prism  30   a  and the inclined surface  23  of the cubical trapezoidal prism  30   d  (for use in passing a light) are mutually bonded together by means of the same adhesive. 
     At this moment, the side surface  24  of the prism  30   a  is arranged to be connected with the top surface  21  of the prism  30   d , whilst the inclined surface  23  of the prism  30   a  is arranged to be connected with the bottom surface  22  of the prism  30   d . In this way, the side surface  24  of the prism  30   c  becomes parallel with and opposed to the side surface  24  of the prism  30   d.    
     Further, the bottom surface  22  of the cubical trapezoidal prism  30   d  and the inclined surface  23  of the cubical trapezoidal prism  30   b  (for use in passing a blue light) are mutually bonded together by means of the same adhesive. On the other hand, a boundary line between the inclined surface  23  and the bottom surface  22  of the prism  30   b  is in coincident with a boundary line between the inclined surface  23  and the bottom surface  22  of the prism  30   d , so that the side surface  24  of the prism  30   b  is arranged in a direction turned 30 degrees from the inclined surface 23 of the prism 30 d.    
     The operation of a projection type picture display apparatus using the decomposing/synthesizing prism assembly  40  may be described with reference to FIG.  2 . 
     Referring to FIG. 2, a light emitted from a light source  10  is reflected by a reflector  11  to form a number of generally collimated beams. The collimated beams are then passed through a polarizing sheet  12   a  and is reflected by a polarizing beam splitter  13  so as to be incident on the side surface  24  of the prism  30   d  of the decomposing/synthesizing prism assembly  40 . 
     Referring again to FIG. 2, the incident light passing through the side surface  24  of the prism  30   d  is incident on and a second dichroic mirror (BDM)  26  at an angle of 30° with the normal line of the dichroic mirror  26  which reflects a blue light but allows the passing of a red light and a green light. The reflecting angle of the reflected blue light is 60° with respect to an incident light. Then, the blue light reflected from the second dichroic mirror (BDM)  26  is passed through a phase differential plate  15   b  so as to be incident on a reflective type liquid crystal display means  16   b  (for use in processing a blue light). 
     On the other hand, a red light and a green light are passed through the second dichroic mirror (BDM)  26  and are incident on a first dichroic mirror (RDM)  25  at an angle of 30° with respect to the normal line of the mirror  25  which reflects a red light but allows the passing of the green light. 
     The green light passing through the first dichroic mirror (RDM)  25  is passed through a phase differential plate  15   c  so as to be incident on a reflective type liquid crystal display means  16   c  (for use in processing a green light). 
     Meanwhile, the red light reflected by the first dichroic mirror (RDM)  25  is passed through a phase differential plate  15   a  so as to be incident on a reflective type liquid crystal display means  16   a  (for use in processing a red light). 
     The various lights (blue, red and green lights) are modulated respectively in the reflective type liquid crystal display means  16   a - 16   c , while the modulated lights are again passed through the phase differential plates  15   a - 15   c  respectively, so as to be individually incident on the decomposing/synthesizing prism assembly  40 , along optical paths shown by broken lines in FIG.  2 . In this way, the individual color lights (blue, red and green lights) are synthesized in the decomposing/synthesizing prism assembly  40  to produce a synthesized picture light. Then, the synthesized picture light is passed through the polarizing beam splitter  13  and the polarizing sheet  12   b  so as to be incident on a projection lens  17 , thereby allowing a picture to be enlarged and projected on the screen  18 . 
     An important significance between the first and second embodiments of the present invention is that the second embodiment employs a decomposing/synthesizing prism assembly which is constructed such that with each of the first and second dichroic mirrors, an angle between an incident light and the normal line of each dichroic mirror is 30°. Since an angle between an incident light and the normal line of a dichroic mirror is set at 30° which is remarkably smaller than 45°, even if an incident angle of an incident light changes undesirably due to some uncontrollable factors, the wavelength characteristic of a reflected or rectilinearly propagated light will not be affected to any extent. 
     Although in the above first and second embodiments it has been described that a decomposing/synthesizing prism assembly is formed by four right-angle or cubical trapezoidal prisms, it is also possible that a decomposing/synthesizing optical system may be comprised by a plate forming a first dichroic mirror and another plate forming a second dichroic mirror, with both plates disposed in an optical case filled with an optical liquid. 
     As is appreciated from the above description, according to the first embodiment of the present invention, since a plurality of reflective type liquid crystal display means  16   a - 16   c  are used together with a decomposing/synthesizing prism assembly  14 , both light decomposition and light synthesizing may be effected within a single decomposing/synthesizing prism assembly by the reciprocating of the individual lights, thereby reducing the total number of optical elements and making it possible to manufacture a picture display apparatus at a low cost. 
     Further, according to the second embodiment of the present invention, since a decomposing/synthesizing optical system is constructed such that an angle between an incident light and the normal line of each dichroic mirror is 30° which is remarkably smaller than 45°, even if an incident angle of an incident light changes undesirably due to some uncontrollable factors, the wavelength characteristic of a reflected or rectilinearly propagated light will not be affected to any extent. 
     While the presently preferred embodiments of the this invention have been shown and described above, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.