Abstract:
In a projector, the precision of color synthesis is improved and the generation of vertical stripes in the projected image is suppressed by a dichroic prism used for color synthesis that employs at least the following aspects. (1) The dichroic prism is formed using triangular prisms, in which projections or grooves are provided that have two faces parallel to two pasted faces of the triangular prisms, the projections or grooves being integrally injection-molded on faces of the triangular prisms that are substantially perpendicular to the light beam (i.e., the faces through which no light beam passes); (2) the dichroic prism is positioned using marks on the pasted faces of the triangular prisms; or (3) the dichroic prism is formed by engaging triangular prisms in which recesses or projections are formed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a projector in which light beams of respective colors that are modulated by liquid crystal light valves are synthesized by a dichroic prism, and the light beams are enlarged and projected onto a screen via a projection lens. 
     Color synthesizing devices used in conventional projectors include dichroic prisms. Ordinarily, in a dichroic prism, four right-angled triangular prisms are pasted to each other in an X-shaped pattern with the right angles of these prisms contacting each other, and reflective films, such as conductive films or the like, that have selective reflectivity for specified colors, are formed on the pasted faces (it will be assumed herein that such reflective films are formed unless otherwise noted). 
     Currently, glass is the main material used for such right-angled triangular prisms; however, in recent years, prisms formed by injection or compression molding of plastics have begun to be used, as described in Japanese Patent Application Laid-Open No. 2001-66694. 
     A method used to manufacture such a dichroic prism will be described with reference to FIG.  3 . 
     First, a first triangular prism  19  and a second triangular prism  20  are pasted together on the same plane using the planes  19   a  and  20   a  (among the planes on either side of the right angles of the prisms) as reference planes. Then, a third triangular prism  21  and a fourth triangular prism  22  are similarly pasted together. 
     Next, the unit formed by the first triangular prism and second triangular prism is pasted to the unit formed by with the third triangular prism and fourth triangular prism. However, in this case, since there is no reference plane, it is difficult to align the positions of the vertex parts in each unit so that the prisms are accurately pasted together. As a result, if a positional deviation δ occurs do to inaccurate alignment of the pasted faces (as shown, for example, in FIG.  3 ), the images of respective colors that are synthesized via the dichroic prism thus obtained will not be aligned on the screen, so that the image quality drops. 
     A method for accurately pasting the units together to align the vertex positions of the respective prisms in order to eliminate such a drop in the image quality is described in Japanese Patent Application Laid-Open No. 8-184793. In this method, as shown in FIG. 4, a pair of prisms  23  and  24  (among four prisms  23 ,  24 ,  25  and  26 ) are first pasted together so that a step is formed between the two prisms. Furthermore, the remaining pair of prisms  25  and  26  are also similarly pasted together so that a step is formed between the two prisms. Finally, the respective pairs of prisms are pasted together using these step faces  23   a  and  25   a  to guide the positioning of the faces so that the vertex parts are in alignment. 
     SUMMARY OF THE INVENTION 
     In cases where right-angled triangular prisms made of glass are used, the manufacture thereof includes a polishing process. Accordingly, faces that are perpendicular to the faces through which the light beams pass can easily be formed to have a perpendicular orientation. Consequently, even in the case of conventional techniques, dichroic prisms can be formed with a high degree of precision. 
     However, in the case of the abovementioned right-angled triangular prisms formed by injection or compression molding of plastics, an inclination of substantially 2 to 5 degrees is generated in connection with the injection from the mold. 
     If an attempt is made to form a dichroic prism by the method described in Japanese Patent Application Laid-Open No. 8-184793, using right-angled triangular prisms that have such an inclination, the faces that have steps can be aligned, but the directions perpendicular to these faces cannot be aligned. 
     Accordingly, since the edge lines of the right angles of the four prisms that are pasted together cannot be accurately pasted, the light beams of the respective colors cannot be synthesized with a high precision for each pixel, so that desired colors cannot be reproduced. Furthermore, the edge lines themselves will be projected and displayed as vertical stripes on the image. 
     It is an object of the present invention to improve the precision of color synthesis, and to suppress vertical stripes in a projected image. 
     In order to achieve the abovementioned object, the present invention includes the following aspects: 
     (1) A projector comprising a light source, color separating means for separating the light beam emitted from this light source into light beams of respective colors, light valves which modulate the separated light beams of respective colors, a dichroic prism which synthesizes the modulated light beams of respective colors that have been modulated via the light valves, and a projection lens which enlarges and projects the synthesized modulated light beam onto a screen, wherein the dichroic prism is a prism formed by pasting together four triangular prisms, which are substantially right-angled triangular prisms as seen in cross section, and triangular prisms in which projections or grooves that have two faces forming planes parallel to the two pasted faces are integrally molded in the faces that are substantially perpendicular to the light beams (i.e., faces through which the light beams do not pass) are used as the abovementioned triangular prisms. 
     (2) A projector comprising a light source, color separating means for separating the light beam emitted from this light source into light beams of respective colors, light valves which modulate the separated light beams of respective colors, a dichroic prism which synthesizes the modulated light beams of respective colors that have been modulated via the light valves, and a projection lens which enlarges and projects the synthesized modulated light beam onto a screen, wherein the dichroic prism is a prism formed by pasting together four triangular prisms, which are substantially right-angled triangular prisms as seen in cross section, and triangular prisms in which marks used for positioning are formed on the pasted faces are used as the abovementioned triangular prisms. 
     (3) A projector comprising a light source, color separating means for separating the light beam emitted from this light source into light beams of respective colors, light valves which modulate the separated light beams of respective colors, a dichroic prism which synthesizes the modulated light beams of respective colors that have been modulated via the light valves, and a projection lens which enlarges and projects the synthesized modulated light beam onto a screen, wherein the dichroic prism is a prism formed by pasting together four triangular prisms, which are substantially right-angled triangular prisms as seen in cross section, and these triangular prisms are engaged with each other by means of projections or grooves formed in the pasted faces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a dichroic prism for use in a first embodiment of the present invention; 
     FIG. 2 is a perspective view of a dichroic prism for use in a second embodiment of the present invention; 
     FIG. 3 is a diagram which illustrates an example of positional deviation of the prism unit; 
     FIG. 4 is a perspective view of a dichroic prism which illustrates the method of pasting employed in a conventional prism unit; 
     FIG. 5 is a perspective view of a dichroic prism for use in a third embodiment of the present invention; 
     FIG. 6 is a perspective view of a dichroic prism for use in a fourth embodiment of the present invention; 
     FIG. 7 is a detailed diagram of the projections and grooves used in the fourth embodiment of the present invention; 
     FIG. 8 is a graph which illustrates the relationship between the amount of positional deviation and the illumination ratio in the present invention; 
     FIG. 9 is a schematic diagram of the optical system used in a conventional projector; 
     FIG. 10 is a diagram of a conventional color synthesizing prism; 
     FIG. 11 is a perspective view of part of a triangular prism showing a modification of the first embodiment; 
     FIG. 12 is a perspective view of part of a triangular prism showing a modification of the second embodiment; 
     FIG. 13 is a diagram showing the conditions of manufacture of the dichroic prism of the first embodiment; and 
     FIG. 14 is a diagram showing the conditions of manufacture of the dichroic prism of the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic structure of a projector, of the type to which the present invention is applied, will be described with reference to FIG.  9 . 
     The white light beam  49  from the light source lamp unit  48  is converted into a uniform white light beam by a first lens array  50 , a cold mirror  51  constituting a first reflective mirror and a second lens array  52 , and this light beam is separated into a red light beam  54  and a blue-green light beam  55  by a blue-green-reflective dichroic mirror  53 . 
     The abovementioned red light beam  54  is reflected by a first reflection-amplifying mirror  56 ; and, this light beam passes through a first focusing lens  57  and is incident on a first liquid crystal light valve  58 . The abovementiofled blue-green light beam  55  is separated into a green light beam  60  and a blue light beam  61  by a green-reflecting dichroic mirror  59 . The green light beam  60  passes through a second focusing lens  62  and is incident on a second liquid crystal light valve  63 . 
     The abovementioned blue light beam  61  is incident on a third liquid crystal light valve  69  via a first relay lens  64 , second reflection-amplifying mirror  65 , second relay lens  66 , third reflection-amplifying mirror  67  and third focusing lens  68 . 
     The red light beam containing image information, that is emitted from the abovementioned first liquid crystal light valve  58 , is incident on a dichroic prism  70  which has the structure shown in FIG.  10 . This light beam enters from the face  701   a , is reflected at an angle of 90 degrees by the pasted faces  701   c  and  704   d , and is emitted from the face  704   b . The blue light beam containing image information, that is emitted from the abovementioned second liquid crystal light valve  63 , enters the abovementioned dichroic prism  70  from the face  703   a ; and, this light beam is reflected at an angle of 90 degrees by the pasted faces  703   c  and  704   c , and is emitted from the face  704   b . The green light beam containing image information, that is emitted from the abovementiofled third light valve  69 , enters the dichroic prism  70  from the face  702   a ; and, this light beam passes through the pasted faces  701   c ,  703   c ,  704   c  and  704   d , and is emitted from the face  704   b.    
     The emitted light  71  containing image information, that has thus been color-synthesized, is projected by a projection lens unit  72  and focused as an image on a screen  73 . 
     The embodiments relating to the dichroic prism  70  described below are used in this basic structure. 
     (Embodiment 1) 
     The dichroic prism  9  shown in FIG. 1 is employed as the dichroic prism  70  shown in FIG.  10 . 
     The dichroic prism  9  is formed in the shape of a square column with a lozenge-form cross-sectional shape, as a result of four triangular prisms made of plastic, which have a triangular column shape and which have equal refractive indices, i.e., a first triangular prism  1 , second triangular prism  2 , third triangular prism  3  and fourth triangular prism  4 , being pasted together using an adhesive agent, so that the edge lines that form approximately right angles (90 degrees±5 degrees) contact each other. Furthermore, since conductive films are formed on the respective pasted faces so that the desired optical characteristics are obtained, there is a gap of approximately 5 μm or less between the triangular prisms; here, however, the dichroic prism is treated as a square column. 
     On the first triangular prism  1 , as shown in FIG. 1, projections  5  and  6 , which respectively have planes  5   a  and  6   a  that are parallel to the pasted face  1   a  and planes  5   b  and  6   b  that are parallel to the pasted face  1   b , are formed on the two faces which have a right-angled isosceles triangular shape and which are not used for pasting and through which no light beam passes. This shape is manufactured by injection-molding or compression-molding a plastic material. 
     In the third triangular prism  3 , as shown in FIG. 1, projections  7  and  8 , which respectively have planes  7   a  and  8   a  that are parallel to the pasted face  3   a  and planes  7   b  and  8   b  that are parallel to the pasted face  3   b , are formed on the two faces which have a right-angled isosceles triangular shape and which are not used for pasting and through which no light beam passes. This shape is manufactured by injection-molding or compression-molding of a plastic material. 
     Next, the method used to manufacture the dichroic prism  9  that is formed by pasting together the four triangular prisms using these projections will be described. 
     First, the first triangular prism  1  and second triangular prism  2  are pasted together so that the pasted faces  1   b  and  2   b  are on the same plane. Next, the third triangular prism  3  and fourth triangular prism  4  are similarly pasted together. In this case, pasting is performed using the faces in which the projections form a part of the plane. Then, the dichroic prism is formed by further pasting together these pairs of pasted prisms. 
     FIG. 13 is a perspective view that illustrates the pasting process. 
     First, one plane (plane  7   b ) of one projection  7  on the one pair of pasted prisms  200  is supported by a positioning jig  500 . Only a portion of this jig  500  is shown in FIG. 13; however, one plane (plane  8   b ) of the projection  8  on the opposite side of the pair of pasted prisms  200  is also clamped and supported by the same jig. The prisms carried on this jig are coated with an adhesive agent  100 , and this adhesive agent is spread over the entire surface that is to be bonded. 
     The projections on the other pair of pasted prisms  300  are also similarly supported in the jig planes (planes  5   b  and  6   b ) by a jig  400 . Temporary fastening is accomplished by pasting the faces of the pair of prisms  200  and the pair of prisms  300 , while fixing the orientation of these prisms by means of the jigs  400  and  500 . 
     Next, the temporarily fastened pairs of prisms  200  and  400  are fit into an external frame assembly positioning guide, as shown in FIG. 14, so that the triangular prisms on which the projections are formed are on the bottom. Afterward, the prisms are further accurately positioned by means of a positioning suction-chucking plate; then, the main bonding process is performed. 
     Even if such projections are formed by injection molding, the areas of the projections are small, so that the projections have little effect on the positioning with the guide; accordingly, highly precise positioning is possible. 
     Furthermore, even in cases where an arbitrary shape, such as a cylindrical shape or the like, is used for the projections, and the projections are not disposed in positions that are parallel to the pasted faces, pasting can be accurately performed as long as the assembly jig is formed so that this jig corresponds to the shape of the projections. However, from the standpoint of facilitating the manufacture of the assembly jig, it is desirable that the projections be formed in a shape that has faces parallel to the pasted faces. 
     Furthermore, if the triangular prisms are formed such that the projections have an inclination θ as shown in FIG. 11, when the triangular prisms are injection—molded, the triangular prisms will have a mold-release taper when the prisms are separated from the prism-holding molds, so that the triangular prisms can easily be removed from the molds. 
     Furthermore, if at least one of the faces of the dichroic prism on which light beams are incident is formed with a lens shape, such as a concave shape, convex shape or the like, then the color magnification aberration of the dichroic prism can be corrected. 
     (Embodiment 2) 
     FIG. 2 is a perspective view of a dichroic prism  18  constituting a second embodiment of the present invention. This dichroic prism  18  is formed in the shape of a square column with a square cross section as a result of four triangular prisms, which have a triangular columnar shape and which have equal refractive indices, i.e., a first triangular prism  10 , second triangular prism  11 , third triangular prism  12  and fourth triangular prism  13 , being pasted together. Furthermore, conductive films are formed on the respective pasted faces so that desired optical characteristics are obtained. In the first triangular prism  10  of this example, grooves which respectively have planes  14   a  and  15   a  that are parallel to the pasted face  10   a  and planes  14   b  and  15   b  that are parallel to the pasted face  10   b  are, formed in the two faces which have a right-angled isosceles triangular shape and which are not used for pasting. Next, the method used for medicinal aerosol formulation of the dichroic prism  18  by pasting together the abovementioned triangular prisms will be described. 
     First, the first triangular prism  10  and second triangular prism  11  are pasted together so that the pasted faces  10   b  and  11   b  are on the same plane. Next, the third triangular prism  12  and fourth triangular prism  13  are similarly pasted together. Afterward, these respective pairs of triangular prisms are pasted together in the form of an X. In the dichroic prism  18  thus obtained by pasting the triangular prisms together, positioning faces  15   a  and  17   a  that are parallel to the positioning faces  14   a  and  16   a  are formed on the lower end, in addition to the positioning faces  14   a  and  16   a , which are similar to the positioning faces used in a conventional prism. Furthermore, in addition to the positioning faces  14   b  and  16   b  that are perpendicular to the pasted faces, positioning faces  15   b  and  17   b  that are parallel to these positioning faces  14   b  and  16   b  are also formed on the lower end. Accordingly, the four prisms can be accurately pasted together by fitting jigs against these faces. 
     Furthermore, the dichroic prism of this example, which is pasted together as described above, can be accurately positioned by using the abovementioned grooves  14 ,  15 ,  16  and  17  formed in the prisms to attach the dichroic prism to the optical unit in a specified position. 
     Here, positioning is possible using only the grooves  14  and  15 , and pasting can be accurately performed in such a case; however, a larger number of positioning references allows more accurate positioning, so that a larger number of grooves is preferable. 
     Furthermore, even in cases where the grooves have an arbitrary shape, such as a cylindrical shape or the like, and are not disposed in positions parallel to the pasted faces, pasting can be accurately performed if the assembly jig is formed so that this jig corresponds to the shape of the grooves. However, from the standpoint of facilitating the preparation of the assembly jig, it is desirable that the grooves have faces that are parallel to the pasted faces. 
     Furthermore, if the prisms are formed with a shape in which the grooves have an inclination θ as shown in FIG. 12, when the prisms are injection-molded, the prisms will have a mold-release taper when the prisms are separated from the prism-holding molds, so that the prisms can easily be removed from the molds. 
     Furthermore, if at least one of the faces of the dichroic prism on which light beams are incident is formed with a lens shape such, as a concave shape, convex shape or the like, then the color magnification aberration of the dichroic prism can be corrected. 
     (Embodiment 3) 
     FIG. 5 shows a perspective view of a dichroic prism  27  constituting a third embodiment of the present invention. The dichroic prism  27  is formed in the shape of a square column with a square cross section as a result of four triangular prisms, which have a triangular column shape and which have equal refractive indices, i.e., a first triangular prism  28 , second triangular prism  29 , third triangular prism  30  and fourth triangular prism  31 , being pasted together. Furthermore, conductive films are formed on the respective pasted faces so that desired optical characteristics are obtained. In the present example, a projection  33  is formed beforehand on the first triangular prism  28 . Next, the method used to manufacture the dichroic prism  27  by pasting these triangular prisms together will be described. 
     First, the first triangular prism  28  and second triangular prism  29  are pasted together. Next, the third triangular prism  30  and fourth triangular prism  31  are pasted together so that steps  32   a  and  32   b  are formed. Afterward, the respective pairs of prisms are pasted together in the form of an X. In the dichroic prism  27  thus formed by pasting the prisms together, positioning faces  32   a  and  33   a , similar to those in a conventional prism, are formed. In addition, positioning faces  32   b  and  33   b  that are perpendicular to these positioning faces are also formed. Accordingly, the four prisms can be accurately pasted together by placing a jig against these faces. 
     Furthermore, the dichroic prism of this example, which is pasted together as described above, can be accurately positioned by using the abovementioned projection formed on one of the prisms to attach the dichroic prism to the optical unit in a specified position. 
     Here, the abovementioned projections formed beforehand on the triangular prisms, the abovementioned grooves formed beforehand in the triangular prisms and the abovementioned steps that are formed when two of the triangular prisms are pasted together may be used as positioning references. 
     Here, positioning is possible using only the projection  33 , and pasting can be accurately performed in such a case; however, a larger number of positioning references allows more accurate positioning, so that a larger number of projections is preferable. 
     Furthermore, even in cases where the projections or grooves have an arbitrary shape, such as a cylindrical shape or the like, and are not disposed in positions parallel to the pasted faces, pasting can be accurately performed if the assembly jig is formed so that this jig corresponds to the shape of the projections or grooves. However, from the standpoint of facilitating the preparation of the assembly jig, it is desirable that the projections or grooves have faces that are parallel to the pasted faces. 
     Furthermore, if the prisms are formed with a shape in which the projections or grooves have an inclination θ, as shown in FIG. 11 or FIG. 12, when the prisms are injection-molded, the prisms will have a mold-release taper when the prisms are separated from the prism-holding molds, so that the prisms can easily be removed from the molds. 
     Furthermore, if at least one of the faces of the dichroic prism on which light beams are incident is formed with a lens shape, such as a concave shape, convex shape or the like, then the color magnification aberration of the dichroic prism can be corrected. 
     (Embodiment 4) 
     FIG. 6 is a perspective view of a dichroic prism  34  constituting a fourth embodiment of the present invention. This dichroic prism  34  is formed in the shape of a square column with a square cross section as a result of four triangular prisms, which have a triangular column shape and which have equal refractive indices, i.e., a first triangular prism  35 , second triangular prism  36 , third triangular prism  37  and fourth triangular prism  38 , being pasted together. Furthermore, conductive films are formed on the respective pasted faces so that desired optical characteristics are obtained. In the present example, respective projections  40   a  and  40   b  and grooves  41   a  and  41   b  are formed on the two faces of the right-angled isosceles triangular shape that are used for the pasting of the first prism  35 , in areas that are not used for the separation or synthesis of the three colored light beams. Next, the method used to manufacture the dichroic prism  34  by pasting together the abovementioned triangular prisms will be described. 
     First, the projection  40   a  and groove  41   a  of the first triangular prism  35  and the groove  43   b  and projection  42   b  of the second triangular prism  36  are aligned, and these prisms are pasted together. Next, the third triangular prism  37  and fourth triangular prism  38  are similarly pasted together, with the projection  44   b , projection  46   a , groove  45   b  and groove  47   a  used for positioning. Afterward, these respective pairs of triangular prisms are pasted together in the form of an X, with the projection  40   b , projection  42   a , projection  44   a , projection  46   b , groove  41   b , groove  43   a , groove  45   a  and groove  47   b  used for positioning. 
     Here, the shapes of the projections and grooves, in a case where the pasting of the first triangular prism  35  and second triangular prism  36  is used as an example, are shown in FIG.  7 . The height h 1  of the projections  40   a  and  42   b  and the height h 2  of the grooves  41   a  and  43   b  are set at values that are within the thickness of the adhesive layer. Furthermore, positioning can be accurately performed by setting the difference between the distance m 1  between the projection  40   a  and groove  41   a  of the first triangular prism  35  and the distance m 2  between the groove  43   b  and projection  42   b  of the second triangular prism  36  at 2 μm or less; setting the difference between the distance m 3  between the groove  41   a  and center line part of the first triangular prism  35  and the distance m 4  between the projection  42   b  and the center line part of the second triangular prism  36  at 2 μ or less; and setting the gap that is formed when the projection  40   a  and groove  43   b  engage at 1 μm or less. Furthermore, the first triangular prism  35  and the second triangular prism  36  have the same shape; accordingly, in cases where triangular prisms that are prepared using the same mold are used in combination, the distances m 1 , and m 2  between the respective projections and grooves and the distances m 3  and m 4  between the projections or grooves and the center line parts are the same, so that more accurate positioning can be accomplished. 
     Furthermore, accurate positioning can be accomplished even in cases where the projections or grooves have arbitrary shapes, such as square shapes or the like; however, since restrictions caused by the mold release direction are increased in such cases, a cylindrical shape is more desirable. 
     Here, if at least one of the faces of the dichroic prism on which light beams are incident is formed with a lens shape, such as a concave shape, convex shape or the like, then the color magnification aberration of the dichroic prism can be corrected. 
     The present invention makes it possible to cause the edge lines of the vertices of the triangular prisms to coincide; accordingly, accurate color synthesis can be performed, and vertical stripes in the projected image can be suppressed.