Abstract:
A color separation prism assembly includes a first prism block and a second prism block. Two dichroic coatings each corresponding to a specific color band are respectively formed on two adjacent prism faces of the first prism block. The second prism block is in contact with both the prism faces of the first prism block having dichroic coatings and is constructed so that the angles of incidence to the dichroic coatings are all restricted within no more than 30 degrees, and that all split light components traveling in the prism assembly have equal optical path lengths.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (a) Field of the Invention  
         [0002]     The present invention relates to a color separation prism assembly, and, more particularly, to a prism assembly applicable to a projection system for a TV camera or a color projection display.  
         [0003]     (b) Description of the Related Art  
         [0004]      FIG. 7  shows a conventional color separation prism that is generally referred to as a Philips prism. The Philips prism  100  separates an incoming light into three light components, namely the red (R), green (G) and blue (B). Referring to  FIG. 7 , the glass elements of the Philips prism includes three component prisms  102 ,  104  and  106 . A dichroic coating  110  that reflects the red component and transmits blue and green components is deposited on a back face of the component prism  102 , and a dichroic coating  108  that reflects the blue component and transmits the remaining green component is deposited between the component prism  104  and the component prism  106 . An air gap  114  exists between the dichroic coating  110  and a front face of the component prism  104 . As in  FIG. 7 , the dichroic coating  110  reflects the red component and transmits blue and green components as the incoming light I enters the Philips prism  100 . The red component is further internally reflected by the component prisms  102  and illuminates a liquid crystal light valve (LCLV)  112 R. Subsequently, the dichroic coating  108  reflects the blue component and transmits the green component. The blue component is further internally reflected by the component prism  104  and illuminates an LCLV  112 B, while The green component directly illuminates an LCLV  112 G.  
         [0005]     Though such design may make the angle of incidence of a incoming beam to the dichroic coating less steep, the twice reflections for both the red and blue components and the obliquely transmission of the air gap for the blue and green components may result in a color deviation. Further, the long back focal length due to such optical arrangement is also disadvantageous.  
         [0006]      FIG. 8  shows a color projection display  200  incorporating a cross dichroic prism (X-cube)  204 . The X-cube  204  is constructed by cross dichroic coatings  204   a ,  204   b ,  204   c , and  204   d  as shown by diagonal lines of a quadrangular cross section. The dichroic mirror  204   a  and the dichroic mirror  204   d  reflect only the blue component; the dichroic mirror  204   b  and the dichroic mirror  204   c  reflect only the red component. Liquid crystal panels  208 R,  208 G, and  208 B of the reflection type corresponds to their respective light components red, green, and blue. The prism  204  separates the white light into the light components red, green, and blue and reflects by the first to third liquid crystal panels  208 R,  208 G, and  208 B, thereby synthesizing the emitted light components. On the other hand, a projection lens  206  is arranged in the direction opposite to the reflecting direction by the polarizing beam splitters  202   a  and  202   b  of the white light. The P polarizing component of the synthetic light which was synthesized by the cross dichroic prism  204  is transmitted through the polarizing beam splitters  202   a  and  202   b  and, after that, it passes through the projection lens  206  and is projected onto a screen (not shown).  
         [0007]     Such projection system may shorten the back focal length and eliminate the disadvantage of the Philips prism  100  mentioned above; however, the angle of incidence of a incoming beam to the dichroic coating is as large as 45 degrees to considerably deteriorate energy efficiency and the color purity after separation, thus lowering the overall performance of the projection system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     An object of the invention is to provide a color separation prism assembly that is able to solve the aforesaid problems existing in conventional designs.  
         [0009]     According to the invention, the color separation prism assembly includes a first and a second prism blocks. The first prism block has a first and second prism faces adjacent to each other, and the first and second prism faces are respectively formed with a first and a second dichroic coatings through which a light beam sequentially travels. The first dichroic coating filters out a first light component of the light beam, and the second dichroic coating filters out a second light component and transmits a third light component of the light beam. The first prism block also has a third prism face constructed to make the third light component be perpendicularly transmitted therethrough. The second prism block is in contact with the first and second prism faces of the first prism block, and the second prism block has a first prism face for reflecting the first light component and a second prism face constructed to make the first light component be perpendicularly transmitted therethrough. The second prism block also has a third prism face constructed to make the third light component be perpendicularly transmitted therethrough. All the first, second and third light components travel in the prism assembly have equal optical path lengths.  
         [0010]     Through the design of the invention, the prism assembly can not only reduce the optical path lengths of all light components to shorten the back focal length, but assure the angles of incidence to the dichroic coatings are restricted within no more than 30 degrees to enhance energy efficiency and the color purity. Further, since no air gap exists in the prism assembly, the obliquely transmission of the air gap for the light components no longer occurs; in addition, the number of times the light components are reflected also decreases. These all improve the image accuracy of the prism assembly according to the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1A  shows a schematic diagram of a prism assembly in accordance with an embodiment of the invention.  
         [0012]      FIG. 1B  shows a perspective view illustrating a triangular prism (a), a right-angle prism (b), and a quadrangular prism (c).  
         [0013]      FIG. 2  shows an optical arrangement of prisms based on the embodiment shown in FIG  1 A.  
         [0014]      FIG. 3  shows a schematic diagram of a prism assembly in accordance with another embodiment of the invention.  
         [0015]      FIG. 4  shows an optical arrangement of prisms based on the embodiment shown in  FIG. 3 .  
         [0016]      FIG. 5  and  FIG. 6  shows schematic diagrams of a prism assembly in accordance with modifications of the invention.  
         [0017]      FIG. 7  shows a schematic diagram of conventional Philips prism.  
         [0018]      FIG. 8  shows a schematic diagram of a color projection display incorporating a cross dichroic prism. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1A  shows a schematic diagram of a prism assembly  10  in accordance with an embodiment of the invention for separating a light beam into several colored light components, such as red, blue and green components.  
         [0020]     According to this embodiment, the prism assembly  10  is comprised of two prism blocks  12  and  14 . The prism block  12  is in a shape of a quadrangular prism that may be constructed by a combination of a triangular prism  16  and a right-angle prism  18 .  FIG. 1B  shows a perspective view illustrating a triangular prism (a), a right-angle prism (b) and a quadrangular prism (c). It should be noted that, as used in this description and in the appended claims, the word “triangular prism” means a prism having a pair of parallel faces and three side faces perpendicular to the parallel faces. Similarly, the word “quadrangular prism” means a prism having a pair of parallel faces and four side faces perpendicular to the parallel faces, and so on. Further, the incoming light I enters the prism assembly by its side face, as shown in  FIG. 1B .  
         [0021]     Referring back to  FIG. 1A , interference films that selectively reflect colors in two substantially non-overlapping wavelength ranges are deposited on two lateral faces with an included angle θ of the triangular prism  16  to form two dichroic coatings  24  and  26 , and the base face of the triangular prism  16  without the formation of the dichroic coating neighbors the hypotenuse face of the right-angle prism  18 .  
         [0022]     Hence, since the dichroic coatings  24  and  26  are formed on two lateral faces of the triangular prism  16 , the optical path lengths of the light components may be shortened, and the angle of incidence to the dichroic coating is easy to manage.  
         [0023]     The prism block  14  is constructed by a right-angle prism  20  and a pentagonal prism  22 , with their faces respectively touching the dichroic coatings  24  and  26  to closely connect the prism block  14  to the prism block  12 , thus forming a hexahedron prism shaped like a cut-diamond.  
         [0024]     Referring to FIG  1 A, as the incoming light I is incident on the dichroic coating  24  at an angle δ 1 , the red component is reflected and filtered out by the dichroic coating  24 . The prism block  14  is constructed so that the hypotenuse face  28  of the right-angle prism  20  may function as a reflecting surface with respect to the red component. Thereby, the red component is reflected at point P′ and exits the right-angle prism  20  by its face  30  in a direction perpendicular to the face  30 .  
         [0025]     Subsequently, the incoming light I with the remaining light components is incident on the dichroic coating  26  at an angle δ 2 , and its blue component is reflected and filtered out by the dichroic coating  26  while the green component passes through the dichroic coating  26  without refraction. The prism block  14  is constructed so that the green component may exit the pentagonal prism  22  in a direction perpendicular to its face  32 , and the prism block  12  is constructed so that the reflected blue component may exit the right-angle prism  18  by its face  34  in a direction perpendicular to the face  34 .  
         [0026]     The design of the prism blocks can make the optical path lengths of the red, blue and green components equal and allow the color components to exit the prism assembly  10  in a direction perpendicular to its respective faces.  
         [0027]     Typically, the reflectivities and transmissivities of the dichroic coatings  24  and  26  may differ for the two polarizations of the incoming light I, because the incoming light I is not perpendicularly incident on the dichroic coatings. Further, as the incidence angle becomes larger, the color deviation due to the spectral curve shift becomes more apparent.  
         [0028]     Hence, it is preferable that the angle of incidence to the dichroic coating is no more than 30 degrees. Under the circumstance, the angle θ formed by the two lateral faces of the triangular prism  16  is set at 60 degrees such that the angle of incidence to the dichroic coating is easy to be confined to no more than 30 degrees.  
         [0029]      FIG. 2  shows an optical arrangement of prisms based on the embodiment given above. Referring to  FIG. 2 , the prism assembly may consist of a plurality of identical sub prisms. The sub prism, according to this embodiment, is a right-angle prism having angles of 30 degrees, 60 degrees and 90 degrees (hereinafter referred to as a 30°-60°-90° prism). The right-angle prism  20  of the prism block  14  may consist of three sub prisms  20   a ,  20   b  and  20   c , and the pentagonal prism  22  may consist of four sub prisms  22   a ,  22   b ,  22   c  and  22   d . The triangular prism  16  may be an equilateral triangle prism consisting of two sub prisms  16   a  and  16   b , with their hypotenuse faces coated with dichroic coatings. Also, the right-angle prism  18 , identical with the 30°-60°-90° prism mentioned above, neighbors the base faces of the sub prism  16   a  and  16   b  via its hypotenuse face. Through such architecture formed by the plurality of identical 30°-60°-90° prisms, the prism assembly according to the invention can not only reduce the optical path lengths of all light components to shorten the back focal length, but assure the angles δ 1  and δ 2  of incidence to the dichroic coatings are restricted within no more than 30 degrees to enhance the color purity. Further, since no air gap exists in the prism assembly, the obliquely transmission of the air gap for the light components no longer occurs; in addition, the number of times the light components are reflected also decreases. These all improve the image accuracy of the prism assembly according to the invention.  
         [0030]     Referring to  FIG. 3 , the prism assembly  40  according to another embodiment is comprised of two prism blocks  42  and  44  cemented together. The prism block  42  is a triangular prism having two equal lateral faces on which two dichroic coatings  50  and  52  are respectively applied. The prism lock  44  is constructed by two right-angle prisms  46  and  48 . In this embodiment, the included angle β between two equal lateral faces of the prism block  42  is twice as large as the vertex angle α of the prism assembly  40 . It is preferred that the vertex angle α is set as 60 degrees and thus β equals 120 degrees.  
         [0031]     According to this embodiment, as the incoming light I is incident on the dichroic coating  50  at point Q, the red component is reflected and filtered out by the dichroic coating  50 . The prism block  44  is constructed so that the hypotenuse face  54  of the right-angle prism  46  may function as a reflecting face with respect to the red component. Thereby, the red component are reflected at point Q′ and exits the right-angle prism  46  by its face  56  in a direction perpendicular to the face  56 .  
         [0032]     Subsequently, the incoming light I with the remaining light components is incident on the face  58  of the prism block at point R, and its blue component is reflected and filtered out by the dichroic coating  52  while the green component passes through the dichroic coating  52  without refraction. The prism block  44  is constructed so that the red and green components exit the prism assembly  40  in a direction perpendicular to the face  56 , and the prism block  42  is constructed so that the reflected blue component exits the prism block  42  by its face  58  in a direction perpendicular to the face  58 .  
         [0033]      FIG. 4  shows an optical arrangement of prisms based on the embodiment given above. Referring to  FIG. 4 , the prism assembly  40  may consist of a plurality of identical sub prisms having angles of 30 degrees, 60 degrees and 90 degrees. The right-angle prism  46  of the prism block may be a 30°-60°-90° prism consisting of three sub prisms  46   a ,  46   b  and  46   c , and the prism block  42  is an isosceles triangle prism consisting of sub prisms  42   a  and  42   b , with their hypotenuse faces applied with dichroic coatings. Also, the right-angle prism  48  is constructed by the same sub prism. All sub prisms are combined together to form the prism assembly  40  shaped as a equilateral triangle prism.  
         [0034]     In this embodiment, though the reflection number is increased by one (reflection with respect to the face  58  of the prism block  42 ) compared to the above embodiment, the occupied space of the prism assembly  40  is reduced.  
         [0035]     In view of the fact that the interference films are very sensible to the angle of incidence, it&#39;s better to minimize the angle of incidence to the utmost so as to enhance the color purity. Hence, another embodiment that provides reduced angle of incidence to the dichroic coating is described below.  
         [0036]     First, referring to the geometry shown in  FIG. 5 , as the angle γ, formed between the base face and the prism face with dichroic coating  52  of the prism block  42 , is increased, the angle of incidence δ 3  to the dichroic coating is decreased. Hence, in case that the vertex angle α equals 60 degrees, the angle of incidence δ 3  is restricted to less than 30 degrees as the angle γ is set more than 30 degrees. That is, the angle of incidence δ 3  is restricted to less than 30 degrees only by changing the shape of the prism block  42  to decrease the angle γ. Further, it should be noted that, as the angle γ is decreased, an additional prism block  62  is needed to compensate the optical path length of the blue component so as to maintain the equal lengths of three light components. Preferably, the prism block  62  may be a right-angle prism such that a prism face constructed to make the blue component be perpendicularly transmitted therethrough is easy to be provided.  
         [0037]     Referring to  FIG. 5 , the prism block  62  may be cemented on a transparent plate  66  via its hypotenuse face, and the transparent plate  66  is spaced apart from the prism block  42  with an air gap. Alternatively, an adhesive  68  may be directly applied on the end portion of the hypotenuse face of the prism block  62  to maintain an air gap between the prism block  62  and the prism block  42 , as in  FIG. 6 .  
         [0038]     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.