Abstract:
A polarization separation device has a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate and an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction. The polarization separation device separates light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components. Here, the refractive index N0 of the grating and the refractive indices Ne and Np of the optically anisotropic material layer with respect to one and the other, respectively, of the linearly polarized light components are defined as follows: 
     
       
         | Ne−N 0|≧0.2, 
       
     
     and 
     
       
         | Np−N 0|≦0.03.

Description:
This application is based on applications Nos. H11-131017 and H11-133918 filed in Japan on May 12, 1999 and May 14, 1999 respectively, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polarization separation device used to illuminate a spatial light modulation device such as a liquid crystal panel that utilizes polarization of light. The present invention relates also to a projection-type display apparatus having such a polarization separation device. 
     2. Description of the Prior Art 
     Conventionally, as projection-type display apparatuses that project an enlarged image of an original image through a projection lens are known those employing a CRT and those employing a light source and a spatial light modulation device. Here, as spatial light modulation devices are known transmission-type liquid crystal panels that use twisted nematic liquid crystal. Liquid crystal panels of this type are in practical use in various applications because they permit miniaturization of projection-type display apparatuses, because they permit projection of high-resolution images simply if provided with sufficient numbers of pixels, and because their mass-production methods have been well established with those manufactured for direct-view purposes. 
     A spatial light modulation device such as one using twisted nematic liquid crystal utilizes polarization of light, and therefore has polarizers provided at its entrance and exit sides. Out of the light that illuminates the spatial light modulation device, the linearly polarized light component that has passed through the entrance-side polarizer then has its polarization state modulated spatially while passing inside the spatial light modulation device. This controls the amount of light that passes through the exit-side polarizer, and thereby forms an optical image. 
     A projection-type display apparatus employing a spatial light modulation device typically uses a lamp that emits natural light to illuminate the spatial light modulation device. If the spatial light modulation device is of a type that utilizes polarization of light, the polarizer provided at its entrance side transmits only about one half of the natural light emitted from the lamp, and the other half of the light is wasted by being reflected or absorbed. 
     To overcome this inconvenience, various techniques have been proposed that are generally called polarization conversion. According to these techniques, the natural light from a light source is separated beforehand into, on the one hand, a polarized light component (hereafter referred to as the primary polarized light component) polarized in the same way as the light that a spatial light modulation device is designed to use and, on the other hand, a polarized light component (hereafter referred to as the secondary polarized light component) polarized perpendicularly thereto. Then, the polarization plane of the secondary polarized light component, which if left intact the spatial light modulation device cannot use, is rotated through 90° so that the primary and secondary polarized light components are, after their polarization planes are thus made identical, together fed to the spatial light modulation device. In this way, it is possible to use both of the two polarized light components. 
     Accordingly, a projection-type display apparatus utilizing polarization conversion needs to be provided with a polarization separation device for separating natural light into two polarized light components polarized in directions perpendicular to each other and a polarization plane rotating device for rotating the polarization plane of one of those two separated polarized light components through 90°. As polarization separation devices, polarization separation multilayer films are widely known that utilize the Brewster angle and interference and that are available in plate-shaped and prism-shaped types. 
     On the other hand, as polarization plane rotating devices, phase films called λ/2 plates are generally known. A λ/2 plate is made by drawing an optically transparent organic film uniaxially so as to give it optical anisotropy. It has its thickness and optical anisotropy so controlled as to give the light passing therethrough a phase difference that corresponds to one half of the wavelength of the light. Accordingly, if linearly polarized light having a polarization plane in a direction 45° with respect to an optical axis enters a λ/2 plate, it exits therefrom as linearly polarized light having a polarization plane rotated further through 90°. 
     A projection-type display apparatus having a polarization separation device and a polarization plane rotating device as described above is disclosed in Japanese Laid-Open Patent Application H6-202094. The construction of this projection-type display apparatus is shown in FIG.  14 . The natural light radiated from a light source  901  is made into a parallel beam by a parabolic surface mirror  902 , and then enters a polarization separation device  903 . The primary and secondary polarized light components exiting from the polarization separation device  903  travel through a first and a second lens array  904  and  905 , and then illuminate a liquid crystal panel  907 . 
     The first lens array  904  separates the beam of the illumination light into partial beams, and the thus separated partial beams are enlarged by the second lens array  905  to an appropriate size. The separated partial beams are then superimposed on each other on the liquid crystal panel  907  by a convex lens  908 . Another convex lens  909  provided in the vicinity of the liquid crystal panel  907  makes the principal ray within each angle of view parallel to the optical axis. 
     The polarization separation device  903  has a structure as shown in FIG. 15. A structure composed by putting together a prism having an isotropic refractive index and a prism layer made of a birefringent material in general is widely known as a Wollaston prism. This structure exhibits, at the interface between the prism and the prism layer, different refraction conditions in different polarization directions perpendicular to each other, and thereby permits two polarized light components to travel in different directions. 
     The polarization separation device  903  has a plurality of such Wollaston prisms arranged in an array. Thus, the polarization separation device  903  is composed of a prism array base plate  911  having a blaze-shaped section, a flat base plate  912 , and a birefringent optical material layer  913  made of an optically anisotropic material. Here, since calcite, which is generally used as an optically anisotropic material, is expensive, a material produced by uniaxially arranging strips of an organic material such as liquid crystal layers, organic films, or monomers is used. 
     Thus, the polarization separation device  903  separates the light  914  entering it into a primary polarized light component  914   a  and a secondary polarized light component  914   b  that exit therefrom traveling in directions θ′ degrees apart from each other. As a result, the light beams that the first lens array  904  makes converge on the second lens array  905  each form separate spots, a predetermined distance apart from each other in the direction of the angle θ′, for the primary and secondary polarized light components  914   a  and  914   b.    
     In the vicinity of the second lens array  905 , a phase difference plate  906  is provided that selectively acts on the spots formed by the secondary polarized light component so as to rotate its polarization plane through 90°. As a result, the polarization planes of the primary and secondary polarized light components exiting from the convex lens  908  are made uniform. By aligning the polarization plane of these polarized light components with the polarization direction of the entrance-side polarizer (not shown) of the liquid crystal panel  907 , it is possible to realize an optical system that permits efficient use of light. 
     The prism array base plate  911  used in this polarization separation device  903  utilizes diffraction of light, and thus has prisms arranged with a pitch P of about 1 mm. This requires a considerably thick birefringent optical material layer  913 . However, it is generally difficult to form a thick birefringent optical material layer  913  by uniaxially arranging strips of an optically anisotropic material. For this reason, a polarization separation device in which a diffraction grating is used instead of a prism array base plate  911  is disclosed in Japanese Laid-Open Patent Application H10-197827. 
     This polarization separation device is shown in FIG.  16 . The polarization separation device  101  is composed of a diffraction gating  102  having a blaze-shaped section and having a pitch D and a height H both of the order of several micrometers and a birefringent optical material layer  103  made of an optically anisotropic material. As an optically anisotropic material having birefringence, a material produced by uniaxially orienting acicular liquid crystal molecules is used. 
     According to known methods, orientation is achieved, for example, by forming an organic orientation film of polyimide or the like on a base plate and then rubbing the film with a rubbing cloth in one direction so as to form fine grooves (this method will hereafter be referred to as “rubbing”), or by vapor-depositing SiO 2  obliquely on a base plate so as to form fine grooves, or by ion-milling a base plate so as to form fine grooves (this method will hereafter be referred to as “grating”). All of these methods cause a force that tends to align the length direction of the liquid crystal molecules with the grooves, and thus the liquid crystal molecules are oriented uniaxially. 
     If the refractive index of the optically anisotropic material for a primary polarized light component is made substantially equal to the refractive index of the diffraction grating and the refractive index of the optically anisotropic material for a secondary polarized light component is made different from the refractive index of the diffraction grating, when the first and secondary polarized light components pass through the polarization separation device  101 , the former is allowed to travel straight and the latter is diffracted. Thus, light  104  entering the polarization separation device  101  is separated into a primary polarized light component  104   a  and a secondary polarized light component  104   b  that exit therefrom traveling in directions apart from each other by the angle of diffraction θ. This helps make the birefringent optical material layer  103  thin and easy to produce. 
     However, in this polarization separation device  101  disclosed in H10-197827, as shown in FIG. 17, the diffraction grating  102  having a blaze-shaped section has both effective surfaces  102   a  that are involved in diffraction and non-effective surfaces  102   b  that has nothing to with diffraction. 
     The diffraction grating  102  is formed by molding using a metal mold. Therefore, to secure allowances for manufacturing errors in the metal mold and to secure drafts for easing the removal of the metal mold in the molding process, the non-effective surfaces  102   b  are formed at an angle α relative to the direction perpendicular to the entrance surface  101   a  of the polarization separation device  101 . As a result, as indicated by hatching in the figure, part of the incident light  104  strikes the non-effective surfaces  102   b,  and this lowers the diffraction efficiency with which the light is diffracted in the desired direction. 
     Moreover, in cases where the liquid crystal molecules are oriented uniaxially by rubbing, since the diffraction grating  102  that serves as a base plate has a blaze-shaped section, the rubbing cloth does not reach to the bottom portions thereof. This makes satisfactory orientation of the liquid crystal molecules impossible. On the other hand, orientation by vapor-depositing of SiO 2  or by grating requires not only extra production steps but also expensive equipment such as a vapor-depositing, ion-milling, or other apparatus, and thus raises the manufacturing costs. 
     Moreover, a polarization conversion optical system requires the provision of a first and a second lens array  904  and  905  and a polarization separation device  903  or  101 ; that is, it requires a comparatively large number of components, and thus raises the costs of the projection-type display apparatus as a whole that incorporates it. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a polarization separation device that offers better diffraction efficiency. 
     Another object of the present invention is to provide a polarization separation device that permits easy orientation of liquid crystal molecules on a diffraction grating and thereby permits reduction of manufacturing costs. 
     Still another object of the present invention is to provide a projection-type display apparatus that offers better diffraction efficiency and thus offers satisfactory brightness in displayed images. 
     A further object of the present invention is to provide a projection-type display apparatus that permits reduction of manufacturing costs. 
     To achieve the above object, according to one aspect of the present invention, a polarization separation device is provided with: a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate; and an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction. Here, the polarization separation device separates the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components. Moreover, the following conditions are fulfilled: 
     
       
         | Ne−N   0 |≧0.2, 
       
     
     and 
     
       
         | Np−N   0 |≦0.03, 
       
     
     where N 0  represents the refractive index of the grating, Ne represents the refractive index of the optically anisotropic material layer with respect to one of the linearly polarized light components, and Np represents the refractive index of the optically anisotropic material layer with respect to the other of the linearly polarized light components. 
     According to another aspect of the present invention, a polarization separation device is provided with: a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate; an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction; and a lens array layer having a plurality of lens surfaces and arranged on the entrance side or exit side of the polarization separation device. Here, the polarization separation device separates the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components. 
     According to another aspect of the present invention, a polarization separation device is provided with: a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate; and an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction. Here, the polarization separation device separates the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components. Here, the grating has minute grooves formed on a surface thereof facing the optically anisotropic material layer along one of the two directions. 
     According to another aspect of the present invention, a projection-type display apparatus is provided with: a polarization separation device for separating the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components, having a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate, and also having an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction; a light source for emitting light; a light condenser for condensing the light emitted from the light source so as to form it into a substantially parallel beam; a first lens array having a plurality of lenses; a second lens array having a plurality of lenses arranged one for each of the lenses of the first lens array in such a way that the two linearly polarized light components exiting from the polarization separation device are made to converge on two different lenses of the second lens array; a polarization plane rotator for rotating the polarization plane of one of the linearly polarized light components having passed through the second lens array so as to make it identical with the polarization plane of the other of the linearly polarized light components having passed through the second lens array; a spatial light modulation device for modulating the light having passed through the second lens array so as to form an optical image; and a projection optical system for projecting the optical image. Here, the following conditions are fulfilled: 
     
       
         | Ne−N   0 |≧0.2, 
       
     
     and 
     
       
         | Np−N   0 |≦0.03, 
       
     
     where N 0  represents the refractive index of the grating, Ne represents the refractive index of the optically anisotropic material layer with respect to one of the linearly polarized light components, and Np represents the refractive index of the optically anisotropic material layer with respect to the other of the linearly polarized light components. 
     According to another aspect of the present invention, a projection-type display apparatus is provide with: a polarization separation device for separating the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components, having a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate, also having an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction, and also having a lens array layer having a plurality of lens surfaces and arranged on the entrance side or exit side of the polarization separation device; a light source for emitting light; a light condenser for condensing the light emitted from the light source so as to form it into a substantially parallel beam; a first lens array having a plurality of lenses; a second lens array having a plurality of lenses arranged one for each of the lenses of the first lens array in such a way that the two linearly polarized light components exiting from the polarization separation device are made to converge on two different lenses of the second lens array; a polarization plane rotator for rotating the polarization plane of one of the linearly polarized light components having passed through the second lens array so as to make it identical with the polarization plane of the other of the linearly polarized light components having passed through the second lens array; a spatial light modulation device for modulating the light having passed through the second lens array so as to form an optical image; and a projection optical system for projecting the optical image. 
     According to another aspect of the present invention, a projection-type display apparatus is provided with: a polarization separation device for separating the light incident thereon into two linearly polarized light components that have polarization planes perpendicular to each other in accordance with the polarization directions thereof by refracting one and simply transmitting the other of those two linearly polarized light components, having a grating having a recurring blaze-shaped grating pattern formed on one surface of a transparent base plate, and also having an optically anisotropic material layer arranged adjacent to the grating and having different refractive indices in the direction in which the grating pattern recurs and in the direction perpendicular to that direction, with minute grooves formed on the surface of the grating facing the optically anisotropic material layer along one of the two directions; a light source for emitting light; a light condenser for condensing the light emitted from the light source so as to form it into a substantially parallel beam; a first lens array having a plurality of lenses; a second lens array having a plurality of lenses arranged one for each of the lenses of the first lens array in such a way that the two linearly polarized light components exiting from the polarization separation device are made to converge on two different lenses of the second lens array; a polarization plane rotator for rotating the polarization plane of one of the linearly polarized light components having passed through the second lens array so as to make it identical with the polarization plane of the other of the linearly polarized light components having passed through the second lens array; a spatial light modulation device for modulating the light having passed through the second lens array so as to form an optical image; and a projection optical system for projecting the optical image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: 
     FIG. 1 is a diagram showing the construction of the projection-type display apparatus of a first embodiment of the present invention; 
     FIG. 2 is a diagram showing the construction of the polarization conversion optical system of the projection-type display apparatus of the first embodiment; 
     FIG. 3 is a diagram showing an example of the polarization separation device of the projection-type display apparatus of the first embodiment; 
     FIG. 4 is a diagram showing how a metal mold for molding the diffraction grating of the projection-type display apparatus of the first embodiment is produced; 
     FIG. 5 is a diagram showing another example of the polarization separation device of the projection-type display apparatus of the first embodiment; 
     FIG. 6 is a diagram showing the first lens array of the projection-type display apparatus of the first embodiment; 
     FIG. 7 is a diagram showing the second lens array of the projection-type display apparatus of the first embodiment; 
     FIGS. 8 and 9 are diagrams illustrating how the maximum height of the diffraction grating causes the lowering of diffraction efficiency; 
     FIG. 10 is a diagram showing how the refractive index and Abbe number of the polarization separation device of the projection-type display apparatus of the first embodiment affects the diffraction efficiency obtained; 
     FIG. 11 is a diagram showing the construction of the polarization conversion optical system of the projection-type display apparatus of a second embodiment of the invention; 
     FIG. 12 is a diagram showing another example of the polarization separation device of the projection-type display apparatus of the second embodiment; 
     FIG. 13 is a diagram showing another example of the polarization separation device of the projection-type display apparatus of the second embodiment; 
     FIG. 14 is a diagram showing the construction of a conventional projection-type display apparatus; 
     FIG. 15 is a diagram showing an example of the polarization separation device of a conventional projection-type display apparatus; 
     FIG. 16 is a diagram showing another example of the polarization separation device of a conventional projection-type display apparatus; 
     FIG. 17 is a diagram showing the problem associated with the polarization separation device of a conventional projection-type display apparatus; and 
     FIG. 18 is a diagram illustrating the principle of a diffraction grating. 
     FIGS. 19 and 20 are chemical formulae of the material of the diffraction gratings of the first and second embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The projection-type display apparatus of a first embodiment of the present invention will be described below with reference to FIG.  1 . This projection-type display apparatus is composed of a light source  121 , a parabolic surface mirror  122 , a UV/IR cut filter  123 , a polarization conversion optical system  120 , dichroic mirrors  137  and  138 , a reflecting mirror  126 , reflection-type liquid crystal panels  130 R,  130 G, and  130 B (spatial light modulation devices), PBS prisms  127 ,  128 , and  129 , a cross dichroic prism  133 , and a projection optical system  134 . 
     The light emitted from the light source  121  is formed into a parallel beam by the parabolic surface mirror  122 . This parallel beam is then passed through the UV/IR cut filter  123  so that ultraviolet and infrared components are removed therefrom. Then, the light beam is passed through the polarization conversion optical system so as to be converted into linearly polarized light polarized in a predetermined direction. 
     Out of the linearly polarized light exiting from the polarization conversion optical system  120 , only the blue light component is reflected from the dichroic mirror  137 , and the red and green light components are transmitted therethrough. The blue light component is then reflected from the reflecting mirror  126 , is then reflected from the reflecting surface  129   a  of the PBS prism  129  that reflects only light polarized in the predetermined direction mentioned above, and then illuminates the reflection-type liquid crystal panel  130 B. The reflection-type liquid crystal panel  130 B reflects predetermined portions of the blue light component, and meanwhile rotates its polarization plane through 90°. 
     The blue light component exiting from the reflection-type liquid crystal panel  130 B has a polarization plane perpendicular to the predetermined direction mentioned above, and is therefore transmitted through the reflecting surface  129   a  of the PBS prism  129 . The blue light component is then reflected from the first reflecting surface  133   a  of the cross dichroic prism  133  that reflects only blue light, and is then projected through the projection optical system  134 . 
     The red and green light components transmitted through the dichroic mirror  137  is directed to the dichroic mirror  138 , which reflects only green light and transmits red light. The green light component is then reflected from the reflecting surface  128   a  of the PBS prism  128  that reflects only linearly polarized light polarized in the predetermined direction mentioned above, and then illuminates the reflection-type liquid crystal panel  130 G. The reflection-type liquid crystal panel  130 G reflects predetermined portions of the green light component, and meanwhile rotates its polarization plane through 90°. 
     The green light component exiting from the reflection-type liquid crystal panel  130 G has a polarization direction perpendicular to the predetermined direction mentioned above, and is therefore transmitted through the reflecting surface  128   a  of the PBS prism  128 . The green light component is then transmitted through the first reflecting surface  133   a  of the cross dichroic prism  133  and then through the second reflecting surface  133   b  thereof that transmits only red light, and is then projected through the projection optical system  134 . 
     The red light component transmitted through the dichroic mirror  138  is reflected from the reflecting surface  127   a  of the PBS prism  127  that reflects only linearly polarized light polarized in the predetermined direction mentioned above, and then illuminates the reflection-type liquid crystal panel  130 R. The reflection-type liquid crystal panel  130 R reflects predetermined portions of the red light component, and meanwhile rotates its polarization plane through 90°. 
     The red light component exiting from the reflection-type liquid crystal panel  130 R has a polarization direction perpendicular to the predetermined direction mentioned above, and is therefore transmitted through the reflecting surface  127   a  of the PBS prism  127 . The red light component is then reflected from the second reflecting surface  133   b  of the cross dichroic prism  133 , and is then projected through the projection optical system  134 . In this way, the red, green, and blue light components are integrated together and projected onto a screen (not shown). 
     As shown in FIG. 2, the polarization conversion optical system  120  has a polarization separation device  101 , a first lens array  124 , and a second lens array  125 . The second lens array  125  has phase difference plates  139  (a polarization plane rotator) attached thereto so as to partially cover one surface thereof. For simplicity&#39;s sake, in the following descriptions, the direction of the optical axis that runs along the axis of symmetry of the parabolic surface mirror  122  is called the x axis, the direction perpendicular to the plane of the figure is called the z axis, and the direction perpendicular to both the x and z axes is called the y axis. 
     Now, the principle of a diffraction grating will be described briefly. FIG. 18 shows a diffraction grating  21  having a recurring pattern of a microstructure having a blaze-shaped section formed on the surface thereof. Here, assume that the blaze shape has a pitch D and a maximum height H. The maximum height H is so determined that light  22  incident on the entrance-side surface  21   a  of the diffraction grating  21  from the direction of a normal thereto and having a wavelength λ is given a phase difference of 2π. Thus, almost all of the incident light  22  is, as diffracted light of order +1, refracted by an angle of diffraction of θ to become exiting light  22 ′. 
     In this case, the maximum height H and the angle of diffraction θ are given 
     
       
           H =λ/( Ns−Na )  (1) 
       
     
     
       
         sin θ=λ/ D   (2) 
       
     
     where Ns represents the refractive index of the diffraction grating  21 , and Na represents the refractive index of the medium surrounding the diffraction grating  21 . 
     A detail view of the polarization separation device  101  used in this embodiment is shown in FIG.  3 . As shown in this figure, the polarization separation device  101  is composed of a birefringent optical material layer  103  (an optically anisotropic material layer) sandwiched between a diffraction grating  102  and a counter flat plate  105 . Here, assume that, of naturally polarized light  104  incident on the polarization separation device  101  along the x axis, the polarized light component (the primary polarized light component  104   a ) polarized in the same way as the light that the reflection-type liquid crystal panels are designed to use is a polarized light component that vibrates in the y-axis direction, and the secondary polarized light component  104   b  is a polarized light component that vibrates in the z-axis direction. 
     The diffraction grating  102  has effective surfaces  102   a,  which are involved in the diffraction of the incident light  104 , and non-effective surfaces  102   b,  which have nothing to do therewith, arranged in a pattern having a blaze-shaped section that recurs in the y-axis direction. The diffraction grating  102  is produced by resin molding so as to allow easy formation of a complex blaze shape. 
     As the material of the resin here, it is preferable to use a polyamide resin such as those having chemical formulae (I) and (II) as shown in FIGS. 19 and 20 because they are resistant to dissolution in nematic liquid crystal and are thus reliable. In the chemical formula (I) shown in FIG. 19, the symbol n represents an integer number from 2 to 6, and the symbols A and B represent a 1,4-phenylele group or a 1,4-cyclohexyl group. In the chemical formula (II) shown in FIG. 20, the symbols l and m represent integer numbers from 1 to 15 that fulfill l+m&lt;16. 
     The birefringent optical material layer  103  is made of an optically anisotropic material such as nematic liquid crystal. The optically anisotropic material, such as liquid crystal, has acicular molecules, which are arranged with their length direction aligned with the z axis so as to exhibit a positive uniaxiality. Around the birefringent optical material layer  103  is applied a sealing of a ultraviolet-curing or thermosetting resin. 
     The diffraction grating  102 , which has a pattern having a blaze-shaped section that recurs in the y-axis direction (the first direction), is formed by injection molding of glass or a resin. The metal mold used here is produced by a method as shown in FIG. 4. A tool  12  made of diamond is fitted to a shank  11  that rotates in the direction indicated as J, and is placed so as to face the metal mold  10 . The shank  11  is movable in the directions indicated as K 1 , K 2 , and L. On the other hand, the work-table (not shown) that holds the metal mold  10  is movable straight in the direction (called the direction M) perpendicular to the plane of the figure. 
     With the work-table retracted in the direction M, the diamond tool  12  is moved in the direction K 1  until it is stopped when it has reached a predetermined depth d with respect to the molding metal  10 . Then, the work-table is moved in the direction M so that the diamond tool  12  actually cuts the metal mold  10  in such a way as to form a V-shaped groove  10   a  in the metal mold  10  in the direction of the width thereof (in the direction perpendicular to the plane of the figure). When the metal mold  10  has been moved past the diamond tool  12 , the diamond tool  12  is retracted in the direction K 2 , and is then fed in the direction L by a predetermined recurrence pitch D. 
     The diamond tool  12  is then moved in the direction K 1  again until it is stopped when it has reached the predetermined depth d with respect to the molding metal  10 . Then, the metal mold  10  is moved in the direction M, but backward this time as compared to when it was moved last time, so that a groove  10   a  is formed. These movements are repeated until a predetermined number of V-shaped grooves  10   a  are formed, and in this way a blaze shape that recurs with a pitch D is formed. 
     On the inclined surfaces  10   b  of the V-shaped grooves  10   a,  the minute irregularities on the cutting surface  12   a  of the diamond tool  12  leave grinding steaks that extend in the direction M. Accordingly, in the injection molding process, those grinding streaks are transferred onto the effective surfaces  102   a  (see FIG. 3) of the diffraction grating  102 , and thus, in those surfaces, minutes grooves are formed that extend in the z-axis direction (the second direction). This causes a force that tends to align the length direction of the molecules of the optically anisotropic material such as liquid crystal with the grooves, and thereby the molecules of the optically anisotropic material are oriented uniaxially. 
     In FIG. 3, the counter flat plate  105  is made of a resin film. On that surface of the counter flat plate  105  which faces the birefringent optical material layer  103 , an orientation film  105   a  made of polyimide or the like is laid that permits the optically anisotropic material to be oriented uniaxially. If the counter flat plate  105  is made of glass, which has a thermal expansion coefficient greatly different from that of the diffraction grating  102  that is made of a resin, a rise in temperature develops distortion in the liquid crystal cell, which may lead to destruction thereof. For this reason, the counter flat plate  105  needs to be made of a resin that has a thermal expansion coefficient close to that of the diffraction grating  102 . It is preferable that the difference between the thermal expansion coefficients of the counter flat plate  105  and of the diffraction grating  102  be held below 2×10 −5 / deg. 
     However, if the resin of which the counter flat plate  105  is made has birefringence, the polarization planes of the primary and secondary polarized light components  104   a  and  104   b  separated by the birefringent optical material layer  103  are disturbed thereby. This hinders the birefringent optical material layer  103  from functioning as a polarization separation device. For this reason, in this embodiment, to prevent disturbance of the polarization planes, the counter flat plate  105  is formed in the shape of a film. The resin film is made of a resin such as polyether sulfone, and is so formed as to have a thickness below 0.3 mm preferably. Alternatively, the counter flat plate  105  may be formed in the shape of a sheet or plate made of a resin having little birefringence such as PMMA or an acrylic resin. 
     The resin forming the diffraction grating  102  is so formed as to be comparatively thick. For this reason, if the diffraction grating  102  has birefringence, it is necessary to dispose, as in this embodiment, the diffraction grating  102  on the entrance side and the counter flat plate  105  on the exit side. This helps avoid the influence of the disturbance of the polarization planes by the diffraction grating  102 . 
     As shown in FIG. 5, it is also possible to attach a thin diffraction grating  102  on a base plate  102   c  made of glass or a resin having little birefringence. This makes it possible to dispose the diffraction grating  102  on the exit side and the counter flat plate  105  on the entrance side. In this case, if the diffraction grating  102  is made one-tenth or less as thick as the base plate  102   c,  it is possible to alleviate the influence of the thermal expansion of the resin and thereby obtain satisfactory reliability. Here, the counter flat plate  105  needs to be made of a material that has a thermal expansion coefficient approximately equal to that of the base plate  102   c  and that has no birefringence. 
     Assume that the birefringent optical material layer  103  has a refractive index N 1  with respect to linearly polarized light vibrating in the y-axis direction and a refractive index N 2  with respect to linearly polarized light vibrating in the z-axis direction, and that N 1 &lt;N 2 . In this embodiment, the refractive index N 0  of the material of the diffraction grating  102  and one N 1  of the refractive indices of the birefringent optical material layer  103  are made substantially equal to each other such that |N 0 −N 1 |≦0.03. This permits the diffraction grating  102  to be regarded as non-existent with respect to the primary polarized light component  104   a,  and thus the primary polarized light component  104   a  is transmitted intact through the polarization separation device  101  and exits therefrom parallel to the incident light  104 . 
     On the other hand, since N 0 &lt;N 2 , the secondary polarized light component  104   b  exits from the polarization separation device  101  after being diffracted thereby. Here, the maximum height H of the diffraction grating  102  is so determined as to fulfill 
       H =λ/( N   2 − N   0 ).  (3) 
     This is a condition, derived from Equation (1) noted previously, that is to be fulfilled to permit the maximum phase difference that the diffraction grating gives with respect to light of a reference wavelength λ to be equal to 2π. Here, the reference wavelength λ is set, for example, at 550 nm as a wavelength representative of the wavelength range of visible light. In this embodiment, N 2 −N 0 =0.2, and hence H= 2.75 μm.    
     Moreover, since the polarization separation device  101  is disposed in the vicinity of the light source  121 , it is necessary to take into consideration the variation of the refractive indices due to a rise in temperature. Table 1 shows the variation of the diffractive indices of the individual materials due to variation in temperature. As this table clearly shows, in a case where a 25° C. temperature rise from normal temperature (25° C.) to 50° C. is expected, if any difference is secured between the refractive index N 2  of the liquid crystal, which is an optically anisotropic material, and the refractive index N 0  of the diffraction grating  102 , which is made of a resin, it is preferable to set the difference (N 2 −N 0 ) to be as great as 
     
       
         (−6+70)×10 −5 ×25=0.016 
       
     
     at normal temperature. 
     With nematic liquid crystal, an extremely great rise in temperature causes phase transition from a nematic phase to an isotropic phase. Accordingly, if the transition temperature is assumed to be T 0 ° C., it is possible to obtain satisfactory reliability by determining the use temperature T° C. as T&lt;T 0 −20. 
     In this polarization separation device  101 , the use of the diffraction grating  102  makes it possible to make the birefringent optical material layer  103  extremely thin. An angle of diffraction θ of about 10° is obtained when the pitch D and the maximum height H are of the order of several micrometers. Accordingly, it is easy to fill the microstructure portion of the diffraction grating  102  with liquid crystal molecules, liquid crystal monomers, liquid crystal polymers, or a uneasily organic material and then orient them uniaxially in a predetermined direction. 
     The first lens array  124  has a structure as shown in FIG.  6 . The rectangular coordinate system indicated in this figure is the same as that indicated in FIG.  2 . The first lens array  124  has eighteen first lenses  131  arranged so as to lie inside the perfectly circular section A of the beam of light exiting from the parabolic surface mirror  122  (see FIG.  2 ). The individual first lenses  131  have their respective optical axis centers at appropriately decentered points as indicated by numeral  131   a.    
     The second lens array  125  has a structure as shown in FIG.  7 . The rectangular coordinate system indicated in this figure is the same as that indicated in FIG.  2 . The second lens array  125  has as many second lenses  135  as the first lenses  131  arranged in such a way as to correspond one to one to the first lenses  131 . The individual second lenses  135  are so arranged that the apertures of the corresponding first lenses  131  are conjugate with the display areas of the reflection-type liquid crystal panels  130 R,  130 G, and  130 B, and that the light beams having passed through the individual second lenses  135  are superimposed on each other on each of the reflection-type liquid crystal panels  130 R,  130 G, and  130 B. 
     The second lenses  135  each have a phase difference plate  139  attached thereto so as to cover about half of their respective area. The phase difference plates  139  rotate the polarization plane of the light incident thereon through about 90°. These phase difference plates  139  are of a λ/2 type with respect to the wavelength representative of the light passing therethrough, and their optical axes are aligned with an appropriate direction. In this embodiment, the optical axis direction of the phase difference plates  139  is aligned with a direction  136  that makes an angle of 45° with the y axis on the y-z plane. 
     In each of the second lenses  135 , the area where no phase difference plate is attached is the aperture area for the primary polarized light component  104   a,  and has its center of mass at a point as indicated by numeral  135   a;  the area where the phase difference plate  139  is attached is the aperture area for the secondary polarized light component  104   b,  and has its center of mass at a point as indicated by numeral  135   b.    
     Of the incident light  104 , which is naturally polarized light emitted from the light source  121 , the primary polarized light component  104   a  travels straight through the polarization separation device  101 , and the secondary polarized light component  104   b  is diffracted so as to travel in a direction a predetermined angle of diffraction θ apart from the primary polarized light component  104   a  in the y-axis direction. Then, the primary and secondary polarized light components are, by the first lens array  124 , made to converge on the second lens array  125  as indicated by numerals  104   a′  and  104   b′  (see FIG.  2 ). 
     The light that has been made to converge by the first lens array  124  forms discrete illuminated spots on the apertures of the second lens array  125 . The polarization plane of the secondary polarized light component  104   b ′ is rotated through 90° by the phase difference plates  139  that are attached selectively to the areas through which this polarized light component passes, and thereby the polarization planes of the primary and secondary polarized light components are made identical. 
     It is preferable to determine the optical axis centers  131   a  of the first lenses  131  in such a way that the primary polarized light component  104   a′  converges in the vicinity of the above-mentioned centers of mass  135   a  of the second lenses  135 . Then, the second polarized light component  104   b′  that has been made to converge by the first lens array  124  passes through the apertures of the second lenses  135  in positions thereon a predetermined distance deviated in the y-axis direction. By appropriately setting the angle of diffraction θ, it is possible to make the convergent secondary polarized light component  104   b ′ pass through the second lenses  135  in the vicinity of the above-mentioned centers of mass  135   b  thereof. 
     The primary and secondary polarized light components  104   a ′ and  104   b ′ having passed through the second lens array  125  then exit from the polarization conversion optical system  120  and then strike the dichroic mirror  137  (see FIG. 1) as described previously. 
     In this projection-type display apparatus constructed as described above, as will be clear from Equation (3) noted previously, the greater the difference N 2 −N 0  between the refractive indices, the smaller the maximum height H of the diffraction grating  102  can be made. As shown in FIGS. 8 and 9, provided that the angle α of the inclination of the non-effective surfaces  102   b  of the diffraction grating  102  is kept identical, by making the maximum height H smaller, it is possible to reduce the proportion of the incident light  104  that strikes the non-effective surfaces  102   b  of the diffraction grating  102 . 
     Accordingly, if the refractive indices are so selected as to fulfill N 2 −N 0 ≧0.1, the maximum height H is sufficiently small, and thus it is possible to reduce the proportion of the incident light  104  that strikes the non-effective surfaces  102   b  and thereby enhance diffraction efficiency. Normally, the value of N 2 −N 0  is roughly 0.3 or smaller. In particular, it is preferable to fulfill N 2 −N 0 ≧0.2, because then the maximum height H is 2.75 μm or smaller at a design wavelength λ of 550 nm, and thus it is possible to increase diffraction enhance greatly. 
     The smaller the recurrence pitch D of the blaze shape of the diffraction grating  102 , the greater the proportion of the amount of light that strikes the non-effective surfaces  102   b.  This lowers diffraction efficiency. The angle of diffraction θ is given by Equation (2) noted previously that uses the recurrence pitch D. The greater the recurrence pitch D, the smaller the angle of separation (the angle of diffraction θ) between the primary and secondary polarized light components  104   a  and  104   b.    
     Accordingly, to permit the incident light to be separated precisely and thereby permit the phase difference plates  139  to receive the intended component of the incident light, the first and second lens arrays  124  and  125  need to be disposed farther apart from each other, and this makes the polarization conversion optical system  120  larger. Thus, if the recurrence pitch D of the blaze shape of the diffraction grating  102  is within a range from 5 to 15 μm, it is possible to make the polarization conversion optical system  120  compact and simultaneously obtain high diffraction efficiency. Moreover, it is also easy to produce the metal mold. 
     Furthermore, by keeping the ratio of the maximum height H to the pitch D in a range H/D&lt;0.5, it is possible to further reduce the proportion of the incident light  104  that strikes the non-effective surfaces  102   b.  This is preferable because higher diffraction efficiency can be obtained. 
     Moreover, it is preferable that the Abbe number vd 2  of the birefringent optical material layer  103  with respect to light vibrating in the z-axis direction be higher than 10. A lower Abbe number will make the difference between its refractive index and that of the diffraction grating  102  greater in a short-wavelength range, and thus lowers diffraction efficiency. 
     Alternatively, the refractive indices are so selected that |N 2 −N 0 |≦0.03 and N 0 −N 1 ≧0.2. In this case, the linearly polarized light component that travels straight through the polarization separation device  101  and the linearly polarized light component that is refracted thereby exit therefrom with polarization planes different by 90° than in the case described above. Also this serves to enhance diffraction efficiency just as in the case described above. 
     Now, the conditions that permit high diffraction efficiency over the entire wavelength range will be described. FIG.  10  and Table 2 show the results of comparison testing that was conducted with the refractive index N 0  of the diffraction grating made equal to each of the refractive indices N 1  and N 2  of the optically anisotropic material under the following conditions: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Inclination angle α of non-effective surfaces (see FIG. 8) 
                 19.5° 
               
               
                 Maximum height H of diffraction grating 
                 2.4 μm 
               
               
                 Pitch D of diffraction grating 
                 8.5 μm 
               
               
                 Length D′ of effective surfaces (see FIG. 17) 
                 7.65 μm 
               
               
                 Refractive index NO of diffraction grating 
                 1.52, 1.72 
               
               
                 Abbe number vd 0  of diffraction grating 
                 60 
               
               
                 Refractive indices of optically anisotropic material 
               
               
                   (Vibration: y-axis direction) N1 
                 1.52 
               
               
                   (Vibration: z-axis direction) N2 
                 1.72 
               
               
                 Abbe numbers of the optically anisotropic material 
               
               
                   (Vibration: y-axis direction) vd 1   
                 30 
               
               
                   (Vibration: z-axis direction) vd 2   
                 20 
               
               
                   
               
             
          
         
       
     
     As will be clear from FIG.  10  and Table 2, making the refractive index N 0  of the diffraction grating equal to the refractive index N 1  of the optically anisotropic material lowers diffraction efficiency in the short-wavelength range. The reason is as follows. The lower the wavelength, the higher the refractive index, and this tendency is stronger with a lower Abbe number. 
     As the above-noted conditions indicate, the Abbe number vd 0  of the diffraction grating  102  is made comparatively high, and the Abbe number vd 2  of the optically anisotropic material is made comparatively low. Thus, if the refractive index N 0  of the diffraction grating  102  is made equal to the refractive index N 1 , i.e. the one having the lower value, of the optically anisotropic material, the difference between the refractive index N 0  of the diffraction grating  102  and the refractive index N 2  of the optically anisotropic material is too great in the shorter-wavelength range. 
     As a result, with the wavelength short and the difference (N 2 −N 0 ) between the refractive indices great, the maximum height that is most appropriate in the short-wavelength range is too small as given by Equation (3) noted previously. Accordingly, the maximum height that is most appropriate in the short-wavelength range differs greatly from the design height (the maximum height H of the diffraction grating  102  formed so as to offer optimum characteristics at the design wavelength λ), and this lowers diffraction efficiency. 
     By contrast, if the refractive index N 0  of the diffraction grating  102  is made equal to the refractive index N 2 , i.e. the one having the higher value, of the optically anisotropic material, the Abbe number vd 0  of the diffraction grating is comparatively high, and the Abbe number vd 1  of the optically anisotropic material is comparatively low, and thus the difference between the refractive index N 0  of the diffraction grating  102  and the refractive index N 1  of the optically anisotropic material is small in the short-wavelength range. As a result, with both the wavelength and the difference between the refractive indices great, the maximum height of the diffraction grating  102  that is most appropriate in the short-wavelength range does not differ so much from the design height, and this helps maintain high diffraction efficiency over the entire wavelength range. 
     In this case, by using nematic liquid crystal as the optically anisotropic material and designing in such a way that the refractive index N 1  is 1.48 to 1.55, the Abbe number vd 1  is 40 or lower, the refractive index N 2  is 1.65 or higher, and the Abbe number vd 0  of the diffraction grating  102  is 50 or higher, it is possible to minimize the lowering of the diffraction efficiency of the diffracted light ( 104   b ) and enhance the transmission efficiency of the straight-traveling light ( 104   a ). 
     FIG. 11 is a diagram showing the construction of the polarization conversion optical system  120  of the projection-type display apparatus of a second embodiment of the present invention. Here, such components as are found also in the first embodiment are identified with the same reference numerals. The polarization conversion optical system of this embodiment is different from that of the first embodiment in that the diffraction grating  102  and the first lens array  124  (see FIG. 2) are formed integrally and thus the diffraction grating  102  has the lens array surface  102   d  provided on its own surface. In other respects, this embodiment is the same as the first embodiment. 
     In this embodiment, the diffraction grating  102  having a lens array surface  102   d  composed of a plurality of lens surfaces arranged in an array is formed by resin molding. This makes production of both a complex blaze shape and the lens array surface  102   d  easy, and also helps reduce the number of components needed. Moreover, whereas conventionally AR coating needs to be applied to both surfaces of the polarization separation device  101  and also to both surfaces of the first lens array  124 , here AR coating needs to be applied to both surface of the polarization separation device  101  alone. This helps reduce production steps and thereby reduce the overall costs. 
     Here, by arranging the lens array surface  102 d on the entrance side, even if the diffraction gating  102  has birefringence, incident light is first transmitted through the diffraction grating  102  and is then separated into two linearly polarized light components. This helps secure satisfactory polarization separation efficiency. As the material of the resin here, it is preferable to injection-mold a polyamide resin such as those having chemical formulae (I) and (II) as shown in FIGS. 19 and 20 described previously, because they are resistant to dissolution in nematic liquid crystal and are thus reliable. 
     As shown in FIG. 12, just as in the case shown in FIG. 5, it is also possible to attach a thin diffraction grating  102  on a base plate  102   c  made of glass or a resin having little birefringence. The lens array surface  102   d  is formed on the base plate  102   c.  Alternatively, as shown in FIG. 13, it is also possible to form the counter flat plate  105  from glass or a resin having little birefringence and form a lens array surface  105   a  on the surface thereof. Also this serves the same purpose. 
     Although both the first and second embodiments described above deal with a projection-type display apparatus that employs three reflection-type liquid crystal panels, it is possible to achieve quite the same purpose by applying the present invention to a display apparatus of a single-panel type or one employing transmission-type liquid crystal panels. 
     
       
         
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Variation in 
               
               
                   
                 Refractive Index 
               
               
                   
                 ×10 −5 /deg 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Resin 
                   
                 −6 
               
               
                   
                 Glass 
                   
                 0.5 
               
               
                   
                 Liquid Crystal 
                 N1 
                 −15 
               
               
                   
                   
                 N2 
                 −70 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Variation in 
               
               
                   
                 Refractive Index 
               
               
                   
                 ×10 −5 /deg 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Resin 
                   
                 −6 
               
               
                   
                 Glass 
                   
                 0.5 
               
               
                   
                 Liquid Crystal 
                 N1 
                 −15 
               
               
                   
                   
                 N2 
                 −70