Conventionally, as a means of displaying images, projection-type optical display apparatuses such as projectors are known. Such optical display apparatuses require an optical illumination apparatus for efficiently and uniformly illuminating the optical image formed on a display panel, such as a reflective liquid crystal display panel. FIG. 10 is a diagram conceptually showing an example of the construction of an optical display apparatus employing a conventional optical illumination apparatus.
In FIG. 10, reference numeral 101 represents a light source, and reference numeral 102 represents a reflector disposed so as to partially surround the light source 101. A PBS (polarizing beam splitter) prism unit 103 is disposed immediately behind the reflector 102, i.e., on the right side thereof in FIG. 10. The PBS prism unit 103 includes a plurality of PBS prisms arranged parallel to one another. The PBS prism unit 103 splits the light from the light source 101 into two differently polarized types of light. Of the individual PBS prisms 103a and 103b, those which let out S-polarized light as described later have half-wave plates 104 disposed immediately behind them.
Behind the PBS prism unit 103 (i.e., on the right side thereof in FIG. 10) are disposed, in order of arrangement, a first lens array 105, then somewhat away therefrom, a second lens array 106, and a superimposing lens 107 immediately behind it. The first lens array 105 has a plurality of lens cells 105a arranged in a rectangular, grid-like array having an aspect ratio substantially identical to that of a display panel 109 to be described later. Similarly, the second lens array 106 also has a plurality of lens cells 106a arranged in a rectangular, grid-like array. However, the shape of the lens cells 106a of the second lens array 106 is not necessarily geometrically similar to that of the lens cells 105a. 
The images from the individual lens cells 105a of the first lens array 105 are, by the second lens array 106 and the superimposing lens 107 disposed immediately behind it, superimposed on one another in the vicinity of the focal point of the superimposing lens 107. The display panel 109 is disposed at the focal point of the superimposing lens 107. The display panel 109 is illuminated in a telecentric fashion by a condenser lens 108 disposed immediately in front of it. The components from the first lens array 105 through the superimposing lens 107 mentioned above together constitute an optical integrator system. It is to be noted that, in all the diagrams referred to in the present specification, irrespective of whether they relate to prior-art examples or to embodiments of the present invention, light beams are represented by their optical axes alone.
The light emitted from the light source 101 is reflected from the reflector 102, and is thereby formed into a substantially parallel beam and directed to the PBS prisms 103a of the PBS prism unit 103. Here, P-polarized light, indicated by solid lines P, is transmitted straight through the PBS prisms 103a. On the other hand, S-polarized light, indicated by broken lines S, is reflected inside the PBS prisms 103a so as to be directed to the outwardly contiguous PBS prisms 103b, and is then reflected again inside the PBS prisms 103b so as to exit therefrom, still as S-polarized light. That is, by the PBS prism unit 103, the light from the light source 101 is split into two differently polarized types of light in the direction of the longer sides of the display panel 109, i.e., in a vertical direction along the plane of the figure.
The S-polarized light exiting from the PBS prisms 103b is transmitted through the half-wave plates 104 disposed immediately behind the PBS prisms 103b and is thereby converted into P-polarized light. That is, a portion of the light from the light source 101 has its polarization converted first by the PBS prisms 103b of the PBS prism unit 103 and then by the half-wave plates 104, and eventually comes out as uniformly P-polarized light. This arrangement constitutes a polarization conversion device. Here, the type of light into which the light from the light source 101 is converted does not necessarily have to be P-polarized light, but can be of other polarizations. The arrangement described thus far, starting with the light source 101 and ending immediately in front of the display panel 109, constitutes an optical illumination apparatus.
The light thus converted into uniformly P-polarized light is then directed through the above-mentioned optical integrator system to the display panel 109. The display panel 109 modulates, pixel by pixel, the light it is illuminated with according to the display data fed thereto, and emits the modulated light. The light thus emitted then enters an optical projection system 110. The display data presented on the display panel 109 is projected, as an image, onto a screen (not shown) through this optical projection system 110. Reference numeral 110a represents an aperture stop disposed in the optical projection system 110.
FIG. 11 is a diagram conceptually showing another example of the construction of an optical display apparatus employing a conventional optical illumination apparatus. In this figure, reference numeral 201 represents a light source, and reference numeral 202 represents a reflector disposed so as to partially surround the light source 201. Behind the reflector 202 (i.e., on the right side thereof in FIG. 11) are disposed, in order of arrangement, a first lens array 203 and, then somewhat away therefrom, a second lens array 204. The first lens array 203 has a plurality of lens cells 203a arranged in a rectangular, grid-like array having an aspect ratio substantially identical to that of a display panel 209 to be described later. Similarly, the second lens array 204 also has a plurality of lens cells 204a arranged in a rectangular, grid-like array. However, the shape of the lens cells 204a of the second lens array 204 is not necessarily geometrically similar to that of the lens cells 203a. 
A PBS (polarizing beam splitter) prism array 205 is disposed immediately behind the second lens array 204. The PBS prism array 205 includes a plurality of PBS prisms arranged in an array. The PBS prism array 205 splits the light from the light source 201 into two differently polarized types of light. Of the individual PBS prisms 205a and 205b, those which let out S-polarized light as described later have half-wave plates 206 disposed immediately behind them.
A superimposing lens 207 is disposed behind the PBS prism array 205. The images of the individual lens cells 203a of the first lens array 203 are, by the second lens array 204 and the superimposing lens 207, superimposed on one another in the vicinity of the focal point of the superimposing lens 207. The display panel 209 is disposed at the focal point of the superimposing lens 207. The display panel 209 is illuminated in a telecentric fashion by a condenser lens 208 disposed immediately in front of it. The first lens array 203, the second lens array 204, and the superimposing lens 207 mentioned above together constitute an optical integrator system.
The light emitted from the light source 201 is reflected from the reflector 202, and is thereby formed into a substantially parallel beam and passed through the first lens array 203 and the second lens array 204, so that the light exiting from the individual lens cells 204a of the second lens array 204 enters corresponding ones of the PBS prisms 205a of the PBS prism array 205. Here, P-polarized light, indicated by solid lines P, is transmitted straight through the PBS prisms 205a. On the other hand, S-polarized light, indicated by broken lines S, is reflected inside the PBS prisms 205a so as to be directed to the contiguous PBS prisms 205b, and is then reflected again inside the PBS prisms 205b so as to exit therefrom, still as S-polarized light.
The S-polarized light exiting from the PBS prisms 205b is then transmitted through the half-wave plates 206 disposed immediately behind the PBS prisms 205b and is thereby converted into P-polarized light. That is, a portion of the light from the light source 201 has its polarization converted first by the PBS prisms 205b of the PBS prism array 205 and then by the half-wave plates 206, and eventually comes out as uniformly P-polarized light. This arrangement constitutes a polarization conversion device. Here, the type of light into which the light from the light source 201 is converted does not necessarily have to be P-polarized light, but can be of other polarizations. The arrangement described thus far, starting with the light source 201 and ending immediately in front of the display panel 209, constitutes an optical illumination apparatus.
The light thus converted into uniformly P-polarized light is then directed through the superimposing lens 207 to the display panel 209. The display panel 209 modulates, pixel by pixel, the light it is illuminated with according to the display data fed thereto, and emits the modulated light. The light thus emitted then enters an optical projection system 210. The display data presented on the display panel 209 is projected, as an image, onto a screen (not shown) through this optical projection system 210. Reference numeral 210a represents an aperture stop disposed in the optical projection system 210.
In the conventional optical illumination apparatus constructed as shown in FIG. 10, polarization conversion is performed immediately behind the light source 101. Therefore, the light emitted from the light source 101 and then reflected from the reflector 102 has its beam diameter enlarged to about twice its original beam diameter as a result of the polarization conversion. This diminishes the f-number of the illumination light Ia that strikes the display panel 109 and thus diminishes the f-number of the projection light Ea that emanates from the display panel 109, making the burden on the optical projection system 110 heavier.
On the other hand, in the conventional optical illumination apparatus constructed as shown in FIG. 11, the light emitted from the light source 201 and then reflected from the reflector 202 experiences no enlargement of its beam diameter. Therefore, no diminishing occurs in the f-number of the illumination light Ib that strikes the display panel 209 nor in the f-number of the projection light Eb that emanates from the display panel 209. Thus, no extra burden is placed on the optical projection system 210. However, the light from the light source 201 is not converted into uniformly polarized light until it has passed through the second lens array 204. Therefore, in this optical illumination apparatus, no space is available for inserting a polarization-dependent color switching device such as those used in the embodiments of the present invention to be described later. That is, this optical illumination apparatus does not permit a so-called color sequential illumination method using such a color switching device.