As a projection type image display apparatus using a reflection type spatial light modulation element, e.g., reflection type TN liquid crystal panel, an image display apparatus constituted as shown in FIG. 1 is conventionally proposed. In this image display apparatus, light beams emitted from a lamp light source 107 are incident on an illumination optical system 108 having functions such as correction of light beam cross sectional shape, uniformization of intensity and divergence angle control, etc. This illumination optical system 108 includes a P-S polarization converter (not shown). This P-S polarization converter has a function to arrange light beams in non-polarization state into any one of P-polarized light and S-polarized light at efficiency of 50% or more.
In the image display apparatus shown here, light beams which have been passed through the illumination optical system 108 are placed in the polarization state where the electric vector vibrates mainly in a direction perpendicular to paper surface of FIG. 1. The polarization direction in this polarization state, which is S-polarized light, is with respect to the reflection surface of a dichroic mirror 109 of red light reflection which is incident next. Namely, in light beams which have been passed through the illumination optical system 108, the traveling direction of only the red light component is deflected by 90° by the dichroic mirror 109 of red light reflection. The light beams of red light component are reflected by a mirror 110, and are incident on a polarization beam splitter for red light (hereinafter referred to as PBS for red) 111. At a dielectric film 111a of the red PBS 111, only the S-polarized light component is reflected, and is incident on a reflection type TN liquid crystal panel 113 for red light as incident polarized light 112.
In the reflection type TN liquid crystal panel 113 for red light, incident light beams are reflected in such a manner that the polarization state is modulated in accordance with the display image. The light beams which have been reflected by the red light reflection type TN liquid crystal panel 113 are incident on the dielectric film 111a of the red PBS 111 for a second type. At this dielectric film 111a, light beams are detected so that only P-polarized light is transmitted therethrough, whereby polarization modulation is converted into luminance modulation. The outgoing light beams which have been converted into luminance modulation in this way are incident on a cross dichroic prism 114.
On the other hand, light beams which have been transmitted through the dichroic mirror 109 of red light reflection are then incident on a dichroic mirror 115 of green light reflection. Here, only the green light component is reflected, and the remaining blue light component is transmitted. In the green light and the blue light which have been separated, similarly to the previously described red light, only rays of S-polarized light are respectively reflected by a PBS 116 for green and a PBS 118 for blue, and are respectively incident on a reflection type TN liquid crystal panel 117 for green light and a reflection type TN liquid crystal panel 119 for blue light.
The light beams reflected in the state where the polarization state has been modulated by the green light reflection type TN liquid crystal panel 117 and the blue light reflection type TN liquid crystal panel 119 are incident on dielectric films 116a, 118a of the green PBS 116 and the blue PBS 118 for a second time, at which the light beams are detected so that only P-polarized light is transmitted therethrough so that polarization modulation is converted into luminance modulation. Outgoing light beams of green and blue which have been converted into the luminance modulation are respectively incident on the cross dichroic prism 114.
Red light, green light and blue light which have been incident on the cross dichroic prism 114 are synthesized at this cross dichroic prism 114, and rays of synthesized light are incident on a projection optical system 120. This projection optical system 120 forms an image of the light beams which have been incident onto a screen 121. On the screen 121, a predetermined image is displayed.
As an illumination apparatus for reflection type spatial light modulation element, there is, e.g., illumination apparatus described in the Japanese Patent Application Laid Open No.1998-48423 publication. The illumination apparatus described in this publication is directed to a hologram color filter in which two transmission type hologram optical elements are laminated so that waveform dispersion of the hologram is utilized.
As shown in FIG. 2, this hologram color filter is caused to be of the configuration in which two holograms 102, 103 having wavelength dependencies of diffraction efficiency different from each other with respect to illumination light 101 having a predetermined incident angle θ are laminated. In this hologram color filter, a bright color filter having less wavelength dependency of diffraction efficiency and such that color balance of three colors of R (Red), G(Green) and B(Blue) has been corrected can be provided.
The wavelength dependency of diffraction efficiency of two holograms 102, 103 in this hologram color filter is set so that spatial wavelength distributions by wavelength dispersion do not coincide as shown in FIG. 3. For this reason, red light which has been diffracted at the hologram 102 of the incident side illuminates red pixel 104 in the state where it is not diffracted by the hologram 103 of the outgoing side, and blue light and green light which are not diffracted at the hologram 102 of the incident side are diffracted and are optically separated by the outgoing side hologram 103, and are respectively converged onto corresponding color pixels 105, 106.
Further, as an illumination apparatus for a reflection type spatial light modulation element, an illumination apparatus using laminated hologram color filters 124r, 124g, 124b are proposed in, e.g., the Japanese Patent Application Laid Open No.1997-189809 publication as shown in FIG. 4.
In this illumination apparatus, read-out light which is radiated (emitted) from illumination light source (not shown) is incident on hologram color filters 124r, 124g, 124b via a coupling prism 126 and a glass base (substrate) 125. These hologram color filters 124r, 124g, 124b are respectively volumetric hologram lenses for red, green and blue. Interference fringes are baked in advance at these hologram color filters 124r, 124g, 124b by laser exposure. These color filters have a function in which respective micro-lenses for color light having areas corresponding to sizes of substantially one pixel (set consisting of three pixels in total of respective color pixels of R, G, B) are laminated.
These hologram color filters 124r, 124g, 124b allow a red light component, a green light component and a blue light component of the spectrum of read-out light RL to be transmitted through a cover glass 123, a common electrode 134, an orientation film 133, a liquid crystal layer 132, an orientation film 131 and a dielectric mirror film 130 of the reflection type liquid crystal panel to respectively converge them onto corresponding color pixel electrodes 129r, 129g, 129b on a pixel electrode layer 129.
This hologram lens has dependency with respect to polarization characteristic of incident light. Namely, among rays of incident light onto the hologram lens, an S-polarized light component is mainly diffracted, and diffraction efficiency of a P-polarized light component is lower than that.
By rigorous solution of the Coupled-wave theory (reference thesis: M. G. Moharam and T. K. Gayload: Rigorous Coupled-wave analysis of planar grating diffraction, J. Opt. Soc. Am. 71, 811–818 (1977), M. G. Moharam and T. K. Gayload: Rigorous Coupled-wave analysis of grating diffraction E-mode polarization and losses, J. Opt. Soc. Am. 73, 451–455 (1983)), it is indicated that, e.g., in the case where the corresponding hologram is a thick hologram of the reflection type and the value (t/Λ) determined by thickness t of hologram and pitch Λ of interference fringe within the hologram falls within the range from 1 to 5, a difference takes place between the diffraction efficiency of TE (S-polarized light) and that of the TM (P-polarized light), and diffraction efficiency of S-polarized light is greater by about 45% at the maximum as compared to diffraction efficiency of P-polarized light.
By this phenomenon, in this illumination apparatus, light of the S-polarized light component of rays of read-out light RL which have been obliquely incident on the hologram color filters 124r, 124g, 124b is mainly diffracted. Further, since light (P-polarized light component) PL reflected in the state where the polarization direction has been modulated by 90° of rays of illumination light which have been caused to be incident substantially perpendicular to the liquid crystal panel 122 has low diffraction effect, most rays of light are emitted vertically from the hologram color filters 124r, 124g, 124b without undergoing diffracting action by the hologram color filters 124r, 124g, 124b. 
As a typical example of the incident polarization characteristic of diffraction efficiency of the transmission type volumetric hologram, in the case where the refractive index modulation degree is 0.04, the thickness is 3 μm, the incident angle within hologram medium is 60°, the outgoing angle is 0°, and the manufacturing wavelength and reproduction wavelength are both 532 nm, diffraction efficiency of S-polarized light SP is 70%, whereas diffraction efficiency of P-polarized light PP is 25% as shown in FIG. 5. As a result, dependency of diffraction efficiency by incident polarized light appears.
As shown in FIG. 6, in order to increase superficial aperture percentage of the transmission type liquid crystal image display element to improve transmission percentage, a transmission type liquid crystal image display element using a micro lens array 137 of the refraction type is proposed.
In this transmission type liquid crystal image display element, an illumination light which has been incident on an incident side polarization plate 135 and has been changed into linear polarized light by the incident side polarization plate 135 is incident from an incident side glass base (substrate) 136, and is transmitted through a liquid crystal layer 138 and is converged onto pixel opening portions 139 of TFT by the micro lens array 137. The polarization state of this incident light is modulated at the pixel opening portions 139. The incident light thus modulated is emitted from an outgoing side glass base (substrate) 140. This illumination light is then transmitted through an outgoing side polarization plate 141, and modulation of the polarization state is converted into luminance modulation at this outgoing side polarization plate 141.
In the transmission type hologram as described above, since diffraction acceptance angle of incident light is narrow, and the diffraction acceptance angle and the outgoing angle are not sufficiently separated, light utilization efficiency is low.
In the image display device using the above-described transmission type hologram as a color filter, since color separation is conducted by making use of wavelength dispersion of the hologram, there is no degree of freedom at the separation angle setting of respective rays of color light. As a result, it is impossible to optimally set the distance between the color filter and color pixels of spatial light modulation element from viewpoints of manufacturing difficulty and/or light utilization efficiency.
In the image display apparatus using such an image display device, in the case where it is the premise that spatial light modulation element of the same pixel pitch is employed, separation angles of respective rays of color light cannot be large. Accordingly, it is impossible to set the distance between the color filter and color pixels of the spatial light modulation element to a short distance, e.g., 50 μm or less. Namely, in this image display apparatus, it is impossible to realize improvement in light utilization efficiency by a broad angle of view and/or a broad-band of illumination light with respect to the color filter. As a result, a bright image cannot be obtained.
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