This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP02/02010 filed Mar. 5, 2002.
The present invention relates to image display apparatus in which high-resolution images are displayed by using a pixel shifting unit for shifting pixels by means of optical wobbling operation.
Among image display apparatus using a liquid crystal display device or the like, an image display apparatus has been disclosed for example in Japanese patent applications laid open No.6-324320 and No.7-7704 in which resolution of the liquid crystal display device is improved by effecting a pixel shifting operation called wobbling where the optical axis of light from the liquid crystal display device is wobbled in predetermined directions.
A description will now be given with respect to the general construction of an image display apparatus in which resolution is improved by such optical wobbling operation. As shown in FIG. 1, a back light 102 for emitting white light is placed on the back side of a color liquid crystal display device 101, and a wobbling device (a pixel shifting unit) 103 for wobbling in predetermined directions the optical axis of light from the color liquid crystal device 101 is placed on the front side of the color liquid crystal display device 101. Here, odd field images and even field images of input video signal are displayed on the color liquid crystal display device 101 at the same pixels thereof through an image display control circuit 104. In accordance with their display timing, the optical axis of light from the color liquid crystal display device 101 is wobbled in predetermined directions by the wobbling device 103.
The wobbling device 103 includes a polarization changing liquid crystal plate 105 and a birefringence plate 106 which is placed on the front side thereof. Here, ON/OFF of voltage across the polarization changing liquid crystal plate 105 is controlled by a wobbling liquid crystal drive circuit 107 based on synchronizing signal of the video signal to be displayed on the color liquid crystal display device 101. The light from the color liquid crystal display device 101 is thereby transmitted without changing its polarization when the voltage is ON, while, when the voltage is OFF, the light from the color liquid crystal display device 101 is transmitted with changing its polarization through 90 degrees, effecting the wobbling operation by changing the location to be emitted from the birefringence plate 106 in accordance with such direction of polarization. It should be noted that, since the color liquid crystal display device 101 retains the image of the preceding field until rewriting of the image of the next field, one of the electrodes of the polarization changing liquid crystal plate 105 is divided into parts each with a plurality of lines such as 5 lines. The other electrode is used as a common electrode and application of voltage is controlled by selecting the one of the electrodes in accordance with the timing of line scan of the color liquid crystal display device 101.
The following operation is performed when alternately displaying odd field images and even field images on the color liquid crystal display device 101. In particular, a case is supposed here as shown in FIG. 2A that the horizontal pixel pitch is Px and the vertical pixel pitch is Py of a pixel group in delta array of the color liquid crystal display device 101. An oblique wobbling operation of 0.75 Px in the horizontal direction and 0.5 Py in the vertical direction, for example, is performed by the above described wobbling device 103 so that the pixel array of the color liquid crystal display device 101 is located at the position as indicated by the broken lines in FIG. 2B when an odd field image is to be displayed, while the pixel array is located at the position indicated by solid lines when an even field is to be displayed. Specifically, if for example Px is 18 xcexcm and Py is 47.5 xcexcm, the wobbling operation is effected so as to achieve an oblique distance of about 27.3 xcexcm, shifted by 13.5 xcexcm horizontally and 23.75 xcexcm vertically.
For this reason, a crystallographic axis 106a of the birefringence plate 106 is set as shown in FIG. 3 in a direction inclined with respect to the XY coordinate of on the color liquid crystal display device surface and Z direction which is normal thereto. Here, when the direction of polarization of incidence agrees with the direction of polarization of light from the color liquid crystal display device, the light from the color liquid crystal display device is transmitted as extraordinary rays so as to shift the pixels. When the direction of polarization of incidence is rotated through 90 degrees with respect to the direction of polarization of light from the color liquid crystal display device, it is transmitted intact as ordinary rays without shifting the pixels.
In this manner, as shown in FIG. 4, when the image of an odd field is to be displayed on the color liquid crystal display device 101, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned ON, so as to transmit the light from such lines intact without rotating the direction of polarization through 90 degrees. The light is emitted by the birefringence plate 106 as extraordinary rays to shift the pixels. On the other hand, when the image of an even field is to be displayed, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned OFF, so as to transmit the light from the lines as rotated in the direction of polarization through 90 degrees, causing the birefringence plate 106 to emit the light intact as ordinary rays without shifting the pixels.
In addition, an image display apparatus is known to be provided with two units of such one-dimensional two-point pixel shifting unit each having a polarization changing liquid crystal plate and birefringence plate so as to achieve a high resolution of two-dimensional four-point pixel shift. These are combined to form a laminate where one of the units is rotated through 90 degrees about the axis of incident light with respect to the other, thereby performing four times of pixel shift in the vertical and horizontal directions within one frame or one field.
On the other hand, Digital Micromirror Device [abbreviated as: DMD (trademark)], referred to as variable form mirror device for example used in the image display apparatus disclosed in Japanese patent application laid open No.8-190072, is known in addition to the above liquid crystal display device (LCD) as a display device in the image display apparatus. Such DMD has an array of several hundred or several thousand small inclined mirrors each representing one pixel. To achieve an inclination, each mirror is attached to one or more hinges placed on a supporting column, and a control circuit thereunder is disposed with an interval from others. An electrostatic force is then imparted by the control circuit, to thereby selectively incline each mirror. When it is applied to a display, image data is loaded to DMD and, in accordance with the data, light is selectively reflected or not reflected from each mirror to the image plane.
Further, in addition to polarization beam splitter (PBS), half-mirror (HM), etc., one as disclosed in Japanese patent application laid open No.9-189809 is known as a control device of optical beam. Specifically, in that publication, a disclosure is made with respect to a color image display apparatus using Holographic Optical Element (abbreviated as: HOE) where an incident light is diffracted/separated into the respective components such as R, G, B so as to obtain convergence at a desired portion by the diffraction/spectroscopic function of a transmitting type hologram.
In high-resolution image display apparatus using a known pixel shifting unit, transmitting type LCD is mostly used as the display device. An effective light transmitting region of pixels of the transmitting type LCD is limited for example by the wiring region that is provided between the pixels. Further, it is necessary to cut off light to avoid an erroneous operation which results from radiation of light on semiconductor switching device for driving LCD. One of the problems due to such reasons is that the aperture rate thereof cannot be increased.
To eliminate the above problems of image display apparatus using a conventional transmitting type display device, it is an object of the present invention to provide an image display apparatus in which a high resolution displaying is possible by using a reflecting type display device which the effective light transmitting region of pixels is not limited.
In accordance with the present invention, there is provided an image display apparatus for displaying an image to a viewer, including: illumination means for emitting an illuminating light; reflecting type display means capable of selectively controlling by each pixel a reflected light amount of the light emitted from the illumination means; pixel shifting means for shifting an optical axis of reflected light of each pixel reflected at the reflecting type display means so as to improve resolution of image observed by the viewer; and optical means for making it possible to display to the viewer an image constituted by light reflected from pixels selected to cause reflection at the reflecting type display means.
By thus using the reflecting type display means such as reflecting type LCD display device and the pixel shifting means of two-point or four-point pixel shift, the image display apparatus can be achieved as capable of high-resolution displaying.
If a polarizing beam splitter is then used as the optical means, the pixel shifting means is preferably placed at a subsequent stage of the polarization beam splitter. It is thereby possible to improve a utilization factor of light as compared to when a half-mirror is used.
Further, if a half-mirror is used as the optical means, the pixel shifting means is preferably placed between the half-mirror and the display means. It is thereby not necessary to use an expensive polarizing beam splitter as the optical means so that it can be formed at a lower cost.
Further, if a holographic optical element is used as the optical means, the pixel shifting means is preferably placed at a subsequent stage of the holographic optical element. Compacting and reduction in weight of the image display apparatus are thereby possible.
Further, the optical means and the pixel shifting means are preferably bonded to each other. By such monolithic construction, it is possible to reduce ghosts and flares due to reflected light between the pixel shifting unit and the optical means.
Further, the pixel shifting means is preferably placed as inclined with respect to the optical axis of the reflecting type display means. By such disposition, the reflected light from the pixel shifting means can be caused to return to the outside of the region of the reflecting type display means so as to reduce ghosts and flares due to re-reflection at the reflecting type display means.
Further, a digital micromirror device is preferably used as the reflecting type display means. By such construction, light from the illumination means can be caused to be directly incident on the digital micro-mirror device, making it unnecessary to use an light dividing device.
Further, a total area of all pixels that can be displayed by the reflecting type display means is preferably xc2xc to xc2xe of the total area of the reflecting type display means, or a mask for cutting off light is preferably placed on the upper surface of the reflecting type display means so that it is xc2xc to xc2xe of the total area of the reflecting type display means. By such construction, the resolution together with contrast can be remarkably improved though the efficiency in using light is somewhat lowered.
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