The present invention relates to a liquid crystal panel type projection display More particularly, the present invention is directed to a compact, high definition, rear projection type display liquid crystal panel display with an improved lens or liquid crystal panel control system.
The construction of a conventional liquid crystal panel type rear projection display is shown in FIG. 2 wherein the numeral 1 denotes a light source such as a halogen lamp or a xenon lamp, numeral 2 denotes a condenser mirror, numeral 3 denotes a dichroic mirror having the property of reflecting red color and transmitting green and blue colors (red, green and blue colors will hereinafter be referred to simply as R, G and B, respectively), and numeral 4 denotes a dichroic mirror for reflecting B and transmitting R and G. Numerals 5, 6, 7 and 8 denote mirrors, and numerals 9, 10 and 11 denote liquid crystal panels for R, G and B, respectively. The liquid crystal panels may be so-called TFT (Thin Film Transistor) type nematic liquid crystal panels. Numerals 12, 1 3 and 20 denote a dichroic prism, a projection lens and a screen, respectively.
In the construction shown in FIG. 2, color images can be projected onto the screen 20 by inputting image signals to the liquid crystal panels 9, 10 and 11. Such prior art construction is described in '86 Television Society Report IPD 109-5, pp. 23-28. Further, Japanese Patent Application Laid-Open No. 19834/87 describes the use of a Fresnel lens for a liquid crystal panel for controlling light input or output therefrom, but is not concerned with viewing angle of a projection system.
The above-noted prior art is advantageous in that the use of only one projection lens suffices On the other hand, however, it has the drawback that it is structurally difficult to set the value of an image projection field angle, .alpha., shown in FIG. 2 at 25.degree. or more. The value .alpha. in the prior art is about 23.degree..
For a front projector, a field angle of 23.degree. or so is appropriate, but for constituting a compact, rear projection type display it is considered necessary to use an optical system having a field angle of about 25.degree. or more, preferably 35.degree. or more. There are two reasons why it is difficult to obtain a field angle of 25.degree. or more in the conventional one-lens, three-panel type construction shown in FIG. 2. The first reason is based upon a physical restriction with regard to the presence of the dichroic prism 12. The second reason is that since the electrooptical conversion characteristic per se of the liquid crystal panels depend on the direction of exit light from the panels, there occurs unevenness in luminance and in color, causing the image quality to be impaired, at diagonal corners on the screen in an optical system having a large field angle of 25.degree. or more.