William E. Glenn
In Japan much of the new display development has been motivated by the high- definition television (HDTV) or "hi-vision" market, which the Japanese see as the next big advance in television. It has been recognized that high-definition images at normal home viewing distances should be at least 36 inches diagonal, and preferably 50 - 60 inches. In these sizes the direct-view shadow-mask tube is quite impractical. It is felt that unless an economical thin display in the 50-inch range can be developed, the future of consumer HDTV will be quite limited. At this time the only feasible options seem to be either direct-view large panels--such as plasma display panels (PDPs) or active matrix liquid crystal display (AMLCD) panels--or projectors. Even though there is an effort by the Giant Technology Corporation (GTC), partially under government sponsorship, to develop economical large AMLCD panels, there is considerable skepticism in the industry that the effort will be economically successful. PDP panels appear likely to be less costly, but in experimental displays they have been marginal in brightness, contrast, and uniformity.
In the short term, only projectors seem to have the cost and performance characteristics for consumer HDTV displays. For large-screen displays, cathode-ray tube (CRT) projectors with good performance have been produced. However, the AMLCD light-valve projector is rapidly surpassing the CRT projector as a technology that can meet the cost and performance objectives.
AMLCD panels are under very active development, primarily for color computer displays. This same technology is suitable for the light modulator panel in light-valve projectors. In fact, the small panels used in projectors are cheaper and easier to produce than direct-view panels. To produce color, direct-view panels need three times as many addressable subpixels as there are pixels in the image. A projector uses three small panels but needs only one addressable pixel per displayed pixel in each panel. Consequently, the yield requirements for high-resolution panels are far less severe for projectors than for direct-view color panels. Furthermore, on a substrate that will produce only one 14-inch direct-view panel, about nine projector panels can be produced--enough for three projectors. On a substrate that will produce one 44-inch panel, enough projector panels can be produced to make 30 projectors.
Since the light-valve projectors are single-lens devices with very accurately produced image plane geometry, the convergence problems that have plagued CRT projectors do not occur.
It is clear that while large hang-on-the-wall panels are a long-range goal for satisfying the display needs for HDTV, the light-valve projector is the prime candidate for the short-range solution. Sanyo, Seiko-Epson, and Sharp have invested heavily in the development of AMLCD projectors. HDTEC, a consortium that is partially supported by the MPT in Japan, is developing a consumer back-projection HDTV display using AMLCD light valves.