Rear-projection television (RPTV) is a type of large-screen television display technology in which the image is projected inside the cabinet from behind onto a diffusing screen at the front. Unlike traditional televisions, a rear-projection television converts electrical video signals into beams of light, directed by an optical lens and mirror system. Until approximately 2006, most of the relatively affordable consumer large screen TVs up to 100 inches (250 cm) used rear-projection technology. A variation is a video projector, using similar technology, which projects onto a screen. Three types of projection systems are used in projection TVs. CRT rear-projection TVs were the earliest, and while they were the first to exceed 40 in (100 cm), they were also bulky and the picture was unclear at close range. Newer technologies include DLP (reflective micromirror chip), LCD projectors, Laser TV and LCoS. They are capable of displaying high-definition video up to 1080p resolution, and examples include Sony's SXRD (Silicon X-tal Reflective Display), JVC's D-ILA (Digital Direct Drive Image Light Amplifier) and MicroDisplay Corporation's Liquid Fidelity.
Background and history
Necessity Cathode-ray tube technology was very limited in the early days of television. It relied on conventional glass blowing methods largely unchanged in centuries. Since the tube had to contain a very high vacuum, the glass was under considerable stress. This, together with the low deflection angle of CRTs of the era, meant the practical size of CRTs was limited without increasing their depth. The largest practical tube that could be made that was capable of being mounted horizontally in a television cabinet of acceptable depth was around nine inches (23 cm). Twelve-inch (30 cm) tubes could be manufactured, but these were so long that they had to be mounted vertically and viewed via an angled mirror in the top of the cabinet. In 1936, the British government persuaded the British Broadcasting Corporation to launch a public high definition (for the era) television broadcasting service. The principal driver for the British government's move was to establish cathode ray tube production facilities which it believed would be vital if the anticipated Second World War was to materialise. The ability to correct the deflection signals for aberrations in tube geometry had not yet been developed, and it was necessary to make tubes that were relatively long compared with their screen size to minimise distortion. However, because the tube face had to be convex to provide resistance to air pressure, this mitigated the problem but only if the apparent deflection centre was more or less at the centre of curvature of the screen. This necessitated a tube that was relatively long for its screen size. The accelerating voltage used for these tubes was very low by later standards and even a twelve inch tube only ran from a 5000 volt supply. The early white phosphors were not as efficient as later offerings and these early televisions had to be watched in subdued lighting.
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