CRT
Until the popularization of flat panel LCD and plasma technologies for home televisions, rear projection TVs were the only option for those looking to have a screen larger than 40 inches in their living rooms. The first of such TVs used cathode ray tube (CRT) technology. Like smaller CRT sets, rear projection CRTs used cathode ray tubes to beam electrons onto a phosphor-coated screen, creating an image. The electron beam maintains focus in the imperfect vacuum of the tube, and the phosphor coating of the screen creates a picture by emitting light. Rear projection CRT televisions added another step to the process by projecting the image from the phosphor-coated screen onto a larger screen using a lamp. This worked much the same way as a motion picture or slide projector. CRT technology dominated the rear projection TV market until the late 1990s, when it fell victim to its own limitations. CRT projection screens were unable to provide as sharp or bright an image as their LCD or plasma competitors. CRTs were also quite bulky and expensive to produce. Today, it is quite rare to find a CRT rear projection TV on the market.
LCD
LCD rear projection TVs offer some advantages over their CRT predecessors. They can be thinner, so they are less costly to manufacture, take up less space in a room and can be wall mounted. They also offer a true high definition display in keeping with modern resolution standards. By projecting an LCD image onto a larger surface, the actual display screen can be made of less expensive materials than a non-projection LCD TV. However, the origin of the image remains much the same as any other LCD display. Light passes through a prism, which separates it into red, green and blue beams. These beams are then directed at three panels each consisting of a polarizing layer, a layer of liquid crystals and an analyzing layer. The liquid crystals can be electrically manipulated to alter the flow of light, creating images of multiple colors and brightnesses. The light from the three panels is then passed through a lens system and projected onto a larger surface.
DLP
Digital Light Processing technology is the third method used in rear projection televisions, and like LCD, it is still in use today. The heart of a DLP TV is its digital micro-mirror device, a chip that contains a matrix of microscopic mirrors comprising the pixels of an image. The mirrors can be adjusted to alter the intensity of the light and are controlled using electrostatic attraction. Light passes through either a spinning color wheel or a red, green and blue-chipped projector before being sent to the micro-mirror chip, which then projects the combined color image onto a flat screen. Like LCD rear projection TVs, DLPs are capable of displaying HD images in a relatively small form factor by enlarging an image created by a smaller source. However, due to the comparatively low cost and high quality images of full LCD and plasma televisions, both rear projection methods are on a steady decline.
Tags: rear projection, onto larger, image onto, larger surface, layer liquid, layer liquid crystals