A projector, sound system, control panel and screen may be configured together at one end of an area or dispersed, for multi-media group presentations.
Projection TV forms an image on a CRT or an LCD (Liquid Crystal Display) that is inside the projector and then reflects this image on a large screen outside the projector. A control panel and a sound system accompany the projector and the screen to complete a complete Projection TV system. The arrangement can be one of 2 types-Rear Projection or Front Projection. Rear Projection systems have all the components in one unit. This may often be in a room or a space behind the main screen that faces the audience in an indoor setting.
Front Projection is dispersed. The projector and screen are at opposite ends of the audience area, or at least far apart. The control panel may be at any location convenient to the operator and is most often a Laptop computer. The sound system will have speakers distributed as per the size of the audience and the acoustics.
Original Projection TV design uses small CRTs to create an image inside the projector. This image is transmitted to a screen through a lens. The CRT may produce all colors, one of the primary colors or may produce a black and white image. A single lens suffices to project the image for a color LCD. A set of color filters are placed between the CRT and the lens for a black and white CRT. The color filters rotate at high speed, projecting separate images of each color. These images change so fast that the human eye perceives them as a unitary colored picture on the screen. A projector with single color cathode ray tubes has 3 of them inside the projector. Each such CTR has its own lens. The projection from each lens coalesces on the screen to form a unitary image. This original design is not very portable and images have relatively poor resolution. A Liquid Crystal Display can remove these drawbacks of a Cathode Ray Tube.
The first models of LCD Projectors work on the principle of transmission. The LCD consists of 2 polarized glasses at right angles to each other, with transparent electrodes and a layer of nematic liquid crystals. Light from the first glass, aligned in a particular direction, passes through the plate of liquid crystals. The latter are substances in-between solid and liquid states and are naturally twisted to allow light waves of a particular orientation to pass through. The light that passes through the nematic liquid crystal layer then emerges through the second polarized glass. This happens when there is no current in the system and results in input light reaching a screen. The nematic crystals change shape reversibly when the electrodes are charged with current from a battery or other power source. The light from the first polarized glass can no longer pass through, and hence a dark patch appears on the screen. Input light in an LCD may either be reflected from an external source or may be visible because of surrounding fluorescent tubes. The latter system is used in computer screens.
Projectors that transmit are more common than the reflective type but the latter have technical advantages and may replace transmission in time. Projection TV has many educational and communication-related applications and their popularity will grow with time. They allow large numbers of people to see and hear the outputs of a single computer or other image system.
