There are three common technologies in use for projection of digital images onto an external screen -- LCD, DLP, and LCOS. All the projection technologies use very bright and quite expensive bulbs that generate large amounts of heat. The picture element selection is usually done by small devices. In some technologies the picture generation device is on the scale of a square centimeter. The combination of high heat and small elements presents can make heat control a problem. The display devices also may exhibit many of the problems of monitors including inability to handle some combinations of resolution and scan rate well (or at all).
Liquid Crystal Displays (LCD) projectors are basically LCD monitors using a very bright light source in order to project an image on a screen.
Digital Light Processing (DLP) displays shine a beam of light through a rotating color wheel. The colored beam is then bounced off a very large array of tiny remotely controlled mirrors that can tilt through as much as 20 degrees in less than a millisecond. Desired colors are reflected onto the screen. Undesired colors are deflected away from the screen. Color intensity is controlled by the percentage of the potential display time that the mirror is aimed at the screen. The color wheel contains filters for three colors as well as (often) a clear segment for white. The mirror devices used in DLP projectors are made by Texas Instruments.
Liquid Crystal On Silicon (LCOS) technology uses liquid crystal material plated onto a silicon microcircuit control matrix. A mirror finish is plated between the microcircuit matrix and the liquid crystals. Three separate matrices are used for the three colors. A newer version has been demonstrated uses a single LCOS matrix illuminated by three scanning color beams that move simultaneously -- each sweeping the entire screen, but phased one third of the screen sweep distance apart. This arrangement delivers roughly three times as much light as DLP and conventional LCOS from the same projection bulb..
DLP technology displays are generally thought to have better contrast than LCD and are immune to convergence problems. Colors -- especially yellow -- may be better with LCD. Because DLP projects colors sequentially rather than simultaneously, the technology is subject to a problem with rapidly moving objects showing fringes of individual colors to the right and/or left of sharp edges
Return To Index Copyright 1994-2002 by Donald Kenney.