Digital projectors are the modern replacement to more traditional transparency projectors. Transparency projectors work by shining light through a semi-transparent material, such as slides or film. Digital ones, by contrast, project a digital image directly from a computer through a lens on to a screen. There are four technologies which may be used in these types of projectors.
Eidophor projectors, used in the 1950s, were the first digital projectors. They used an oily surface in a rotating disk with light shining through it and employed an electron beam to disrupt the oil in a very precise manner. Although Eidophor projectors were a very primitive form of projector, they were nonetheless capable of producing color images. In the wake of much simpler, cheaper, and higher-fidelity devices, however, Eidophor projectors are no longer used.
Cathode Ray Tube (CRT) digital projectors operate along the same general principles as a CRT television set. There are three main configurations for CRT projectors: the projector may have one RGB (red, green and blue) color tube with a single lens, one black-and-white tube with a rotating color wheel, or three RGB color tubes with three lenses. In all of these configurations, an image is produced at a relatively small size ranging from six to twelve inches (15-30 cm), and then magnified with a lens onto a large screen. These devices are quite bulky and heavy, making them suitable primarily for fixed locations.
Liquid Crystal Display (LCD) digital projectors are much lighter and more portable than their CRT cousins, making them widely popular. With the advent of new LCD technologies, digital projectors have been developed which have very clear and crisp fidelity even at large projection sizes. The LCDs used in these machines are approximately the size of a small color slide, and in fact the projectors operate very much like a traditional slide projector. The main difference is that the slide is constantly changing.
Both CRT and LCD projectors are known as transmittive projectors, meaning that light shines through the image to project it. There is another class of digital projectors, however, known as reflective projectors, which provide a much higher quality of image.
Texas Instruments produces the chip for the only currently viable reflective digital projector using a technology known as DLP (Digital Light Processing). These have an array of tiny mirrors, one for each pixel. As these mirrors reposition themselves to either place light on the screen or not, they produce shading which creates the illusion of a complete image. DLP technology is used in theatres as they make the transition to digital projectors, due to its extremely high quality and fidelity and its lack of pixilation even at high image sizes.