Digital projection is a method of displaying moving images such as television shows or movies onto a large viewing screen. This type of projection has largely replaced analog projection, which must use a translucent film in order to project the image. Digital projection uses a computerized display to create a visible image, and does not require expensive film in order to operate.
All digital projectors are able to generate a projected image directly from a video signal. Past projection technology, such as that of slide projectors, require a pre-produced image to be printed on film before a picture can be viewed. Digital projection units, however, can be directly connected to a computer or video player. This advantage makes digital projectors popular for business presentations and home theater systems where a wide variety of different images or videos need to viewed.
The first digital projectors were built in the 1950s, and were named "Eidophor projectors" after the Greek term for "image carrier." These early digital projectors used a rotating disk covered in transparent oil. An electron beam is aimed at the disk, causing the transparency of the oil to change based on electrostatic charges. Eidophor projectors are famous for their use as large information displays at NASA's Mission Control Room, but have since become obsolete due to their large size and high cost.
Cathode ray tube (CRT) is the digital projection that replaced Eidophor technology. CRT digital projectors employ blue, red, and green cathode ray tubes that create colors on a flat screen, similar to a traditional television. A CRT projector uses a special lens to expand the screen image onto a large area. These units are bulky and can be inconvenient to move, but are relatively inexpensive.
Liquid crystal display (LCD) digital projectors have replaced CRT units in many settings. This type of projection uses several semi-transparent crystal panels. Each panel contains many different pixel areas, which are controlled by a computer. Light is sent through these panels and past a lens, which expands the image from the small pixels to create a large viewable picture. LCD units typically have more contrast and brightness than CRT devices.
Digital light processing (DLP) projectors use extremely small mirrors in place of crystal panels. These mirrors reflect light through a rotating color wheel. This approach allows a DLP projector to create a very wide range of different colors, without the jagged "pixelation" image effect that is created by LCD units. These projectors are lightweight and produce a high-contrast image, but are often more expensive than other digital projection technologies.