Rear-projection Television - Types of Rear-projection Technologies

Types of Rear-projection Technologies

A projection television uses a projector to create a small image from a video signal and magnify this image onto a viewable screen. The projector uses a bright beam of light and a lens system to project the image to a much larger size. A front-projection television uses a projector that is separate from the screen, and the projector is placed in front of the screen. The setup of a rear-projection television is in some ways similar to that of a traditional television. The projector is contained inside the television box and projects the image from behind the screen.

The following are different types of projection televisions, which differ based on the type of projector and how the image (before projection) is created:

• CRT projector: Small cathode ray tubes create the image in the same manner that a traditional CRT television does, which is by firing a beam of electrons onto a phosphor-coated screen and then the image is projected to a large screen. This is done to overcome the limit of size of cathode ray tube which is about 40 inches. Normally 3 CRTs are used, one red, one green and one blue, aligned so the colors mix correctly on the projected image.

• LCD projector: A lamp transmits light through a small LCD chip made up of individual pixels to create an image. The LCD projector uses mirrors to take the light and create three separate red, green, and blue beams, which are then passed through three separate LCD panels. The liquid crystals are manipulated using electric current to control the amount of light passing through. The lens system takes the three color beams and projects the image.

• Digital Light Processing (DLP) projector: A DLP projector creates an image using a digital micromirror device (DMD chip), which on its surface contains a large matrix of microscopic mirrors, each corresponding to one pixel in an image. Each mirror can be rotated to reflect light such that the pixel appears bright, or the mirror can be rotated to direct light elsewhere and make the pixel appear dark. The mirror is made of aluminum and is rotated on an axle hinge. There are electrodes on both sides of the hinge controlling the rotation of the mirror using electrostatic attraction. The electrodes are connected to an SRAM cell located under each pixel, and charges from the SRAM cell drive the movement of the mirrors. Color is added to the image-creation process either through a spinning color wheel (used with a single-chip projector) or a three-chip (red, green, blue) projector. The color wheel is placed between the lamp light source and the DMD chip such that the light passing through is colored and then reflected off a mirror to determine the level of darkness. A color wheel consists of a red, green, and blue sector, as well as a fourth sector to either control brightness or include a fourth color. This spinning color wheel in the single-chip arrangement can be replaced by red, green, and blue light-emitting diodes (LED). The three-chip projector uses a prism to split up the light into three beams (red, green, blue), each directed towards its own DMD chip. The outputs of the three DMD chips are recombined and then projected.