How Video Works - pág.51

Projection Systems

Projection systems are available as front-projection and rear-projection types. In a front-projection system, a video projector is used to illuminate a screen in much the same manner as film is displayed. The screen size can be as large as one hundred inches measured diagonally. Rear-projection systems generally have an internal projector and a mirror to reflect the image on to the viewing screen and are viewed in much the same manner as a direct-view CRT. The viewing screen is generally forty to eighty inches in size, measured diagonally. In the older type of projection systems, both front and rear systems used three CRTs-one red, one blue and one green-that are aligned to create a single image on the viewing screen.

The newer types of displays include Digital Light processing (DLP), liquid crystal display (LCD), liquid crystal on silicon (LCoS) and plasma.

DLP

The DLP uses an optical semiconductor called a Digital Micromirror Device developed by Texas Instruments. The device is composed of millions of microscopic mirrors arranged in a rectangular array on a silicon chip that rotate or move thousands of times a second (Figure 7.1). They direct light toward or away from specific pixel spaces. DLP projectors can use three individual chips, one for each primary color.

Figure 7.1
DLP Microscopic Mirrors Technology

There is also a single-chip version that works by focusing a high-intensity lamp through a condensing lens. The condensed light then passes though a six-panel color wheel that filters the light into red, green and blue. Each color appears twice on the filter wheel. The light then passes through a shaping lens. This shaped light source focuses on the chip, is reflected by the microscopic mirrors, and then passes through a projection lens onto a screen.