1 Projection and imaging systems
1.2 LCD projectors

Basic principle of an LCD projector

Light leaving the device’s lamp is incident on a system of dichroic mirrors (that is, semipermeable mirrors), which are generally used in optical prism to split the white light into the base colors of the RGB (Red-Green-Blue) model. The first dichroic mirror passes through the red light component, the others (i.e. the green component and the blue component) are reflected. The light in the complementary color (i.e., cyan color) goes on to the next dichroic mirror. There is a reflection of the green light on it, while the blue light passes forward. These mirrors are produced by depositing a thin reflective layer on a glass plate that reflects light of the given color only (i.e., the light of a specific wavelength).

The optic light-processing system from the projector’s lamp goes on through the polarizing filters and the LCD panel. An electrical current (corresponding to the television signal) is carried to the LCD panel, which produces an image corresponding to this electrical current in the LCD panel. Then, the light of the three basic colors of the RGB model (carrying information about the generated image) is reflected on the dichroic prism in which they compose the resulting light (and the image). Finally, the light passes through the lens directly to the screen.

The LCD projector principle

One advantage of such projectors is that they create a stable image that is not shattered (in comparison with the DLP technology). The color of the entire projected scene can be adjusted with the throughput of each LCD panel. This allows for a fairly high contrast and true color display.

One disadvantage is that, in particular, the first models of these types of projectors displayed a visible grid on the screen as a result of the use of LCD panels. The individual display points that are to be displayed on the screen, are positioned side by side on the LCD panel, but they do not touch one another. It is necessary to bring the conductors with electric current to these display points controlling particular pixels. Newer models with better LCD panels no longer suffer from this defect; the production technology makes possible to bring individual pixels closer to each other. Another disadvantage is the so-called pixel aging. Transistors that control the brightness of individual pixels work at high temperature and are burning over time, and some of them stop working.