1 Projection and imaging systems
1.3 DLP projectors

Basic principle of a DLP projector

White light from the projector’s lamp goes through a rotating circular color filter. It is symmetrically divided into three color parts corresponding to individual base colors of the RGB model. The light of red, green, and blue color is gradually coming to the chip. The video signal we want to project is fed to the chip’s controlling electronics. The processor connected to the chip, based on a digital signal in which the projected image is encoded, transmits pulses for individual mirrors of the DMD (Digital Micromirror Device) technology. Thus, flipping the DMD mirrors between their ON and OFF position is controlled by the video input to the chip with respect to the color that the given mirror has to produce on the screen (i.e., we need to assign a given pixel the desired color in a projected image). An important role here is played by the ratio of the times in which the mirrors are in ON and OFF state. This way, up to 16.7 million colors can be created on the screen from the RGB model base lights. The colors we see on the screen are created by additive mixing the lights of these three basic colors at the appropriate intensity and brightness.

The DLP projector principle

One advantage of the DLP technology is high contrast and image brightness. There is also no grid in the projected image (as in some models of LCD technology), because the individual DMD mirrors are located close to each other. It can also be mentioned that the DLP technology means a simpler design, higher dust and smoke resistance and longer life.

Image flicker and poor color rendering are shortcomings, compared to LCD. The light that cannot be modulated when it passes through a rotating color filter is incident on a mirror and, after reflection, it falls on the projection screen with full light intensity that is emitted by the projector’s lamp. Therefore, the brightness of individual pixels cannot be adjusted individually. On the contrary, in LCD it is possible to set the intensity for each point individually. In DLP technology, the image is slightly blurred and there is a so-called "rainbow effect". This is due to the rotation of the color filter.