How do battery powered pico-projectors work?

Once upon a time, very long ago, the projector world was ruled by the intense light of arcs. As they were rather unwieldy, xenon lamps took their place. With the unrelenting march of innovation, the era of OHPs or overhead projectors that could project images of transparencies, came into existence. These soon became obsolete as computers evolved and could be directly connected to projectors with LCD screens. The latest in line is the Pico-projector, which uses tiny batteries and the light from LEDs to project large displays.

Although Pico-projectors are small – as small as mobile phones, and sometimes even smaller – they can project large displays, sometimes up to 100 inches. Even though their brightness and resolution is not up to the mark of their bigger brethren, Pico-projectors are relatively new in the innovation chain, and as the market expands, they are expected to develop further.

Several companies have developed their own methods of producing battery-powered Pico-projectors. Of them, the three major technologies are DLP or Digital Light Processing, LCoS or Liquid Crystal on Silicon and LBS or Light Beam Steering. DLP and LCoS use a white light source and a system of filtering techniques to create different color and brightness of each pixel. On the other hand, LBS uses a small liquid crystal display to control the amount of light going to each pixel.

Digital Light Processing or DLP is pioneered by Texas Instruments (TI). Their idea is to use tiny mirrors on a chip to direct the light. Each mirror controls how much light goes onto each pixel of the display. The mirror can be turned on or turned off on command many times a second, and the on to off time ratio defines the brightness of the pixel. For color, there is a color wheel in front of the light source, splitting the beam into red, green and blue. Each mirror controls all the three light beams.

Liquid Crystal on Silicon or LCoS, as the name suggests, uses an LCD to control the amount of light reaching the pixel of the display. For color, two techniques are used. One is the Color Filter where three sub pixels are used, and they each have their own color, Red, Green and Blue. The other is the FSC or Field Sequential Color that requires a fast LCD and a color filter to split the image into RGB, the three main colors sequentially. The LCD is refreshed three times, once for each color. For LCoS, the light source could be an LED or a diffused Laser.

Laser Beam Steering or LBS creates the image one pixel at a time. The technique uses three directed laser beams, red, green and blue. The three beams are combined using optics and are guided using mirrors. So that the eye does not notice the pixel-by-pixel design, the image is scanned at over 60Hz.

LBS has some advantages over the other two techniques. The size is small and power consumption lowest, as the darker pixels require less energy, while the black pixel does not require any energy at all. The image from an LBS system is always focused, even on curved surfaces. On the other hand, lasers are expensive, cause random intensity patterns and are a concern for eye safety.