How LCoS Works

Review of LCD and DLP

A DLP system using one DMD and a color wheel to provide color.
A DLP system using one DMD and a color wheel to provide color.
Photo courtesy Texas Instruments

The most common use for LCoS is front- and rear-projection televisions. The setup is a lot like what you find in a DLP system. DLP uses a digital micromirror device (DMD) to create a picture using a process that's like making a mosaic out of small, square tiles. The DMD contains millions of microscopic mirrors that reflect light from a lamp. Each mirror creates one pixel of the final image.

The mirrors flip back and forth between their "on" and "off" positions very rapidly. When mirrors are on, they point toward a projection lens. The longer a mirror is in the on position, the brighter the pixel it creates. Mirrors creating black pixels remain off. In most DLP televisions, a color wheel spins between the lamp and the DMD, adding red, green and blue light to the picture. The viewer's eyes combine these colors to create the finished image.

­ LCoS uses a very similar idea. As with DMDs, LCoS devices are tiny -- most are less than one inch square. Both technologies are also reflective -- the devices reflect light from a source to a lens or prism that collects the light and displays the image. But instead of tiny mirrors that turn on and off, LCoS uses liquid crystals to control the amount of reflected light.

A liquid crystal is a substance that is in mesomorphic state -- it's not exactly a liquid or a solid. Its molecules usually hold their shape, like a solid, but they can also move around, like a liquid. Nematic liquid crystals, for example, arrange themselves in loose parallel lines. Most LCDs use twisted nematic (TN) crystals -- with the application of an electrical charge, the twisted crystals straighten out.

When placed between two polarized panels, the twisted crystals guide the path of light. By changing the direction of the light, the crystals allow or prevent its passage through the second panel. The crystals' ability to change the path of the light is central to its use in LCDs and LCoS systems.

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In their twisted state, liquid crystals direct the light

so it can pass through the second polarized panel.

Ferroelectric liquid crystals (FLCs), sometimes used in LCoS devices, are crystals which align themselves at a fixed angle away from the normal into orderly rows. They also develop electrical polarity when they come into contact with an electrical charge. Ferroelectric chiral smectic C crystals can switch their orientation very quickly. You can learn more about smectic and nematic liquid crystals at Kent State University's Liquid Crystal Institute.

The liquid crystal layer in an LCoS microdevice controls the amount of light for each pixel, like the mirrors do in a DMD. But making the picture requires more than just the microdevice -- it also requires lenses, mirrors and prisms.