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What is a Light Valve?

Ron Davis
Ron Davis

A light valve is a device that either reflects light or allows light to pass through the shutter. Light valves have a wide range of uses from TV sets to top secret military applications. A light valve can be either liquid crystal (LC) or a nanomechanical device. Nanomechanical devices include the digital micromirror device (DMD) and grating light valve (GLV).

Liquid crystals, a naturally occurring substance, were originally identified in 1889 by Friedrich Reinitzer who was working at Charles University in Prague. LCs remained little more than a novelty until 1969, when important steps were made towards a commercially viable material. The property of LCs that allows them to function as light valves is their ability to polarize light. LCs are used with a polarizer, which either lets light pass through or reflects it, according to its polarity. An electrical current applied to the device determines the polarity.

A color wheel shows colors that contrast and complement each other.
A color wheel shows colors that contrast and complement each other.

GLVs were developed at Stanford University by Dr. David Bloom and some of his graduate students. One of Bloom’s students, Raj Apte, outlined the prospective commercial viability of the technology in his 1994 doctoral thesis. Dr. Bloom founded a start-up company the same year to commercialize the technology. GLV devices are mechanical gratings that modulate the amount of diffraction light experiences when it encounters them. These devices are found in television sets and also used in military, technology, and industrial applications.

DMDs were created by Texas Instruments in 1987 under the direction of Larry Hornbeck. The original concepts were aimed at detection of military objects, such as tanks and armored personnel carriers, during surveillance. A DMD is a light valve consisting of arrays of up to 1.3 million hinge-mounted microscopic mirrors. Each chip corresponds to one pixel on a screen, and each can be either off or on, passing light or reflecting it. Many applications, such as TV, home theater systems and business video projectors use a one chip system combined with a color wheel, and the speed and duration of time that each color is passed through is coordinated according to the color to be displayed.

Very high image quality requires a three chip DMD unit. One chip is used for each primary color, with the same effect as the one chip system. These very high-end units are used in movie theaters, some TVs, and some military settings where very high resolution is needed. DMDs have a much faster response time than LCs, but are measurably slower than GLVs.

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    • A color wheel shows colors that contrast and complement each other.
      By: WavebreakMediaMicro
      A color wheel shows colors that contrast and complement each other.