Digital Light Processing and Other Party Tricks
The Varied Uses of TI’s MEMS Device
The transistor has been hailed as the greatest invention of the 20th century. Developed at Bell Laboratories in 1947, this solid-state device uses electrons to switch the flow of other electrons on or off. Additionally, it can modulate the amount of electric current in much the same way as the dimmer switch in my dining room — but without the big beige knob. The transistor permitted the development of the integrated circuit at Texas Instruments (TI) in 1958 and thus was born our modern-day industry of microelectronics.
Forty years after the invention of the transistor, TI developed an electronic device capable of switching and modulating a beam of photons. While working in TI’s Central Research Laboratories in 1987, Larry J. Hornbeck invented a micro-electromechanical system (MEMS) device known as a Digital Micromirror Device (DMD). Each element of this MEMS structure consists of a 16-micrometer square cantilevered aluminum mirror monolithically integrated over a silicon address circuit. When powered with a bias voltage, each mirror can switch between two deflection angles of plus or minus 10 degrees by writing a logical one or zero to the mirror’s address. At these angles, the mirror reflects a beam of light into (logical 1) or out of (logical 0) the desired optical path. Changing the duty cycle between the two angles of each DMD pixel modulates the photon flux.
DMD development was spurred by the needs of an all-digital image processing system that could be inserted between the original analog image and the analog detection system of the human eye. The analog-to-digital converter of a digital camera could be used to transform the analog image into a digital representation that could be processed, analyzed, compressed, transmitted, received, and decompressed digitally. However, in the late 1980s there were few components available to interface the digital image directly to the analog human eye. So digital-to-analog converters (DACs) such as the cathode ray tube (CRT) or photographic projection film were used to present the image to the eye. The original DMD was used to imprint a digital image on the electrophotographic printer drum of a copy machine. Recently, TI has produced a 7056 x 64 element DMD capable of producing 11.7 inch-wide hardcopy at 600 dpi. Even though the original mission of the DMD has been fulfilled, like all good technology, the applications don’t stop here.
The light reflected by the DMD can be projected onto a screen and viewed directly by the eye without the intermediary hardcopy. The digital mirror switching speed can be increased above the perception threshold so that the DAC can be performed entirely by the eye, removing the CRT or film. Used in this mode, the DMD-based projection system is known as Digital Light Processing (DLP) technology. Single-chip, grayscale-DLP projection systems reflect the white light of a projector bulb while single-chip, color-DLP systems incorporate a synchronized rotating red-green-blue optical filter wheel. DLP-based meeting room projectors, high-definition projection televisions, and digital movie theaters are beginning to displace their analog cousins.
The DMD also is displacing the liquid crystal display (LCD) for its utility as a spatial light modulator (SLM). The all-reflective DMD has advantages in contrast ratio, throughput, switching speed, and wavelength insensitivity over the LCD. DMD-based SLMs are being developed as network switches in Dense Wavelength Division Multiplexing optical networks. InPhase Technologies has recently announced a holographic optical storage system incorporating a DMD-SLM that is capable of holding 100 gigabytes of data in the form of 1.3-megabyte holographic images burned into an industry standard 4.75-inch optical disk. The U.S. Air Force Research Laboratory has published research exploring the utility of the DMD-SLM to a 3-D cockpit threat display system. Rather than encoding a 3-D image into a 2-D holographic recording, the futuristic holographic display system would use the DMD-SLM to output a computed 3-D image as an actual free-space 3-D image for the pilot to analyze.
Researchers at the Massachusetts Institute of Technology also have demonstrated the ability of the DMD-SLM to manipulate laser light for the purpose of trapping and holding microscopic objects such as biological cells in an application known as “optical tweezers.” The object of the research is to gain the ability to move and sort multiple targets in two dimensions through the use of a computer-controlled system. Like its electrons-controlling-electrons transistor cousin, the electrons-controlling-photons DMD continues to facilitate new innovations inconceivable at its inception.
This material originally appeared as a Contributed Editorial in Scientific Computing and Instrumentation 19:8 July 2002, pg. 16.
William L. Weaver is an Associate Professor in the Department of Integrated Science, Business, and Technology at La Salle University in Philadelphia, PA USA. He holds a B.S. Degree with Double Majors in Chemistry and Physics and earned his Ph.D. in Analytical Chemistry with expertise in Ultrafast LASER Spectroscopy. He teaches, writes, and speaks on the application of Systems Thinking to the development of New Products and Innovation.
