US firm sees the light via micromachined projectionTexas Instruments and Daewoo have been joined by Californian firm Silicon Light achines with a micromachined light projecting pixel the Digital Light Machine. Steve Bush takes a look
Just when you thought it couldn’t be done, someone has developed another way to make a micromachined light projecting pixel.
Texas Instruments has its Digital Micromirror Device, Daewoo has its Thin-film Micromirror Array, now Sunnyvale, California-based Silicon Light Machines is entering the fray with its Digital Light Machine (DLM).
DLMit significantly different from either the Texas or Daewoo devices in that it does not use a conventional mirror, however small. Instead, it uses light wave interference techniques to turn pixels on and off. Micromachines projector silicon at a glance Company
Silicon Light Machines
Voltage or time
1 x n array
m x n array
Voltage or time
m x n array
The technology behind DLM was originally developed and patented by Professor David Bloom and his students at Stanford University where it was called a grating light valve.
Each DLM pixel consists of a number of parallel reflective strips held above a flat surface. When the strips are all relaxed, they form a flat mirror, reflecting light that impinges upon them.
However, if every second strip is pulled down towards the surface by a quarter of a wavelength, destructive interference occurs above the strips and the pixel does not reflect.
The surrounding optics can actually be arranged to treat either of these states as ‘on’ and a grey-scale can be developed by deflecting the moving strips less than a quarter wavelength.
Switching speed is fast: “Ithas been demonstrated at 20ns,”said company CEO Dave Corbin. This give an alternative route to a grey-scale using temporal modulation.
As the pixel action depends on wavelength, they are colour sensitive. For a full colour projector, “you would use three devices for red green blue. They would be identical, but operated at different deflections by the driver circuitry. Alternatively, one device could be operated with the colours in a time sequence,”said Corbin.
Several devices have been prepared. A typical pixel consists of three pairs of strips each 100?m long and 4?m wide. Claimed reflection efficience of the complete pixel, including diffraction and inter-strip gaps, is around 70 per cent says the company. This can be raised somewhat by the use of more complex optics which collect more of the diffracted light.
The first demonstration of a digital light machine was a 1920 x 1080 pixel HDTV projector. This was generated using a 1 x 1080 pixel array and optically scanning the image to get the second dimension.
This technique, says Corbin, is easy to implement and gets the company away from the problems of yield when producing reflective devices on silicon. “We get over 50 candidates from a six inch wafer,” he said, adding:”And it is only a seven mask process.”