Binary spatial light modulator produces full-color images

Images of boats and an astronaut floating in space were produced by a full-color display based on ferroelectric-liquid-crystal (FLC) spatial-light-modulator (SLM) technology. The device was developed at the University of Edinburgh (Edinburgh, Scotland) by a team led by David Vass of the department of physics and astronomy and Ian Underwood of the department of electrical engineering.

Binary spatial light modulator produces full-color images

Laurie Ann Peach

Images of boats and an astronaut floating in space were produced by a full-color display based on ferroelectric-liquid-crystal (FLC) spatial-light-modulator (SLM) technology. The device was developed at the University of Edinburgh (Edinburgh, Scotland) by a team led by David Vass of the department of physics and astronomy and Ian Underwood of the department of electrical engineering.

The pictures show color images displayed on the SLM using a color-sequential-field technique. This involves cycling through a series of binary images, with each image illuminated by one of three high-intensity light-emitting diodes (LEDs) emitting at 635 nm (red), 568 nm (green), and 450 nm (blue).

"Our technology is not unique, but it is unusual. Our system uses an active matrix that is entirely digital. This means the backplane is purely digital," says Underwood. An image is split into primary colors, say red, green, and blue. Each color is assigned a multibit binary value at each pixel.

The key to the project is the very short time it takes to display a single binary image or bit plane, says Underwood. The SLM exhibits a high switching speed of 100 µs because of the FLC. This means that binary states "off" and "on" can be sequenced rapidly for each pixel--faster than the eye can perceive. By modulating the proportion of time for which each pixel is off and on, the perceived intensity of light reflected by each pixel can be varied. If three colors of light--red, green, and blue--are sequenced very fast, the viewer sees the average of these--or full-color images.

"Because color is produced in sequence (red, green, blue) at each pixel, no matter how highly a display is magnified no color separation occurs--color separation is when you look close enough to see the individual red, green, and blue dots on a conventional display," says Underwood. "In our display every single pixel produces a full range of color."

A conventional CMOS integrated circuit carries all the drive electronics, with additional surface structures to complete its optical interface. The pixel drive circuits are electronically connected to an optical surface--an array of 1024 ¥ 768-pixel mirrors. Each mirror is 10 µm square, arranged on a 12-µm grid. "We had to place very small, very flat mirrors on the surface of the silicon. We used a chemical-mechanical polishing process that was 100% developed at the University of Edinburgh to produce these," says Underwood.

The 1024 ¥ 768-line display corresponds to XGA graphics. "The optical system we use magnifies the display to the equivalent of a 14-in.-diagonal display distance of 4 ft," says Underwood.

GEC Marconi markets an SLM consisting of a 176 ¥ 176 array of single-transistor DRAM-style pixels of 30-µm pitch. The SLM has an overall active area size of 5.28 ¥ 5.28 mm, with a pixel flat factor of 81% achieved by planarization. The device is filled with a surface-stabilized FLC material.

Using this fast-switching liquid crystal and flexible high-speed drive electronics, the reflective SLM has been operated with a binary frame rate of 1.5 kHz. A pixel appears to be on when the voltage between the SLM front electrode and the pixel DRAM bit changes the FLC molecular orientation so that the incident polarized light is rotated on reflection and can pass through an analyzer to the viewer.

According to Underwood, there are many appliations for these devices. The most obvious and probably the largest market in terms of device numbers, however, is in displays. These devices are not only of high pixel count, but also very small and light in comparison with competing technologies. Niche markets currently targeted include projection displays and head-mounted displays.

Admit Design Systems (Edinburgh, Scotland) is preparing to launch a head-mounted SLM device in 1998. The head-mounted display was the result of a joint program led by Mike Worbeys at GEC Marconi, the University of Edinburgh, and Admit Design.

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