Electronic Design

ASIFs Enable New Projection Systems

What's an ASIF, you ask? According to Jonathan Waldern, CEO at DigiLens, Sunnyvale, Calif., an ASIF is an application-specific integrated filter. Based on holographic technology, the device can fabricate complex optical properties into thin, active elements that can be combined to filter, focus, correct, or redirect light in optical systems. Such elements might become the key enabling technology behind new projection systems.

At the recent Society for Information Displays (SID) conference, Waldern reported progress, like work on an ASIF to replace the rotating color wheel in a one-chip digital light-processing (DLP) projector. In such projectors, white light passes through red, green, and blue filters in rapid succession to create a full-color image. An ASIF, essentially a solid-state color wheel, could provide a quieter, lower profile and a more reliable solution for generating sequential light.

Last fall, DigiLens demonstrated a prototype system that used a projector whose color wheel was replaced by an ASIF. At SID, the company demonstrated a much improved version of its retrofitted projector. In a side-by-side demo with an unmodified system, the visual quality of the ASIF-based system wasn't too bad. It certainly operated fast enough (50-µs switch speed) so that no motion or color break-up artifacts were readily apparent. The color balance, however, isn't quite right yet.

Waldern also noted that this retrofitted demonstration isn't ideal for showcasing the technology. In a properly designed system, the ASIF works best with 15° off-axis illumination, producing high contrast and low black levels. The retrofitted projector didn't allow for this optimized configuration, though. By the third quarter of 2000, DigiLens hopes to offer new ASIF samples optimized for one-chip DLP systems.

Furthermore, DigiLens has been developing a solution for one-panel projection systems using liquid-crystal-on-silicon (LCOS) light valves. The company has a new scrolling technique that allows the use of twisted-nematic LCOS panels. Most one-panel LCOS systems so far have used ferroelectric LCOS panels, which have much faster switching speeds. If the slower-switching twisted-nematic LCOS panels are implemented, then the system suffers in overall optical efficiency, producing a much dimmer display. So, DigiLens developed a scrolling technique similar to the several-year-old falling raster method of Philips Research Labs, Briarcliff Manor, N.Y. A mechanical scanner was used to scroll horizontal bands of information vertically.

DigiLens replaced the mechanical scanner with a scrolling ASIF. It consists of a stack of electrically switchable Bragg gratings (ESBGs), each optimized to reflect red, green, and blue light. These color filters are each subdivided into multiple electrically addressable hologram "stripes" that can be electrically switched off and on to be reflective and nonreflective, respectively. The stripes can be switched to create a rectangular pattern of reflected color light that "scrolls" down the ESBG layer in a semicontinuous set of discrete jumps. As the pattern scrolls off the bottom of the ESBG, it starts to reappear at the top. By superimposing individual color scrolling ESBGs, rectangles of colored light scroll down the ASIF. Drive signals to the single LCOS panel are synchronized with these color bands, resulting in a solid-state mechanical scanner with much higher optical efficiencies.

The company is now crafting the computer-generated holograms for this new scrolling ASIF. Waldern thinks it will have optical efficiencies close to those of a ferroelectric one-chip LCOS system, but by using twisted-nematic-based LCOS panels, it will provide greater selection in LCOS display supply. DigiLens is seeking partners to further develop this technology.

ColorLink, Boulder, Colo., is developing an alternative approach to the solid-state color wheel. Named ColorSwitch, the device consists of three active color filters with crossed polarizers on each end of the assembly. Every active filter is composed of input and output polarization retardation stacks with a single-cell liquid-crystal switch in between. The three filters function as additive color element filters optimized to transmit either red, green, or blue light. Control voltages activate each filter to pass white light or else the specific color for which it's optimized, enabling color-sequential light to flow through this 0.25-in.-thick device.

The company says that ColorSwitch recently achieved over 90% transmissivity at f/2.5 in all three primary color bands. This marks a significant improvement and overcomes the performance limitations encountered with previous products. To reach the 90% transmissivity value, the company developed a new bonding technique for assembling the thin polycarbonate stacks that comprise each retarder stack. Previous pressure-sensitive adhesive materials were too glossy and added a yellowish tint to the stack. The new technique also uses index-matched materials to eliminate reflections.

To obtain proper color balance, designers must adjust several parameters. One is to control the voltage on each color element in the switch, thus varying the amount of colored light passing through the device. This can help compensate for the red spectrum deficiency of metal halide lamps, for instance, but sacrifices efficiency. Fast switching speed also is required to avoid color mixing. The latest devices switch at 240 to 390 µs, depending on the color band. Don't be surprised if an ASIF or ColorSwitch helps to power projection systems in board rooms and living rooms in the next few years.

TAGS: Components
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