Color-changing Glasses Block Laser Light

By LCD-based spectacles activate protective filters automatically, make possible sunglasses with changeable lens colors -- Design News, May 8, 1995

Foster City, CA-Surgeons and researchers who work with lasers might soon replace their collections of protective goggles with pairs of high-tech spectacles. Sporting a pair of compound LCD lenses, the new glasses change from transparent to tinted in microseconds in response to harmful levels of laser light. Lightweight, low-powered, and potentially inexpensive, they offer a convenient and powerful alternative to existing protective glasses that employ mechanically switched filters.

During surgery doctors frequently use a variety of lasers, CO₂, Excimer, Argon, Krypton, and Nd:YAG, to name a few. Each produces a different monochromatic beam of light and offers unique advantages to the surgeon. Trouble is, operations often require more than one laser, and surgeons therefore need more than one set of eye filters to protect them from scattered laser light.

Developed by Michael Black, president and chief designer at Reliant Technologies, these glasses consist of a pair of LCDs, a photocell, a watch-type battery, and driver electronics. Optionally, solar cells in the frame above the lenses provide power for outdoor applications.

The photocell mounts in the frame and measures the intensity of incident radiation. It incorporates interference filters designed to selectively transmit one or more narrow bands of light, bands that correspond exactly to those the LCD lenses filter out. Should the magnitude of radiation within one of the transmittance bands exceed a specified value, the electronic driver will trigger the LCDs to change color and block the harmful light.

Black designed the glasses to work with any of several LCD designs. Each offers unique advantages. None, as yet, has proved versatile enough to be specified in all situations.

For protection from intermittent bursts of scattered laser light-a frequent phenomenon in surgery-he chose fast-acting ferroelectric liquid crystals (LC). These crystals possess a rise time of just 10 to 50 æsec. But their voltage requirements, -15V to +15V, necessitate an external power source. By contrast, steady-state applications benefit from super-twisted nematic (STN) liquid crystals. They require a switching voltage range of just 1.8V to 2.2V, but have a rise time of 50 msec.

Both the ferroelectric and STN lens designs are similar and contain several monolithic, unbroken layers consisting of colored and neutral polarizers, liquid crystals, and glass plates. Reliant calls these "tunable multilayered cells." By varying the number of layers, the color of the polarizers, and the thickness of the liquid crystals, they can be made to selectively block one, two, or several wavelengths of light. Black is also working on a fully tunable version. "You can mix color of polarizer with different levels of polarization to achieve any color you want," he says. The current design's greatest advantage, however, may be that the lenses become completely transparent when unpowered.

Other Applications
Tinted windows Camera filters

A third lens design, patented by Black, takes a dot-matrix approach similar to that found in a portable computer's color display. It uniquely includes a fourth, transparent pixel to the usual triad of blue, green, and orange (or red) pixels. This addition allows for a transparent-albeit dimmed-view when needed. A dot-matrix approach permits thousands of filter choices, instead of just a few, yet it suffers from low brightness and poor contrast.

While the glasses might prove a hit in the $1-billion-a-year market for sunglasses, Black emphasizes his design's roots as a protective device for scattered light. "When moving from one laser environment to another you don't need 20 pairs of glasses, you can have just this one," he says.

Additional details...Contact Michael Black, President, Reliant Technologies, 248 Lincoln Center Drive, Foster City, CA 94404, (415) 573-5346.

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