Film Selectively Darkens Light

Aaron J. Hand

Intense light sources can cause damage to eyes and other optical sensors: a welding arc to a welder's eyes or a robot's vision system, a laser to a sensor, or even the glare of the sun or headlights to a driver's eyes. A group of researchers is tackling this problem, exploiting the optical properties of nematic liquid crystals to keep that light from being a hazard.

Liquid crystal materials already exist that darken when their electronics sense a bright light. For example, welding helmets that darken in a split second when they sense the blinding welding arc have been readily available for years. However, these materials darken the entire scene, making it difficult to view objects around the bright light.

A new liquid crystal material, if used in welding masks, would darken a bright welding arc while leaving the rest of the scene light and visible.

By doping nematic liquid crystal films with a small amount of methyl red dye, researchers at Pennsylva-nia State University have created a material that selectively darkens a bright spot, leaving the rest of the scene light. And it does so with-out the use of photoconductors or sensors.

Detailing their research in the May 24, 1999, issue of Optics Express, the researchers contend that the doped nematic films may be the most nonlinear optical materials known. Their unusually large nonlinearity gives them significant photorefractive reorientation capabilities. The more intense the light, the less light the liquid crystal lets through. For example, the film can reduce a 140-mW light to just 5 µW. Also, the more intense the light, the faster the material responds.

Although the researchers have no immediate plans to commercialize their discovery, there are several potential applications for the film's nonlinear effects, said I.C. Khoo, professor of electrical engineering and a researcher on the project. Besides welders, communications satellites could use protection from intense laser light, rearview mirrors could be coated with the material to protect drivers from headlight glare without unnecessarily darkening the entire view, and imaging devices could reduce problems associated with glare.

Khoo said that because the material's operation does not require embedded sensors or other circuitry, the films could be made inexpensively when compared with standard liquid crystal devices.