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Lighting the Dark

by Kathleen Cason


When a power outage darkened dozens of Northeastern cities this past August, New Yorkers trapped in subways groped along pitch-black tunnels, hunting for exits. Had William Yen’s invention been available then, glowing arrows painted on walls might have illuminated the commuters’ paths to safety.

Yen, the University of Georgia’s Graham Perdue Professor of Physics, has invented and patented a new breed of glow-in-the-dark pigments or “long-persistence phosphors.” After just a few minutes’ exposure to light, these inorganic compounds emit a bright glow for more than 20 hours twice the time of similar substances.

Yen’s invention advances a technology that first appeared more than 2,000 years ago. During the Han Dynasty, Chinese artists used phosphors to create luminescent paintings. Today phosphorescent materials have become standard on clocks, watch faces and instrument panels.

These new phosphor technologies offer several advantages over those available in the marketplace. Yen’s materials are low-cost, non-toxic and come in any color imaginable. (Previously only green and blue were available.) He can even engineer how long his material will emit light. Plus, his phosphors do not contain many of the troublesome substances commonly used to make compounds glow, such as radioactive elements, heavy metals or sulfurs, which smell like rotten eggs when wet.

“There are many possible applications: emergency signage, paintings, military uses, toys, clothing, even luminescent nail polish,” said Yen, who is co-editor of the Phosphors Handbook, a technical manual of phosphor properties and uses.

The uses are limited only by imagination. Phosphors could be added to paints, fabrics or other materials. Exit signs powered by phosphors would need no electricity, saving money and energy. Color-coded phosphors added to the plastic sleeves on wires might make maintenance easier in low-light environments. In fact, Yen’s research group, which includes former graduate students and postdoctoral fellows, has developed specific uses incorporating these phosphors.

“The worldwide market for phosphor technologies could be around $600 million to $800 million dollars a year, if not a billion dollars,” said Gennaro Gama, a technology manager for the UGA Research Foundation.

For more information, contact Gennaro Gama at


Research Communications, Office of the VP for Research, UGA
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