Mysteries of Light

The sphere has been used to test such things as paint, sealant, caulk, the material in CDs, firefighters' coats, body armor and outdoor furniture. Of all things, body armor did not do well, Chin said. It was "highly susceptible to UV degradation, highly susceptible," she said. It can, however, be protected by a UV-resistant liner, she said.

UV light tends to be absorbed by certain things, such as plastics and human skin. When that happens, the light breaks molecules apart. "It actually physically takes the chemical bond and just slices it in half," Chin said. "That's how you get damage in your skin" and in plastics, which have similar chemical makeups, she said.

The sphere has a series of boxes attached to its exterior. Substances to be tested are placed in the boxes. The box doors are closed. Doors to the sphere are then opened, and the box fills with ultraviolet light.

Although NIST does not do product development, Chin said the sphere has been able to tell industry how long a given material will hold up in the sun.

Chin said that because UV is the most damaging part of sunlight, the sphere filters out almost everything else.

Indeed, Chin and Byrd said, sun light is an imprecise term for the complex spectrum of energy on Earth that is affected by a variety of weather and climate conditions.

"I don't like the word sun," Byrd said. "What's it mean? Is it 'sun' when it's cloudy?"

Chin agreed: Is it the kind sun you have in Miami? she asked. Or "the sun in Phoenix?"

"Exactly," Byrd said.

"We try to be more technical here at NIST," Chin said. Sun is "not a very NISTy kind of term."

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Up on the building's sun-baked, white-gravel roof -- site of the agency's photovoltaic test facility -- A. Hunter Fanney is not so picky.