A Breakthrough in Capturing Camera-Shy Ultraviolet Light

The increasingly ubiquitous digital camera has taken another step into the future with the development of the first such camera that senses only ultraviolet light.

Ultraviolet light has shorter wavelengths than visible light. It is sometimes called black light because it causes some materials to glow in the dark.

Normal digital cameras “see” light that is visible to the human eye, sometimes called white light. But many objects, ranging from stars to biological warfare materials, also emit ultraviolet light that we cannot see.

Now scientists at North Carolina State University’s Solid State Physics Laboratory say they have built a camera that can take pictures of anything that emits ultraviolet light. “It’s truly an advancement,” said the lab’s Jan F. Schetzina, a physicist who has pioneered the field of photodiodes--light-sensitive, electrochemical devices.

The research, sponsored by the Army Research Office and the Defense Advanced Research Projects Agency, has many applications beyond some obvious military uses, he said. The university has teamed with the Honeywell Technology Center to develop the technology for uses ranging from environmental monitoring to astronomy.

Like other digital cameras, the ultraviolet camera works in much the same way as an array of solar cells. Light falling on each of the cells generates an electric current that varies according to the intensity of the light.

But unlike traditional digital cameras, which use silicon for the cells because it is sensitive to visible light, the ultraviolet camera uses aluminum gallium nitride in its photodiode, which is sensitive only to ultraviolet light.

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There is much excitement in the scientific community these days over diodes, which can be structured to work in opposite ways: An electric current applied to a diode can generate light, and light captured by a diode can generate electricity.

Using gallium nitride as the principal material in a diode is a hot topic because scientists have figured out how to use it to create short wavelengths of light, which we see as blue or green light. These small devices are expected to revolutionize everything from household lighting to computer monitors.

Diodes can produce light very efficiently, using far less energy than even fluorescent lights, and have a life span of up to 100,000 hours. They also can be used for brilliant outdoor displays and are already used in Japan, which has electronic billboards as sharp as any computer monitor.

Cree Research Inc., of Durham, N.C., and Hewlett-Packard of San Jose are among the industrial leaders in this area. “That’s a very, very big market,” said Schetzina, who is one of the leaders of a large team of researchers at the university.

The North Carolina camera uses an array of 1,024 diodes, each working as a separate sensor. Light collected by each of the diodes generates a current that is fed into a computer, which reconstructs the image. The first photo was of an ultraviolet-light-emitting sign that read “NCSU” (North Carolina State University.)

“Then we took it outside, and it was very easy for us to get an ultraviolet image of the sun,” Schetzina said.

All stars emit ultraviolet light, so compact digital cameras sensitive to that wavelength could open a new window on the universe. By studying ultraviolet emissions, astronomers hope to learn more about the birth and evolution of stars.

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There are many potential applications. In industry, for example, an ultraviolet camera could precisely follow a welding process. All visible light would be screened out since the camera would not be sensitive to anything but ultraviolet.

Since welding produces an intense display of ultraviolet light, the camera could zero in on just the welding, thus permitting real-time monitoring and evaluation.

For many applications, the diodes could be used as sensors, not cameras.

“One big threat we all have to worry about is chemical and biological warfare,” Schetzina said. Many biological agents emit ultraviolet radiation if excited by a laser. So a series of ultraviolet sensors in a high-risk area could detect if biological agents are present. It could even tell which kind because each substance emits light of a slightly different wavelength.

Such a device could also monitor the amount of ozone in the atmosphere. Ozone absorbs harmful ultraviolet radiation from the sun, so if the ozone is depleted the radiation increases.

The device could even have a more down-to-earth application. Ultraviolet light blisters the skin, causing sunburn and even skin cancer. So maybe the bikini of the future will come equipped with a tiny sensor to tell the wearer when to head indoors.

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Lee Dye can be reached at leedye@compuserve.com.