UV Detectors: Camera detects power-line corona in daytime

Aerial inspection offers an efficient way to survey vast areas of power lines for potential problems, such as cracks on insulators, which can eventually lead to costly power outages. The corona discharges from high-voltage lines and equipment indicative of possible faults can be detected with ultraviolet (UV) cameras, but Andrew Phillips of the Electrical Power Research Institute (Palo Alto, CA) says, "Until recently, this option only worked at night when the solar radiation in the UV-B and UV-A regions does not blanket the weak corona effect in those bands."

One possible solution is a new camera that takes advantage of the absorption of UV wavelengths below 280 nm by the ozone layer (see images). In this "solar blind" region, the radiation from corona is very weak. However, there is no solar background to mask the effect or cause false signals.

The DayCor system built by the Israeli firm Ofil Ltd. is essentially a combination of two cameras, one of which uses a solar-blind filter that transmits in the solar-blind UV band and totally rejects other wavelengths. The other is a standard visible charge-coupled device (CCD). The video output from the UV camera is combined with output from the visible CCD.

"The UV channel has an intensified CCD with a so-called solar-blind photo cathode," says Jeremy Topaz of Ofil. "This device has considerable sensitivity outside the solar-blind region. Even at 350 nm, it may be a few percent of the peak quantum efficiency. The solar radiation reflected from an insulator there could be 1010 times greater than the UV output from corona."

Malka Lindner, head of Ofil, says that the solar blind filter transmits between 240 and 280 nm and, together with special mirror coatings, blocks radiation above 280 nm to the required degree. "To avoid parallax effects that could interfere with registration," says Lindner, "we built the device with cameras mounted side by side on a common base and the visible optical axis shifted by a beamsplitter and mirror. To allow the two images to be brought into accurate registration, the mirror is adjustable in two axes, and the zoom lens on the visible CCD can be set so the fields of view are equal."

Topaz added that the UV intensified CCD is sensitive enough to record individual photons from the corona discharge as separate spots. "Although the tube and noise background is very low, some spurious spots appear, and signal processing is used to distinguish effects concentrated at one point from those that are randomly scattered over the image. The weak UV image can also be enhanced by extending the integration time over a number of TV frames, but only at the expense of image smearing when the scene is not static," says Topaz.