An oil-slick tracker invented by a Southampton University scientist had its baptism by fire this week when it was called into action in the clean-up operation involving stricken tanker Sea Empress off Milford Haven, South Wales.

Simon Boxall, of the Southampton Oceanography Centre, is checking data picked up by a camera that he developed to detect whether oil sprayed with detergent from planes had broken up in the sea or whether it was moving away.

The camera, which filmed the clean-up process from a plane, is an improvement on radar which cannot detect oil once it sinks below the surface. Experts "were not sure how efficient the detergents are or how much oil is going beneath the surface," said Dr Boxall.

When an oil spill floats on calm seas it can be tracked by radar. But when it encounters rough seas or detergent, which cause it to mix with the water, it stops floating and sinks, where radar cannot detect it. Yet it needs to be tracked because it can be sucked into water cooling systems for power plants or damage fish farms and marine life.

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During the 1993 Braer oil disaster in the Shetlands, Dr Boxall and his colleagues developed an imaging spectrometer - a camera that can detect 288 different colours. Different particles in the water reflect slightly different colours, so the spectrometer gives a precise and instantaneous picture.

"The Braer spill showed that we could track optically the cloud of oil in the water," said Dr Boxall. "We weren't sure that we would be able to pick it up. But if we fly just behind the aircraft that is dropping the dispersant we can detect it. It looks like pernod in water."

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Dr Boxall said he could see up to 100 metres, but typically ten metres, below the surface depending on the conditions.

In future satellites will be able to pick up oil slicks. But these would pass over the affected area less than once a day and give a resolution of less than a kilometre. A European satellite with better resolution is due for launch in about ten years.