Oil spills in water present a special challenge for imaging-based detection methods. In the visible-light waveband (VIS for short), ocean water is often murky and appears dark when looking straight down. At high angles, ocean water reflects the sun, horizon or sky and can appear very bright. Against this highly variable surface brightness, any thin film of liquid floating on the surface can be hard to see with the naked eye or with a color video camera.
Crude oil or diesel spills typically rise to the surface and float there for awhile because of their lower density. Although spills can form well-defined films, especially in still waters, there is often not a strong visual contrast between the film and the water surface, at least to the unaided eye. Particularly at low incidence angles, both the water and the oil film tend to look dark. Detecting the oil becomes even more difficult in choppy or wavy water since the undulating water surface alternately appears dark or light depending on how it reflects the sky or the sun above it, masking the low contrast oil-film areas to an even greater extent.
But visible light is just one method of imaging. Alternative wavebands of the spectrum hold out the promise of increased contrast between petrochemicals and water in a variety of different sea states and lighting conditions. Recent investigations by FLIR engineers have determined that there are at least three fundamental reasons why longwave infrared imaging is a powerful tool for spill detection in field conditions.