Through a Fish Eye, Lightly

Tuesday, October 01, 1996
RELATED TAGS: LIGHT
The green sunfish, a freshwater fish native to the Northeast, can spot prey--and predators--even in murky, algae-filled waters. To find out how it manages to see in such conditions, Mickey Rowe, a neuroscientist at the University of Pennsylvania, has been studying its eyes. But what began as an attempt to understand piscine vision has evolved into an imaging technology that may one day help airline pilots make safer landings in dense fog.

The sunfish, like some insects and unlike all humans, can apparently see polarized light. Normal light consists of a wave that vibrates up and down, side to side, and every direction in between. When such light hits particles floating in cloudy water, it scatters in random directions. But when light reflects off some solid, uniform surface, say the scales of a fish, some of the light is polarized--it vibrates preferentially in one direction. Polarized light itself eventually scatters as it travels through the murk. But for fish that can see it before it has been entirely scattered, polarized light--and the object it reflects off-- still stands out against the general background of scattered, nonpolarized light.

The sunfish, Rowe suspects, detects such polarized light with unique retinal cells. Neighboring retinal cells in the sunfish have their long axes--the cells are elliptical--aligned perpendicular to one another. Each cell probably detects polarized light vibrating in a direction parallel to the long axis of the cell. Rowe wondered if the fish might somehow be combining two different polarized images to get a clearer view of its environment.

To test this idea, Rowe and Scott Tyo, an electrical engineer at Penn, immersed a small aluminum disk etched with one patch of vertical scratches and one patch of horizontal scratches in a tank of water mixed with milk. They placed lights on one side of the tank and a camera with a rotatable, polarized filter on the other. The camera fed its images to a computer. Without the polarized filter, the scratches on the disk were invisible. Even when the filter on the camera was aligned with either group of scratches so it could pick up polarized light reflected off the scratches, the scratches were still invisible. They showed up only when one polarized image was subtracted from another by the computer. This subtraction, says Rowe, eliminates any remaining nonpolarized light that seeps through the filter, leaving a sharper polarized image.

Rowe and Tyo aren’t sure if they’ve duplicated the way the sunfish’s eye works, but their research has suggested to them at least one practical application. If airport runways were equipped with polarized lights, an imaging system mounted on a plane could locate the runway, guiding the plane to a safe landing even in thick fog. Because we’re blind to polarized light, says Rowe, we just don’t tend to think about it, and we don’t have a sense of what its usefulness would be if we could detect it.
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