02 July 2004

Eye shine and red eye

Forrest M. Mims III

How many times have you taken a flash photograph of a friend or relative in which the subject's eyes appear bright red?

Red eye in photographs is the same phenomenon that causes the eyes of cats, rabbits, deer and other animals to glow brightly when they are illuminated by the headlights of a car at night. Only only those in the car actually observe the phenomenon.

Both phenomena are known as eye shine, and it can be observed only when the viewing conditions are just right.

Only people in or very near a car see eye shine caused by the car's headlights. Similarly, only the camera sees the eye shine of your friend when you take a flash photograph.

Here's why. The front of an eye is a carefully designed, precision optical system complete with a lens, focusing mechanism and a variable aperture.

Things are visible only because they scatter light from natural or artificial sources. The lens of an eye focuses this light onto the light sensitive cells in the eye's retina.

Some of the light passes into the retina, where it is detected and sent to the brain for processing.

The remainder of the light is reflected from the lens back to where it came from.

If the light came from a camera flash or a car headlight, the light will be returned directly to its source.

When you take a flash photograph of someone, most of the light reflected from their eyes returns to the flash unit. If the lens of the camera is close to the flash, some of this reflected light will enter the lens of the camera and cause the eyes to appear bright red.

The same effect is why the eyes of animals shine so brightly when they are caught in your headlights.

The scientific name for eye shine is retroreflection.

Tiny glass beads are reflectors. After highway signs are painted, glass beads are sprayed or sprinkled across the wet paint. This is why highway signs appear so bright when your headlights illuminate them.

Glass beads are applied to the paint that separates highway lanes for the same reason.

Car, bicycle and mailbox reflectors are also retroreflectors. They are based on a different principle that returns light to the source in a very narrow beam. In a future column we'll look at these kinds of reflectors and find out why astronauts left some of them on the Moon.

Forrest M. Mims III and his science are featured online at http://www.forrestmims.org/.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.