Why is light reflected off animal eyes at night, but not humans?

While on my way home yesterday, two eyes were staring at me out of the bushes. What is the reason for this scary phenomenon?


The phenomenon of 'eye-shine' is seen in a variety of species, and is thought to be due to the presence of an intraocular reflecting structure, the tapetum lucidum. This structure was described as early as 1854 in the Medical Lexicon by Robley Dunglison:

"TAPETUM, Ta'petry, Tape'tum choroidea. A shiny spot, on the outside of the optic nerve, in the eyes of certain animals, which is owing to the absence of the pigmentum nigrum, occasioning the reflection of a portion of the light from the membrana Ruyschiana."

Raccoons in a tree with their tapeta lucida reflecting camera flash

The Tapetum Lucidum
And he wasn't too far off back then. The tapetum lucidum is basically a biological reflector system that is a common feature in the eyes of vertebrates. Some species, usually diurnal animals (primates, squirrels, pigs), do not have this structure. Depending on the species, location and composition may differ; the tapetum lucidum can either be located within the retina or within the choriod behind the retina. The reflecting material in the tapetal cells also varies from guanine crystals (in some teleosts, reptiles) or lipid (in some teleosts, some mammals) to zinc cysteine/Riboflavin/collagen (most mammals). Therefore, not all animals eyes glow the same color, ranging from yellow, to green, to blue, and everything in between. Age and light angle can also affect the color of the eye shine. The tapetum functions like a mirror and provides the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels.

The Red Eye Effect
But wait a second... what about the 'human' red eye effect in pictures? Interestingly this effect rarely occurs in animals that have a tapetum lucidum; however, as we all know, it often does in humans. The cause of it is that the light flash occurs too fast for the pupil to close, allowing much of the very bright light from the flash to pass through the pupil into the eye. It then reflects off the fundus at the back of the eyeball and out through the pupil. The red to orange color which we then observe on photographs is due to the reflection of light from choroidal blood vessels.


Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away? For questions like this, just mail us your question (cns-newsletter@charite.de) and Dr. Harebrained will give us his explanation!

Dunglison, Medical lexicon, p. 843., 1854
Ollivier, Vet Ophthalmol, 2004

By Veronika Lang, originally published March 2012 in Volume 5, Issue1, "Mental Health Disorders"