Veterinary ophthalmologist J. Phillip Pickett of the Virginia-Maryland Regional College of Veterinary Medicine explains:
"Red eye," the nemesis of amateur photographers, occurs when someone looks directly at the camera while a picture is taken. If the flash is on the same axis as the visual axis of the camera, the reflection off the blood vessels in the person's retina can produce an eerie, satanic look, the so-called red reflex.
Dogs, cats and almost all domestic animals have a special reflective layer in the back of the eye termed the tapetum. Incoming light passes through the animal's retina and is then reflected back through the retina a second time from the tapetal layer. This double stimulation helps these species to see better in dim light. The color of this tapetal layer varies to some extent with an animal's coat color. A black Labrador retriever, for example, will usually have a green tapetal reflection. A buff-colored cocker spaniel will generally show a yellow, tapetal reflection. Most young puppies and kittens have a blue tapetal reflection until the structures in the back of the eye fully mature at six to eight months of age. "Color dilute" dogs and cats, such as red Siberian huskies and blue point Siamese cats, may have no tapetal pigment and may therefore exhibit a red reflex just like human beings.
From Scientific American, Volume 285 Number 3, September 2001, Page 104
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