MadSci Network: Zoology

Re: Why do the eyes of cat's and dogs change color when photografed?

Area: Zoology
Posted By: Rick Huneke, D.V.M./M.P.H. Faculty, Division of Comparative Medicine, Washington University School of Medicine
Date: Fri May 9 11:22:54 1997
Area of science: Zoology
ID: 862685970.Zo

Dear Phil:

When you photograph a dog or cat with a flash, you will see a reflection
of color from the tapetum lucidum.  You can also see this effect when 
your headlights shine into an animal's eye. The tapetum lucidum (or "bright
carpet") is a layer of cells behind the rod and cone layer of the retina.  
These reflective cells serve to intensify light in nocturnal situations.
This layer acts as a mirror, reflecting light back to the rods and cones.
In dogs this layer is present in one third of the back of the eye.  As the
dog matures, the color of the tapetum changes from slate gray to violet to
red-orange at about 4 months of age.  Some dogs, like huskys and collies
with blue irises (subalbinotic) lack a tapetum lucidum.

In man and other animals without a reflective tapetum, the back of the eye
is pigmented to absorb of light once it passes the rods and cones.  We
day-vision animals need to see sharp images and a reflection from the 
back of the eye would blur the primary image.  Sometimes, however, when
you take a photograph of a person you get the "red-eye" effect.  This is 
a reflection from the back of the eye and is red because of the blood
vessels in that area.

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