A while back there was a question about what causes red-eye in flash
photographs and we agreed that (a) the retroreflection causes the light
scattered from the retina to be focused back onto the film and (b) the
reason why it was red was probably to do with the blood vessels concentrated
in the retina. I put a question as to what colours appear in the eyes of
other animals in similar circumstances.
Since then a friend has sent me the most interesting clippings from the
September 96 issue of Popular Photography, in the first place, showing a
photograph of two cats side by side. The one on the left shows glowing red
eyes while the one on the right shown glowing green eyes. The accompanying
text quotes Dr Michael Ringle, veterinary ophthalmologist at the animal eye
clinic in Red Bank NJ: "Behind the retinal membrane of a cat's eye is a
cellular structure called the tapetum. It's function is to enhance low-light
sensitivity, enabling the cat to see in very dim light. Normally the tapetum
is greenish yellow in colour, and that's why you often get green-eye when
taking flash pictures of cats. However, a small percentage of cats have what
is known as an albinotic fundus, a lack of green pigment in the tapetum. In
such cats the flash is reflected back by the retinal membrane itself... so
you get red-eye just as you do with humans."
So far so good - this agrees with what I read on this list about alligators
eyes too. In a subsequent issue of Popular Photography (I have the clipping
but not the date) two letters appeared, accompanied by photographs of cats
showing the most splendid traffic-light eyes: one eye red and the other
green! A case of monocular albinotic fundus??
Does anyone have a physical explanation for how the tapetum enhances vision
in low light?