Re: retro-reflection from cats' eyes

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

Thanks, Mark, for sharing some more factual documentation on this red-eye
issue. I, too, was dissatisfied with the usual statements that the red-eye
seen in flash photographs was simply due to the blood vessels in the eye.
Many textbooks say this, without explanation. It fails to explain:

a) Why the effect has different colors in some animals.

b) *Which* blood vessels are responsible. Surely not the very coarse and
very visible net of them which lies on the *surface* of the retina, and is
seen in most photos of the retina.

c) One book simply says it's "because the retina is reddish". That's true,
and doesn't speak of blood. As seen in photos, the retina is reddish,
quite independently of the blood vessel net which overlies. it.

So I laid low on this issue for a while, seeking more authoritative
information than was forthcoming from this discussion. I went to the
library, but found very little. A discussion with a biologist was helpful.
A call to Eastman Kodak was a waste. They didn't know anything about the
eye. So then I called Geissinger Hospitals' Ophthalmology department.
Bingo! I talked to a ophthalmologist who found the question quite
interesting, and knew something about it. He, too was unhappy with the
usual flippant answers.

Below the retinal receptors is a layer of pigment, below that is the
choroid, which is reddish because of its rich net of capillaries. That's
the reason the back of the eye looks reddish. As we look through the pupil
into the eye we are seeing it *through* the layer of retinal cells.

Now that pigmented layer between the retinal receptors and the choroid is
interesting. Its pigmentation varies. In dark skinned individuals it is
darker, so the choroid appears less red. In albinos it has no
pigmentation. Presumably in other animals it is other colors, and this
guy's wife has a researcher-friend who takes photos of animal's eye
structures, and she will get back to me. As Mark notes below, in some
animals, particularly nocturnal ones (deer, raccoon, cats, etc.) the
pigmented tapetum lucidum is reflective. My biologist friend wasn't sure,
but thought he'd learned in school that in those cases the cell layer had
a 'scaly' structure.

We are following up on this. Here's a case where the internet was of
little use. We'll have to use such old-fashioned techniques as searching
the journal literature, and making phone calls to find the folks who
actually know about these things from experience.

When do we dig out more detailed info on the tapetum layer, I suspect
we'll find there's some very interesting physics involved. Now this is
what textbooks ought to do better when they want to show the applicability
of physics to other fields, instead of those lame examples from sports,
and cop-out superficial explanations like "Red-eye results from flash
light reflecting from the blood vessels in the eye."

Someone else on this list commented on the eyes of animals being suited to
the environmental conditions in which they live. That's so, to an extent,
for an ill-adapted eye doesn't enhance the individual's survival. But
evolution doesn't produce the best and most efficient design, only a
'good-enough' functionality for continued survival. For example, our
eyes' peak sensitivity is in the yellow-green, just about where the black
body curve of the sun peaks. But other animals, which live in the same
bath of sunlight, have sensitives peaked in different portions of the
spectrum, and they get along just fine. 

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
......................................................................


On Thu, 9 Jan 1997, Mark Sylvester wrote:

> 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?
>
> Buon Anno to all. Mark.
>
> Mark Sylvester
> UWCAd, Duino, Trieste, Italy.