MadSci Network: Zoology

Re: How much 'hotter' can eyes be as compared to the rest of the body?

Date: Mon Jul 17 15:24:11 2000
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Zoology
ID: 962342223.Zo

Hey there,
  Although I wasn't able to pin down any 
scientific data to answer your question, I'm just 
going to tell you what I know and extrapolate 
from that.
  As you know, when you shine visible light onto 
many animals' eyes, you get a very shiny, often 
greenish reflection.  This reflection is from a 
layer behind the retina called the tapetum 
lucidum, which basically is a living mirror.  In 
humans, light that passes through the retina 
without being seen is absorbed so no stray light 
goes bouncing around in our eyes to cloud our 
vision.  Cats, deer, and other wild and/or 
nocturnal animals depend on night vision to 
live, though.  The tapetum lucidum gives them 
another chance to "see" light that has already 
been through their retinas once.  Because the 
light doesn't always bounce straight off the 
tapetum, they don't know exactly where it came 
from. So, their vision isn't as sharp, but it is 
much more sensitive at night.
  The infrared spectrum begins where the visible 
spectrum ends in the reddish colors.  The cut-
off for infrared and visible red isn't exact.  But 
the animal needs to see reds at the far end of 
the spectrum, so I imagine the reflectance of 
the tapetum extends a ways into the infrared 
also.  Unfortunately, I couldn't find a 
reflectance profile to tell you exactly what 
wavelengths of the electromagnetic spectrum 
are reflected by the tapetum.  I imagine it also 
reflects ultraviolet quite well too.
  Anyway, the tapetum may be slightly hotter 
than core body temperature, but not much 
hotter.  What's happening is that it's just 
reflecting a lot more infrared than the animal's  
body is.
  Hope that helps answer your question,


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