Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shades
Date: Tue, 9 Jan 2001 21:48:35 GMT

In article <u0lmskd2fg.fsf@orville.dfrc.nasa.gov>,
Mary Shafer  <shafer@orville.dfrc.nasa.gov> wrote:
>I suspect they don the sunglasses when they take off their helmets, to
>save their night vision.  There's a compound (rhodapsin is how I
>remember the name) that gets used up in the sunlight and until more
>can be generated, which is a slow process, night vision is impaired.
>Someone who knows more about the physiology of the eye can jump in
>here any time and explain this better.

To elaborate a little...  Most of the adaptation of the eye to bright vs.
dim light is done by chemical changes in the retina, not by changes in the
size of the pupil.  The pupil can vary the amount of light reaching the
retina by perhaps a factor of ten or so... but the intensity of noon
desert sunlight is *ten thousand times* the minimum needed to read by, and
of course it's possible to see quite well -- for less demanding tasks --
in light that is much dimmer yet.

The bleaching of the retinal pigments that adapts them to bright light
happens almost instantaneously, but if I recall correctly, they don't
unbleach -- dark adaptation relies on the regeneration of the original
pigments, which is a slow process, taking half an hour or more for full
effect.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)