Re: ATM - Color sensitivity of the eye

[tmasnyk@surgery.bsd.uchicago.edu (Taras Masnyk)]

>On Jul 7,  1:29pm, Jim McKay wrote:
>> Subject: ATM - Color sensitivity of the eye
I beg your indulgence for the following longish post, but it may be of
interest to those of you looking around in the dark.
As a student of the clinical neurosciences, I wish to correct several
misconceptions about rods and cones.
First, there are actually three different types of cone cells, maximally
responsive to blue, yellow-green and orange light , known as blue, green,
and red cone cells.  They are responsible for daytime, color vision; they
are relatively low in sensitivity (need several to many photons to be
activated), but of quite high resolution- i.e., are able to discern very
small angular separations, allowing us to see fine details.  The resolution
is in part do to the small receptive fields of the cone cells (each cone
responsible for a small area of the retina), and in part due to the dense
packing of cones in the area of maximal vision, the macula.
Rods are responsible for night-time vision (in a crude way of thinking).
They are very sensitive to light (theoretically capable of detecting a
single photon, though various physiologic constraints limit this to a few
photons), but the resolution of rods is lower than for cones; that's why you
can't see very many details at night- everything is a little fuzzy.  In
addition, while rods respond to a variety of wavelengths of light, they are
not programed to transmit information about the wavelength (and are not
really capable of doing so anyway)- they just transmit "light present"- so
you see essentially in black and white (and shades of grey) if you are using
rods.

Darl adaptation comes from from a variety of mechanisms, some purely
neurological.  Importantly, though, the chemical reactions responsible for
the detection of light are "recharged" in the dark.  After a while in the
dark, more receptor protiens are available for light detection than in
daylight, when they get bleached.  This is true for both rods and cones,
though rods are significantly more sensitive than cones.  This is why DIM
light is as important for dark adaptation as the color of light- a red
search light will destroy dark adaptation as surely as a flashbulb.  The
reason RED light works well is because neither rods nor cones are very
sensitive to it.  Even red cones are only half (maybe less) as sensitive to
red light as to orange light, and blue and green cones are essentially
insensitive to red light.  Rods respond to many wavelenghts, most strongly
in the yellow-green range, but respond least to red light; violet and blue
light is intermediate in effect on rods.  Therefore, no receptor is strongly
stimulated by red light, though a dim red light will be seen well enough by
only the red cones to allow you to read your star chart, but not enough by
any other cones or the rods to destroy your appreciation of M42 (or insert
your favorite Messier object here).

In summary, yes, red light is best for preserving nigh vision, though it
should be relatively dim as well as red.  If any one wants any more details
(MORE? whatever for...) consult a physiology text or you may email me-

tmasnyk@surgery.bsd.uchicago.edu

Thanks for your patience and bandwidth!
mud-fud