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(meteorobs) Fwd: "Believing is Seeing" (Human low-light detection)
NOTE: Ed is not a current 'meteorobs' subscriber. If you choose to follow
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Lew Gramer <owner-meteorobs@jovian.com>
------- Forwarded Message #1
Date: Fri, 2 Mar 2001 13:12:47 -0500
From: Edward P Wallner <epwallnr@world.std.com>
To: Amateur Telescope Makers of Boston <atmob-discuss@jovian.com>
Subject: (ATMoB:Discuss) Believing is Seeing (fwd)
This is a comment on an article in the NEJS. Thought it might be
ofinterest to some.
Ed Wallner (epwallnr@world.std.com)
------------------------------------------------------------------------
The New England Journal of Skepticsm
To the editor;
The article "Believing is Seeing" by Robert Novella points out the
amazing ability of the human mind to recognize patterns and to detect
signals buried in noise (whether they're there or not.)
Besides the many "miraculous" appearances of religious figures, there is
also the famous case of the "canals" on Mars. When the planet made a
relatively close approach to the earth in 1877, the Italian astronomer
Schiaparelli perceived long, straight narrow dark lines on the surface
which he called "caneli." This can mean canal or simply channel and
Schiaparelli had not asserted that they were water carriers. Other
astronomers also claimed to see some of these controversail caneli though
few saw the frequent doubled caneli which Schiaparelli began observing and
plotting in 1881.
The field was given a further boost in 1892 when the French popularizer
of astronomy Camille Flammarion wrote "The Planet Mars and its
Conditions of Habitability." [1] This espoused the theory that the
caneli were artifical channels used to distribute scarce water from the
polar caps to other parts of the planet.
"Percival Lowell, a wealthy New England aristocrat, enthusiastic, gifted
and well versed in many fields, in 1894 founded an observatory at
Flagstaff, in the bright desert climate of Arizona, at a height of 6000
feet, especially to study Mars and its inhabitants." [1] Lowell wrote
several books describing life on Mars.
Unfortunately this is another case where the sceptics, such as the
French astronomer Antoniadi were right.
Lowell is associated with another "Believing is Seeing" episode. One of
the greatest triumphs of celestial mechanics was the calculation of the
approximate location of the planet Neptune from the minute perturbations
it causes in the orbit of Uranus. Lowell attempted to apply the same
technique to the residual errors in Uranus' orbit after accounting for
Neptune and predicted a new planet. Eventually Pluto was found near the
predicted longitude after an extensive search. However the mass of Pluto
is small and Lowell was essentially working with noise.
One claim in Novella's article, that the human eye can detect a single
photon, requires a response. The only documentation I can find for such a
claim is a statement about "our rods, specialized cells in the retina
of the eye that can respond to as little as a single photon of light,
triggering neural responses that will eventually relay an image to our
brains." This appeared in an article in the News Focus section of
Science, v.282 p225, 9 October 1998. No reference was given.
Having worked on low-light-level television at RCA for several years, this
is an area with which I have some familiarity and in which I maintain an
interest.
As far as I can find, the most carefully designed and well documented
experiments on threshold vision were done by Hecht, Shlaer and Pirenne
in 1942. [2] A detailed discussion of the expereimenmts is given in [3].
In the experiments, a flash was detected 60% of the time with between 48
and 148 quanta incident on the cornea. The estimated number of quanta
interacting with the rods was 5 to 14. It was extremely improbable that
more than one photon fell on a single rod so it is possible to say that
a single photon is adequate to initiate a nerve impulse though several
are required to generate the perception of a flash.
The experimental data can be roughly matched by assuming that the
quantum efficiency of the eye (in terms of impulses per quantum at the
cornea) is 6% and the threshold of detection is about 6 impulses.
If there is later work indicating a different threshold, I would
appreciate references to it.
Ed Wallner
32 Barney Hill Road
Wayland, MA 01778-3602
508-358-7938
epwallnr@world.std.com
[1] "A History of Astronomy", A. Pannekoek, Interscience, 1961
[2] "Relation between the average energy (in number of quanta) of a flash
and frequency of seeing for three subjects." J. of General Physiology,
v.25 pp.819-840, 1942.
[3] "Vision and Visual Perception", C.H. Graham, Ed., Wiley, 1965,
Chapter 7.
1PHOTON.013
------- Forwarded Message #2
To: Amateur Telescope Makers of Boston <atmob-discuss@jovian.com>
Cc: epwallnr@world.std.com
Subject: Re: (ATMoB:Discuss) Believing is Seeing (fwd)
Date: Fri, 02 Mar 2001 13:29:05 -0500
From: Lew Gramer <dedalus@latrade.com>
Ed, this was a genuinely fascinating post. Down at WSP last week, I heard
a fine talk by Stephen James O'Meara, on the eye's ability to detect very
low light levels. O'Meara made one of his points by citing a reference that
I believe matches your citation, and contradicts the above: I do not have
that ref. to hand (you could contact him), but the gist of it was that the
human eye CAN detect extraordinarily low light levels - but not fewer than
six photons over some non-trivial time (say a few seconds), and only that
small a number if they happen to fall in one or more of the eye's highest
spectral-response ranges. (What wavelengths are IN these highest-response
ranges may actually vary from individual to individual, according to both
O'Meara's citation and my own anecodotal experiences.)
In any case, I wonder if you'd mind my forwarding your fine email to a list
which I administer, on meteor observing? As you can imagine, the eye's exact
capabilities and limitations as a light detector are of significant interest
to visual meteor observers. Can I have your permission to do the forward?
Clear skies!
Lew Gramer
------- Forwarded Message #3
Date: Fri, 2 Mar 2001 14:11:31 -0500
From: Edward P Wallner <epwallnr@world.std.com>
To: Lew Gramer <dedalus@latrade.com>
Subject: Re: (ATMoB:Discuss) Believing is Seeing (fwd)
Feel free to forward my note to anyone who might be interested. Maybe
someone will know ofmorerecent work.
In the Hecht, Shlaer and Pirenne experiments, a 10 arcminute circle 20
degrees temporally on the dark adapted (30 min.) retina was illuminated
with a 1 millisecond flash of 510 nm light.
I had some correspondence with O'Meara in 1985 trying to relate limiting
magnitude to quantum response. As I recall I converted the scale of the
"standard" scotopic response curve to quanta per lumen-second and
integrated it times black body output for stellar temperatures. I don't
remember the results but I have a microfiche of my letter which I'll have
to print out.
[To be continued --I hope!]
Ed Wallner (epwallnr@world.std.com)
------- End of Forwarded Messages
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