From: firstname.lastname@example.org (William J. Beaty)
Subject: Re: Make infrared goggles inexpensively (like $10!!!)
Date: 18 Sep 2002 11:21:46 -0700
References: <email@example.com> <4Aoh9.15391$1C2.firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 18 Sep 2002 18:21:46 GMT
Ian Stirling wrote in message news:...
> Obtain a prism that will pass IR. (or at least near IR)
> Arrange so that you can look through it at the sun, from a distance, so
> you can observe various wavelengths by moving your head.
> (a mirror may be handy to reflect the prisms output into a horizontal
> Now, get a dense neutral-density filter, and compare transmission at
> various points in the prisms output with your filter, to see if it passes
> any green, or if what you are seeing is truly 'IR' (in the loosely defined
> sense of bits of the spectrum that are almost always overwhelmed by
> wavelengths to which the eye has a greater response)
I don't think I have a prism. I'll stop at the science shop
across town. I do have a spectrophotometer which is on my
repair/calibration list. As soon as it's working I'll run my
filter stack through it.
When wearing the goggles, the world is deep, deep ruby red. If
my eyes were mostly responding to green, etc., then things would
look orangey or yellow. The question is not whether I'm seeing
some green light. The question is whether my eyes' red cones
are mostly seeing light below 700nM wavelengths, or mostly seeing
longwave above 700nM.
Here's a crude and simple test. Stack up two large sheets of Congo
Blue filters and one sheet of Primary Red. Tape the filter stack
to your wall, and turn on some nearby incandescent lights to provide
some red/IR illumination. The filter sheets look opaque black.
Now place an image such as a newspaper BEHIND the filters on the
wall. You can't see the newspaper since the filters look nearly
opaque. Now strap the "IR" filter-goggles to your face. Suddenly
you can see the newspaper! The sheets of filters taped to the wall
longer appear black, but instead look light grey. By wearing the
goggles you can change "black" filters into nearly transparent
But were those large sheets of filters actually passing red light,
or passing IR light? The question itself depends on how we
define "IR." Since we can see 680nM, and we can see bright
700nM, and can see 720nM if it's even brighter, etc., etc., there
is no solid boundary between "visible" frequencies and "invisible"
frequencies. We could arbitrarily define IR as the -3db point at
around 650nM. Or arbitrarily define it as the -20db point around
700nM. This isn't very satisfying though.
For this application (viewing the outside world,) it might be
better to base our definition upon the eye's dynamic range in bright
sunlight (obviously not including the extra range which comes from
dark adaptation.) Does anyone know this value? In other words,
if perfect white paint is 1.00, how black can a surface be before
further increases in absorbtion are invisible to the eye? 10-3?
10-4? Knowing this value would let us pick a point on the human
visual sensitivity curve. That point would give a frequency which
separates "visible colors" seen by the eye, from "infrared colors"
which cannot be seen even when superposed over black or white
The filter-goggles would then remove all the incoming shortwave
light, which cranks up the AGC, which makes the shortest
wavelengths of the "Infrared Colors" be seen as various shades
of deep red.
Another test: find some fabric which looks black to human eyes,
but looks white when viewed with an infrared CCD security camera.
(These fabrics are pretty common, just look at a crowd of people
with an IR converter scope.) Write a message on that fabric with
India Ink or other carbon-base ink which appears black in both
the visible and NIR bands. Human eyes cannot see this black-on-
black writing, but IR cameras see it as black writing on a white
background. Now take the "secret message" out into the sunlight
and wear the IR filter-goggles. Guess what. The "black" fabric
now looks light grey, and the India Ink writing is totally visible.
No IR-scope needed. (But as always, it only works in bright
sunlight or with very bright incandscent illumination.)
Please read the original article for other simple tests:
After all that, do people here seriously think that I'm seeing
visible light below 680nM? If so, that's sad, since I'm going
to have fun playing with other infrared "science demonstrations,"
and you're not. Nyaaa!
Skepticism in science is all about requiring evidence before making
a decision. This has nothing to do with adopting an initial belief
or disbelief, and then defending it to the death. In other words
there is a very large difference between skepticism and disbelief.
"Doubt everything or believe everything: these are two equally
convenient strategies. With either we dispense with the need for
reflection." - Henri Poincare
"New and stirring things are belittled because if they are not
belittled, the humiliating question arises, 'Why then are you
not taking part in them?' " - H. G. Wells
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William J. Beaty SCIENCE HOBBYIST website
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