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Subject: Re: Make infrared goggles inexpensively (like $10!!!)
Date: 21 Sep 2002 20:43:14 -0700
References: <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 22 Sep 2002 03:43:15 GMT
Jonathan Kirwan wrote in message news:<email@example.com>...
> I took a 5800 K blackbody radiation curve (Planck's Law for Sol) and
> passed it through three Congo Blue filters and one Primary Red filter
> (Leefilters was kind enough to send me data spaced on 1nm, ranging
> from 250nm to 800nm -- sadly, I don't have information on those
> filters longer than that.) This information was then further combined
> with the x, y, and z filters from CIE and plotted. I also computed
> the xbar and ybar values of (.6863,.2440). [Using 5555 K for Sol, you
> get (.6903,.2458), in case that's useful.]
> The combined filters yield two peaks: One at 444nm and one at 718nm.
From an earlier message:
firstname.lastname@example.org wrote in message news:<email@example.com>...
> A stack of blue filters gives a dim blue-greyish-violet world
> with bright red foliage. Adding one layer of Lee #106 red
> filter eliminates most of the blue-purple part yet doesn't affect
> the bright foliage. A second layer of #106 red will remove all
> leftover traces of purple, but additional layers of red don't
> give any visible effects. The red filter apparently removes
> all of Congo Blue's "blue peak", while leaving the lowpass IR
> part of the curve unaffected.
So, if you want to view "pure" infrared, you need several layers of
Congo Blue (three is good), plus *two* layers of Primary Red. One
layer of the red filter takes out most of the 440nM blue peak, but
not all of it. Any visible-only reflectors then appear as dim blue.
This is actually useful: during overcast days when the IR light is
dimmed more than the blue, the tiny leakage of blue light through the
goggles still allows you to see bright scatterers such as streets and
sidewalks. The better to walk without tripping! That's why my
"goggles" article instructs hobbyists to use only one layer of primary
red filter. I still need to add a paragraph about experimenting with
various numbers of red layers in the stack.
> What I'm curious about is whether the remaining light levels are low
> enough that scotopic vision takes over.
Nope, not with 3 blue and 2 red on a sunny day. However this
brings up an interesting perceptual effect. Note that the
curve for scotopic (rods) vision has an IR cutoff which is much
less deep in the IR than the photopic, so as your eyes dark-adapt
they gradually lose about 70nM worth of IR sensitivity. If I use
IR filters with much deeper IR cutoff, at first I can barely see
anything... then that extremely tiny bit of vision *goes away*.
For some reason this just feels deeply Wrong. It feels like a
malfunction, like your eyes going blind. Your immediate impulse
is to tear off the goggles and make sure your eyes still work
normally. We're so accustomed to our vision in darkness improving
with time, that when the opposite happens it's vaguely upsetting.
> Sounds like fun playing, though.
More IR: here's Famous Trails FT-300 as disassembled by a
friend: http://home.covad.net/~fhmundy/ These scopes can be
had on eBay for under $50. Red/Congo-Blue filters easily convert
them to NIR, but for deeper IR you might want to order some actual
Kodak Wratten IR-pass filters. The FT-300 is actually TOO sensitive.
If you want to observe the IR world in full daylight, you need to
include a aperature to stop down the lens. Of course a B&W CCD
camera with an IR filter works just as well, and has a video out jack
to boot... but then you don't get to play with any high voltage.
William J. Beaty http://staff.washington.edu/wbeaty/
firstname.lastname@example.org UW Chem Dept, Bagley Hall RM74
Research Engineer Box 351700, Seattle, WA 98195-1700
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