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From: firstname.lastname@example.org (William J. Beaty)
Subject: Re: Make infrared goggles inexpensively (like $10!!!)
Date: 15 Sep 2002 09:58:37 -0700
NNTP-Posting-Date: 15 Sep 2002 16:58:37 GMT
"Prai Jei" wrote in message news:...
> No comprendo.
Read the article. Sometimes that helps.
> If you can see it, it is (by definition) not IR.
These goggles let you see "IR colors" which would otherwise
be visible to unaided eyes. By your definition of IR, they
actually TRANSFORM infrared light into visible light. (What
a great advertizing claim!)
Think a bit more: where is the cutoff freq of ANY highpass
filter? The filter response rolls off logarithmically, so
depending on the signal frequency, you just have to increase
the applied signal a few orders of magnitude in order to get
that signal through the rejected frequency band... so you've
just "proved" that filters don't have cutoff frequencies?!!
Defining infrared as "invisible" means that there's no such
thing as IR light at all (if the wavelength is too long to
see, we just have to crank up the brightness.) If you imagine
that 800nM light is IR, why that just means that you've never
observed REALLY BRIGHT 800nM light.
No, that doesn't make for clear thinking. For the term "IR
light" to have meaning, we have to pick a relative cutoff
amplitude, then look at the HVS curve and note what frequency
that gives. E.g. declare that "IR light" is longwave light to
which your eyes' response is, say, 0.0001 down from your eyes'
best response. Obviously the cutoff frequency will vary with
our choice of amplitude, so we have to choose our amplitude
according to the particular application. These goggles let
you see "infrared colors." Go outside on a bright sunny day.
Can you see stripes of color where the colored materials'
absorbtion band is entirely above 700nM? Or above 725? 750?
The filter goggles let you see "IR colors" in the environment
which would otherwise be invisible to unaided eyes.
Arguments aside, try making a pair of these goggles and take
a walk outdoors. You'll see the same thing that NIR film sees:
extremely bright pink vegetation against an extremely dark sky.
You can see dark colored birds hiding in shrubbery which
looks dark in visible light but looks frosty-white in the IR.
Some types of clothing will appear white, even though it looks
completely black to normal eyes. (You can scribble on such
clothing with carbon-based ink, and the scribbling will be invisible
to normal humans but visible when wearing the goggles in sunshine.)
If you take a hunk of black IR filter plastic outdoors with you,
you'll find that it looks transparent when you're wearing the
goggles. Indoors it's tough to see anything... unless you
build an incandescent spotlight which uses the same IR filter
Right now I'm looking for a dye which has an absorbtion band
starting at 700nM, but doesn't absorb anything in the visible
band. This will let me put invisible graffiti all over the place;
graffiti which is plainly visible only to people who wear
these easily-made goggles.
Heh. How about "Goth warchalking", where you write on black
paper with black markers, and the resulting sign is only
visible to people who go around with black goggles strapped
to their faces.
> "William J. Beaty" wrote in message
> > New article just added, check it out:
> > Make your own passive IR filter-goggles
> > http://amasci.com/amateur/irgoggl.html
> > Human eyes can see IR light if it's very bright. Sunny days
> > have quite a bit of IR in them.
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William J. Beaty SCIENCE HOBBYIST website
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