From: email@example.com (William J. Beaty)
Subject: Re: Make infrared goggles inexpensively (like $10!!!)
Date: 17 Sep 2002 00:59:18 -0700
References: <firstname.lastname@example.org> <3D848F5E.91EC2A2F@xympatico.ca>
NNTP-Posting-Date: 17 Sep 2002 07:59:18 GMT
Jonathan Kirwan wrote in message news:...
> Yes, I have the same information. For example, the three color
> sensitive cone pigments show a response factor of 1.0254E-5, 3.97E-6,
> and 0 for 800nm.
> But 5-6 orders down sensitivity is still asking too much. The flux of
> the sun in the visible range as compared to only 100nm or 200nm
> further out, will be rather similar in magnitude. Yet the filter
> response curves aren't 5-6 orders down in the visible range where they
> only "mostly" block. Not even at three-ply.
From eyeballing some crude values for Lee filter curves and for the
human visual sensitivity function, the 3-congo/2-red goggles give
a peak around 710nM. I have a dead spectrophotometer at work.
Once I get it up and calibrated, I can get some proper numbers for
those Lee filters and see if my 710nM estimate is right.
> >Bill Beatty's descriptions more or less match the appearance of objects
> >viewed with infrared cameras.
> The simplest explanation for what our poster observed comes from
> simply convolving the two filter responses he talked about.
Huh? Convolving the filters will smooth out the sharp passband
edges in the graphs, while actually stacking up some bandpass
filters will sharpen the edge of the passband. You don't want to
convolve, instead you just want to multiply each filter's absorbtion
at a particular frequency by the absorbtion of all the other filters
in the stack.
If you want to do convolution too, you could assume that the original
instruments which produced those absorbtion curves had 10nM windows
of accuracy, then use convolution to include that smoothing effect
(but what if the spectrometer slits were actually 1nM?)
> curves shown are in linear scale response, not logarithmic, and there
> is probably substantial flux remaining almost across the spectrum,
> though most of it will be in the 650+nm area. Certainly, there will
> be plenty of 680nm light available, given the curves I saw. And that
> is visible.
If a particular filter response is down 99.0% at a certain frequency,
and you stack three of those filters up, then it's down six orders of
magnitude at that frequency. (If I'm wrong, can you please explain
> There is no need to pull out some incorrect analogy to explain being
> able to see some odd looking things. It's enough to use traditional
Declaring my analogy to be incorrect, yet not saying WHY, is a
typical debating trick. It's wise to withhold judgement before
investigation. But simply declaring something to be wrong is just
as irrational as declaring something to be right.
How is the rolloff in the human eye sensitivity function at the
IR end fundamentally different than the rolloff of human ear
sensitivity at the ultrasonic end? If you think my analogy is
misguided, please show this. Are you that confident that you
can't hear a pure 30KHz tone no matter how loud the signal?
Are you confident that you can't see a narrow 800nM emission line
no matter how bright the light?
> If anyone has the exact filter response data, over wavelength, for
> these filters, I'd be more than happy to convolve them with the human
> photopic vision curves to find the resulting response. I think we
> will find, from that, where the dominant is at. And I rather doubt it
> will be in the longer than 800nm area.
We agree. For eyes wearing "congo blue" goggles, the sensitivity
probably peaks out at around 710 to 720 nanometers. I also made up
some goggles with four layers of Lee #120 "deep blue" which has a
much deeper IR cutoff. They make the sky look even blacker, but the
light is so dim that it takes about 20 seconds before you can see
anything at all, even in full sunlight. I figure that these
"deep blue" goggles peak out your eye's response at around 720 to 730nM.
To see an outdoor scene at 800nM, the sun would have to be many orders
of magnitude brigher, and I'd have to use much deeper IR filters
(like Wratten 87B for example.)
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William J. Beaty SCIENCE HOBBYIST website
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