From: Clay Harmon (wcharmon@wt.net)
Date: 04/02/02-07:57:47 PM Z
on 4/2/02 11:09 AM, Katharine Thayer at kthayer@pacifier.com wrote:
> Richard M. Koolish wrote:
>>
>
>>>
>>
>> Color is very misleading, especially when your eyes can't see the part of
>> the spectrum involved. I'm sitting here with a copy of the Kodak filter
>> publication B-3. If I look through the various filters for ones that block
>> UV to 400 nm, I find that among them are 2A (pale yellow), 8 (yellow), 21
>> (orange), 25 (red), 40 (light green), 45 (blue green).
>
>
> Do you have any more specific information on the wavelengths blocked by
> the various colors? Surely they don't all block the same wide range of
> wavelengths (360-400?) or if they do, then the answer to my question
> would be that the color isn't all that important; any color but blue
> would work, which would explain why both yellow-green and orange seem to
> do fine.
> kt
>
Katherine:
First of all, don't think of the color wheel. Visualize a line or continuum
going from the infrared the ultraviolet. I think the wrong mental 'model'
may be confusing you.
All of the colorsyou mentioned act as filters, and do not cut out a part of
the spectrum sharply. Most filters have a ramp or slope that gradually
transmits less and less radiation going either way from the wavelength that
it DOES transmit. Most filter's effects look like the classic Gaussian
lognormal distribution that put everyone to sleep in statistics, and not a
sharp-cutting 'boxcar' filter.
Think of a red negative as having its mean or average at the red wavelength,
but sloping off and gradually transmitting less and less radiation as you go
further away from the mean. With that image in mind, it is obvious that the
color whose 'mean' is furtherest away from the UV side of the spectrum will
transmit the least amount of UV light. That said, any filter whose 'mean' is
biased toward the red and infrared will be the most effective at cutting out
UV radiation and blocking it from the paper. That's why Dan's technique
works so well.
Sorry about the wordiness. I'm dusting off a lot of old physics and wave
theory classes here, and time has filtered a lot of that VERY effectively.
Check out the Kodak web page on their now-discontinued Infrared film. It has
lots of cool graphs and stuff that my be illuminating.
Clay
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