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Infra-Red Filter
Name: Ricordo A.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: November 2002
Question:
I am a life scientist trying to set up a red light source
that does not include infrared radiation. Other than using LEDs or
special lamps, is there a common material one can use as a filter? Water
absorbs infrared, but it also absorbs red. Same with copper sulfate
solutions. I have heard that gold-coated polycarbonate safety goggles
might do the trick, but I cannot find any "absorption" spectra for gold.
Do you have any suggestions?
Replies:
I would think that a simple prism or optical grating from Edmunds
Scientific or other suppliers of science "stuff" would work well. A beam of
"white" light would be diffracted by the prism or grating according to
wavelength and with an appropriate arrangement of slits you could choose
which wavelength you wanted. The infrared radiation would be scattered at a
different angle than red radiation (~700 nm) and could be blocked from the
target you are illuminating.
Vince Calder
It appears that a gold thin film might not quite work. You can see a
reflectance spectrum for gold at
http://www.thermo.com/eThermo/CDA/Technology/Technology_Detail/1,1213,-10333
-109,00.html; note that the reflectance shoots up between about 500 and
600 nm, so it will take out the red with the IR.
It is true that water absorbs red, but not strongly. A column of water even a
few feet thick will transmit most red light. If you are willing to spend
some money on a filter material, the absorption bands in heavy water (D2O)
are shifted from those of water so that its minimum occurs in the red rather
than the blue.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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