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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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