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thanks a lot for your reply. you mentioned two light sources are used in addition to the IR source. The laser I understand, as it will allow you to keep track of the position of the moving mirror to within less than a wavelength of the laser light. What is the other light source? Also I have heard that KBr or NaCl crystals are used for IR beam splitters. It seems like the moisture in the air would attack these crystals so the whole interferometer would have to be sealed from the atmosphere. Is that true? Or maybe they coat crystals with something to protect them.

How hard would it be to make your own IR-FT spectrometer? (Thats an Infrared Fourier Transform spectrometer -- used for ID'ing organic ompounds.) I checked around and even used ones are several thousand dollars. It seems possible that one could make one, the hardest part would be the interferometer I think: would nee to have two mirrors one moveable and the other precisely adjustable. The light source would be easy (but does an interferometer work with a non-coherent light source?). And the IR detector you would have to buy. The FT is easily done in software. Does any body have experience these things and know why one would be hard to build?