Parallel to the thread on IR thermometers is the oxygen saturation
sensors (pulse oximeters) used in hospitals. It clamps on a patient's
finger and reads the relative oxygen saturation of the patient's
blood and counts the pulse rate. It uses two different wavelengths of
light. Wikipedia says the LEDs used as light sources are red (660 nm)
and infrared (905, 910 or 940 nm). BTW, this matches what I remember
from a journal article I read several years ago. These two
wavelengths correspond to two wavelengths where the oxygenated and
deoxygenated hemoglobin spectra differ significantly. By using two
wavelengths, the calculational algorithm does not need to worry about
differing pathlengths and general absorptivity differences between
different patient's fingers. Interesting how a basic understanding of
physics (and chemistry and biology) goes into what appear to be very
simple electronic devices.
Richard Bowman
Dept. of Physics/Bridgewater College
Bridgewater, VA 22812 USA