 

Emissivity of Human Skin in the Waveband between 2µ and 6µ DAVID J. WATMOUGH & R. OLIVER Department of Radiation Physics, Churchill Hospital, Oxford. THE emissivity of human skin ε(λ) in the range 2µ to 6µ has recently assumed considerable importance because of the increasing medical use of infrared scanners to measure skin temperature (t). Several commercially available scanners utilize indium antimonide detectors which are sensitive in the range 2µ to 5.4µ. Such machines measure the energy (Q) radiated by the skin and, being calibrated against a standard black body, changes in Q are represented as variations in skin temperature. Dreyfus^{1} has shown that Q is related to t by an equation of the form, where k is a constant, and where the index n depends on λ as λ_{max} being the wavelength corresponding to the maximum in the emission curve. The importance of variations in emissivity can be seen by differentiating equation (1) for constant Q. We obtain which simplifies to give for Δt For a skin temperature of 27° C (t=300° K), n is about 12.5. It follows from equation (2) that if ε(λ) were to vary by as much as 5 per cent over the skin surface, this would be interpreted by the scanner as a temperature variation of about 1° C. A fairly accurate knowledge of ε(λ) is thus necessary, for hot spots with temperature elevations of only 2° C are considered to be of clinical significance.
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