From a Nature Article of the same title:

By DAVID J. WATMOUGH & R. OLIVER, Department of Radiation Physics, Churchill Hospital, Oxford.
Citation: Nature 219, 622 – 624 (10 August 1968); doi:10.1038

Abstract:

The emissivity of human skin epsilon(lambda) in the range 2micro to 6micro 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 2micro to 5.4micro . Such machines measure the energy (Q) Bactrim radiated by the skin and, being calibrated against a standard black body, changes in Q are represented as variations in skin temperature. Dreyfus1 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 lambda as lambdamax 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 Deltat For a skin temperature of 27° C (t=300° K), n is about 12.5. It follows from equation (2) that if epsilon(lambda) 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 epsilon(lambda) is thus necessary, for hot spots with temperature elevations of only 2° C are considered to be of clinical significance.

References

1. Dreyfus, M. G., Appl. Optics, 2, 1113 (1963). | ISI |
2. Hardy, J. D., and Muschenheim, C., J. Clin. Invest., 13, 817 (1934).
3. Elam, R., Goodwin, D. W., and Lloyd Williams, K., Nature, 198, 1001 (1963). | ISI |
4. Watmough, D. J., and Oliver, R., Nature, 218, 886 (1968).
5. Derksen, W. L., Monahan, T. I., and Lawes, A. J., J. Opt. Soc. Amer., 47, 995 (1957). | ISI |
6. Hardy, J. D., Hammel, H. T., and Murgatroyd, D., J. Appl. Physiol., 9, 257 (1956). | PubMed | ISI | ChemPort |
7. Hardy, J. D., and Muschenheim, C., J. Clin. Invest., 15, 1 (1936). | ChemPort |

NOTE: In the last Reference, the online description at ChemPort from the 1936 paper states:

(Paper Title:) Radiation of heat from the human body. V. The transmission of infrared radiation through skin

About 95% of infrared rays are absorbed within 2 mm. of the surface. The therapeutic effect is confined to the surface. The absorption spectrum of human wet skin is essentially that of water. Upon drying other absorption bands appear.