Measurement of the thermal inertia of the skin using successive thermograms taken at a stepwise change in ambient radiation temperature

Abstract

Skin thermal properties are difficult to measure in vivo in the steady state because there is a constant temperature gradient across the skin surface. However, measurement of skin thermal properties is postulated in quantitative evaluation for thermographic observation. Here, imaging of the thermal inertia of the skin was attempted by thermographic measurements at a stepwise change in ambient radiation temperature achieved by quickly switching 2 hoods maintained at different temperatures. Using this technique, a total of 65 thermograms were sequentially recorded at intervals of 0.5 s beginning 2 s before the stepwise change. The image of skin thermal inertia was estimated by applying statistical curve fitting at each pixel of the thermograms. In addition, the emissivity and true temperature of the skin were also determined, together with thermal inertia, in a single measurement. Measurements were made at different sites on 10 subjects. The average values of thermal inertia of normal skin were scattered throughout a range from 1.4*10³ to 2.1*10³ W s^1/2 m^-2 K^-1. Investigations of the relationship between skin blood flow and thermal inertia were also made by imaging thermal inertia when skin blood flow was changed by applying a vasodilator or vasoconstrictor on the skin surface. In a comparison with the data measured by laser Doppler flowmetry, the average slope of skin blood flow versus thermal inertia was 2.88*10^-4 V per W s^1/2 m^-2 K^-1, and the thermal inertia of the skin with no blood flow was 1.03*10³ W s^1/2 m^-1 K^-1. The results also show an almost linear correlation between skin blood flow and thermal inertia in each individual, but interindividual differences were also observed. The results suggest that skin blood flow distribution can be estimated by noncontact imaging of thermal inertia.