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Correction of the angular emissivity of human skin for clinical thermal imaging

Boaz Arnon, Kahana Oria, Yoel Arieli*

Clinical thermal imaging is based on the postulation that cancerous tissue is warmer due to enhanced metabolism and increased blood supply. The thermal map of a skin is obtained by imaging the skin by a thermal camera where the amounts of heat energy received by the camera's detectors are interpreted in terms of a distribution of the skin temperature. However, it is well known that the amount of heat energy emitted by a surface also depends on the emissivity of the surface. Theoretical and experimental results have shown that the emissivity depends on the viewing angle of the surface, i.e., the angle between the imaging axis and the normal to the surface. These results indicate a significant fall in the emissivity when the viewing angle is increased beyond 60°, which leads to inaccurate temperature mapping. In our study, several human bodies were thermal mapped in incrementally varying angle from 0° to 90° and the thermal maps were compared. The acquired experimental data was processed to obtain a correction formula for calibrating the measurement error due to the emissivity dependence on the viewing angle. Using the correction formula, the measurements at different viewing angles were calibrated, compared and statistically tested to assess the empirical correction formula.