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In clinical thermography the amount of heat energy received by the detector is interpreted in terms of a distribution of skin temperature but it also depends on the emissivity of the surface. It has been concluded previously that the emissivity in the range 2-5 μm for skin at normal incidence is about 0.98, and that variations are not likely to represent a difference in apparent temperature of more than ±0.5°C. However, theoretical considerations are presented for the variation of emissivity with the angle at which the surface is viewed. These indicate a significant fall in emissivity as the angle to the normal is increased beyond 90°, corresponding to a reduction of 4°C or more in apparent surface temperature. Thus it would be possible for a 'hot spot' associated with significant pathology to remain undetected on a surface viewed obliquely. Examples of this obliquity effect in clinical and experimental thermographs are demonstrated.
Watmough et al. (Thu,) studied this question.