Abstract
In clinical thermography the amount of heat energy received by thedetector is interpreted in terms of a distribution of skin temperature but it alsodepends on the emissivity of the surface. It has been concluded previously that theemissivity in the range 2-5 μm for skin at normal incidence is about 0.98, and thatvariations are not likely to represent a difference in apparent temperature of morethan ±0.5°C. However, theoretical considerations are presented for the variationof emissivity with the angle at which the surface is viewed. These indicate a significantfall in emissivity as the angle to the normal is increased beyond 90°, corresponding toa reduction of 4°C or more in apparent surface temperature. Thus it would bepossible for a 'hot spot' associated with significant pathology to remain undetectedon a surface viewed obliquely. Examples of this obliquity effect in clinical andexperimental thermographs are demonstrated.
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