some of it arrives at the other surface, through which most of it escapes; part of it is
reflected back again. Although the progressive reflections become weaker and
weaker they must all be added up when the total emittance of the plate is sought.
When the resulting geometrical series is summed, the effective emissivity of a semi-
transparent plate is obtained as:
When the plate becomes opaque this formula is reduced to the single formula:
This last relation is a particularly convenient one, because it is often easier to measure
reflectance than to measure emissivity directly.
178 Publ. No. T559597 Rev. a554 – ENGLISH (EN) – September 27, 2011
33 – Theory of thermography
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