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 effec-
tive 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.
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18.4 – Infrared semi-transparent materials
