Abstract

The emissivity of a unit area of the opening of a cavity c is a function of its geometry as well as of the emissivity of its walls 0.

If fn is a fraction of incident radiation which is reflected n times before emerging from the cavity, then ρc=∑fnρ0n is the reflectance, and c=1−ρc is the effective emissivity. Under diffuse reflection f1=∫F2dA/∫FdA, where F is the viewfactor from cavity wall to open space. If fn=(1−f1)n−1f1 then c=0/[0+f1(1−0)]. The approximation is good in the domain of low reflectivity. The f1 have been calculated for a variety of cavity shapes.

© 1963 Optical Society of America

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