Abstract

For thermal emission from particles the conventional size parameter X = 2πa/λ does not distinguish between small and large particles. We show that the opacity parameter Ω = 4πka/λ = 2kX is a more accurate means of demarcating the two emission regimes. Ω is approximately equal to the particle’s mean optical depth, and it can be derived from both scattering theory and geometrical optics.

© 1999 Optical Society of America

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