Size parameter for thermally emitting particles

摘要

For thermal emission from particles the conventional size parameter X = 2πα/λ does not distinguish between small and large particles. We show that the opacity parameter Ω = 4πkα/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.