For thermal emission from particles the conventional size parameter <i>X</i> = 2π<i>a</i>/λ does not distinguish between small and large particles. We show that the opacity parameter Ω = 4π<i>ka</i>/λ = 2<i>kX</i> 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
David K. Lynch and Stephan Mazuk, "Size Parameter for Thermally Emitting Particles," Appl. Opt. 38, 5229-5231 (1999)