Modified Planck function for different column densities from 10²⁰ to 10²⁵ cm⁻² at a temperature of 100 K. The solid gray curves are calculated based on the opacities of Ossenkopf & Henning (1994), while the dashed lines are obtained from the extrapolation of the two lowest frequency values in Ossenkopf & Henning (1994). The Planck function or perfect black body at a temperature of 100 K is shown with a black solid and thick line.