Consider the case where the emission coefficient j is fueled purely by scattering. An example is the reflection of sunlight to Earth from another planet. The reflected sunlight is I (cos ), where is the scattering or phase angle between the light of sight and the incident radiation (approximately zero for the giant planets).

Scattering removes radiation from a particular direction and redirects it onto another path. If photons undergo only one encounter with a particle, the process is referred to as single scattering; multiple scattering refers to multiple encounters. The angular distribution of the scattered radiation is given by the scattering phase function P (cos ), which is by definition normalized such that, when integrated over a sphere,

We define as the albedo for single scattering; it represents the fraction of incident radiation lost due to scattering. It is equal to unity if the mass absorption coefficient is equal to zero. The source function S can be written as

Here are a few common scattering phase functions.

• Isotropic scattering: in which case

• Rayleigh scattering: the scattering of sunlight by air molecules,

• First-order anisotropic scattering: in this case, the scattering is isotropic if q_ph = 0; the radiation is back-scattered if q_ph > 0; the radiation is scattered forwards if q_ph < 0. Flux is scattered predominantly in the forward direction if the particles are similar in size or slightly larger than the wavelength of the scattered light.