Consider a source of radiation hidden behind a non-emitting cloud along the line of sight. In this case, the emission coefficient j is zero, and thus the source function S is zero. The incident light I is attenuated by the absorption of the cloud, and we can evaluate its properties in the case of an optically thin or an optically thick cloud.

This relation is often called Lambert's exponential law of absorption. The specific intensity decreases by the exponential of the absorption coefficient (optical depth) along the line of sight. If is constant,

Consider as well the parallel case of a non-absorbing cloud. In this case is equal to zero, and the increase in specific intensity is the integral of the spontaneous emission along the line of sight.