 In cases of a discrete spectrum, the observed formula_29 at a particular frequency formula_30 can be treated as a sum of contributions from each transition, 

where formula_32 is the contribution at formula_30 from the formula_34 transition, formula_35 is the absorption coefficient for the formula_34 transition, and formula_37 is a bandshape function (formula_38).Because eigenstates formula_21 and formula_22 depend on the applied external field, the value of formula_41 varies with field.It is frequently useful to compare this value to the absorption coefficient in the absence of an applied field, often denoted 

When the Zeeman effect is small compared to zero-field state separations, line width, and formula_43 and when the line shape is independent of the applied external field formula_44, first-order perturbation theory can be applied to separate formula_29 into three contributing Faraday terms, called formula_46, formula_47, and formula_48.