We present a theoretical treatment of the fundamental perturbations caused when continuous wave tunable laser radiation is absorbed by the low-temperature plasma of a noble gas positive column. For the first time we present a full theoretical description of the contributions due to the induced changes in the electric field, electron density and radiation trapping, as well as the primary laser perturbations. Comparison of our theory with experimental observation shows that the primary perturbations are generally dominant but in some instances it is the field perturbation which has the greatest effect.