The theory of the self‐absorption of resonance photons within a luminescent airglow layer is developed and expressions obtained analogous to the Van Rhijn equation for the variation of intensity as a function of zenith angle. The measured flux is shown to be a function ofL0/T, whereL0is the vertical thickness in atoms/cm2andTthe absolute temperature of the layer. The attenuation at large zenith angles in the case of the sodiumDlines is shown to be large enough whenL0is between 1 and 2×1010cm−2andTbetween 160° and 220° to cause a layer at 70 km to simulate a non‐absorbing (Van Rhijn) layer between 200 and 300 km. If self‐absorption is important, it is shown that the ratio ofD1toD2intensity should vary with zenith angle and different apparent heights found for the two components. Accurate measurements ofL0andTare shown to be necessary for even good approximate height deductions where resonance radiation (for example, Na and OI) is