Sodium (Na) self absorption is a natural phenomena.
When Na atoms are heated in the flame they become excited and then relax into their ground state.
The energy released is by light being emitted which is what we can see as the yellow light.
If there are very many atoms in close proximity ( i.e. higher concentration) the light itself can cause an adjacent
atom to become excited so the light is absorbed by the sodium atom instead of falling on the detector.
Thus the higher the concentration, the less light proportionally falls on the detector.
All manufacturers correct for this as the algorithm is well known. On the model 410 we add
a separate module (lineariser module) whereas on the Model 420 we have built-in this module.