| "getter" is a chemical intended to absorb atoms and molecules contaminating fill of sealed structures.
On incandescents a red phosphorus was used to absorb any remaining traces of moisture from the lamp internal atmosphere.
A barium is used to absorb mainly oxygen mainly in vacuum devices - in vacuum tubes it use to have the form of a coating on the glass surface, applied by evaporating the pellet containing Ba after the tube was evacuated (either heating the cup with Ba pellet with laser, or earlier filling thin tube by the BA, welding this tube into a loop and then firing it by an induction heater so thethin tube bursts releasing the Ba) and letting it condense on the tube glass wall. During this it reacts with most of the remaining air in the tube, but then the coating on the glass was absorbing the gasses that got released from the internal materials over the lifetime, so helps to maintain the good vacuum there. It became the characteristic mirror-like finish on the top of the tubes. You may find the same form also in some Na lamp outer bulbs - usually the mirory like patches around the bulb base.
On discharge outer bulbs the vacuum does not have to be that good, but the "mirror like" form was unwanted (as it obstructs light, or even reacts with the IR reflector coating on SOX), it is just put into a cup welded on some stem wire and let it act from there.
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