Getters

During the exhausting process in which the lamp is evacuated and perhaps also gas-filled, it is impossible to remove all traces of air even with high speed vacuum pumping equipment. Additionally small amounts of air and moisture will adhere to internal glass and metal components, and if nothing was done to remove these, the lamp would have an unacceptably short and inefficient life. It is for this reason that a special chemical called the GETTER is included inside every lamp. A getter is a chemical which will remove specific impurities form the lamp atmosphere both to assist in the achievement of vacuum during manufacture, and to maintain purity during lamp life.

One of the most damaging phenomena to plague the incandescent lamp is the water cycle. This will occur if there is insufficient getter to remove all remaining moisture inside the bulb. When the filament is lighted the high temperature breaks the water vapour down into hydrogen and oxygen:
     2H₂O     >     2H₂     +     O₂

The free oxygen then reacts with the hot filament to produce tungsten oxide:
     2W       +     3O₂     >     2WO₃

The tungsten oxide has a low vapour pressure and boils off the surface of the filament, then being deposited on the cooler bulb wall or the filament support wires. It is then reduced by the hydrogen which was freed when the water decomposed. Effectively, the hydrogen takes the oxygen back from the tungsten, producing water again.
     WO₃     +     3H₂      >     W     +     3H₂O

The tungsten metal remains as a dark film over internal components, and the water can go back to the filament and attack it again. The water cycle causes rapid darkening of incandescent lamp bulbs, and when enough of the filament has been removed it will burn out causing premature failure.

Red phosphorus was the first getter, introduced by Malignani in 1893 to remove oxygen and water vapour from the lamps made in his Italian laboratory. Today it helps to combat the water cycle, a chemical process which destroys the filaments of poorly processed incandescent lamps (see below). Immediately before sealing into the bulb, the filament is dipped into or sprayed with getter suspended in an organic solvent.

When the filament is first lighted, the phosphorus is evaporated and reacts with any residual oxygen while it is travelling away from the filament. This forms phosphorus pentoxide, sometimes seen as a ginger coating on the glass bulb. This chemical is hydroscopic and goes on to remove water vapour. The getter is only active for the first few minutes after the initial light up. It is not an especially good getter, but is very cheap and is adequate for GLS lamps which are processed well. Many inferior manufacturers still have to rely on additional getters to make up for poorly controlled stem making, mounting and exhausting operations.

If the getter is added in excess (or lamps of unusually high purity are made, such that not all of the phosphorus pentoxide is consumed) then a ginger coating will appear on the bulb wall. This is especially problematic in miniature lamps since it absorbs a significant proportion of the already small light output. Cryolite is often added to the phosphorus mixture for vacuum and miniature lamps, and its reaction with the getter produces a coating that is much more transparent, but still just as effective.

Red phosphorus cannot always stand the high temperatures encountered on Sealex machines, and its alcohol solvent can also carry water into the lamp. Thus today, it has been largely superseded by the new series of Phosphorus Pentanitrile (P3N5) getters, which are more stable and are based on amyl acetate solvent. They are also less flammable, and reduce the occurrence of spontaneous fires on the lampmaking machinery that can sometimes erupt from old deposits of dried-out red phosphorus getter.

Lastly, in the case of some high value GLS lamps a second getter may be employed. This consists of zirconium-aluminium paint which is applied to the lead wires, or some other component which will operate at about 400-500°C. It forms an effective hydrogen getter which further helps to fight the water cycle. It is often required for inside white-coated lamps. The surface area of the powder particles can be as much as 2000 square metres in a typical GLS lamp, and this is very difficult to thoroughly outgas on high speed lines.