The wattage at which a lamp operates will vary from its nominal rating depending on the ballast, the supply voltage, and the lamp voltage. Limits for wattage variation and lamp voltage for many lamp types appear later in technical information section.
The arc tube design of a Low Pressure Sodium lamp features a series of
reservoirs, or dimples, that help assure even distribution of the sodium during cooling and
condensation of the metallic vapors after the lamp is switched off. As a Low Pressure
Sodium lamp ages, however, there is a tendency for the sodium to migrate toward
the bend in the arc tube, which is cooler than the rest of the arc tube
structure. This will normally be accompanied by a slight rise in arc voltage and a
corresponding rise in wattage.
As an additional means of controlling sodium migration and minimizing wattage
rise in a Low Pressure Sodium lamp, some manufacturers place an insulating cap
over the arc tube bend. This diagram indicates approximate rise in lamp voltage during lamp life with
and without the insulating cap.
