Early experiments showed that for long positive column discharge lamps, maximum lamp efficacy occurs when the sodium vapour pressure is 0.4 Pascals. This occurs above molten sodium at 260°C, and is illustrated in Figure S9. Even a slight deviation from this temperature will cause a drastic decrease in light output, so it is clear that accurate temperature control of the discharge tube is vital to maintain high efficacy.
It is important to realise that the sodium vapour pressure is determined only by the temperature of the coldest spot in the discharge tube. Local hot spots have no effect in increasing the sodium vapour pressure, because the metal will preferentially evaporate from those hotter regions and condense at the coldest spot. In other words, the sodium migrates over time to the coldest part of the lamp, and it is the temperature at this region that determines the vapour pressure and which must be kept as close as possible to 260°C.
Because some energy is required to heat the discharge tube to this temperature, the most efficient lamp will be the one that achieves this temperature for the lowest energy input. As will be seen later, many techniques have been combined to produce a lamp having excellent thermal insulation which heats itself to the required temperature with the lowest electrical energy input.
Figure S9 - Effect of sodium cold spot temperature on the luminous flux