||The first reported high pressure sodium lamps were in fact mercury-free types. However mercury was added to the first commercialised lamps in the form of an Na/Hg amalgam. It serves to raise efficacy, owing to the reduced thermal conductivity and losses from the Na-Hg-Xe plasma. It also acts as a buffer gas, decreasing electrical conductivity so that shorter arc tubes can be used. Finally it allows the colour point to be brought on to the blackbody locus. Owing to the low plasma temperature it is scarcely excited and contributes no direct radiation of its own.
Recently it has become desirable to eliminate mercury for environmental reasons. Also with normal sodium loss during life, the discharge in the increasingly Hg-rich plasma causes a lamp voltage rise, which leads to cycling and eventual failure.
The development of the high Xenon pressure lamp in the 1980s made it possible to replace the Hg content with a similar pressure of xenon. However since mercury was used to decrease the electrical conductivity of the plasma, its removal means that longer and thinner arc tubes are needed to control the volt drop, such that standard lamps can be retrofitted. This results in a small loss of efficacy compared to "Super" HPS lamps with mercury, but the Hg-free lamps remain more efficient than conventional standard HPS types.
The mercury in standard lamps causes some broadening of the red-wing of the sodium spectrum, and this is absent for the Hg-free lamp (see spectrum below). Consequently the they appear slightly yellower than standard types. The colour temperature is around 100K higher with CRI a little lower, and colour is shifted somewhat away from the blackbody locus.