Scientific aspects of the high-pressure sodium lamp

A description is given of the basic physical mechanisms operating in the high-pressure sodium (h.p.s.) arc with particular emphasis on the role of self-absorption. This is followed by reviews of the measured properties of the arc, and of the work that has been done on the modelling of the arc´s behaviour. A well substantiated picture emerges of an arc in l.t.e. with a centre temperature of ~ 4000 K in which power is dissipated mainly by thermal conduction and self-absorbed radiation from the sodium D lines and the non-resonance lines. Finally methods of designing the h.p.s. arc to give optimum efficacy are compared.