Progress in high pressure sodium lamp technology

This paper reviews the most important technological advances in high pressure sodium lamp technology in the past decade. The basic effects of increasing the xenon starting gas pressure on the electrical and photometric lamp characteristics are discussed. The implications for a practical high xenon pressure product are also briefly reviewed. The basis of colour improved lamps, both the AC and the pulsed current versions, is explained. It is then demonstrated how the different colour improved commercial lamps are derived. The basic difference between saturated and unsaturated HPS lamps is explained by considering different dosing schemes, then the advantages of an unsaturated HPS lamp over a saturated one is demonstrated and finally, a brief description of the effects of omitting mercury from the HPS discharge vessel is given and it is suggested how a practical mercury-free lamp with good efficacy and life could be derived.