High-pressure sodium lamp

A saturated high pressure sodium discharge lamp exhibiting unsaturated characteristics, said lamp comprising:

• a) an outer envelope sealed in a gas-tight manner;
• b) a gas filling within said outer envelope including at least 70 mol.% nitrogen;
• c) a discharge device within said outer envelope, said discharge device comprising a ceramic discharge vessel, said discharge vessel including a pair of discharge electrodes between which a discharge is maintained during lamp operation, a pair of current supply conductors each connected to a respective discharge electrode and issuing from the discharge vessel in a gas-tight manner, said discharge vessel including a central portion between said electrodes with a volume V and a length L, and a filling of an amalgam with a quantity of m.sub.NA of sodium by weight and a quantity m.sub.Hg of mercury by weight,
• the quantity of mercury and sodium each being selected such that mercury and sodium each remain present in the liquid phase during lamp operation throughout lamp life,
• m.sub.Hg, V and L satisfy the relation ##EQU12## and the ratio between sodium and mercury in the amalgam satisfies the relation ##EQU13## in which m.sub.Hg and m.sub.NA are expressed in mg, L in cm and V in cm.sup.-3, whereby said saturated high pressure sodium lamp is highly insensitive to external temperature changes.