Metal halide lamp containing ThI.sub.4 with added elemental cadmium or zinc

A high intensity metal halide arc discharge lamp comprising an envelope of fused silica,

• inleads sealed into said envelope and electrically connected to electrodes positioned to define an arc gap therein, at least one of said electrodes serving as cathode comprising a tungsten portion on which thorium may deposit and be continually renewed by a transport cycle involving iodine, said thorium serving as an electron emitter allowing said cathode to achieve the electron emission required for the current through said lamp at a lower temperature,
• a discharge sustaining filling in said envelope provided by inserting therein at manufacture a charge comprising mercury, NaI, ScI.sub.3, ThI.sub.4 and an inert starting gas,
• and a getter in said envelope selected from the metals Cd, Zn, or mixtures thereof, the quantity of said getter being at least sufficient to provide the stoichiometric equivalent M' of any iodine released in said envelope as a result of impurities picked up during manufacture plus the stoichiometric equivalent M" of the iodine resulting from decomposition of the ThI.sub.4 in said charge, and the quantity of said getter not exceeding approximately three times the stoichiometric equivalents M' plus M".