The quality and useful life of regenerative-cycle type incandescent lamps that employ iodine as the halogen are enhanced by dosing the lamps with controlled amounts of SnI.sub.4 in combination with elemental iodine. Alternative dual-additives that provide iodine-bromine type lamps are SnI.sub.4 in combination with a compound such as methylene bromide (CH.sub.2 Br.sub.2) or mercury bromide which decompose within the energized lamp and release free bromine, and mercury iodide in combination with CH.sub.2 Br.sub.2. The combined use of two different additives or dosing materials to provide either an iodine or an iodine-bromine atmosphere within the operating lamp permits elongated tubular lamps having singly-coiled filaments to be operated in various non-horizontal burning positions at voltages above their nominal voltage ratings for much longer periods of time than lamps of the same rating and construction that are dosed with a single additive such as elemental iodine, SnI.sub.4 or CH.sub.2 Br.sub.2.