Electric lamp having optical interference filter of alternating layers of SiO.sub.2 and Nb.sub.2 O.sub.5 --Ta.sub.2 O.sub.5

Thin film interference filter consists of alternating first and second layers, the first layers being silica, the second layers being a mixture of tantala and niobia. The filter may be used on a high temperature lamp in an inert atmosphere if the niobia in the mixture is less than 40% by weight. For higher percentages a partial pressure of oxygen has been found to prevent blackening of the layers