Abstract: The electric lamp comprises a lamp vessel (1) which is transparent to visible light, and which lamp vessel accommodates a light source. The lamp vessel (1) is covered with an optical interference film (5; 5′) which includes layers of alternately a first layer (51, 53) of silica and a second layer (52) of a material having a relatively high refractive index. According to a first aspect of the invention, one of the second layers (52) includes an odd number of sub-layers (52a, 52b, 52c) of alternately a first sub-layer (52a, 52c) of the material with a relatively high refractive index and a second sub-layer (52b) of a further material with a further relatively high refractive index. Preferably, the material of the second sub-layer (52b) is Ta2O5.

Abstract: A high pressure sodium lamp includes discharge vessel (20) which is enclosed with intervening space (1) by an outer bulb (10), which space contains a gas-fill with at least 70 mol. % nitrogen gas. Electrodes (30a, 30b) are positioned in the discharge vessel (20) and are each connected to a current supply conductor (40a, 40b) which issues from the discharge vessel (20) at a respective end (21a, 21b) thereof. The discharge vessel (20) has between the electrodes (30a, 30b) a central portion (22) with length L and volume V. The discharge vessel (20) contains an amalgam containing a weight m.sub.Hg of mercury and a weight m.sub.Na r. of sodium, whereby the following relations are fulfilled: ##EQU1## in which m.sub.Hg and m.sub.Na are expressed in mg, and L and V are expressed in cm and cm.sup.3, respectively.