Electric lamp with optical interference coating

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. According to a second 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 the relatively high refractive index and a second sub-layer (52b) of silica with an optical layer thickness smaller than 20 nm. Preferably, the physical layer thickness dph of the second sub-layer (52b) is less than 25 nm. Preferably, each high-refractive index layer (52) includes three sub-layers (52a, 52b, 52c). The electric lamp according to the invention exhibits reduced diffuse scattering.