A linear tungsten halogen lamp which can be burnt at any desired angle is made by selecting the gas filling and the halogen or halogen-containing compound to satisfy certain conditions which can be mathematically expressed. If these conditions are satisfied, the separation of halogen which occurs in conventional linear tungsten halogen lamps when burnt at angles inclined to the horizontal is reduced to an acceptable level. It is found that the maximum degree of separation of the halogen, herein denoted by .gamma. max, can be calculated from lamp parameters as a function of the thermal diffusion coefficient of the halogen in the filling, the tube length, the temperature difference between the filament and the tube wall, the average gas temperature, the radius of the coiled filament and the inside radius of the tube.