Halogens

Slow evaporation of the helix allowed the use of tungsten-halogen lamps. In the bulb is injected a small amount of the halogen iodine or bromine. It forms a volatile tungsten compound, which is deposited on the helix, and not on the walls of the flask. However, iodine and bromine also have a chemical effect on the glass, and therefore the flask has to be made of expensive quartz. Tungsten-halogen lamps withstand the high temperature of the filament, which has no effect on their longevity, and emit a brighter light, close to natural spectrum.
Gas discharge lamps are used since the early 1930s. the First lamps were filled, instead of the downloaded air, a small amount of neon. High voltage sent to the electrodes, which were located at both ends of the tube.
Between the electrodes an electric discharge, and the lamp radiated a reddish light. Such tube bending, forming different shapes or letters through which quickly spread in advertising. Thus began the era of neon lights that flashed everywhere in urban centers.
Experiments with other gases has opened a wide range of different colors. Sodium low pressure lamps emit yellow light and are used for street lighting. First, their efficiency did not exceed 70 lumen/watt, but now it has increased to 200 lumens/watt.