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The average durability of the light sources is the time after which half the number of lamps ceases to light.

The nominal durability is the value of the durability declared by manufacturer. It is generally equal to the average durability.
The durability of bulbs, not taking into according mechanical damages such as breakage of bulbs, mainly depends on evaporation of wolfram from the filament. In bulbs electric current flows through the filament in form of wolfram coil, heated to a temperature in which it starts emitting visible radiation. This process is accompanied by slow vaporization of wolfram from the filament surface. Glass bulb would start to darken due to settlement of wolfram elements evaporated from the filling on it. This process reduces the luminous flux of the bulb. The evaporation reduces the diameter of the wolfram filament until final it breaks at its thinness point. This causes the disconnection of electric circuit and constitutes to the end of lamp usage.
In 1960 the then ordinary light bulb was improved by adding chemical compounds, so-called halogens, for example, iodine that increased the exploitation time of the wolfram filament. Thanks to halogen proprieties, use of more thermal resistant materials for the bulb and reduction of the sizes it was possible to increase the pressure of the gas inside the bulb. In this way the evaporation of wolfram filament was slowed down and thanks to this the bulb’s life was extended
The most significant factor limiting the life of bulb light sources is the exploitation under a voltage different from the nominal voltage that is for which the lamp was designed.
The drawing shows the dependence of the life of the bulb on the supply voltage. As can be seen with a voltage 120% of the nominal voltage the bulb’s durability is shorten by just 10-15% of the nominal life, but lowering the supply voltage to about 90% of the nominal value increase the life by up to 4 times. This phenomenon is used by designers when designing lamps used in street lighting as well as any other type display lamps where long life and reliability are of utmost importance. There is a possibility of increasing the life of the lamps by designing them for nominal voltage higher in which they are to actually work. It is possible, for example, to increase the resistance of the bulb filament by increasing its length and this cools it. This in addition allows for the increment of the diameter of the wolfram wire. Thanks to the lowering of the operation temperature and use of a thicker wire it is possible to prolong the life. The side effect unfortunately is the drop in lighting efficiency and light flux. Street lamps have average life nominal expectancy of 8000 hours thanks to designing their voltage higher than the nominal exploitation voltage.