The tungsten filament is used most often with incandescent lamps because it has a high melting point (around 3419°C) and a low rate of evaporation. The bulb is filled with an inert gas to slow the rate of evaporation even further. If a filament was used without the glass cover and inert gas it would evaporate in a matter of minutes, rather than the average 1000 hours of useable lamp life that we get from standard bulbs.
Incandescent lamps have a positive temperature coefficient, which means that when the lamp is cold it draws more current than when it is warm. As most materials heat up, their electrical resistance increases. So as the filament heats up, its resistance increases, allowing less current to pass through, meaning that incandescent lamps have an “in-rush current” that is much higher than their rated current.
For example, a 120V 100W incandescent lamp may have a cold resistance of 10.4Ω, and a hot resistance of 144Ω. It draws a rated current of 833.3mA and an in-rush current of 11.54A
Normal Running Current:
It is because of this in-rush current that most general use switches will have a “T” rating, meaning they are designed to handle this in-rush current. If they do not have this “T” rating, they must be oversized to handle the in-rush current.
If the lamp is run at a higher voltage than it is rated for, it will have a greater light output at the cost of a shorter life span, and the same is true in reverse, lamps operated at less than their rated voltage will be dimmer, but last longer.
For example, a 100W bulb rated 120V will burn brighter at 130V but will have a shorter than rated lifespan, while the same bulb operated at 110V will burn dimmer, but have a longer than rated lifespan. Most incandescent lamps have an average rated life of 1000 hours.
As the tungsten filament burns, it slowly evaporates and leaves deposits of tungsten on the inside of the glass. This darkening of the glass is called Lamp Lumen Depreciation and reduces the amount of light (lumens) emitted from the lamp, while still drawing its rated current.
This evaporation process continues until the filament is weakened enough that it finally breaks. Incandescent lamps usually fail if they suffer severe shaking or sudden movement while the filament is hot, or during the in-rush current when they have just been switched on.
In addition to Lamp Lumen depreciation, Luminaire Dirt Depreciation, or the accumulation of dust and dirt on lamps, will also reduce the illumination levels of the light source. Good ventilation and regular cleaning of lamps can help reduce this effect.