Regular incandescent light bulb are filled with a mixture of argon (93%), and nitrogen (7%) Argon is chemically inert. The higher vapor pressure decreases the evaporation rate of the tungsten filament and the low thermal conductivity lowers the filament’s conductive cooling. To prevent filament support arcing, some argon is mixed with nitrogen, due to its low breakdown voltage. Pure nitrogen may be used by high-intensity project bulbs to prevent arcing between close-spaced lamp electrodes. The filament’s diameter would be reduced over time by evaporation, which will also redeposit a darker layer tungsten on the bulb’s inner surface. These phenomena could reduce the lamp’s useful life.
Some high-brightness “halogen” bulbs use a combination of argon and some halogens, such as bromine or iodine. The gaseous compound of tungsten and halogen reacts with evaporated, tungsten. This tungsten-halogen compounds then comes into contact with hot filaments, redepositing the molten onto the filament. This is called the “halogen cycling”, and allows the filament to operate more reliably at higher temperatures than an ordinary incandescent lamp. The envelope is usually made of quartz, as opposed to glass, due to the higher operating temperatures.