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Three 100-Watt light bulbs. They are (left to right): halogen, pearl envelope and clear envelope. The halogen bulb has a tungsten filament surrounded by low-pressure iodine gas and a quartz glass inner envelope. These effectively prevent the metal from the filament from subliming onto the inner glass, prolonging the bulb's life. This is needed as the halogen bulb runs at higher operating temperatures to give a more natural quality of light. The other bulbs are conventional domestic types, with a tungsten filament surrounded by an inert mixture of nitrogen and argon. They run at lower temperatures, giving a warmer colour. Their average life is 1, 000 hours.