Recently in a thread on the future of incandescent lamps, I tried to explain these terms in a way that was quick and easy. It was also late at night and I was in the middle of tech week for Cabaret. In the process I oversimplified and didn't get some of the information right. Rather than edit and bury the topic in another thread. It thought it was a topic worthy of it's own thread so here we go. Incandescence: When an object is heated to the point that it glows, it is producing light by incandescence. A modern incandescent lamp has a metal filament through which electricity is passed. Due to electrical resistance in the filament, the metal heats up and begins to glow. The Filament: In 1879, Thomas Edison invented the "light bulb". His first filament was made of partially charred bamboo... it only lasted a few seconds before it burned up. It took 28 years of experimenting until the perfect filament material was discovered, Tungsten. Tungsten, and it's alloys, provide the maximum combination of both light emission and durability. (Useless fact: the other wires inside the lamp that hold the filament in place are typically molybdenum). The "Glass" Envelope: The outer bulb part of a lamp is called the envelope. Some household lamps are still made of glass. However the high temperature of theatrical lighting is too much for typical glass. Our lamps are made of a much stronger fused silica material called quartz. Quartz doesn't soften until it reaches 1600 degrees Celsius... it's very tough stuff. Touching the lamp with your bare fingers can leave enough oil that it will boil the surface of the lamp and cause it to fail very rapidly. Some people keep a pair of thin cotton gloves just for changing lamps, sometimes you can use the plastic or foam material the lamp is packed in to hold it. If you do touch the lamp clean it with alcohol. I keep a box of the small medical alcohol wipes they use when giving you an injection near bye. They are only a few dollars for 100 individually wrapped pads. Gases inside the Lamp: Edison knew that normal oxygen rich atmospheric gas caused the rapid destruction of the filament, so his experimental lamps were usually vacuum sealed. In 1913, it was discovered that filling the lamp with a inert gases not found in typical atmospheric gas could actually prolong the life of the filament. Inert gases like argon and Krypton don't react to the tungsten and cause deterioration. (Useless fact: Philips HPL's use Krypton in them.) Later in time it was discovered that adding a Halogen to the gas mix had a unique property. As the lamp heats up, tungsten molecules are driven off of the filament by the intense heat. In a non-halogen lamp, these molecules eventually bump into the envelope and are stuck there since it is much cooler than the rest of the lamp. Over time, a dark build up on the envelope occurs and eventually the filament weakens and fails. In a halogen lamp, the Tungsten molecules combine with the halogen to make a form of salt. This new molecule floats around the lamp and once it comes back near the filament, the heat breaks the molecule apart and the tungsten is deposited back on the filament. This prolongs the life of the lamp significantly. This cycle doesn't work when lamps are dimmed, so it's good idea to run your Halogen lamps on high for a few minutes at the end of the show. So that the process can kick in and the tungsten molecules can be relocated back to the filament.