The way a filament lamp works is very simple. Take a very thin wire (the filament) and force an electric current through it. Doing so causes the wire to get hot and if you make it hot enough it will become incandescant - light emitting. The amount of heat and light given off depends on the voltage across the filament and the current flowing through it and is determined by multiplying the Volts by the Amps which will give you the electrical power in Watts.

The incandescant lamp was simultaneously invented by Thomas Edison in th US and Sir Joseph Swan in the UK, precipitating a fight over patent rights eventually leading to the two inventers joining forces to form the Ediswan Company. The first lamps had carbon filaments sealed into glass bulbs with the air removed to prevent the hot filament from burning away. Carbon filaments were difficult to make, short lived and very fragile. Tungsten replaced carbon once the difficulties of working with such a hard metal was overcome.

Tungsten is the most refractory (has the highest melting point) of all metals and tungsten filaments could be run at white heat and still have a reasonable life expectancy. The hotter the filament the hotter the more light you get per Watt of power.To avoid rapid destruction the filament still needed to be run in an evacuated tube. The first tungsten filament bulbs suffered from a problem. After a while the inside of the glass bulb became blackened due to material boiling off the filament, then condensing on the comparatively cool bulb. It was found that filling the bulb with an inert gas, usually Argon, prevented this from happening.

The research into this phenomenon resulted in the discovery of the thermionic valve and electronics as we know it today.

As the gas filling allowed the filament to run at higher temperatures, alloys of tungsten were developed with even higher melting points, one of which was osram (derived from the chemical names osmium and wolfram - the latter being another name for tungsten and the reason for its chemical symbol W)

While this conventional technology is just about OK for indicator and stop / tailamps, the search for ever brighter headlamps resulted in the development of what was known then as the quartz iodine lamp, now known as the tungsten halogen lamp. Although the new filament alloys allowed temperatures high enough to give an intense white light, the bulb blackening problem returned with a vengence despite the gas filling. This loss of material led to severe erosion of the filament resulting in rapid failure. Researchers found that a small amount of iodine introduced into the bulb combined with any tungsten vapour present. In practice any free tungsten vapour reacts with the iodine in the cooler parts of the lamp to form tungsten iodide. Convection currents in the filling gas eventually sweep the tungsten iodide into contact with the hot filament where it breaks down to deposit tungsten onto the filament, releasing the iodine for further use. In a conventional lamp the temperature of the envelope is unimportant, but as tungsten iodide condenses at 25 degrees Centigrade it's important for the bulb to be maintained at a higher temperature than this. This is achieved by using a relatively small bulb, but due to the necessary high temperature and extreme thermal shocks as the filament is switched on and off, ordinary glass with its comparativly high rates of expansion and poor tolerance of thermal shock cannot be used.

Instead the enevlope is made from synthetic quartz (aka silica). For this reason it's also necessary to run a quartz lamp at a minimum of 90% of of its rated voltage to ensure an adequate operating temperature. Over running both conventional and halogen bulbs is very harmfull. 10% over voltage reduces the life by 50%.

The reason that you must not touch the envelope of a quartz halogen bulb is the risk of grease from your skin contaminating the glass causing the quartz to develope minute cracks which eventually allow air in. Should you accidentally touch the envelope it can be cleaned with methylated spirits and a soft cloth.

Xenon bulbs are another story.