326
Lighting history and terminology
Bob Owen (Blaze 1625) from Hampshire provides some useful notes for those who might be interested. (Apr 05)

Most lighting developments come from GE, the US successor to Edison's original company. They developed sealed beam units in the 1940s and halogen bulbs in the late 1950s although Philips also have some claims. The terms Halogen, Quartz Halogen, Tungsten Halogen and Quartz Iodine are largely synonymous. They refer to a bulb with a small quartz glass envelope and tungsten filament with a halogen gas filler. Halogen is the name of a chemical family comprising Iodine, Chlorine and Bromine. Halogen lamps use either Iodine or Bromine. They allow a tungsten filament to be run very hot but still not fail. In a conventional lamp, attempts to get more light by running the filament hotter result in evaporation of tungsten from the filament - this deposits on the cool glass envelope (blackening) and the resulting thinner parts of the filament run even hotter, so evaporating more and causing run-away thinning and early failure. GE found that if halogen vapour is present in the lamp it combines with the particles of tungsten that have
been evaporated from the filament and redeposits them back on the filament- magic! But for this process to take place, bulb wall temperatures should not be below 250 C. To achieve this, the bulb must be small to be close to the filament and cannot be of ordinary glass as it would soften, so it is made of quartz glass. Further efficiencies can be achieved by adding other gasses, for example xenon, to pressurize the envelope and further discourage evaporation. The result is a bulb which gives at least 50% more light per watt than a conventional bulb and still has a long life.

Notes:
Halogen lamps with Xenon should not be confused with "Xenon head lights" which are high voltage discharge lamps now appearing on upmarket models and give at least twice the light per watt as standard bulbs.

It's interesting that the regulations appear to define the power of the headlight bulbs NOT the light output - ie they are implicitly based on assumptions from old technology. All efforts are therefore in getting more light for a given power. We could have had more light with old technology if higher power levels were allowed!