With winter upon us, it becomes doubly important that the lights on our vehicles are in tip-top condition. That includes all the lights we need in order to see and to be seen - headlights, tail-lights, brake lights, indicators and so on.
There have been revolutionary developments in car headlights in recent years, but on the vast majority of vehicles the headlights will still be either of the type where a replaceable bulb fits into a holder at the back of the reflector, or of the "sealed beam" variety.
The first type has the disadvantage that we cannot completely prevent dust and moisture from getting in and tarnishing the reflector or settling on the inside of the front glass.
This happens gradually, so we don't notice the fall-off in headlight performance until we look closely at the headlight and spot the blotchiness on the reflector or the film of dirt on the inside of the glass. Such a headlight should be removed, and cleaned as best we can.
On sealed beam headlights, so popular on American cars for decades, there is no separate bulb. The entire headlamp is in effect a huge bulb.
The tungsten filament is fixed precisely at the optimal position in front of the reflector, the space around it is filled with inert gas to exclude oxygen, and the whole unit is permanently sealed. No gradual fall-off of light output, no replacement bulbs slightly out of alignment.
But they also have drawbacks: relatively high cost, and immediate light failure if the front glass is cracked in service (picture a dark gravel road, stone kicked up by a passing car, poof - no more light from that sealed beam).
A headlight can also be dead, as opposed to merely dull. That would indicate a broken filament or a problem in the electrical circuit feeding current through the bulb. The temperature of a glowing tungsten filament is about 2300C, and that is hot enough for some tungsten molecules to escape from the spiral of wire forming the filament.
On the earlier bulbs (with the spherical glass envelope filled with inert gas like argon or krypton), these molecules would settle on the cooler, inner bulb wall, causing the bulb to blacken slowly. This drastically reduces the light output, but it also means the filament wire is slowly getting thinner and thinner until, one day, it breaks at the thinnest point.
In the early 1960s scientists made a remarkable breakthrough on this problem when they discovered that if a trace of vapour of halogen (a highly reactive group of chemical elements that include iodine and bromine) was added to the inert gas inside the bulb, the escaping tungsten molecules would be "captured" in a chemical reaction and instead of settling on the glass wall, they would be returned to the filament.
This process - known as the tungsten-halogen cycle - maintains a constant light output throughout the life of the bulb. But the cycle will only work if the bulb surface is very hot, generally above 250C.
Since a halogen bulb is fairly strong, it can be filled with gas to a much higher pressure than previously, which in turn slows down the evaporation of tungsten from the filament.
Halogen bulbs last twice as long and produce light that is whiter and brighter.