Even the best of car lamps will eventually burn out. Some do that very quickly, whereas others take years. That’s no coincidence though. There are physical reasons for both behaviors.
It does exist, the nigh-on everlasting light bulb. At a fire station in California one has been burning for 116 years. Now someone could get the idea that lamp manufacturers would intentionally design their products in such a way that they fail earlier. Other than the fact that no car lamp needs to last more than a hundred years, the wonder lamp from the American West Coast is a bad example. Because above all it produces heat and only very little light. Its filament is made of carbon. It’s true that this very early technology, invented by Mr Edison in person, no less, is known for its long life, but a lamp like that only reaches it if handled with kid gloves. Even the slightest shock or vibration will break the carbon filament. The operators of the above-mentioned lamp don’t dare turn it off even for a short time. The resulting mechanical stress would mean the kiss of death for the filament.
Carbon filament lamps are therefore completely unsuitable for vehicles. Car lamps have filaments made of metal – tungsten to be precise. It can get a lot hotter than carbon and therefore not only provides whiter light but also a lot more per watt.
Unfortunately there’s always a tiny amount of tungsten that vaporizes when the filament glows. In classic household bulbs, which will soon be extinct in Europe, and in conventional car lamps, the vaporized metal is deposited on the glass bulb in the form of a layer ranging from silver to shiny black. One day, so much will have vaporized at the hottest point of the light bulb that the filament will melt through. The lamp will then be dead.
In early headlight lamps this point was reached as early as after 50 to 100 hours. Halogen technology improved things dramatically. That’s what it’s called because the lamps are filled with a gas which, for the sake of simplicity, we’ll call halogen gas. To begin with, it prevents the vaporized tungsten from being deposited on the inside of the bulb. The second effect of the pressurized gas filling is even better. It ensures that the tungsten particles return to the filament and once again melt back into it. Physicists call it the halogen cycle, which in practice is a bit more complex than what we’ve described here. And unfortunately the tungsten doesn’t only return to the place from which it set off on its journey through the bulb, but also to other parts of the filament. At some stage even in halogen lamps the hottest point of the filament will become too thin and will melt through.
Nevertheless their lifetime is many times longer than for lamps in which the filaments glow in a vacuum. A most pleasant side-effect is that the metal wire can get a lot hotter in the gas filling without burning out too quickly. So a halogen lamp provides more light and once again it’s quite a bit whiter.
The hotter the light the whiter it is. And now we will learn the reason why even high-quality lamps such as the ones from Osram have different lifetimes. In high-performance lamps such as the Nightbreaker series, the filament is operated at a significantly higher temperature than for instance in standard or even longlife lamps. What’s more, top light sources are designed in such a way that the portion of the filament that glows is as small as possible. The smaller this portion the better the headlight can focus the light and direct it to the right place on the road. Smaller parts that glow, however, also result in more heat and higher evaporation rates, which can’t be entirely offset by the halogen cycle. So one thing is clear: more light means a shorter life. The same applies to more blue, the generation of which also produces extremely high temperatures.
So much for the unavoidable physical reasons for lamps burning out. Especially in cars you then also have to factor in the brutal conditions, vibrations above all. Or sometimes also the fact that car users can be unwittingly reckless. For example, slamming shut the trunk lid or rear door is a first-class stress test for any lighting that is installed in these places and is switched on. That’s when the hot filaments are subjected to extremely high loads. Many a lamp will stop working as a result, especially in the license plate lights. The weaker a lamp the thinner the filament wire needs to be – once again physics has a hand in this. And that will make it less durable.