Numerous questions and comments have indicated to us that a lot of drivers are interested in the service life of halogen lamps in car headlights. We’ve received comments like this: “You recommended I buy an expensive high-performance lamp, but it only lasted six months. Before that, I had had two carefree years with a cheap no-name product.” So okay, let’s tackle this topic in short series of articles.
Tall people will be most familiar with the problem: the short blanket on your bed gives you the choice between cold feet and an exposed neck. But what the heck does that have to do with automotive lighting? Replace the blanket with “physics,” the feet with “lamp life” and the neck with “light performance” and you’ll start getting the picture.
When engineers want to squeeze more light out of a lamp, a lot of the common methods are not available to them, such as higher electrical output in watts or stronger luminous flux in lumens. Both of these are limited by law. One adjustment they can make is the luminous density of the filament. The more concentrated the light (on a smaller area), the more is projected on the right spot on the road. But developers have to be careful, because now the physics “blanket” enters into the picture again. The high luminous density of a thin wire also is associated with a lot of concentrated heat that puts a strain on it. And that – not surprisingly – leads to premature failure. Conversely, a thick tungsten wire stays cooler and lasts longer, but unfortunately produces less light. The basic physical explanation is plain and simple: more light equals a shorter service life and vice versa. Just like with warm feet and a freezing neck.
But physics does have a few tricks up its sleeve. Shivering sleepers often solve their thermal problems quite simply: they use the diagonal length of the blanket. The neck gets a bit more coverage and the feet are still well-wrapped. But the space on the sides gets a little skimpy in the process, meaning it has to be positioned more precisely. In the case of a halogen lamp, greater precision helps to a certain extent to generate more light without compromising on lamp life. Other possibilities along these lines are available through the gas inside a halogen lamp: one is its makeup, the other a higher pressure. Of course there are limits to those, too, especially the pressure. It must not get so high that the blanket slips off, er, I mean the bulb bursts!
Fine-tuning like this can achieve a longer operating time at the same performance level. Osram went this route with its standard lamps, for instance, which today last much longer than 20 or 30 years ago. In contrast, for high performance light, such as that of the Night Breaker Plus lamps, we have to accept a compromise. The same goes for particularly blue lamps. In this case, the blue filter in the glass prevents heat dissipation; the lamps get hotter.
But most readers will be wanting to know at this point: so just how long does a lamp like that last? We will talk about this question in our next post!