Those of you who have replaced failed car lamps in the past probably remember strange stains on the glass. For one, there’s a black, often opaque layer inside the glass bulb. It mainly occurs in smaller signal lamps, so basically the ones for rear lights, license plate lights and indicators. The deposit also has a metallic shimmer depending on the angle you’re looking at it. And that explains how it developed.
We’re talking about tungsten, a metal the filament in the lamp is made of. During the course of its hopefully long life, some of it vaporizes from the surface of the spiral wire which is only a hundredth of a millimeter thick; after all, it reaches a temperature of well over 2000 degrees. And because the “vapor” can’t escape from the lamp it is deposited on the inside of the glass, where it produces a black layer.
The vaporized tungsten is then missing from the filament, which will get thinner and thinner with time and will eventually burn out.
There’s no black discoloration in halogen lamps. Inside these lamps the effect dubbed as the halogen cycle by engineers ensures that vaporized tungsten is redeposited on the filament. That’s no guarantee for an everlasting life though. The deposit is not uniform, and even in halogen lamps parts of the filament will get thinner and eventually burn out.
Halogen lamps take on a different color when they fail though – white or beige. In brand lamps it’s very rare, and in Osram lamps even rarer because it’s an indication of low quality. The glass bulb takes on a creamy color if air gets inside and is mixed with the filler gas. That can happen if the bulb is not completely tight, mostly at the penetrations of the filament. The filler gas which is under pressure escapes and its residue reacts with the air. At the same time, the oxygen from the air is virtually burning the filament. The coloring is not a deposit as in the black lamps, but the normally invisible gas has changed color.
These failures known as air leakages normally occur pretty soon after using the lamp, often even when switching it on for the first time. Lamps from Osram are virtually immune to this because they are switched on several times during production. No-name manufacturers won’t normally go to such lengths.