If you stroll into an auto accessory store and begin looking for “a 55-watt lamp”, you will initially run into a problem. Several different headlight lamps produce this output level. But they differ widely from one another. You need to know the exact type of lamp you are looking for. For halogen technology, it all starts with the letter “H.” The reason is pretty clear. The number that follows the letter gives you some idea about the date that the lamp type was introduced to the marketplace. It all began in 1961 with H1. By the 1970s, the industry had reached the number “4.” In the 1990s, things moved from H7 to H11.

The last letter is important, too

So, the range of halogen lamps is fairly clear. A letter and a number. That is all it takes. If you set off to find xenon headlights, you will come across designations that begin with the letter “D.” Why not “X,” for xenon? The “D” stands for “discharge,” that is (gas) release. This is the technology used by xenon lights. A number is placed after the “D.” But D1 is actually the newer lamp type than D2. History comes into play here. The D2 was the first replaceable xenon lamp. Originally, the non-replaceable light source of BMW’s first xenon headlight was designated D1. A short time later, this designation was no longer permitted for use with new models. Its number then became available for use with the replaceable version.

The designation of xenon lamps also includes a second letter that follows the number: a “R” or “S.” This letter identifies the base type of headlight for which the lamp is designed. For instance, the D1S is designed for a headlight with lenses. Experts describe it as a projection system. Its sister, the D1R, delivers light for a lensless headlight with a large reflector, called a reflection system. The “R” stands for reflector as a result. Now – just what does the “S” mean? It stands for shutter. A shutter is used to produce low beam light. The device slides in front of the lamp and creates the low beam in the process.

The S and R lamps differ from each other. As a result, a S lamp cannot be used to replace a R lamp or vice versa. To prevent this from happening, the bases are designed differently.

A trademark as a type designation

In addition to H and D lamps for headlights, the industry uses the designation R2. But the “R” does not stand for reflector by any means. It is a reference to a regulation, namely ECE Regulation No. 2. What is this all about? A very old headlight lamp, that’s what. Only a few insiders actually call it R2. Everybody else who is familiar with this type of lamp uses Osram’s trademark name for it: Bilux. For roughly 25 years now, it has not been permitted to be used in new designs. Long before this time, the more modern H4 had practically replaced the R2 anyway. But it is still available for really old cars. Strictly speaking, there is actually more than one Bilux. Osram also uses the word “Bilux” to designate state-of-the-art halogen lamps with two (“Bi”) coiled filaments like the H4 and H15.