Sometimes the fault lies with the generator and voltage regulator. There is some truth in this: the malfunctions of these devices are noticeable by the "movement" of the light flux, as if subject to the number of engine revolutions. It is clear that poor operation of the generator and voltage regulator affects not only the efficiency of the headlights - first of all, the battery suffers, either not receiving a full charge, or, on the contrary, receiving it in excess, which causes the electrolyte to "boil". In this case, the headlights play only the role of an indicator, although excess voltage greatly reduces the service life of the lamps. Constant dim radiation occurs for other reasons: either the reflector is heavily contaminated or corroded, or a dark coating appears on the inner surface of the lamp bulb.
If the reflector's durability depends on many factors, then even a perfectly serviceable lamp "accumulates" a dark coating in some 30-50 thousand kilometers. Its efficiency in this case is 20 percent lower than a new one.
The reflector "lives" longer. In modern cars, the reflector and the headlight glass are, so to speak, an indivisible whole, which first of all ensures the most optimal light beam, and secondly - tightness. It was the lack of tightness that contributed to the penetration of dust into the headlight, which then burned on the reflector and reduced its light output. The same thing happened with moisture, actively condensing on the reflector, which caused corrosion.
About five years ago, they tried to combat corrosion in some incredible way, pouring red brake fluid into the headlight. The fashion for this idea, which was more harmful than useful, has passed, but another extreme has appeared - plexiglass "glasses" on the glass. In addition to the fact that such protection reduces light output, "smearing" the direction of the rays, it causes strong heating and burning of the reflector. What is the use of stone-protected glass in this case, if the reflector suffers - it is not clear, especially since, as has already been said, the glass and the reflector are now an indivisible whole? By the way, this is why there is no point in buying a "lens" separately: you will never get good light from such a headlight.
Finally, regarding adjustments. Even the best headlights, equipped with halogen lamps, may be useless on the road if they are adjusted poorly. Not everyone, and not always, has the opportunity to make adjustments on a stand. Therefore, old, well-tried methods of "home" adjustment give quite good results. The car is placed on a level area, 30 meters are measured and a brick is placed on the mark, for example. One of the headlights, turned on low beam, is covered with something, and in the second, the adjustment screws are turned so that the illumination boundary is vertically brought to the placed brick. The same is done with the other headlight. High beam is adjusted horizontally, but without a brick: here it is important to ensure that the beams from both headlights do not bifurcate and do not converge in the middle.
[Material taken from a web resource Mercedesman.ru]