Editing Headlamp (section)

====== European system ======
The traditional European method of achieving low and high beams from a single bulb involves two filaments along the axis of the reflector. The high beam filament is on the focal point, while the low beam filament is approximately 1&nbsp;cm forward of the focal point and 3&nbsp;mm above the axis. Below the low beam filament is a cup-shaped shield (called a "[[Graves shield]]") spanning an [[circle|arc]] of 165°. When the low beam filament is illuminated, this shield casts a shadow on the corresponding lower area of the reflector, blocking downward light rays that would otherwise strike the reflector and be cast above the horizon. The bulb is rotated (or "clocked") within the headlamp to position the Graves shield so as to allow light to strike a 15° wedge of the lower half of the reflector. This is used to create the upsweep or upstep characteristic of [[World Forum for Harmonization of Vehicle Regulations|ECE]] low beam light distributions. The bulb's rotative position within the reflector depends on the type of beam pattern to be produced and the [[traffic directionality]] of the market for which the headlamp is intended.

This system was first used with the tungsten incandescent Bilux/Duplo R2 bulb of 1954, and later with the [[halogen lamp|halogen]] H4 bulb of 1971. In 1992, US regulations were amended to permit the use of H4 bulbs redesignated HB2 and 9003, and with slightly different production tolerances stipulated. These are physically and electrically interchangeable with H4 bulbs.<ref>{{cite web|url= http://candlepowerinc.com/pdfs/H4_9003.pdf |title=H4 vs. 9003/HB2 bulbs }}&nbsp;{{small|(52&nbsp;KB)}}</ref> Similar optical techniques are used, but with different reflector or lens optics to create a US beam pattern rather than a European one.

Each system has its advantages and disadvantages. The American system historically permitted a greater overall amount of light within the low beam, since the entire reflector and lens area is used, but at the same time, the American system has traditionally offered much less control over upward light that causes glare, and for that reason has been largely rejected outside the US. In addition, the American system makes it difficult to create markedly different low and high beam light distributions. The high beam is usually a rough copy of the low beam, shifted slightly upward and leftward. The European system traditionally produced low beams containing less overall light, because only 60% of the reflector's surface area is used to create the low beam. However, low beam focus and glare control are easier to achieve. In addition, the lower 40% of the reflector and lens are reserved for high beam formation, which facilitates the optimization of both low and high beams.