Editing Headlamp (section)

===== Dual-beam reflector headlamps =====
Night driving is difficult and dangerous due to the blinding [[glare (vision)|glare]] of headlights from oncoming traffic. Headlamps that satisfactorily illuminate the road ahead without causing glare have long been sought. The first solutions involved resistance-type dimming circuits, which decreased the intensity of the headlamps. This yielded to tilting reflectors, and later to dual-filament bulbs with a high and a low beam.

In a two-filament headlamp, there can only be one filament exactly at the focal point of the reflector. There are two primary means of producing two different beams from a two-filament bulb in a single reflector.

====== American system ======
One filament is located at the focal point of the reflector. The other filament is shifted axially and radially away from the focal point. In most 2-filament sealed beams and in 2-filament replaceable bulbs of type [[Automotive lamp types|9004, 9007, and H13]], the high-beam filament is at the focal point and the low-beam filament is off focus. For use in right-traffic countries, the low-beam filament is positioned slightly upward, forward, and leftward of the focal point, so that when it is energized, the beam is widened and shifted slightly downward and rightward of the headlamp axis. Transverse-filament bulbs such as the 9004 can only be used with the filaments horizontal, but axial-filament bulbs can be rotated or "clocked" by the headlamp designer to optimize the beam pattern or to affect the traffic-handedness of the low beam. The latter is accomplished by clocking the low-beam filament in an upward-forward-leftward position to produce a right-traffic low beam, or in an upward-forward-rightward position to produce a left-traffic low beam.

The opposite tactic has also been employed in certain two-filament sealed beams. Placing the low beam filament at the focal point to maximize light collection by the reflector, and positioning the high beam filament slightly rearward-rightward-downward of the focal point. The relative directional shift between the two beams is the same with either technique – in a right-traffic country, the low beam is slightly downward-rightward and the high beam is slightly upward-leftward, relative to one another – but the lens optics must be matched to the filament placements selected.

====== 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.

====== Developments in the 1990s and 2000s ======
Complex-reflector technology in combination with new bulb designs such as H13 is enabling the creation of European-type low and high beam patterns without the use of a Graves Shield, while the 1992 US approval of the H4 bulb has made traditionally European 60% / 40% optical area divisions for low and high beam common in the US. Therefore, the difference in active optical area and overall beam light content no longer necessarily exists between US and ECE beams. Dual-beam HID headlamps employing reflector technology have been made using adaptations of both techniques.