Modern headlights are electrically operated, positioned in pairs, one or two on each side of the front of a vehicle. A headlamp system is required to produce a low and a high beam, which may be achieved either by an individual lamp for each function or by a single multifunction lamp.
High beams cast most of their light straight ahead, maximizing seeing distance, but producing too much glare for safe use when other vehicles are present on the road. Because there is no special control of upward light, high beams also cause back dazzle from fog, rain and snow due to the retro reflection of the water droplets.
Low beams have stricter control of upward light, and direct most of their light downward and either rightward (in right-traffic countries) or leftward (in left-traffic countries), to provide safe forward visibility without excessive glare or back dazzle.
Low beam headlights provide a distribution of light designed to provide adequate forward and lateral illumination with limits on light directed towards the eyes of other road users, to control glare. This beam is intended for use whenever other vehicles are present ahead. The international ECE Regulations for filament headlamps and for high-intensity discharge headlamps specify a beam with a sharp, asymmetric cutoff preventing significant amounts of light from being cast into the eyes of drivers of preceding or oncoming cars. Control of glare is less strict in the North American SAE beam standard contained in FMVSS/CMVSS 108.
High beam headlights provide a bright, centre-weighted distribution of light with no particular control of light directed towards other road users’ eyes. As such, they are only suitable for use when alone on the road, as the glare they produce will dazzle other drivers. International ECE Regulations permit higher-intensity high-beam headlamps than are allowed under North American regulations.
Dual-beam reflector headlights
Night driving is difficult and dangerous due to the blinding 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.
Automatic beam switching
Even when the high beam is warranted by prevailing conditions, drivers generally do not use them. There have long been efforts, particularly in America, to devise an effective automatic beam selection system to relieve the driver of the need to select and activate the correct beam as traffic, weather, and road conditions change. General Motors introduced the first automatic headlight dimmer called the Autronic Eye in 1952 on their Cadillac and Oldsmobile models; the feature was offered in other GM vehicles starting in 1953. The system’s photo resistor and associated circuitry were housed in a gunsight-like tube atop the dashboard. An amplifier module was located in the engine compartment that controlled the headlight relay using signals from the dashboard-mounted tube unit. This setup gave way in 1958 to a system called GuideMatic in reference to GM’s Guide lighting division. The GuideMatic had a more compact dashtop housing and a control knob that allowed the driver to adjust the system’s sensitivity threshold to determine when the headlamps would be dipped from high to low beam in response to an oncoming vehicle. By the early 1970s, this option was withdrawn from all GM models except Cadillac, on which GuideMatic was available through 1988. The photo-sensor for this system used an amber lens, and the adoption of reflectoriszed yellow road signs, such as for oncoming curves, caused them to dim prematurely – possibly leading to their discontinuation
Ford- and Chrysler-built vehicles were also available with the GM-made dimmers from the 1950s through the 1980s. A system called AutoDim was offered on several Lincoln models starting in the mid-1950s, and eventually the Ford Thunderbird and some Mercury models offered it as well. Premium Chrysler and Imperial models offered a system called Automatic Beam Control throughout the 1960s and early 1970s
Daytime running lamp
Some countries require automobiles to be equipped with automatic daytime running lamps (DRL), which are intended to increase the conspicuity of vehicles in motion during the daytime. DRL may consist of the manual or automatic illumination of the low beams at full or reduced intensity, or the high beams at reduced intensity, or may not involve the headlamps at all. Countries requiring DRL include Albania, Argentina, Bosnia and Herzegovina, Bulgaria, Canada, Colombia, Croatia, Czech Republic, Denmark, Estonia, Finland, Hungary, Iceland, Israel, Kosovo, Latvia, Lithuania, Macedonia, Norway, Poland, Republic of Moldova, Romania, Russia, Serbia, Slovak Republic, Slovenia, Uruguay, and Sweden.
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