A headlight has to light the road and spare everyone else's eyes. The rulebooks, the beam shapes and a century of dipping tricks explain why that is so hard.
A low beam avoids dazzling oncoming drivers with a sharp, asymmetric cut-off, flat and low on the oncoming side. UN rules produce less glare than the US standard, and high beams have no glare control. LED headlights arrived in the 2000s and use less power and last longer than incandescent sources.
Nearly half of all traffic-related deaths happen in the dark, according to the US National Highway Traffic Safety Administration, even though only 25% of traffic travels after nightfall. That gap has driven headlamp development for over a century, and it sets a brutal brief: light the road far enough ahead to be useful, and do it without blinding the people coming the other way.
What does a low beam have to do?
The low beam, also called the dipped, passing or meeting beam, must give enough light forward and to the sides without dazzling other road users. The rules call for it whenever other vehicles are in front of you.
The trick is geometry. United Nations ECE rules shape a dipped beam with a crisp, lopsided cut-off. On the side facing oncoming drivers the beam is held flat and low; on the side facing the outer edge of the road it rises towards the near side. Glare for oncoming drivers is largely prevented, and the driver can still see pedestrians, road signs and hazards on their own side.
The high beam, also called main, driving or full beam, is a different animal. It throws an intense, centre-weighted spread of light and makes no attempt to control glare, so it suits only a driver who is alone on the road. Anyone else in its path is dazzled.
The low beam is a piece of geometry: flat and low towards oncoming eyes, sloping up towards the verge.
Why do US and European rules differ?
The United States and Canada do not use UN ECE regulations. They have their own FMVSS and CMVSS standards. These also demand a sharp, asymmetric cut-off with a flat, low oncoming half, though it sits higher than the UN rules require, and the outer half is flat too but rides higher than the half facing oncoming traffic. The result is substantially more glare for oncoming drivers and poorer lighting of the near side of the road than headlights built to the UN rules provide.
| Feature | UN ECE rules | US and Canadian rules |
|---|---|---|
| Dipped beam on the oncoming side | Flat and low | Flat and low, though set higher |
| Glare to oncoming drivers | Substantially prevented | Substantially increased by comparison |
| High-beam intensity | Higher intensity permitted | Lower limit |
| Daytime running lights | White only, 400 to 1200 candela | White or amber, up to 7,000 candela |
| Cornering lights | Traditionally prohibited; now allowed below 40 km/h (about 25 mph) | Provisions exist for front and rear cornering lights |
The gap was wider in the past. High-beam peak intensity was capped at 140,000 candela per side in Europe, while the US limit was 37,500 candela per side until 1978, when it was raised to 75,000. Taking advantage of the higher allowance needed a move to halogen technology, so sealed-beam lamps with internal halogen bulbs reached American cars for 1979 models.
Why do headlights seem too bright?
Several threads from the rulebooks and the technology meet here. A low beam that is not as low on the oncoming side, as in North America, gives oncoming drivers more glare. A high beam has no glare control at all. And daytime running lights, which exist to make a car visible rather than to light the road, are allowed to reach 7,000 candela in the US against a ceiling of 1200 candela under UN Regulation 87.
Auxiliary lights add to the mix. Driving lights are extra high beams that let a driver see further than the main high beams do, and they are most common on rally cars and in places with large stretches of unlit road or few winter daylight hours, such as the Nordic countries. Rules on their use vary widely: Russia caps a vehicle at three pairs, and Paraguay requires them to be switched off and covered in towns.
Front fog lights are another source of confusion. They give a wide, bar-shaped beam with a sharp cut-off at the top, mounted low, for use at low speed in fog, dust or snow. In most countries there is no legal requirement for them, and their main purpose is frequently cosmetic. An SAE study found that in the United States more people use them wrongly in dry weather than properly in poor weather, and in Australia the law bars their use unless the driver is in fog, mist or other conditions that restrict visibility.
How have headlights tried to adapt?
Dipping a headlamp was once a chore. The Guide Lamp Company introduced dipping headlamps in 1915, but it took the 1917 Cadillac system to let a driver dip the light with a lever inside the car instead of stopping and getting out. Next came the Bilux bulb of 1924, the first modern unit, which produced both low and high beams from a single bulb, and a foot-operated dip switch appeared in 1927 and stayed standard for much of the century. The final vehicles to carry one were Ford F-Series and E-Series vans in 1991.
Some designs tried to do more than two beams. Packards of 1933 and 1934 had three filaments in each bulb, giving three beams with the names country passing, country driving and city driving. Nash fitted a three-beam arrangement of its own in 1934, with an intermediate setting that put the low beam on the driver's side and the high beam on the passenger's side, widening the view of the roadside while limiting glare for oncoming traffic.
Then came automation. Cadillac's 1952 Autronic Eye system automated the choice between high and low beams. Steering-linked lighting arrived on the 1947 Tucker Torpedo, whose centre-mounted headlight turned with the steering wheel and was later popularised by the Citroen DS, so the light could follow the direction of travel. A rare, one-year-only 1935 Tatra had a switch and an electromagnetically shifted reflector that lit only the curbside.
Cornering lights carry on the idea. On some models they cast steady white light to the side the car is about to turn or change lanes towards, generally working with the turn signals. Some modern cars switch one on when the steering reaches a set angle, even without a signal.
From acetylene to LED
The first headlamps burned acetylene gas or oil from the late 1880s, and acetylene was popular in the 1900s because it made a bright white flame that wind and rain could not put out. The first electric headlamps appeared in 1898 on the Columbia Electric Car. Filaments did not last long in the harsh environment of a car and small dynamos were hard to make, so electric lamps spread slowly, but Peerless made them standard in 1908.
In the United States, a standardised 7-inch round sealed beam was required from 1940, which virtually froze lighting technology until the 1970s. Elsewhere it moved on. A European consortium introduced the first halogen lamp for headlamp use, the H1, in 1962. High-intensity discharge systems arrived in the early 1990s, debuting on the BMW 7 Series. The first LED tail lights went onto mass-production cars in 1993, and LED headlights followed in the 2000s, once more powerful LEDs could be made.
The American sealed beam had its logic. By sealing reflector and bulb together, it ended the problem of tarnished reflectors, made aiming the beams simpler and removed non-standard bulbs. Federal Motor Vehicle Safety Standard 108 of 1968 obliged every vehicle to use a twin or quad round sealed beam system, and a 1974 amendment let rectangular units in. Only in 1983 did the US allow replaceable-bulb headlamps of non-standard shape with polycarbonate lenses, which produced the 1984 Lincoln Mark VII, the first American-market car with replaceable bulbs since 1939.
How do LED headlights compare with halogen?
An LED is a semiconductor that emits light when current passes through it. Electrons and electron holes recombine inside it and release energy as photons, and the colour depends on the energy needed to cross the semiconductor's band gap. Against incandescent sources, LEDs offer lower power consumption, less waste heat, a longer life, better physical toughness, smaller size and faster switching.
There are drawbacks. LEDs are limited to low voltage and generally to DC power, they cannot give steady illumination from a pulsing supply, and their maximum operating and storage temperatures are lower. White light normally comes from a blue LED coated with a yellow phosphor, which is why a white LED looks yellow when it is off. The first white LEDs went on sale in autumn 1996, and the blue LEDs that made them possible, developed in the early 1990s by Shuji Nakamura, Hiroshi Amano and Isamu Akasaki, won the 2014 Nobel Prize in Physics.
Small size and fast switching are the qualities that suit a lamp meant to be controlled finely, which helps explain why LED headlights arrived in the same period as more automatic lighting. The colour of the light, meanwhile, is fixed by convention: forward-facing lights must be white or selective yellow, a convention set down in 1949 by the Geneva Convention on Road Traffic and spelled out further by the 1968 Vienna Convention.
What are daytime running lights?
Many countries permit or require them. Under UN Regulation 48, they are compulsory on passenger cars and small delivery vans that gained type approval on or after 7 February 2011, and on trucks and buses approved since August 2012. The rules call for dedicated lights, so using the headlights, front indicators or fog lights for the job is not allowed. Several countries, among them Canada, Sweden, Norway and Denmark, require hardwired automatic systems.
What this means for drivers
The practical lessons follow from the rules. Use the dipped beam whenever another vehicle is ahead, and keep the high beam for when you are alone on the road. Treat fog lights as a tool for fog, dust and snow rather than a styling feature, and check the local rules on driving lights before fitting any. A car built for North America carries a dipped beam designed to a different standard from one built to UN rules, which is part of why the same road can feel different depending on what is coming towards you.
Where this goes next
The direction of travel shows in what makers are dropping and adding. As early as the 2020s, a number of car makers began leaving front fog lights off many of their latest models, judging them redundant now that daytime running lamps and LEDs linked to automatic high-beam systems exist. Some makers that still fit them have given them a second job, lighting the surroundings and kerbs after the car is parked. Hidden headlamps, popular on sports cars in the 1960s and 1970s, have not appeared on a modern volume-produced model since 2004, because they struggle to meet pedestrian-protection provisions.
