What are asymmetrical headlights?

Answer 1

Low beam headlamps, all around the world, produce an asymmetrical light distribution: instead of shooting out straight ahead (like a high beam), the highest-intensity light in the low beam light pattern is concentrated slightly downward and slightly toward the direction of travel. In a country where traffic flows on the right-hand side of the road, the low beam light is concentrated slightly downward and slightly right. In a country where traffic flows on the left-hand side of the road, the low beam is focused slightly downward and slightly left. That's what makes a low beam different from a high beam; the main concentration of light is directed away from oncoming drivers' eyes to allow cars to meet and pass each other in two-way traffic without drivers being exposed to dangerously high levels of glare.

There are two different kinds of asymmetrical low beam. The type developed in Europe has sharp cutoff (boundary with light below and dark above) at the top of the low beam light pattern. The cutoff is horizontal (flat) from the center of the low beam pattern outward toward the side of the low beam that faces towards oncoming traffic. The cutoff sweeps upward (or steps upward like a stairstep) from the center of the low beam pattern outward toward the direction of travel. For example, a low beam of this type designed for use in right-hand traffic has a cutoff that is flat on the left and sweeps or steps upward-rightward. This type of asymmetrical low beam has been in use in Europe since the mid-1950s, in the United Kingdom, Australia, Canada, and most of the world in general since the 1960s-'70s, in Japan since the early 1990s, and in the United States since the late 1990s.

Another type of asymmetric low beam, developed in the United States in the late 1930s, has a beam pattern that is more or less symmetrical but is shifted slightly downward-rightward relative to the straight-ahead. There may or may not be a straight-across flat cutoff at the top of such a low beam pattern. This kind of asymmetric low beam is still in use in the United States and Canada. In the past it was common in Japan (with downward-leftward shift because Japan drives on the left) as well as in the United Kingdom, Australia, and other parts of the British Commonwealth, but is generally no longer used outside North America.

Four important things to understand about the matter:

1. Most all low beams are asymmetrical. Just because the cutoff at the top of some US-specification low beams is flat (straight across) without an upstep or upsweep, does not mean the low beam is safe to use in both left- and right-hand traffic. All US-specification headlamps are suitable only for use in traffic flowing on the right-hand side of the road.

2. The asymmetrical light distribution is produced by the optics of the lamp. It is independent of the aim adjustment, and no aim setting or other adjustment can turn a left-traffic low beam into a right-traffic low beam or vice versa.

3. Both low beams on a vehicle have substantially the same asymmetrical light distribution. There is not a light distribution specified for the left headlamp different from the specification for the right headlamp.

4. Both low beams on a vehicle must be aimed to the same height. It is incorrect to set the right lamp higher than the left lamp or vice versa. Similarly, if there is a horizontal aim adjustment (most US-spec vehicles don't have it), each headlamp must be aimed only with respect to its own centerline; it is incorrect to try to cross-eye the headlamps so the upsweeps or upsteps in the left and right cutoffs coincide with each other.