Low beam headlamps, all around the world, produce anasymmetrical light distribution: instead of shooting out straightahead (like a high beam), the highest-intensity light in t
…he lowbeam light pattern is concentrated slightly downward and slightlytoward the direction of travel. In a country where traffic flows onthe right-hand side of the road, the low beam light is concentratedslightly downward and slightly right. In a country where trafficflows on the left-hand side of the road, the low beam is focusedslightly downward and slightly left. That's what makes a low beamdifferent from a high beam; the main concentration of light isdirected away from oncoming drivers' eyes to allow cars to meet andpass each other in two-way traffic without drivers being exposed todangerously high levels of glare. There are two different kinds of asymmetrical low beam. The typedeveloped in Europe has sharp cutoff (boundary with light below anddark above) at the top of the low beam light pattern. The cutoff ishorizontal (flat) from the center of the low beam pattern outwardtoward the side of the low beam that faces towards oncomingtraffic. The cutoff sweeps upward (or steps upward like astairstep) from the center of the low beam pattern outward towardthe direction of travel. For example, a low beam of this typedesigned for use in right-hand traffic has a cutoff that is flat onthe left and sweeps or steps upward-rightward. This type ofasymmetrical low beam has been in use in Europe since themid-1950s, in the United Kingdom, Australia, Canada, and most ofthe world in general since the 1960s-'70s, in Japan since the early1990s, and in the United States since the late 1990s. Another type of asymmetric low beam, developed in the United Statesin the late 1930s, has a beam pattern that is more or lesssymmetrical but is shifted slightly downward-rightward relative tothe straight-ahead. There may or may not be a straight-across flatcutoff at the top of such a low beam pattern. This kind ofasymmetric low beam is still in use in the United States andCanada. In the past it was common in Japan (with downward-leftwardshift because Japan drives on the left) as well as in the UnitedKingdom, 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 atthe top of some US-specification low beams is flat (straightacross) without an upstep or upsweep, does not mean the low beam issafe to use in both left- and right-hand traffic. AllUS-specification headlamps are suitable only for use in trafficflowing on the right-hand side of the road. 2. The asymmetrical light distribution is produced by the optics ofthe lamp. It is independent of the aim adjustment, and no aimsetting or other adjustment can turn a left-traffic low beam into aright-traffic low beam or vice versa. 3. Both low beams on a vehicle have substantially the sameasymmetrical light distribution. There is not a light distributionspecified for the left headlamp different from the specificationfor the right headlamp. 4. Both low beams on a vehicle must be aimed to the same height. Itis incorrect to set the right lamp higher than the left lamp orvice versa. Similarly, if there is a horizontal aim adjustment(most US-spec vehicles don't have it), each headlamp must be aimedonly with respect to its own centerline; it is incorrect to try tocross-eye the headlamps so the upsweeps or upsteps in the left andright cutoffs coincide with each other. ( Full Answer )