Originally Posted By: DBMaster
Ever since we switched to composite headlights in the U.S. (at least, what, thirty years ago?) it's been a losing battle against glare.
In my opinion, it has as much to do with our allowable regulations as anything else. Better optics have just allowed the engineers to push right up to those regulations.
For example, you aim a typical VOR headlamp so that the cutoff is AT the horizon. The VOR specification basically defines a flat cutoff on the right side of center, and nothing above that cutoff on the left side...it can dip down or be straight across. Typical implementation, here, is a completely flat cutoff straight across. This allows for NO error or situational variance (such as a loaded vehicle). Add in some partially fogged lenses, and you create the situation we have -- a good amount of light at or above the horizon for oncoming drivers.
The VOL specification is one I like a little better. It defines the cutoff on the left side of the lamp, and what's to the right of center can kick up, stray up, or even be straight across. The common implementation, here, is for an angled sweep up on the right or a "stair step" or angled step at the center, to a higher horizontal cutoff on the right side. The key to this, however, is the left side is aimed BELOW the horizon, so there is less glare to oncoming drivers.
Read more about aiming procedures here:
https://www.fhwa.dot.gov/publications/research/safety/humanfac/04148/aiming.cfm
The worst case scenario for glare, in my opinion, is a VOR headlamp that is already less than stellar for glare control, where the owner has installed an HID or LED retrofit. The best case scenario for glare, in my opinion, is a properly-aimed VOL with the proper light source in a clean housing. Any variation of either will put you in the middle somewhere.
Both North American and European regulations allow for both flat and stepped/sweeped cutoffs. But the North American regulations allow for more light near the horizon on the driver's side of the vehicle than do European regulations. The benefit is more visibility of pedestrians on the driver's side of the vehicle, and better down-road light on low beam. The drawback is increased glare to oncoming drivers. This is somewhat reversed (speaking in very general terms, here) with European lamp regulations. Low beam down-road light is generally reduced, as is driver's side visibility, but there is less glare to oncoming drivers.