The Meeting Beams of Headlights;Effects of Deterioration and Misaim

Abstract

The most important factor in the design of the typical meeting beam of headlights, so far as the range of direct seeing is concerned, is the sharpness and form of the cut-off near the horizontal. But the effect which the cut-off will have on the likelihood of being dazzled, i.e., of being rendered incapable of seeing more than a short distance, when meeting other vehicles at night depends enormously on the accuracy with which meeting beams are aimed. The effect can be calculated when the standard of aiming is known; the basis of the calculation and some results are given in this paper. Curves are provided from which may be found the sharpness of cut-off required to give any desired level of freedom from dazzle. It is shown that if the standard of aiming is too low it will be impossible to design a beam to fulfil the required conditions. The necessary improvement in aiming can, however, be determined from the curves. The effect of deterioration in increasing the liability to dazzle is also considered. The pitching motion of the vehicle, and its effect on seeing distance and on intermittent glare, have had to be omitted from this analysis; the effect will be more important the sharper the cut-off employed.