The literature on headlamp aiming is surveyed in detail to pinpoint the various sources and magnitudes of aim variance. Four major sources of variance are identified (differences between beam and mounting plane, photometric changes in use, long axis alignment, and human factors), along with a number of others of lesser consequence. Illustrations are offered showing the expected population variance under a variety of conditions. It is apparent that, at the present state-of-the-art, a substantial percentage of the lamp population can be expected to be beyond the limits recommended in SAE J599c. It is further apparent that this would be true regardless of whether or not a vehicle inspection program is in operation.
Recommendations are given regarding research emphasis in headlighting. Ways of reducing variance from the most significant sources are considered and recommendations offered.