8 ref. tables.This study evaluated the relative effects of horizontal and vertical misaim of low-beam headlamps. The approach involved analyzing light-output matrices of 150 production low beams, manufactured for sale in the U.S, Europe, and Japan. The specific analysis involved computing, for 225 locations in the central part of each beam pattern, the ratios of nominal intensity to intensity for horizontal and vertical misaim of up to 1.5°. The resultant intensity ratios were then interpreted in terms of their likely effects on visual performance and discomfort glare. Simultaneous horizontal and vertical misaim were not considered. The results indicate that horizontal misaim of the U.S., European, and Japanese lamps of up to 1.5° in either direction has no practical significance. Vertical misaim of American and Japanese lamps is of practical significance when misaim reaches 1.5°, and is of likely significance at misaim of 1°. European lamps are more sensitive to vertical misaim, with vertical misaim of 1° of practical significance, and misaim of 0.5° of likely significance. The relative difference in the importance of horizontal and vertical misaim is greatest for European lamps.Michigan University, Ann Arbor, Industry Affiliation Program for Human Factors in Transportation Safetyhttp://deepblue.lib.umich.edu/bitstream/2027.42/64073/1/84475.pdf