Automobile headlamp alignment by visual aim: Photometric correlates of the visual cutoff and aim variability

Abstract

An automobile headlamp projects a beam pattern that exhibits a relatively abrupt drop off in intensity near the lamp's horizontal axis. This apparent edge is used as a reference line in visual alignment of the headlamp. This laboratory experiment was conducted to identify the photometric parameter that best represents the location of this visual edge; the relationship between edge sharpness and variability of beam pattern alignment was also investigated. Subjects in the experiment vertically aligned each of 14 illumination patterns so that the most salient visual edge of the pattern overlaid a horizontal line on the projection screen. On average, the vertical position chosen as the most prominent edge was closest to the point of largest intensity contrast. The variability in aiming performance was closely related to the peak intensity contrast value of the beam pattern.

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