This paper presents a mathematical model that predicts the effect of lighting conditions and visual characteristics of a target on the visual search time. It also outlines some implications for vehicle lighting design. Human vision differs significantly between photopic (daylight), mesopic (twilight) and scotopic (night) conditions. The project aimed to measure visual search performance for a range of light adaptation levels from low daylight through twilight to high night-time ranges. The experimental technique had to be applicable to a wide range of activities. A computer-controlled display presented targets to an observer, who had to identify a target in a field of distractors. The experiments were repeated for six levels of screen luminance for 15 subjects. The mathematical model gives a formula for the search time in terms of the photopic and scotopic contrast, the target luminance, and the background luminance of the computer display. An accurate model of the human visual system in mesopic conditions is outlined. The model can be applied to vehicle and traffic lighting design in several ways. Dashboard lighting can be designed to minimise head-down time. The design of lighting clusters, for example between red brake lights and fog lights, can be modified. The design of street lighting and signing can be improved.
Abstract