Visual recognition functions, such as acuity and contrast sensitivity, deteriorate rapidly over the declining luminances found during civil twilight. Thus, civil twilight, a critical part of the transition between daylight and darkness, represents lighting conditions that may be useful to describe artificial illumination. Automotive headlamps project a 3-D beam that ranges from illumination levels comparable to daylight at the vehicle to the dark limit of civil twilight (3.3 lx) at some distance ahead. This twilight envelope is characterized as a distance beyond which foveal visual functions are severely impaired, and thus it provides a general, functional description of the useful extent of the headlamp beam. This user-centered approach to describing illumination is useful for characterizing visibility when driving at night or in other artificially lit environments. This paper discusses the twilight envelope approach and its application to intervehicle variations in headlamp systems.
Abstract