Abstract

Civil twilight, when the sun is six degrees below the horizon, has been described as a period when all normal daylight activities can be carried out without supplementary illumination. While this was an appropriate description during the previous centuries when this definition was formulated, the nature of visual demands accompanying the introduction of modern technology in this century suggests that this criterion should be reexamined. Figure 1 presents the relation between visual acuity and luminance (1). This function is typical of a number of visual functions including resolution, vernier, single line and stereoscopic acuities, oculomotor responsiveness, contrast sensitivity and reaction time. It should be noted that performance begins to decline at about 10 millilamberts and that at .01 millilambert, foveal vision is no longer possible.

© 1990 Optical Society of America