4. Illuminance level for visual tasks without intrinsic luminance

As we have just seen, in the case of visual tasks without intrinsic luminance, adaptation luminance depends essentially on the illuminance for a given visual task. Insofar as it is possible to define one or more standard visual tasks for each particular type of activity, it is also possible, after evaluating these tasks, to recommend an illuminance level that will lead to a suitable adaptation luminance and visual performance. These basic values can be weighted to take account of deviations from the reference situation (particular constraints, duration, age of workers, etc.).

Basic values are generally deduced from laboratory experiments. The first values used in British and French recommendations were derived from measurements by H.C. Weston (Sight, light and efficiency, London, Lews, 1949). The type of test chosen consisted in crossing out, from a sheet of Landolt...