Inverse Square Law Approximation of Illuminance

The paper examines the accuracy of inverse square law approximations of task illuminance as a function of the following parameters: (1) Size of luminaire element, (2) distance of luminaire element from task, and (3) location of luminaire element with respect to task. Graphs are presented which show percent error and percent weighted error in task illuminance to be expected from a given size luminaire element at a given location. From these graphs, a chart of suggested element sizes is composed, which can be used in formulating approximate calculation procedures for determining illuminance.