Sumpner's principle: A discussion

Sumpner's principle interrelates mean room surface illuminance and its direct and indirect components, and offers a flexible approach to estimating illuminances. Designers may identify parameters of interest and generate expressions for their calculation, and this is illustrated by examining illuminance as it affects room surface appearance and object appearance. Procedures are given for estimating average illuminances of selected object surfaces and room planes, including that of the horizontal working plane.