The ability of a photo-electrically controlled lighting system to maintain a constant total lightslevel on a task surface by responding to changing daylight levels is affected by the controlsalgorithm used to relate the photosensor signal to the supplied electric light level and by thesplacement and geometry of the photosensor. We describe the major components of astypical control system, discuss the operation of three different control algorithms, andsderive expressions for each algorithm that express the total illuminance at the task as asfunction of the control photosensor signal. Using a specially designed scale model, wesmeasured the relationship between the signal generated by various ceiling-mounted controlsphotosensors and workplane illuminance for two room geometries under real skysconditions. The measured data were used to determine the performance of systemssobeying the three control algorithms under varying daylight conditions. Control systemssemploying the commonly-used integral reset algorithm supplied less electric light thansrequired, failing to satisfy the control objective regardless of the control photosensor used.sSystems employing an alternative, closed-loop proportional control algorithm achieved thescontrol objective under virtually all tested conditions when operated by a ceiling-mountedsphotosensor shielded from direct window light.