The ability of a photo_lec_eally controlled lighting system to maintain a constant total light level on a task surface by responding to changing daylight levels is affected by the control algorithm used to relate the photosensor signal to the supplied electric light level and by the placement and geometry of the photosensor. We describe the major components of a typical control system, discuss the operation of three different control algorithms, and derive expressions for each algorithm that express the total illuminance at the task as a function of the control photosensor signal. Using a specially-designed scale model, we measured the relationship between the signal generated by variousceiling-mounted control photosensors and workplane illuminancefor two room geometries underreal sky conditions. The measureddata were used to determine the performanceof systems obeying the threecontrol algorithms undervarying daylight conditions. Control systems employing the commonly-used integral reset algorithm supplied less electric light than required, failing to satisfy the controlobjective regardless of the control photosensor used. Systems employing an alternative,closed-loop protxmional control algorithm acheived the control objective undervirtuallyali tested conditions when operatedby a ceiling-mounted photosensor shielded fromdirect window light.°1

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