The ability of daylight-following lighting systems to provide a minimum specified lightslevel at the task surface is influenced by 1) the control algorithm used, 2) the spatialsresponse of the ceiling-mounted control photosensor and 3) the location of the photosensorsrelative to task and window. Best performance was obtained with a closed-loopsproportional control system controlled by a photosensor, with a large field of view butsshielded from direct light from the window. A minimum specified illuminance level couldsbe maintained at specific points on the task surface regardless of daylight condition or roomsgeometry provided that the system gain was properly calibrated to account for the localsluminous environment.

Open-loop proportional control also performed adequately but offered less precise controlsthan closed-loop systems due to the necessity of using a photosensor that was not shieldedsfrom direct window light. Integral-reset systems that were tested performed poorly, butsperformance could be improved slightly by completely shielding the photocell from directswindow light.