Photoelectric control

Where lights are operated electronically according to natural light levels, the systems can be either closed or open loop. A closed loop system controls the lighting to top up the daylight to achieve a given minimum acceptable illuminance level. Open systems measure external incident daylight to dim lights but with no feedback of the actual levels of realised illuminance.

Blinds can override the controls of both open and closed systems. Complaints at the lack of finesse of such systems can result in management abandoning photoelectric control altogether. Where sensors in closed systems are near windows, it is not uncommon to find occupants closing the blinds to activate the lights. Furthermore, in many cases there are not enough sensors and lighting zones to take account of localised variations in daylight due to orientation or shading. As a rule of thumb, to avoid the need to make small-scale adjustments, lights should not go off until the illuminance level is about twice the design level.

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