Daylight harvesting

Daylight harvesting systems use daylight to offset the amount of electric lighting needed to properly light a space, in order to reduce energy consumption. This is accomplished using lighting control systems that are able to dim or switch electric lighting in response to changing daylight availability. The term Daylight Harvesting has become the standard in the fields of lighting, sustainable architecture, and active daylighting industries.

System design and components

Daylight harvesting systems are typically designed to maintain a minimum recommended light level. This light level will vary according to the needs and use of the space; for example, the commonly recommended light level for offices is 500 lux (or around 50 foot-candles) on the desktop.

Photosensors

All daylight harvesting systems use a light level sensor, a photosensor, to detect the prevailing light level, luminance or brightness, in open-loop or closed-loop systems. Photosensors are used to adjust electric lighting based on the available daylight in the space. In an open-loop system, the photosensor detects the amount of available daylight only, and can be positioned on the building's exterior wall or roof, or inside the building facing the window or skylight. In a closed-loop system, the photosensor detects the total photometric amount of light, from both daylight and electric sources in the space. For example, in an office a closed-loop photosensor can be positioned on the ceiling facing the desktops in order to detect the amount of light on the work surface, as placing the sensor on the desktop itself would be impractical.