How to Light, Office

How to Light: What is the difference between presence and absence detection in occupancy sensors?

What is the difference between the presence and absence detection of an occupancy sensor?


• Presence detectors will turn on automatically when the a sensor detects someone in an area
• The sensor will then automatically switch off after a set period when no movement is detected. The delay time can either be set on the sensor itself (for standalone sensors). This can be done via an IR transmitter or rotary switch on the sensor, or via  rogramming software if the sensor is protocol dependent, like DALI or KNX. 



Placing occupancy sensors in multiple rooms will reduce energy bills. 


• In this case, the lighting has to be manually switched on by the user, rather than automatically detecting the movement.
• And the lighting will automatically turn off when leaving the area. Of course the lights can also be turned off manually.

In both cases the occupancy sensors use the same detection method: PIR, microwave or ultrasonic.

More advanced occupancy sensors utilize photocells. This will measure the daylight amount in the space and switch off the lights at the set value of the light needed (also referred to as bright out value), or dim the lights if the lights have dimmable control gears.

Using occupancy sensors in an area can provide energy savings in different parts of a building. For example, corridors, storage rooms and toilets can benefit from a potential energy saving of up to 80 percent.

Answered by: Mohammad Darwish – Head of solutions and controls at Fagerhult Middle East