How do motion sensors work? 21181 back to overview

How do motion sensors work?



How does a motion sensor operate? What is the difference between the various technologies and for which application are they useful?

The lighting experts will give you the answers!



Motion sensors are electronical detectors which react to movements in the near surrounding and then operate as an electronic switch. Basically, we can differentiate between active and passive motion sensors.

Active motion sensors work with electro-magnetic waves or with ultrasound. In lighting technology HF (High Frequency) sensors are used most commonly. Passive sensors like PIR (Pyroelectric Infrared)-sensors act on changes in temperature, i.e. on heat radiation of persons and objects.

Which kind of sensor should be taken for your application?

To give a response to that question, we will explain functions and restrictions of both types in more detail:


Passive motion sensors:

This is how PIR-sensors view their environment

They work with infrared technology, thus the name PIR-/IR– (Pyroelectric InfraRed) sensor. Light beams are focused with a lens which simultaneously functions as grid for the detection area.  This sensor technology reacts to changes of temperature of the surrounding. As soon as a person or an object with a certain temperature appears within the detection area, i.e. the grid, the lighting connected or similar switches on automatically.

Therefore, the ideal scope of these sensors is exterior areas, warehouses or production areas.


However, infrared sensors have some sources of error which limit their application in certain cases.

As the sensor reacts to temperature, floor heating may reduce the detection. The contrast between human heat and heated floors is potentially not high enough. Furthermore, warm air turbulences may release default actions. Conditions of environment should therefore be well analyzed.

A further potential issue is the installation of PIR-sensor technology in garages or carports. Lighting may not switch on when a retracting vehicle does not give off engine (necessary for detection).


Active motion sensors:

This shows how high frequency sensors work : comparable to radar measurements

They release radar-/ high frequency rays being reflected by objects and detected by the sensor. The temporal distance of „transmitting – receiving“ measures whether circumjacent objects are in movement. When such movement is detected, the HF-sensor activates.

It is a major advantage, that the radar-/ high frequency rays are also able to detect movements trough thin glass, plastic layers or wood. Thus, they are the ideal solution for interior areas, toilettes or washing rooms, as they can already detect persons through thin doors.  They are also well applicable for installation in luminaires as they detect movements through the cover if mounted directly behind.


Typically, default actions may be released in exterior areas: the motion sensor may already react to bushes or trees in the surrounding moving by wind. Also, small animals scurrying within the detection area may provoke an undesired reaction of the sensor.


Hence, the choice of right motion sensor depends on the area of application. You will find a resume of all information in our video:


We offer both types of sensors – and would enjoy rendering advisory service to you if you are still uncertain which kind of motion sensor serves best to your purposes.