Presence sensors also detect people who are seated, and are mostly used inside buildings. The most popular areas to use presence sensors include classrooms, large offices, meeting rooms, sports facilities, halls and industrial installations. The different designs make the devices suitable for surface mounting, flush mounting or for mounting in suspended ceilings. Lights come on only where needed, i.e. when the lighting conditions make it necessary and only while there is anybody present. Energy is only consumed when actually needed. The high-frequency presence detectors are sturdy and temperature-resistant, meaning they can be used in extreme temperatures. It actively emits high-frequency waves that register even minute movements and transmit these back to the sensor by capitalising on the Doppler radar effect.