Occupancy sensors use different technologies — including passive infrared ‎(‎PIR‎)‎‎,‎ ultrasonic and dual‎-technology — to detect the presence or absence of occupants in a space‎.‎ Some sensors also use acoustic detection‎.‎

PIR sensors detect occupants’ presence by sensing the difference between heat emitted by moving people and background heat‎.‎ Ultrasonic sensors detect the presence of people by sending out ultrasonic sound waves into a space and measuring the speed at which they return‎.‎ They look for frequency changes caused by a moving person‎.‎

PIR sensors require a direct line of sight between the sensor and occupants in a space‎.‎ Because of this requirement‎,‎ managers can strictly define the sensor’s coverage‎.‎ Ultrasonic sensors cover the entire space and do not need a line of sight‎.‎ As a result‎,‎ they can detect people behind obstacles‎.‎ They also are more sensitive to minor motions‎,‎ such as hand movements‎.‎

PIR sensors are very suitable for enclosed spaces‎,‎ wall‎-switch replacements‎,‎ high‎-ceiling areas‎,‎ spaces with high air flow‎,‎ areas with direct line‎-of‎-sight viewing‎,‎ and spaces in which it is necessary to mask unwanted detection in certain areas‎.‎ Examples of these spaces include private offices‎,‎ lobbies‎,‎ warehouse aisles‎,‎ hallways‎,‎ computer rooms‎,‎ laboratories‎,‎ library book stacks‎,‎ conference rooms‎,‎ storage closets and outdoor spaces‎.‎

Issues that might complicate their application include low levels of motion by occupants‎,‎ obstacles blocking the sensor’s view‎,‎ and sensors mounted on sources of vibration or within 6‎-8 feet of air diffusers‎.‎

Ultrasonic sensors‎,‎ meanwhile‎,‎ are highly suitable for spaces in which a line of sight is not possible‎,‎ such as partitioned spaces‎,‎ and in spaces requiring a higher level of sensitivity‎.‎ Examples of such spaces include restrooms‎,‎ open offices‎,‎ enclosed hallways and stairways‎.‎

Issues that might complicate their application include‎:‎ ceilings higher than 14 feet‎;‎ high levels of vibration or air flow‎,‎ which can cause nuisance switching‎;‎ and open spaces that require selective coverage‎,‎ such as control of individual warehouse aisles‎.‎

Dual‎-technology sensors use both PIR and ultrasonic technologies‎,‎ activating the lights only when both technologies detect the presence of occupants‎.‎ This setup virtually eliminates the possibility of false‎-on problems‎,‎ and requiring either technology to hold the lights on significantly reduces the possibility of false‎-off problems‎.‎

Appropriate applications include classrooms‎,‎ conference rooms and spaces where a higher degree of detection is desirable‎.‎