An occupancy sensor is a lighting control device that detects occupancy of an area by people and turns the lights on or off automatically, using infrared, ultrasonic or microwave technology. An occupancy sensor may have a photoelectric cell that could be used to inhibit light from turning on when there is sufficient natural light in the area. Occupancy sensors are typically used to save energy, provide automatic control, and comply with building codes.
Sensing technology used in an occupancy sensor include:
Passive infrared (PIR) sensor, which detects body heat movement. The device is typically made of 2, 4 or 8 pyroelectric elements (pyros) calibrated to detect infrared radiation. A Fresnel lens on the device which is a multi-segmented lens array focuses the infrared radiation to the pyros. A microprocessor in the device reads the infrared signal to determine presence of occupant. PIR sensor requires line of sight between the sensor and the detection zone to operate effectively
Ultrasonic sensor, similar to radar works on Doppler shift principle. It sends high frequency sound wave in the area and checks for the reflected pattern. Ultrasonic sensor does not require line of sight and could detect movement of occupant around a barrier, e.g. office partition. Ultrasonic sensor could be triggered by airflow from ventilation outlet and inanimate moving object, thus making it more susceptible to false detection
Microwave sensor, similar to ultrasonic sensor also works on Doppler shift principle. It sends high frequency microwave in the area and checks for the reflected pattern. Microwave penetrates concrete, wood and plaster and it does not require line of sight. It is more susceptible to triggering due to movement beyond the intended detection zone
An occupancy sensor may use a combination of sensing technology to compensate for weaknesses in each type of technology. It may also apply signal processing logic to reduce false triggering.