designated area will cause the sensor to assume that the area is unoccupied and
turn the lights off, even when it isn’t.
Careful placement of occupancy sensors is required to prevent false alarms.
Occupancy sensors should not be mounted in the direction of a window. Although
the wavelength of infrared radiation to which the chips are sensitive does not
penetrate glass very well, a strong infrared source such as a vehicle headlight or
sunlight reflecting from a vehicle window can overload the chip with enough infra-
red energy to fool the electronics and cause a false alarm. A person moving on the
other side of the glass however would not be “seen” by the device.
Devices should not be placed in such a position that an HVAC vent would blow
hot or cold air onto the device surface. Although air has very low emissivity (emits
very small amounts of infrared energy), the air blowing on the plastic window
cover could change the plastic’s temperature enough to “fool” the electronics.
Other technologies and approaches to motion detection include sensing
audible noise. Hybrid sensors (PIR and ultrasound, PIR and audible) offer
the most effective occupancy detection and have maximum sensitivity without
triggering false detections.
Figure 4.3 Occupancy sensor.
52 Smart Building Systems for Architects, Owners, and Builders