Motion detectors or motion sensors, as the names suggest, are devices that detect movement. They are generally divided into the categories of active and passive and each category is further subdivided according to the technology employed.
Active devices transmit a signal and then monitor the returned signal for changes in that signal caused by movement. Ultrasonic and microwave motion detectors are examples of active devices.
Passive devices do not transmit a signal; they monitor an existing environmental condition and detect motion based upon changes in that condition. The infrared detector is the most common example of passive devices.
The common alarm system passive infrared (PIR) detector monitors the background infrared signature of the room and when a rapid change is detected, combined with movement across the detector's coverage pattern, it is detected as movement.
Dual technology detectors combine two separate detectors in the same housing, the most common being microwave and PIR. They require both detectors to be triggered within a prescribed time before the device is activated. Generally these detectors are more tolerant to conditions that would cause a traditional PIR to false.
A recent enhancement of the dual technology detectors RCR (range controlled radar) provides the ability to adjust the range of detection thereby ignoring any movement beyond that distance.
Motions have undergone many refinements in the years past to make them more immune to sources of false alarms, however they are still susceptible to environmental factors, which could trigger a false alarm.