Effects of Seasonal Variation in the Thermal Background Scene on the Detection Capability of Passive Infrared Instrusion Detection Systems

The interaction of a passive infrared intrusion detection system (IDS) with its environment has been studied by continuously monitoring a voltage indicative of the sensor's proximity-to-alarm status to determine its behavior as the thermal background scene changes diurnally and seasonally. Alarm voltages caused by the detection of a low-threat human intruder have also been recorded occasionally. Concurrently, site characterization data such as solar illumination, air and soil surface temperature and wind speed have been recorded for correlation with the recorded system voltages. This approach identifies situations when the background scene dynamics render the detection capability of this type of IDS unreliable, either by masking the presence of an intruder or by causing frequent non-intruder alarms. In conjunction with a performance model for a specific passive infrared IDS, such as described by J. Lacombe, an understanding of the environment-dependent variation in thermal background scene permits security personnel to set appropriate alarm criteria for the IDS on a daily or seasonal basis, as required.