Passive Infrared Overview & Advantages
Outdoor Sensors for Perimeter Protection 
Integrated Security Corp offers passive infrared sensors manufactured in Switzerland by Xtralis and distributed in North America by the ADPRO subsidiary of Xtralis.
This high quality family of detectors measures thermal radiation contrast against a background. A person moving into or through a detector's field of view generates an input signal. Advanced digital signal processing minimizes nuisance alarms through adaptive threshold decoding (ATD) and signal shape analysis. In other words, background noise in the field of view is filtered out. Automatic temperature compensation and heated high quality optics further reduce the chance of false alarms caused by changes in environmental conditions. Think sunrise/sunset.
Flexible Application
Models are available with a variety of coverage patterns.
Many models provide "curtain coverage", meaning they have a very narrow detection field of view. The longest range model has detection field that is only 11' wide at is nominal maximum range of 500'. This allows you to install these sensors along fences, roads or walkways without nuisance alarms being created by incidental traffic near the perimeter.
If you need to cover a wide area and not just the perimeter, we have models with fields of view up to 90o wide. And perhaps the most intriguing model has dual curtains and allows you to set the direction of movement that represents and alarm. That is, you can set the detector to alarm only on entry or exit, but not both.
What Advantages Does a Passive Detector Have?
Because there is no transmitter/receiver pair as with an active system, a sensor can be pointed outward from the perimter to detect a person approaching.
Alignment, while important, is not as critical as with active detectors.
When mounted at the recommended height of 8' - 13' and angled toward the ground as recommended, the sensors will detect movement through minor depressions in the ground that are invisible to active systems that require line of sight between the transmitter and receiver.