These sensors look for thermal radiation that is projected against a cooler background. Problems occur when the temperature of the environment approaches the same temperature as the intruder, who may not be detected under these circumstances. Since wide temperature changes occur frequently in outdoor environments, passive infrared sensors are not an ideal perimeter monitoring solution.

• Heat emitted by small animals can cause false alarms
• Vehicle headlights and other sources of focused light can trigger nuisance alarms
• Sensors require electricity

• Sensors do not require electricity
• Can be calibrated to eliminate nuisance alarms
• Not affected by changes in temperature or humidity
• Extraneous light will not trigger false alarms
• Economical to purchase
• Easy to install