Thermal cameras are especially useful for detecting intruders in the outdoors for a few key reasons.
One, they “see” heat rather than visible light, so they work in total darkness as well as poorly lit areas at night when most of the “bad” stuff is likely to happen. In order to present heat in a way our eyes can see, thermal cameras convert the temperature of an object into shades of gray which are darker or lighter than the background. Because background objects tend to be cooler at night or hotter during the day than a person at 98.6 degrees, they represent a really good “human detector” at almost all times, even in bad weather like rain, snow or fog.
Today’s imagers, when backed with good video processing, can determine temperature variations down to 1/12th of a degree, so thermal cameras provide what’s probably the most important function for security – they almost never miss an intruder. They’re a very reliable way to detect people outdoors.
They are also good at the other problem most security systems face, which is nuisance alarms. Thermal cameras ignore movement which triggers alarms for visible cameras, like headlights sweeping through a scene, reflections off water or windows, or small animals and blowing debris. Nuisance alarms are almost as bad as missed detections, because if you get too many, like the boy who cried wolf, you stop paying attention when a real intrusion happens.
At one time, thermal cameras were relegated to more expensive use, like for the military or really critical sites, like airports or refineries or chemical storage. But lots of innovation in the sensitivity and manufacturing of thermal imagers has brought their costs way down, so now commercial sites like car dealers, scrap yards, industrial parks, and other common facilities can take advantage of thermal cameras for about same price as a quality visible camera.