Every object warmer than absolute zero (that's -459.67°F, the coldest temperature possible) emits invisible (to humans but not alien predators) infrared radiation. Things like humans, coyotes, car engines, and even a footprint left in grass all give off tiny heat signatures. One of the useful characteristics of thermal is that it doesn't care if it's dark, foggy, or smoky. It "sees" heat patterns anyway and translates them into visible images.
Inside a thermal monocular like the NocPix Lumi P13, a sensor array made up of thousands of microscopic detectors measures those minute temperature differences -- from a distance. Think of each detector acting like a tiny thermometer. Together, they build a heat map, which the device's processor then converts into the vivid images you see on its AMOLED screen. That's Active Matrix Organic Light-Emitting Diode by the way, but all you need to know is it's bright, responsive, and power efficient.
The base model Lumi P13's sensor resolution -- 256 × 192 pixels -- may sound modest compared to your smartphone camera, but each of those pixels represents a different temperature zone. Its Reality+ image processing algorithm sharpens contrast so subtle details jump out: a rabbit hiding under brush, or the faint warmth left on a doorknob after someone walked by. The sensitivity rating (known as NETD, noise-equivalent temperature difference) is ≤18 mK, which means the P13 can distinguish temperature differences of less than two-hundredths of a degree Celsius. That kind of sensitivity is why you can see things like breath from an animal on a cold night.