Thermal imaging
A thermal device sees heat, not light. Every warm animal glows against cooler ground, day or night, with no light needed.
What it means
Everything gives off infrared energy, and warmer things give off more. A thermal sensor reads that energy and turns small temperature differences into a picture. It needs no moonlight, no starlight and no illuminator, which is why a coyote bedded in grass or a hog at the edge of a black tree line shows up as a bright shape.
Thermal devices work in what is called long-wave infrared, a band of energy well beyond what the eye can see. Ordinary glass blocks it, so thermal lenses are made from other materials, usually germanium. That is one reason thermal costs more than night vision with a similar-sized lens.
What it changes in the field
Thermal is the best tool for finding animals. You can scan a pasture and spot a coyote at distances where a light or night vision would show nothing. It works in daylight too.
It is weaker at telling one animal from another. A thermal picture shows shape and heat, not color, eyes or markings, so a fox, a bobcat and a small coyote can look alike past a certain range. See identification distance.
Thermal cannot see through glass, walls or thick brush. Fog, rain and high humidity cut its range. Hot summer evenings are hard: when the ground, rocks and animals sit near the same temperature, contrast drops and the picture washes out. Cold, dry nights give the cleanest image.
What to look for
The specs that decide what you see are sensor resolution, focal length and pixel pitch. Together they set how many pixels land on an animal. NETD matters most in damp air.
Compare thermal with digital night vision and image intensifier tubes, which amplify light instead of reading heat. Whether you may hunt with thermal at all varies by state and species: check the thermal and night vision laws page.