Heat signatures in inanimate objects such as trees, rocks and manmade structures are attained by absorbing heat from the sun. Living things such as humans and animals create their own heat internally, as do vehicles and aircraft. Because different materials absorb and lose heat at different rates, they render a distinct pattern and signature when rendered through a thermal imager. As a device, thermal imagers are as usable during the day as they are at night time.
Thermal imagers are rapidly replacing or supplementing traditional night vision for surveillance and hunting, particularly when the quarry is dangerous game that are effective at hiding in brush or planted fields, regardless of lighting conditions.