Unlike a night vision scope that relies upon ambient light to render a usable image the power of a thermal scope is derived internally and works well in reduced visibility scenarios as well as total darkness. Thermal imaging allows the shooter to see his target through smoke, dust, and fog in any lighting conditions.
Heat signatures in inanimate objects such as trees, rocks and man made 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 scopes are as usable during the day as they are at night time.
Thermal scopes 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.