[catermos]

Are you saying that taking cover behind bushes, etc, will block heat sigs in these viewers?

What I'm saying is that if you cannot see the ('hot') object with your naked eyes, then your IR imaging device won't 'see' it either - it really is that simple (e.g. a thermal blanket will provide 'cover', HOWEVER if the aluminum on the thermal blanket is exposed to the IR imaging device then it will shine like a beacon, just like with the naked eyes; cover the thermal blanket so the aluminum is blocked and that's sufficient). Surrounding objects picking up radiated heat will give an indication of a nearby very hot object (but not animals, only very hot objects like internal combustion engines). The best example I can give you is that of an automobile that has been in operation and is now parked - the exhaust system will heat up the ground enough to indicate the heat (but not to a great extent, it's very subtle), and the automobile will show heat mostly at the engine compartment (it's the body sheet metal showing the heat radiating into it, NOT the engine itself) and at the wheel assemblies (depending upon the ambient temperature). A car that has been parked (running or not, depending upon conditions) and then moved will have the ground indicating heat for a minute or three after it is moved (or maybe not, depending upon the ambient temp. again).