thermodynamics - Is there a portable method of preventing all IR radiation from being emitted?

Me and some friends were recently disuccing "How cool it would be to be invisible", from a military viewpoint, I said that Camouflage, when done correctly, works surprisingly well, so invisibility was unnecessary. They countered with the argument that an infrared camera could see you easily. Apart from several warm coats, are there any non-restictive ways of preventing IR radiation being emitted from your person?

Answer

The usual way to achieve minimal radiative losses (and thus a low thermal signature) is using MLI - multilayer insulation. The use of multiple layers of reflectors creates a "thermal series" - each layer gets a temperature that is closer to the radiative temperature of the environment, and it allows you to achieve a low thermal emission. This is very important in cryogenic systems where a small amount of radiative heat transfer can be devastating (it's really hard to pump even one watt of energy from near absolute zero to room temperature).

A nice description can be found here. A very simple (single layer) example is the radiative emergency blanket - typically a thin sheet of metallized plastic that helps reduce heat losses in cases of exposure. These are sometimes known as "space blankets".

Note that the ultimate goal of such insulation is that the outermost layer is at the same temperature as the environment. This means that the rate at which it gets heated (by whatever is inside) must be relatively low compared to the rate at which it loses heat (by radiation, or by convection). This opens up another avenue - make the "blanket" bigger.

For the simple case of a small source of heat with a thermal output of $W$, you can see that the temperature of a black body shell of radius R will depend on the size of the shell. Heat that needs to be dissipated is constant, but as the area increases, the temperature needed for thermal dissipation decreases. If you assume radiative losses only, the expression becomes

$$W = \sigma T^4 4 \pi R^2\\ T = \sqrt[4]{\frac{W}{4 \sigma \pi R^2}}$$

More importantly, if you assume that there will be convective heat losses, then the increased area again plays a role - but now $T$ will scale with $1/R^2$ and therefore go down substantially more quickly.

Note that multilayer insulation doesn't just have to be reflector-void-reflector; it could equally be reflector-insulator-reflector, and for the purposes of camouflage you might want the outermost layer to be similar to the background (rather than a reflector which would presumably stand out in daylight or under spot illumination). A video showing the effect of some approaches to insulation / thermal invisibility is given here - with a tip of the hat to Dirk Bruere who made me aware of it.