So if we focus on thermal IR: shiny metal surfaces hardly emit or absorb it. A black surface emits and absorbs as much as possible. We quantify this via the emissivity (and also reflectivity, shiny metal is a mirror). Emissivity of 1 = theoretical maximum of a prefect absorber/emitter (both are directly linked), everyday stuff like skin or wood is about 0.95, shiny metal surfaces roughly 0.1, a rusty or painted surface again about 0.95.
You only emit and absorb the same in thermal equilibrium. If the surface is hotter, it emits more. If it is colder, it absorbs more.
So with that last part, I get that in theory, but practically, you can only really absorb IR temporarily, right? Because it all gets released again as heat eventually. Like, say you have a mostly inert object, that isn’t creating any meaningful amount of heat. If you hit that object with 10 joules of thermal IR, over time, wouldn’t nearly all of that energy be either reflected or emitted off it as IR radiation?
I guess it depends on the context, initially I was making comparisons to heat seeking missiles. In that context, one might use the radar absorbent materials to defeat radar guided missiles, but that trick just doesn’t work with heat, as you can’t “permanently” absorb heat the way you can other kinds of energy. If your vehicle is also generating heat, hiding that is quite difficult.
So if we focus on thermal IR: shiny metal surfaces hardly emit or absorb it. A black surface emits and absorbs as much as possible. We quantify this via the emissivity (and also reflectivity, shiny metal is a mirror). Emissivity of 1 = theoretical maximum of a prefect absorber/emitter (both are directly linked), everyday stuff like skin or wood is about 0.95, shiny metal surfaces roughly 0.1, a rusty or painted surface again about 0.95.
You only emit and absorb the same in thermal equilibrium. If the surface is hotter, it emits more. If it is colder, it absorbs more.
So with that last part, I get that in theory, but practically, you can only really absorb IR temporarily, right? Because it all gets released again as heat eventually. Like, say you have a mostly inert object, that isn’t creating any meaningful amount of heat. If you hit that object with 10 joules of thermal IR, over time, wouldn’t nearly all of that energy be either reflected or emitted off it as IR radiation?
That is how heat works in general, but that has nothing directly to do with reflecting or absorbing IR.
I guess it depends on the context, initially I was making comparisons to heat seeking missiles. In that context, one might use the radar absorbent materials to defeat radar guided missiles, but that trick just doesn’t work with heat, as you can’t “permanently” absorb heat the way you can other kinds of energy. If your vehicle is also generating heat, hiding that is quite difficult.