This only works because the sky does not transmit much energy at the particular infrared frequency this operates at.
In contrast ordinary objects (the computer case, a tree, a wall), do transmit those frequencies.
This would have no effect - it's absorbing the frequencies emitted by the wall, and transmitting to the wall, the net effect is zero.
i.e. it only works under open sky. I suspect even clouds would block this from functioning. (Not sure about this - maybe water has a "window" at this frequency?)
8 to 14 microns is called the "atmospheric window", and a large portion(up to around 80%, from what I've seen) of energy at these wavelengths escapes the atmosphere. This window is a result of weak absorption by carbon dioxide and water, thus the IR emissions will pass right through clouds.
In contrast ordinary objects (the computer case, a tree, a wall), do transmit those frequencies.
This would have no effect - it's absorbing the frequencies emitted by the wall, and transmitting to the wall, the net effect is zero.
i.e. it only works under open sky. I suspect even clouds would block this from functioning. (Not sure about this - maybe water has a "window" at this frequency?)