Wavelength Choice when working Outdoors

Here is a question we get from time to time at the Infrared Training Center.

"Am I correct in thinking that the solar reflections only become a problem when using shortwave IR equipment that detects the reflective IR spectrum below 3 micrometers? That is, would solar reflections be seen at all using equipment that, say, detects 5-20 micrometers?"

Unfortunately no. The sun is a powerful infrared radiator and emits energy in all IR wavebands. However, the relative amount the sun emits reduces as the wavelength increases, just as Planck's law says it should. So, as we move out to longer wavelengths the amount of energy that reflects from the sun compared to the energy emitted by our targets gets smaller. This means that IR cameras that view in the longer wavelengths would be "bothered" less by solar reflections or artifacts. So the LW (8-13 µm) IR cameras see the least amount of reflected solar energy. This makes surveys easier for the operator as the true "hot spots" are easier to find.