We need to be careful when we say the technique for reading the temperature only measures photons. When I read the documents from the camera vendor site I came away with the understanding that the detectors that they use in their instruments actually respond to heat directly. The heat is in the form of IR radiation, and the photons from the visual portion of the spectrum would have little effect.
It's all photons. Visible, Infra-red, ultra-violet, radio ....
