A beginner's guide to radiation thermometers: Pt. 1

It is a matter of common experience that hot objects emit invisible radiation and, if sufficiently hot, visible radiation. The invisible radiation can be detected by the heat it produces when it is absorbed. It is not quite obvious that even cold surfaces also emit radiation, through at a much lower level, and that all surfaces emit not only these readily observable forms but also those covering an infinitely wide range of wavelengths