Photon-in-photon-out spectroscopy refers to all kinds of scattering of electromagnetic radiation, e.g. X-ray emission spectroscopy. Spectroscopy is a means to obtain information from atomic systems (matter) by recording their response to an external excitation. One can classify the type of spectroscopy by the means of excitation (e.g. photons or electrons) and the kind of "particles" that are detected. An important example for a photon-in-electron-out spectroscopy is photoelectron or photoemission spectroscopy.