In isochromat mode, the incident electron energy is ramped and the emitted photons are detected at a fixed energy that is determined by the photon detector. Typically, an I gas filled Geiger-Müller tube with an entrance window of either SrF or CaF is used as the photon detector. The combination of window and filling gas determines the detected photon energy, and for I gas and either a SrF or CaF window, the photons energies are ~ 9.5 eV and ~ 9.7 eV, respectively.