 Briefly, the full width of the photoelectron spectrum (from the highest kinetic energy/lowest binding energy point to the low kinetic energy cutoff) is measured and subtracted from the photon energy of the exciting radiation, and the difference is the work function.Often, the sample is electrically biased negative to separate the low energy cutoff from the spectrometer response.UPS has seen a considerable revival with the increasing availability of synchrotron light sources which provide a wide range of monochromatic photon energies.