Abstract: Apparatus for controlling operation of a number of radiation generator assemblies comprises a store for storing calibration values for respective assemblies, respective switches, a digital to analogue converter (D/A) and control means. The switches are coupled between the radiation generator assemblies and D/A. The control means selectively addresses the store and controls the switches such that when a switch is closed a corresponding calibration value is supplied to the D/A to energize the generator assembly. Such apparatus is typically for use in monitoring of passage of sheets such as in banknote sorting.