Abstract: A digital representation of the time-integral of the illumination-dependent electric current flowing in a photo-conductive diode (1) is produced by feeding the current initially to the first capacitor (3a) of a series of capacitors (3a-3d). When the voltage across this first capacitor reaches a predetermined value the current is subsequently directed to the next capacitor (3b) of the series, and so on for the remaining capacitors (3c, 3d) sequence succession, by means of a voltage level responsive coupling cirucit (6). At the end of the integration period the voltage level in that capacitor which is only partly charged at that time is converted into digital form by means of a corresponding analog-to-digital converter (16) and applied to an output (18) together with a code identifying from which capacitor the digital output has been derived.