A thin film system of PC-EL type composed of two elements which are connected in series was constructed. The first element, a photoconductive element (PC), was a copper - and chlorine doped CdS thin film evaporated under vacuum. The second one was a thin film elec-troluminescent cell (EL) with a copper, chlorine and manganese - doped ZnS phosphor layer. This PC-EL system was supplied with sinusoidal voltage. Moreover a light signal in the shape of rectangular pulses, illuminating the PC element as being input, was applied. The output signals in the form of the luminance of light, emitted from the electroluminescent cell at various values of the supplying voltage frequency (ranging from 50 Hz to 10 kHz) were measured.