An electroluminescent (EL) element is connected to the power supply and is intermittently operated as a lighting device and as a brightness sensor. For this purpose, the processing unit induces the controlled AC voltage source to reduce the voltage for a short period. The current intensity is measured by current measuring devices and the corresponding measuring signal is digitized using the analog/digital converter (A/D). A processing unit uses the signal and the effective voltage to determine, by means of empirical calibration data stored in the memory or by using a calibration function, the brightness, i.e. the luminance of the light that is incident on the EL element. The processing unit associates, using desired values stored in the memory, the detected brightness with an operating voltage level which ensures the desired emission brightness of the EL element when operated as a lighting device. Optionally, the determined operating voltage is corrected in order to take into consideration any brightness loss of the EL element caused by aging. The optionally corrected value is output via the control channel as the desired operating voltage and is maintained by the power supply unit until the next measuring interval.