The measurement of photo-stimulated currents (PSC) or thermally stimulated currents (TSC) in ACTFEL devices is sometimes used as a method to identify energy levels of trap centers in the phosphor layer. These methods are based on the fact that optically or thermally generated charges are able to move in the polarization field in the phosphor layer of an EL device, which remains after charging the device with a voltage pulse. Similar information about space charge can be obtained by the measurement of loss factor and capacitance of uncharged EL devices under illumination during a low ac-voltage excitation. This new method avoids one of the major disadvantages of the other methods, namely the continuous change of the field situation throughout the measurement. The measurement of low-voltage photo-stimulated currents (LVPSC) creates an identical and reproducible field situation at each wavelength, which makes interpretation more reliable. Moreover, this new method allows to extract information about the low-field conduction behavior of EL thin films.