A method to characterise the wall voltage distribution in a three-electrode AC PDP cell is discussed. The method makes use of a firing voltage loop in a two-dimensional voltage plane. From this "fingerprint," data on the relative wall voltages as well as on the non-uniformity of the wall voltages can be inferred. The properties of the loop are explained using a simple numerical model based on field line tracing. The fingerprint method is applied to analyse ramp waveforms on the scan and data electrode of a surface discharge PDP. Many features of the measurements can be understood in terms of uniform wall voltage distributions on the dielectrics covering the electrodes. A more detailed analysis, however, shows that considerable wall voltage non-uniformities can exist, which play an important role in the firing behaviour of the cell.

The cell structure of surface discharge ACPDPs with a long gap between the sustaining electrodes achieves high luminous efficiency. However, the long gap cell structure causes high firing voltage and thus makes driving more difficult than with the conventional gap cell structure. The rise in firing voltage in the long gap cell structure could not be explained by Paschen's scaling law. We derived a new governing equation for firing voltage, involving the influence of a non-uniform electric field, to investigate this deviation from Paschen's law. From the calculated results we found that changing the gap length corresponds to the change in the degree of distortion of the electric field between the sustaining electrodes.