Ac and dc electroluminescence in insulating polymers and implication for electrical ageing

Electroluminescence detected in many different polymers under uniform 50 Hz ac and dc fields exhibits a threshold-like character, i.e. the light emiss ion increases supralinearly with the applied voltage above a critical value called the emission threshold. The significance of this threshold is not s traightforward. It could be a true physical threshold corresponding to the onset of the luminescence excitation as opposed to a sensitivity-limited th reshold without specific significance. This paper discusses this issue on t he basis of electroluminescence analysis in several polymers used in electr ical engineering, such as polyethylenes, polyesters and polyimides. The ana lysis relies on the consideration of the field dependence of the electrolum inescence and its imaging in the sample plane. It is shown that the thresho ld-like character under a dc field reveals the onset of the electroluminesc ence excitation whereas it has no specific meaning under ac stress where th e detection is sensitivity-limited. The significance of the luminescence-vo ltage characteristic, as, regards the electrical ageing and its possible us e for diagnostic purposes, is further considered.