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Electroluminescence (EL) of insulating polymers is now recognized as a warning sign for electrical aging. In order to further understand the emission process, we have compared the luminescence spectra of the same material when the emitting states are created by different excitation sources. Additive free and carbonyl containing low density polyethylene films were used. Light emission was stimulated by photon irradiation in the ultraviolet range (photoluminescence), by the interaction between a cold plasma and the polymer surface leading to a post-discharge emission (plasma induced luminescence), and by the electric field in the absence of any gaseous discharge (EL). Plasma induced luminescence and electroluminescence exhibit similar spectral features in the wavelength range between 500 and 650 nm. It is concluded that the same kind of chromophores is involved in both processes. They could be conjugated groups acting as recombination centers because charges are selectively trapped on these groups. This explains why photoluminescence does not show similar emission features since all the material chromophores are excited upon UV irradiation. Moreover, it is shown that the EL-features are sensitive to structural relaxation of the polymer probably through the interaction between emitting species and the surrounding molecules.

Electroluminescence associated with ac voltage aging of polyolefins has been reported under divergent field configuration. Neither the mechanism of excitation nor the nature of luminescent centers are well understood in these polymers. Homogeneous field experiments are desirable since they are easier to run and more light can be collected, thereby allowing spectral analysis to be made. We describe a suitable experimental configuration for electroluminescence detection from polyethylene and polypropylene films submitted to a homogeneous field and report on surface roughness, field, and time dependence of the light emission. Field and time dependence of the light spectra are also investigated. It is shown that the light originates from the polymer bulk and is controlled by the surface roughness of the films. The light spectrum appears to consist of two components which exhibit different behavior as a function of field and stressing time. A tentative explanation is given in terms of competition between two different excitation processes.