An expression of the optical efficiency for alternating current thin film electroluminescence (ACTFEL) devices is given by a model of an optical microscopic cavity, and the optimization of the structure of the ACTFEL device is discussed. The model is reasonable since the radiation patterns developed by it coincide with the experimental results. The model indicates that in the ACTFEL device, the wide-angle interference is obvious, but the multiple-beam is slight. (C) Elsevier Science SA.