AC-coupled thin-film electroluminescent devices employ insulating layers to limit the current density through the active layers. The requirements and limitations in driving such devices are then dependent upon the properties of the dielectric layers. We define an external efficiency for an ac thin-film electroluminescent device and consider two limiting cases, current response 1) fast, and 2) slow, compared to an applied voltage change. Relationships are then derived among external efficiency, relative luminance, and the two principal characteristics of the insulating layer, specifically charge density at breakdown and thickness. It is shown, for example, that charge density at breakdown for the insulators should be at least three times the corresponding quantity for the active layer. Copyright © 1977 by The Institute of Electrical and Electronics Engineers, Inc.