Previous experiments showed that the luminescent emission of ZnS(Cu) excited with an a.e. electric field is strongly affected by the simoultaneous excitation with UV radiations (1.2). As a matter of fact, the luminescent intensi ty turns out greatly enhanced with respect to the one observed in the absence of UV radiations. Moreover, the light consists of a succession of pulses (brightness waves) with the same frequency of the applied field, while, when the UV radiations are absent, the pulse frequency is twice that of the field. This state of affairs induced us to perform measurements on the emission of an electroluminescent (EL) phosphor by applying the a.c. field just after the flux of UV radiation is removed. This in order to gain information on the EL process when a density of UV excited carriers is still present, but avoiding the troubling presence of the photoluminescent emission. The experimental set-up consisted of a ZnS(Cu) quartz condenser-shaped EL cell (3) optically connected through two shutters to a 250 W d.c. low-pressure Hg lamp and a photomultiplier (EMI 9804/B). Initially, in the absence of the a.c. electric field, the shutter in front of the UV lamp is open to allow the cell excitation, while the shutter in front of the photomultiplier is closed to shelter it from the intense photoluminescent emission of the cell. At a given instant, the UV lamp shutter becomes closed, thus ending the UV excitation of the cell and originating (by means of a photoresistance and a Schmidt trigger) a signal which, after a suitable delay (about 0.05 s), causes the opening of the photomultiplier shutter. The same signal feeds the start input of an electronic t imer (s) which, when a pre-established time t D is spent, operates an oscillator (Wavetek 116) thus exciting the EL cell with a 103 Ha a.c. field of 10 a V,.m.8./cm. The photomuitiplier output is sent to a digital transient recorder (Data lab DL 905). In this way, the initial behaviour of the EL brightness waves was recorded for various values of t n. Several runs were performed for each tD to reduce the experimental error.