Electroluminescent (EL) film panels of ZnS: Mn, Cu, Cl operated by a.c. electric field are studied at room temperature. The emission spectrum consists of a single peak at 590 nm. The EL emittance B varies with frequency f of the applied a.c. electric field as B = Bs − Bm exp − f/fc where Bs, Bm and fc are constants. This equation indicates that B approaches a saturation value Bs when f≫fc and a linear relation between B and f when f≪fc. At a fixed frequency f, B is found to depend on the applied voltage V as B = A exp [-G/F + V12)] where A, G and F are constants. This formula is valid at all stages of the operating life of the film panel.