The electroluminescence spectrum of CaS:Er phosphor is found to exhibit nine bands covering the ultraviolet, visible and near-infrared regions. The most intense bands, which occur at 530 and 550 nm, are split into as many as seven sharp lines. Calculations based on the theory of crystal-field splitting indicate that Er³⁺ ions can occupy the sulphur-ion vacancy position as well as the Ca²⁺ substitutional site.