In this thesis photoluminescence measurements were made on ZnS:Mn thin-films with different concentration of Mn in order to observe the phenomenon of concentration quenching. A mathematical model, based on migration of energy among Mn centres and subsequent transfer to traps, is proposed to explain it. Probability of energy transfer to traps is considered independent of Mn concentration. It is consistent with experimental evidence reported in literature. Using this model we can give physical interpretation to the empirical decay-law for luminescence of Mn centres n(t) =n(0)exp[ - (t/r)a ] , where j and a change with Mn concentration. We are able to quantify the probability of energy loss to traps using Dexter's equation for dipole-dipole transfer. It is suggested that energy loss to the traps can be influenced by growth technique. / Thesis / Master of Engineering (ME)