A possible alternative to current active matrix liquid crystal displays (AMLCD) and conventional cathode ray tube (CRT) displays are field emission displays (FEDs). It works on a similar principle as CRTs, producing light by cathodoluminescence (CL). but has millions of tiny metallic tips situated near the phosphor screen that act as electron emitters. During prolonged electron beam irradiation, a non-luminescent ZnO layer forms on the surface of the ZnS phosphor. The low energy electrons used in FEDs have a short penetration depth and since the CL is dependent upon the energy loss in the phosphor, growth of the ZnO layer significantly degrades the CL intensity. An expression was derived to quantify the CL generated during electron irradiation using electron energy loss profiles simulated by the Monte Carlo technique. These profiles were used to construct a curve relating the CL intensity to the ZnO thickness. For the ZnS:Cu,Al,Au phosphor the calculated oxide thickness compare extremely well with experimental measurements. However, using the same simulation parameters, the oxide thickness measured on the ZnS:Ag,Cl phosphor is much thinner than the calculated value. This difference is attributed to charge trapping over the range of the primary electrons during electron irradiation of the phosphor which influences the electron-hole (e-h) recombination and rate of oxide growth.