Abstract
The well‐known deterioration of phosphors in fluorescent lamps consists essentially of two parts, a short‐time degradation over roughly one hour possibly due to color center formation, and a long‐time effect extending over thousands of hours explained by a slow build‐up of a disordered and non‐luminescent layer on the phosphor particles. Such layer has been observed on deteriorated zinc silicate, it is very thin. Its optical absorption is the main reason of decreasing lamp brightness. Mercury buried in, but not absorbed on, the dead layer may contribute to the optical absorption. The brightness‐time function of the long‐time degradation is 
with τ as a time constant. This model agrees well with observations. Conclusions are drawn concerning the stability of phosphors in lamps and its dependence on some parameters.