It has been reported that the efficiency of certain ZnS: Ag phosphors decreases as the exciting current density is decreased. This would cause the light gain of a cascade image intensifier to vary with the intensity of the incident illumination.Measurements were performed on P11 phosphor screens of a cascade intensifier to investigate whether such a change in efficiency with current density occurs. It was found that the efficiency of these phosphors remains constant as the exciting current density is decreased from 10−5 to 10−15 A cm−2.