A theoretical development has been applied to intensifier image tubes which is similar to that used for ir photodetectors. In essence, a differential equation applicable to intensifier image tubes has been used to obtain exact solutions for signal and noise. These expressions account for factors such as phosphor time constant, area, frequency response, resolution, and efficiency. They can be inserted into the definitions of Noise Equivalent Power, Detectivity Star, and Detective Quantum Efficiency to provide exact expressions indicating the dependence of these quantities on basic factors governing tube operation and performance. An experimental arrangement is described to measure the light signal and noise from the intensifier, utilizing a traveling test wave rather than a stationary TV type of test pattern. This permits image tube evaluation on the same basis as photodetectors.