Three experimental methods for measuring the noise‐equivalent absorption of an x‐ray image intensifier are described. In all cases, the image intensifier is irradiated with a source of x rays or &ggr; rays, and the output is monitored with a photomultiplier. In one method the rms value of the photomultiplier current is observed as a function of the bandwidth of the measuring circuit. In the second method, the scintillation pulse spectrum is recorded, and its moments computed to give the required information. In the third method, the response to a series of short bursts of radiation is measured. The noise‐equivalent absorption is determined from the standard deviation of a large number of observations, automatically recorded. The three methods are described and compared, and typical results are shown.