For monoenergetic x rays, the detective quantum efficiency (DQE) of an x‐ray image intensifier (XRII) obtained by measuring the ratio of signal to root‐mean‐square (rms) noise is compared with the DQE obtained by the scintillation spectrum method. To obtain agreement between the two methods, the rms noise must be measured at a very low frequency because of the temporal responseRXRII(f) of the XRII. This takes a long time and the measurement of the DQE from the scintillation spectrum is much quicker. Measurement ofRXRII(f) shows that it is dependent on x‐ray flux rate.RXRII(f) measured at the same flux rate as DQE permits correction of DQE measurements to zero frequency. In order to estimate DQE for polyenergetic beams, we need to know as a function of energy not only the DQE but the amount of energy deposited per absorbed x ray. The latter is also obtained from the rms measurement.