When the signal in an x-ray image system is formed by integrating the scintillation pulses rather than by counting them, the signal-to-noise ratio is reduced by a factor which depends on the shape of the pulse-height distribution. The signal-to-noise ratio cannot be related directly to either quantum absorption or energy absorption, and a new quantity called noise-equivalent absorption is defined which bears a simple relationship to the signal-to-noise ratio. Quantum, energy, and noise-equivalent absorption are calculated as a function of thickness and x-ray energy for CsI, Gd2O2S, LaOBr, Zn0.6Cd0.4S, and CaWO4.