In storage phosphor radiography, a portion of the x-ray energy transmitted through the subject is absorbed by a storage phosphor plate. When stimulated by visible light of the right wavelength, the phosphor plate emits light (at another wavelength) in proportion to the absorbed energy. To obtain the radiographic image, the plate can be scanned in a raster fashion and the emitted light can be photoelectrically detected. The detected signal is then amplified, digitized, processed and finally printed on film, or displayed on a cathode ray tube (CRT). Setting the amplification gain of this scanning process, the so-called final scan (or final read-out) gain, is of interest in maximizing the information extracted from the phosphor. One method of setting the final scan gain invokes the use of a preliminary read-out (pre-scan). A pre-scan can be conducted on the phosphor plate by use of a stimulating ray having a stimulating energy lower than the stimulating energy in the final scan. The present invention provides a technique that uses the pre-scan image histogram to automatically determine the final scan gain in storage phosphor radiography.