The psychophysical gradient δ was obtained for various output devices of X-ray radiographic systems (Ex. Film/Viewer system, CRT and LCD). The δ can express more suitable visual sense than physical gradient. The shape of δ curve of CRT was flatter than that of film/viewer system. This means that the CRT gives more stable visual sense for wide dynamic range than film/viewer system. The Moon & Spencer model corresponding to VRF (Visual Response Function) was in good agreement with our measured value.