The total evaluation theory of image quality by detectivity derived from spectral signal-to-noise ratio was applied to radiographic screen-film systems. Detectivity of radiographic image equals to root integration square of spectral signal-to-noise ratio. A 5.45 detectivity value of a radiographic bead image on TF-2+RP screen-film system a comparatively better estimation at granularity, where as, a value of 4.28 on Par+RP/R screen-film system which has comparatively better estimation at MTF. Detectivity value of radiographic edge image of a needle was 559.5 of root integration of NEQ (u) on the TF-2+RP, and 596.8 on the Par+RP/R screen-film system. As result of above it is considered that TF-2+RP adapts to the detection of radiographic bead image, and the Par+RP/R screen-film system to the detection of radiographic edge image of a needle. This result of ours agreed with David J. Goodenough's conclusions. A 4.61 detectivity of radiographic contact bead image on Lanex Regular+OG screen-film system was remarkably increased to 9.08 at radiographic bead image of magnification factor 2. Evidently the radiographic magnification technique is one of the best method to increase detectivity.
