1. A superposition processing method for a radiation image, comprising the steps of: i) obtaining a plurality of image signals representing one or more radiation images of an object, each of the plurality of image signals having different respective frequency characteristics for noise, each of the plurality of image signals having respective image signal components, ii) for each of a plurality of different frequency bands, weighting the respective image signal components with weight factors to provide weighted image signal components, wherein, for each of said image signals, all the respective image signal components are identically weighted, and iii) adding all of the weighted image signal components to one another to obtain an addition signal representing the superposition of said plurality of image signals, wherein values of the weight factors used for ones of the plurality of image signal components having a low signal-to-noise ratio are smaller than values of the weight factors used for ones of the plurality of image signal components having a high signal-to-noise ratio, and wherein said weight factors are determined, at each of said plurality of different frequency bands, based on said different respective frequency characteristics for noise for said respective image signal.