In any alloy exists a region in which the constitution of structural elements corresponds to a mean content of metals forming the alloy. A region for which we can prove a presence of all alloy structural elements in the amount corresponding to the mean concentration is named a specific region. In this paper, the rules of choice of the split size for optical density measurement are presented and a thesis constating that a split size for the optical density measurement should be equal to the specific region size is verified. The thesis verification was accomplished using pattern autoradiographs.