SHADING CORRECTION SYSTEM FOR IMAGE SCANNING APPARATUS

A shading correction system is used in an image scanning apparatus which stores shading data which are obtained by scanning a white substrate and carries out a shading correction on the image information which is obtained by scanning an intended image by use of the stored shading data. The white substrate is scanned a plurality of times at different locations thereof and previous shading data stored in a memory are compared with present shading data so as to renew the contents of the memory depending on the compared result. The renewal of the contents of the memory is carried out in N stages, where each of the N stages carry out the comparing and renewing operation for one of N different groups of scans over the white substrate for one of N different picture element groups each comprising 1/N a total number of picture elements in each scan.