METHOD FOR RECOGNIZING AN IRRADIATION FIELD, AND METHOD FOR JUDGING THE CORRECTNESS OR INCORRECTNESS OF PROSPECTIVE CONTOUR POINTS OF AN IRRADIATION FIELD

A method for recognizing an irradiation field on a recording medium comprises the steps of, on each of radial lines each of which connects a point located in the irradiation field with an edge of the recording medium, detecting prospective contour points, each of which is considered to be an intersection of each line and a contour of the irradiation field, together with their prospectiveness ranks, based on the image signal components corresponding to the picture elements arrayed along each line. When the prospective contour points thus detected are present in both peripheral and center regions of the recording medium, and a prospective contour point standing in the first rank is present in the peripheral region, the first rank is given to a prospective contour point which is in the highest rank among the prospective contour points located in the center region. The irradiation field is recognized based on the first-ranking prospective contour points thus determined on the lines.