Correction of color defects during printing of photographs

摘要

The gray balance of a color copying apparatus is adjusted by generating three sets of test signals respectively representing red, green and blue components of a test image. The test image is modified by operating on each set of signals with a respective characteristic line. The characteristic lines are stored in look-up-tables constituting part of the apparatus. Each characteristic line has a first point corresponding to a first location of the modified test image and a second point corresponding to a second location of the modified test image. A first copy of the test image is made and the red, green and blue densities of the copy at the first location determined. If any of these densities deviates from a reference value, the corresponding characteristic line is adjusted so that the respective density assumes the reference value. A second copy of the test image is now made and the red, green and blue densities of the copy at the second location determined. In the event that one or more of the densities at the second location deviates from a reference value, the associated characteristic line or lines are further adjusted in such a manner that the first point of each characteristic line remains unchanged.