Image forming apparatus

An image forming apparatus, comprising:

• a print engine configured to physically print an image to be printed on a print sheet;
• a sheet transportation unit configured to transport the print sheet along a predetermined transportation path;
• an optical sensor arranged so as to face the transportation path, and configured to detect a color of a test chart on a colored print sheet transported along the transportation path;
• said test chart being comprised of patch images of plural gradation levels for a toner color;
• a control unit;
• a storage device configured to store a surface color correction table and a reference color data that indicates a reference color of the test chart;
• the reference color data containing reference colors corresponding to the plural gradation levels of each toner color;
• and
• an image processing unit configured to perform color correction of the image to be printed on the colored print sheet, on the basis of the detected color of the test chart on the colored print sheet so as to restrain color change of the image due to a surface color of the colored print sheet;
• wherein the image processing unit (a) determines a color correction characteristic corresponding to the detected color of the test chart in accordance with the surface color correction table that indicates a relationship between a color of the test chart and a color correction characteristic, and (b) performs the color correction in accordance with the determined color correction characteristic;
• (c) generates a new surface color correction table on the basis of the detected color of the test chart and the reference color;
• (d) calculates a difference value between the new surface color correction table and the stored surface color correction table;
• and (e) renews the surface color correction table stored by the storage device with the newly generated surface color correction table when the difference value is higher than a predetermined value.