Gamma correction for apparatus using pre and post transfer image density

An image forming apparatus for performing gamma correction based on an image density on a photosensitive body or an intermediate transfer body, comprising:

• test pattern generator means for generating a test image on said photosensitive body or said intermediate transfer body;
• a toner transfer section;
• first density detector means provided upstream of said toner transfer section with respect to a rotation direction of said photosensitive body or said intermediate transfer body, for detecting a pre-transfer image density on said photosensitive body or said intermediate transfer body before an image transfer at said toner transfer section;
• second density detector means provided downstream of said toner transfer section with respect to the rotation direction of said photosensitive body or said intermediate transfer body for detecting a post-transfer image density on said photosensitive body or said intermediate transfer body after said image transfer at said toner transfer section;
• corrector means for correcting image data;
• and
• control means for calculating correction data for use in correcting said image data based on said pre-transfer image density data outputted from said first density detector means and said post-transfer image density data outputted from said second density detector means and for setting said correction data in said corrector means.