Correcting density unevenness in image forming apparatus

An image forming apparatus that forms an image on a sheet, the apparatus comprising:

• an image forming unit configured to form an image on a photosensitive member that rotates;
• an intermediate transfer member to which the image is transferred from the photosensitive member;
• a sensor configured to receive reflected light from a test image on the intermediate transfer member passing through a detection position and output an output signal based on a reception result of the reflected light, the test image being formed by the image forming unit;
• and
• a controller configured to:
• reduce density unevenness of the image to be formed in a rotation direction of the photosensitive member based on correction data;
• and
• determine a reference output signal used to generate the correction data among output signals output from the sensor in time series based on a timing at which the test image reaches the detection position by performing the steps of:
• detecting, based on a comparison of the output signals with a threshold value, a timing at which a level of a signal binarized by the comparison changes;
• and
• determining the reference output signal based on the detected timing.