Patent ID: 7652790

Claim:
An image forming apparatus comprising: an image forming unit for forming a correcting image for correcting gradations of an output image, on a bearing body; a sensor for measuring a reflected light quantity of the correcting image formed on the bearing body; a gradation correcting unit for correcting the gradations of the output image, based on a measurement result of the measured reflected light quantity of the correcting image; and a timing correcting unit for detecting a shift of measurement timing at which the correcting image is measured by the sensor, based on the measurement result by the sensor, and for correcting the detected shift of the measurement timing; wherein: the gradation correcting unit corrects the gradations of the output image using the measurement result which is measured at the timing corrected by the timing correcting unit, the correcting image comprises a gradation pattern comprising a plurality of gradations; the sensor measures the reflected light quantity of the correcting image at a fixed interval timing; and the timing correcting unit detects a timing of measurement of a measured value having a largest change of measured light quantity value between two adjacent sampling points in a vicinity of a specified timing, as a starting timing at which a measurement of a head part of the gradation pattern has actually been started, based on measured values measured at the fixed interval timing, wherein the specified timing is prescribed in advance as a timing at which the measurement of the head part of the gradation pattern is started, and wherein the timing correcting unit detects a shift between the specified timing and the start timing as the shift of the measurement timing.