Patent ID: 8723906

Claim:
An image-forming apparatus, comprising: a plurality of photosensitive drums provided for every color; a plurality of laser scanning units for scanning and exposing the corresponding photosensitive drum, each of the laser scanning units including a laser-light-emitting unit for switching a laser beam on and off in accordance with image data; a polygon mirror for reflecting, while rotating, the laser beam emitted by the laser-light-emitting unit and scanning and expose the corresponding photosensitive drum, the polygon mirror having a plurality of reflective surfaces; a polygon motor for rotating the polygon mirror, the rotational speed of the polygon motor changing in accordance with the frequency of a provided drive signal; and a light-receiving unit for outputting a detection signal having an output value which changes when the laser beam is received, the light-receiving unit being provided within a range of irradiation with the laser beam by the polygon mirror; a motor control unit for: providing the drive signal of a predetermined base period to the polygon motors and causing the polygon motors to rotate such that each of the polygon motors rotates at the same speed; detecting the time difference between a change point of the detection signal of the laser scanning unit of a reference color and the change points of the detection signals of the laser scanning units of colors targeted for phase correction other than the reference color; and, when the absolute value of the time difference is greater than a predetermined threshold, carrying out a rough adjustment process in which the drive signal with which the base period has been shortened or lengthened by a first period change amount causes the polygon motors of the colors targeted for phase correction to rotate and the time difference to decrease, or, when, as a result of the rough adjustment process, the absolute value is not greater than the threshold, carrying out, successively to the rough adjustment process, a fine adjustment process in which, until the time difference falls within a predetermined acceptable range, the drive signal with which the base period has been shortened or lengthened by a second period change amount smaller than the first period change amount with respect to the base period causing the polygon motors of the colors targeted for phase correction to rotate and the time difference to decrease; and one correction current supply unit for changing, in a similar manner for each of the laser-light-emitting units, the current flowing to each of the laser-light-emitting units in accordance with the scanning position in a main scanning direction, and for changing the light emission level of each of the laser-light-emitting units and correcting the difference in energy received by the photosensitive drums in the main scanning direction.