Image forming apparatus

An apparatus for forming a multi-color image, comprising:

• a rotatable image carrying member having a peripheral surface;
• a plurality of image forming means each having a charging device so that the plurality of image forming means form the multi-color image during a single rotation of the image carrying member on the peripheral surface;
• each scanning exposure device of the plurality of image forming means including a beam generator and a polygonal mirror and being controlled such that the polygonal mirror is rotated at a same rotational speed as the other scanning exposure devices and such that a beam scans the peripheral surface in an axial direction of the image carrying member;
• a first sensor for detecting the beam passing at a start reference position on the peripheral surface and for outputting a start reference signal;
• a second sensor for detecting the beam passing at an end reference position on the peripheral surface and for outputting a stop reference signal;
• measuring means for receiving the start reference signal and the stop reference signal sequentially for each scanning exposure device and for measuring a time between the start reference signal and the stop reference signal;
• and
• control means for controlling a frequency of clock pulses to transmit image signals for each scanning exposure device based on the measured time of each scanning exposure device;
• wherein each of the first sensor and the second sensor comprises a window member having one of a light reflecting member and a light transmitting member and detects the beam by light from the window member;
• and
• wherein one of a light reflecting member and a light transmitting member has a first edge at the scanning start side and a second edge at the scanning stop side and the first edge and the second edge are not parallel to each other, and wherein the measuring means receives two signals corresponding to the first edge and the second edge and measures a time between the two signals and the control means controls a timing in terms of a sub-scanning direction based on the measured time between the two signals.

An apparatus for forming multi-color image, comprising:

• a rotatable image carrying member having a peripheral surface;
• a plurality of image forming means each having a charging device so that the plurality of image forming means form the multi-color image during a single rotation of the image carrying member on the peripheral surface;
• each scanning exposure device of the plurality of image forming means including a beam generator and a polygonal mirror and being controlled such that the polygonal mirror is rotated at a same rotational speed as the other scanning exposure devices and such that a beam scans the peripheral surface in an axial direction of the image carrying member;
• a first sensor for detecting the beam passing at a start reference position on the peripheral surface and for outputting a start reference signal;
• a second sensor for detecting the beam passing at an end reference position on the peripheral surface and for outputting a stop reference signal;
• measuring means for receiving the start reference signal and the stop reference signal sequentially for each scanning exposure device and for measuring a time between the start reference signal and the stop reference signal;
• and
• control means for controlling a frequency of clock pulses to transmit image signals for each scanning exposure device based on the measured time of each scanning exposure device;
• wherein each of the first sensor and the second sensor comprises a window member having one of a light reflecting member and a light transmitting member and detects the beam by light from the window member;
• and
• wherein one of a light reflecting member and a light transmitting member is provided two pieces so as to have a first edge at a scanning start side and a second edge at the scanning start side and the first edge and the second edge are not parallel to each other, and wherein the measuring means receives two signals corresponding to the first edge and the second edge and measures a time between the two signals and the control means controls a timing in terms of a sub-scanning direction based on the measured time between the two signals.