Exposure unit for a multicolor image forming apparatus capable of better registration of the multicolor images

An apparatus for forming a color image by irradiating a rotatable photoreceptor with a plurality of laser beams to write plural color component latent images and by developing the latent images with a plurality of developing devices to form plural color component toner images, comprising

• said rotatable photoreceptor having a given width in a direction perpendicular to the rotating direction thereof and having a first side and a second side opposite to the first side in the width;
• exposure means for scanning said photoreceptors with a plurality of laser beams to write plural color component latent images, including
• a laser generator to generate the plurality of laser beams,
• a rotatable polygon mirror having a mirror at each edge sides thereof to reflect a laser beam so as to scan the photoreceptor in the width direction, wherein the polygon mirror receives two laser beams at two mirrors thereof which are arranged diagonal to each other and reflects the two laser beams in the opposite directions to each other so that a first laser beam of the two laser beams scans from the first side to the second side on the photoreceptor and a second laser beam scans from the second side to the first side;
• wherein a laser beam position most upstream than the other laser beam writes at the first side of the photoreceptor a latent image of a reference position mark which is developed into a visible reference position mark;
• signal means for outputting first and second position signals independently when the first and second laser beams pass respectively the reference position mark provided at the first side on the photoreceptor;
• and
• control means for controlling exposure means in accordance with the position signals so that the start timing of the first laser beam to write a latent image is determined on the basic of the first position signal which is obtained in the current scanning line of the first laser beam and the start timing of the second laser beam to write a latent image is determined on the basis of the second position signal which has been obtained in the previous scanning line of the second laser beam.