Color image forming apparatus and method of obtaining color images with decreased image positional deviation

A color image forming apparatus, comprising:

• a conveying belt on which a single recording sheet is conveyed;
• a plurality of electrophotographic processing sections positioned along a moving direction of said conveying belt, each of the electrophotographic processing sections configured to form and transfer a different colored image onto the single recording sheet by transferring the images, said different colored images being superimposed on one another on said single recording sheet;
• and
• a detector comprising a light source, a slit and a light accepting element, wherein
• at least two of said electrophotographic processing sections is configured to form on said conveying belt at least first and second image positional deviation detecting marks of at least two colors in a same pattern,
• each image positional deviation detecting mark comprising,
• a first line in a main scanning direction, and
• a second line positioned at an inclined angle with respect to said first line,
• said light source of said detector successively illuminating each image positional deviation detecting mark, and light from said light source being passed through said slit and to said light accepting element where the light is detected after having been exposed to each image positional deviation detecting mark, said detector configured to detect a positional deviation of images, in the main scanning direction and a sub-scanning direction F, formed on a recording sheet from a conveying speed of said conveying belt and respective times said detector detects said first line and said second line of each image positional deviation detecting mark, wherein the positional deviation in the sub-scanning direction F is determined based on the following calculation:
• F={(TM1-TK1)-TO}V
• wherein TM1 is a time of detecting the first line of the first mark, TK1 is a time of detecting the first line of the second mark, TO is an ideal interval time, and V is the conveying speed of said conveying belt.

An apparatus for forming a color image on a recording sheet, comprising:

• conveying means for conveying the recording sheet;
• means for forming at least first and second color image positional deviation detecting marks having a common pattern, including:
• means for including a first line of said first mark on said conveying means in a main scanning direction by operating a first electrophotographic processing means disposed at a first position along said conveying means,
• means for including a second line of said first mark on said conveying means positioned at an inclined angle with respect to said first line,
• means for including a first line of said second mark parallel with said first line of said first mark on a common axis by operating a second electrophotographic processing means disposed at a second position along said conveying means;
• means for detecting said first line of said second mark and said first line and said second line of said first mark with a single detecting means comprising:
• means for illuminating at least one of said first line and said second line with light, which interacts with at least one of said first line and said second line,
• means for passing said light to said means for detecting, and
• means for detecting a positional deviation of images, in the main scanning direction and a sub-scanning direction F, formed on a recording sheet from a conveying speed of said conveying means and respective times when said single detecting means detects said first line and said second line of each of said first mark and said second mark, wherein the positional deviation in the sub-scanning direction F is determined based on the following calculation:
• F={(TM1-TK1)-TO}V
• wherein TM1 is a time of detecting the first line of the first mark, TK1 is a time of detecting the first line of the second mark, TO is an ideal interval time, and V is the conveying speed of said conveying belt.