Method for detecting quantity of laser scanning positional deviation on photosensitive body, correcting method thereof and laser color image forming apparatus

摘要

A laser color image forming apparatus uses a plurality of laser beams, forms a latent image on a photosensitive belt by laser scanning having a fixed scanning frequency and a pixel forming clock frequency within one scanning, develops the latent image on the photosensitive belt by toner absorption using toner of a color corresponding to each laser beam, and then transfers the developed image to a printing medium such as a paper sheet or the like. Regarding image patterns for the detection of positional deviation, specifically designed to detect the quantity of printing positional deviation of a color to be measured, zonal patterns arrayed on a fixed cycle for a reference color and zonal patterns having shapes similar to the former and designed for the color to be measured are superposed to form a plurality of image patterns on the photosensitive belt. For the respective image patterns, the zonal patterns for the reference color and the color to be measured are shifted from each other based on the mutually different quantities of deviation. The reflection density of each image pattern is measured, and an image pattern having a minimum reflection density is detected. Based on the quantity of deviation of this detected pattern, the quantity of printing positional deviation of the color to be measured is detected. To eliminate the printing positional deviation, based on the detected quantity of printing positional deviation, the scanning start timing of laser scanning and the pixel forming clock frequency are changed.