Image forming apparatus which corrects main scanning misregistration

Abstract

An image forming apparatus includes a light source which illuminates in response to image data, and a deflector to deflect an optical beam output from the light source into a scanning beam running along a main scanning line across an image forming area in a main scanning direction. There are plurality of beam detectors to detect the scanning beam at a plurality of different positions along the main scanning line, the plurality of beam detectors including first and second beam detectors detecting the scanning beam at positions in front-end and rear-end sides, respectively, in the main scanning direction.

1 . An image forming apparatus, comprising:
 a light source configured to illuminate in response to image data;   a deflector configured to deflect an optical beam output from the light source into a scanning beam running along a main scanning line across an image forming area in a main scanning direction;   a plurality of beam detectors to detect the scanning beam at a plurality of corresponding positions along the main scanning line, the plurality of beam detectors including first and second beam detectors detecting the scanning beam at positions in front-end and rear-end sides, respectively, in the main scanning direction;   a clock generator to generate writing clock signals for controlling an illumination of the light source;   a phase variable control unit configured to vary a phase of the writing clock signals generated by the clock generator, in units of 1/n of a writing clock cycle, where n is an integer having a value of 2 or greater;   a measurement mechanism to measure a scanning period between detections of the scanning beam by the first and second beam detectors;   a frequency corrector to correct a frequency of the writing clock signals so the count number measured by the measurement mechanism becomes substantially equivalent to a predetermined reference count number; and   a scanning period corrector to correct a scanning period at a scanning area where phase variable control is not available.   
   
   
       2 . The image forming apparatus is as claimed in  claim 1 , wherein:
 the scanning period corrector stores phase variable amount data for the scanning area where phase variable control is not available and corrects the scanning period based on the phase variable amount data at the scanning area where phase variable control is not available.   
   
   
       3 . The image forming apparatus is as claimed in  claim 1 , wherein:
 the scanning period corrector is configured to calculate a phase variable amount at the scanning area where phase variable control is not available based on a phase variable amount at the scanning area where phase variable control is available, and is configured to correct scanning period based on the calculated phase variable amount at the scanning area where phase variable control is not available.   
   
   
       4 . The image forming apparatus is as claimed in  claim 1 ,
 the scanning period corrector is configured to correct the scanning period based on a frequency at an area where phase variable control is available and based on a frequency at an area where phase variable control is not available.   
   
   
       5 . A method of controlling an image forming process, comprising the steps:
 scanning a scanning beam along a main scanning line across an image forming area in a main scanning direction using a clock signal;   measuring a scanning period between a first beam detector and a second beam detector;   correcting a frequency of the clock signal so that the scanning period which has been measured becomes substantially equivalent to a predetermined reference count number; and   correcting a scanning period at a scanning area where phase variable control is unavailable.   
   
   
       6 . A method according to  claim 5 , wherein the correcting of the scanning period comprises:
 storing phase variable amount data for the scanning area where phase control is not available, and correcting the scanning period based on the phase variable amount data at the scanning area where phase variable control is not available.   
   
   
       7 . A method according to  claim 5 , wherein the correcting of the scanning period comprises:
 calculating a phase variable amount at the scanning area where phase variable control is not available based on a phase variable amount at the scanning area where phase variable control is available, and correcting the scanning period based on the calculated phase variable amount at the scanning area where phase variable control is not available.   
   
   
       8 . A method according to  claim 5 , wherein the correcting of the scanning period comprises:
 correcting the scanning period based on a frequency at an area where phase control is available and based on a frequency at an area where phase control is not available.