Patent attributes
A scanning unit of an image-forming device has a polygon mirror for deflecting a laser beam across the surface of a photosensitive member. A disc that rotates together with the polygon mirror is provided with a reference mark. When the reference mark passes a photointerrupter, the photointerrupter detects the rotating position of the polygon mirror. A counter measures the number of pulses in a reference clock from the moment the photointerrupter detects the reference mark until a sensor provided in a measuring device detects the laser beam deflected by the polygon mirror, and the value of the counter is stored in memory as timing data. When the scanning unit is subsequently mounted in an image-forming device, the count value stored in memory is used to control the exposure timing for irradiating the photosensitive member in the image-forming device with the laser beam.
