Optical scanning device provided in electrophotographic image forming apparatus, image forming apparatus, and optical scanning method

An optical scanning method implemented in an optical scanning device including:

• a rotary polygon mirror having a reflection surface that reflects incident light and configured to scan, in a predetermined scanning cycle, the light reflected on the reflection surface;
• a plurality of light sources configured to emit, toward the reflection surface, a plurality of light beams that are separated from each other in an axis direction of a rotation shaft of the rotary polygon mirror;
• and a light detecting portion configured to detect, on a light scanning path of the rotary polygon mirror, a light amount of each of the light beams emitted from the light sources and reflected on the reflection surface,
• the optical scanning method comprising;
• a first step of obtaining a switching timing to switch a light emission amount of each of the light sources in the scanning cycle, based on light amounts of the light beams detected by the light detecting portion;
• and
• a second step of switching the light emission amount of each of the light sources based on the switching timing obtained in the first step.