Image forming apparatus

An image forming apparatus, comprising:

• a light source configured to emit a light beam;
• a rotary polygon mirror having a plurality of reflection surfaces and being configured to deflect the light beam so that the light beam emitted from the light source scans on a surface of a photosensitive member;
• a beam detector configured to receive the light beam reflected by each of the plurality of reflection surfaces to output a pulse;
• a pulse interval measurement unit configured to measure a pulse interval of pulses output from the beam detector respectively corresponding to the plurality of reflection surfaces;
• a reflection surface identification unit configured to identify each of the plurality of reflection surfaces;
• a storage portion configured to store a reference pulse interval of each of the plurality of reflection surfaces;
• a correction amount calculation unit configured to calculate a correction amount based on the pulse interval and the reference pulse interval for each of the plurality of reflection surfaces identified by the reflection surface identification unit;
• and
• a light source control unit configured to control the light source based on the correction amount calculated by the correction amount calculation unit.