Optical scanning unit and image forming apparatus

An optical scanning device comprising:

• a light source;
• a rotatable multi-faceted mirror having a plurality of reflection faces reflecting light flux emitted from the light source to scan a scanning area on a scanned object, a width of the light flux entering the rotatable multi-faceted mirror is smaller than a length of a reflection face of the rotatable multi-faceted mirror in a main scanning direction;
• and
• a controller configured to control the optical scanning device, wherein:
• when a center portion of the scanning area of the scanned object is scanned, the controller controls light emission timing of the light source such that an entire width of the light flux emitted from the light source enters a first reflection face of the rotatable multi-faceted mirror, and controls a width of the light flux entering the rotatable multi-facet mirror, and a width of light flux reflected on the first reflection face and directed to the center portion of the scanning area of the scanned object to have a same width,
• when at least one of end portions of the scanning area of the scanned object is scanned, the controller controls the light emission timing of the light source such that the light flux emitted from the light source enters the first reflection face and a second reflection face adjacent to the first reflection face of the rotatable multi-faceted mirror, and controls the width of the light flux reflected on the first reflection face and directed to the at least one of the end portions of the scanning area of the scanned object to be smaller than the width of the light flux entering the rotatable multi-faceted mirror,
• a number of reflection faces of the rotatable multi-faceted mirror is eight or less, and
• when the width of the light flux reflected by the first reflection face and directed to at least one of the end portions of the scanning area of the scanned object is defined as a light flux width d1, and the width of the light flux reflected by the first reflection face and directed to the center portion of the scanning area of the scanned object is defined as a light flux width d2, d2/d1 is set to range from 1 to 1.2.