An optical scanner includes a single deflector to optically scan a plurality of target surfaces to be scanned. The deflector has a common rotary axis for deflecting reflective surfaces and is shared by all the beams from a plurality of light sources. The optical scanner includes photodetectors arranged to receive the beams deflected at the deflector. The beams traveling toward the deflector have an open angle in a deflecting rotation plane. A scanning optical system for guiding the deflected beam to the corresponding target surface includes two or more scanning lenses. A scanning lens proximate to the target surface passes only the beams traveling toward the same target surface. Scanning lenses proximate to the target surfaces for guiding the beams to different target surfaces have optical actions different from each other.
A scanning optical system leads the light fluxes deflected by the polygon mirror to a photosensitive drum. An absolute value of a lateral magnification in a main scanning direction is larger than an absolute value of a lateral magnification in a sub-scanning direction. Moreover, a beam diameter in the sub-scanning direction on a surface of the photosensitive drum is equal to or smaller than a beam diameter in the main scanning direction and larger than a scan line interval.
A common photodetecting unit detects a plurality of beams scanned by a plurality of polyhedral reflection mirrors provided in a multiple stages with a common rotation axis, and generates a synchronization detection signal based on detected beams. The reflection mirrors make a predetermined angle .theta.1 in a direction of rotation of the reflection mirrors. When time between two consecutive synchronization detection signals on a time line generated by the common photodetecting unit is ti, where i is a positive integer equal to or smaller than number of split beams, at least one of ti is different from others.
A common photodetecting unit detects a plurality of beams scanned by a plurality of polyhedral reflection mirrors provided in a multiple stages with a common rotation axis, and generates a synchronization detection signal based on detected beams. The reflection mirrors make a predetermined angle .theta.1 in a direction of rotation of the reflection mirrors. When time between two consecutive synchronization detection signals on a time line generated by the common photodetecting unit is ti, where i is a positive integer equal to or smaller than number of split beams, at least one of ti is different from others.
An optical scanner includes a light source, an optical coupler, an optical line image unit, a deflector, and an optical scanning unit. The optical scanning unit includes scanning lenses that guide the beams to a surface to be scanned. A surface on the deflector side of the scanning lens closest to a deflection reflecting surface has a negative power in a vertical scanning direction, and is a special toric surface in which a radius of curvature in a vertical scanning changes from an optical axis of the lens surface toward a periphery of the horizontal scanning direction. An F number of the beams toward the surface to be scanned of the scanning lens in the vertical scanning direction is larger in a peripheral part than in a central part in an effective scanning width.
