An exposure unit comprises a polygon mirror configured to include a plurality of reflection surfaces; first to fourth light sources arranged at different angle positions along a rotation direction of the polygon mirror; two photo detectors configured to selectively detect first laser light and fourth laser light respectively at a scanning start end side of a main scanning direction among first to fourth laser light which are emitted from the respective light sources and reflected by the polygon mirror to be scanned in the main scanning direction; and a control section configured to control light emitting timing of the first to the fourth light sources on the basis of detection results of these photo detectors.

An exposure unit comprises a polygon mirror configured to include a plurality of reflection surfaces; first to fourth light sources arranged at different angle positions along a rotation direction of the polygon mirror; two photo detectors configured to selectively detect first laser light and fourth laser light respectively at a scanning start end side of a main scanning direction among first to fourth laser light which are emitted from the respective light sources and reflected by the polygon mirror to be scanned in the main scanning direction; and a control section configured to control light emitting timing of the first to the fourth light sources on the basis of detection results of these photo detectors.

A light source module includes a first light source, a second light source, a beam combiner, a substrate, and an adjuster. The first light source emits a first light beam. The second light source emits a second light beam parallel to the first light beam. The beam combiner combines and emits the first light beam and the second light beam. The first light source and the second light source are mounted on the substrate. The adjuster adjusts a mounting height of at least one of the first light source or the second light source on the substrate to vary at least one of (a) a first optical path length of the first light beam or (b) a second optical path length of the second light beam.

A light source module includes a first light source, a second light source, a beam combiner, a substrate, and an adjuster. The first light source emits a first light beam. The second light source emits a second light beam parallel to the first light beam. The beam combiner combines and emits the first light beam and the second light beam. The first light source and the second light source are mounted on the substrate. The adjuster adjusts a mounting height of at least one of the first light source or the second light source on the substrate to vary at least one of (a) a first optical path length of the first light beam or (b) a second optical path length of the second light beam.