Patent ID: 7791633

Claim:
An optical scanning unit having a polygon mirror irradiated with a plurality of light beams emitted from a light source according to image data, the optical scanning unit converting the plurality of light beams to scanning lights by a rotation of the polygon mirror, the optical scanning unit scanning and exposing a plurality of photoreceptors simultaneously with the plurality of scanning lights to form latent images on the respective photoreceptors, the optical scanning unit comprising: a primary optical system unit including the light source and an optical system that emits the plurality of light beams emitted from the light source toward the polygon mirror, the light source including a plurality of laser diodes mounted to a base in the primary optical system unit; and a secondary optical system unit including the polygon mirror and another optical system that emits the light beams reflected by the polygon mirror toward the photoreceptors mounted to a chassis in the secondary optical system unit, wherein the primary optical system unit is fitted removably as a unit to the secondary optical system unit to permit simultaneous removal of all of said laser diodes without removal of any individual one of said laser diodes and wherein a combined unit of the primary optical system unit and the secondary optical system unit is fitted removably as a unit to an image forming apparatus comprising the photoreceptors, wherein optical parts supported by the base of the primary optical system unit and supported by the chassis of the secondary optical system unit are removable to change the optical parts in accordance with changes in specifications of the image forming apparatus, and wherein the optical scanning unit is completed by combining the primary optical system unit with the secondary optical system unit in accordance with the specifications of the image forming apparatus, and wherein the primary optical system unit includes a first laser diode, a second laser diode, a third laser diode, and a fourth laser diode, which act as the light source, wherein the primary optical system unit further includes: first mirrors that reflect the light beams emitted from the second to fourth laser diodes; a second mirror that reflects the light beam emitted from the first laser diode and the light beams reflected by the first mirror; a primary optical system cylindrical lens that acts on the light beams emitted from the second mirror; and a third mirror that reflects the light beams emitted from the primary optical system cylindrical lens toward the polygon mirror, and wherein the secondary optical system unit includes the polygon mirror, an fθ lens that acts on the light beams emitted from the polygon mirror, and a secondary optical system cylindrical lens that makes the light beams emitted from the fθ lens converge on the surfaces of the photoreceptors.