Optical scanning system

Laser scanning apparatus comprising:

• laser means for providing a collimated beam of light;
• a cylindrical drum having a photosensitive outer surface and a central longitudinal axis;
• means for controllably rotating the cylindrical drum about its longitudinal axis;
• rotating mirror means for reflecting and repeatedly scanning the collimated beam of light longitudinally along the drum surface, in synchronism with rotation of the drum about its longitudinal axis, such that the beam of light traces a raster scan pattern on the surface;
• an aperture plate positioned immediately adjacent the cylindrical drum, in the path of the scanning beam of light, the plate including means defining an elongated aperture oriented substantially parallel with the longitudinal axis of the cylindrical drum, the aperture having a substantially uniform width in a cross-scan direction, perpendicular to the scan direction, that is narrower than the cross-scan width of the scanning beam, such that the aperture transmits to the drum surface only a limited portion of the scanning beam;
• wherein imperfections in the rotating mirror means causes the scanning beam of light to deviate in the cross-scan direction from its desired longitudinal scan direction, the deviation being less than the width of the beam at its point of impingement on the aperture plate;
• wherein the intensity profile of the beam of light is sufficiently uniform that the intensity of the portion of the beam transmitted through the aperture to the drum surface is substantially unaffected by the deviation;
• and
• wherein the aperture plate is controllably rotatable about the axis of the elongated aperture, to adjust the width of the aperture in the cross-scan direction and thereby adjust the cross-scan width of the portion of the beam of light transmitted to the drum surface.

A method for repeatedly scanning a beam of light longitudinally along an outer photosensitive surface of a cylindrical drum, the method comprising steps of:

• providing a collimated beam of light;
• controllably rotating the cylindrical drum about a central longitudinal axis;
• repeatedly scanning the beam of light longitudinally along the outer photosensitive surface of the cylindrical drum, in synchronism with rotation of the drum about is longitudinal axis;
• limiting the width of the scanning beam of light in a direction perpendicular to the longitudinal scanning direction using an aperture plate positioned adjacent to the surface of the cylindrical drum, in the path of the beam, the plate including an elongated, narrow aperture oriented substantially parallel with the drum's longitudinal axis and having a width less than the width of the beam in the cross-scan direction;
• and
• controllably rotating the aperture plate about the axis of the elongated aperture, to adjust the width of the aperture in the cross-scan direction and thereby adjust the cross-scan width of the portion of the beam of light transmitted to the drum surface.