Optical scanning device and image forming apparatus

An image forming apparatus comprising:

• an image forming unit that includes an image carrier, a charging unit for charging the surface of the image carrier, an optical scanning device for forming an electrostatic latent image by exposing the surface of the image carrier charged by the charging unit to a light, and a developing unit for forming a toner image by transferring toner to the electrostatic latent image on the image carrier;
• a transfer unit that transfers the toner image formed on the image carrier onto a recording medium;
• and
• a fixing unit that fixes the toner image on the recording medium, wherein
• the optical scanning device including:
• a light source having a plurality of luminous points;
• a light deflecting unit that deflects and scans a plurality of beams emitted from the light source;
• a pre-deflection optical system that is arranged anterior to the light deflecting unit along a beam path, and adjusts the shape of a plurality of beams emitted from the light source and then focuses the beams roughly to a deflecting surface of the light deflecting unit as a linear image extending in a deflection scanning direction;
• and
• a scanning optical system that focuses the beams deflected and scanned by the deflecting surface of the light deflecting unit onto a scanned area, wherein the pre-deflection optical system includes:
• a first optical element that adjusts the shape of a plurality of beams emitted from the light source;
• and
• a second and third optical elements that are arranged such that the second optical element is arranged closer to the light source than the third optical element is, both of the second and third optical elements having no refracting power in the deflection scanning direction and having positive refracting power only in a direction perpendicular to the deflection scanning direction,
• an interval between scanning lines formed on the scanned area and a deviation of the scanning-line interval between scanning positions are adjusted by displacement of the second and third optical elements in a direction of an optical axis of the pre-deflection optical system and displacement of at least one of the second and third optical elements in the direction perpendicular to the deflection scanning direction,
• the second and third optical elements form a zoom lens and are adjacent to one another along the beam path, without any optical element disposed between the second and third optical elements, so that the beams emitted from the light source pass through the second optical element and subsequently pass through the third optical element, and
• when an interval between scanning lines at a scanning center of scanning lines formed on the scanned area from beams emitted from two most alienated luminous points in the deflection scanning direction out of the plurality of luminous points of the light source is denoted by PC, a variation in the scanning-line interval PC caused by displacement of any one of the second and third optical elements due to rotation by an angle Δγ around the optical axis of the pre-deflection optical system is denoted by ΔPC(Δγ), ΔPC(Δγ) at the scanning center caused by displacement of any one of the second and third optical elements having larger refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCM(Δγ), and ΔPC(Δγ) at the scanning center caused by displacement of the other optical element having smaller refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCm(Δγ), the following inequality is satisfied:
• ΔPCM(Δγ)/ΔPCm(Δγ)≧2.5.

An image forming apparatus comprising:

• a plurality of image forming units, each of the image forming units including an image carrier, a charging unit for charging the surface of the image carrier, an optical scanning device for forming an electrostatic latent image by exposing the surface of the image carrier charged by the charging unit to a light, and a developing unit for forming a toner image by transferring toner to the electrostatic latent image on the image carrier;
• a transfer unit that transfers the toner images formed on the respective image carriers of the plurality of image forming units onto a recording medium;
• and
• a fixing unit that fixes the toner images on the recording medium, wherein
• the optical scanning device comprising:
• a light source having a plurality of luminous points;
• a light deflecting unit that deflects and scans a plurality of beams emitted from the light source;
• a pre-deflection optical system that is arranged anterior to the light deflecting unit along a beam path, and adjusts the shape of a plurality of beams emitted from the light source and then focuses the beams roughly to a deflecting surface of the light deflecting unit as a linear image extending in a deflection scanning direction;
• and
• a scanning optical system that focuses the beams deflected and scanned by the deflecting surface of the light deflecting unit onto a scanned area, wherein the pre-deflection optical system includes:
• a first optical element that adjusts the shape of a plurality of beams emitted from the light source;
• and
• a second and third optical elements that are arranged such that the second optical element is arranged closer to the light source than the third optical element is, both of the second and third optical elements having no refracting power in the deflection scanning direction and having positive refracting power only in a direction perpendicular to the deflection scanning direction,
• an interval between scanning lines formed on the scanned area and a deviation of the scanning-line interval between scanning positions are adjusted by displacement of the second and third optical elements in a direction of an optical axis of the pre-deflection optical system and displacement of at least one of the second and third optical elements in the direction perpendicular to the deflection scanning direction,
• the second and third optical elements form a zoom lens and are adjacent to one another along the beam path, without any optical element disposed between the second and third optical elements, so that the beams emitted from the light source pass through the second optical element and subsequently pass through the third optical element, and
• when an interval between scanning lines at a scanning center of scanning lines formed on the scanned area from beams emitted from two most alienated luminous points in the deflection scanning direction out of the plurality of luminous points of the light source is denoted by PC, a variation in the scanning-line interval PC caused by displacement of any one of the second and third optical elements due to rotation by an angle Δγ around the optical axis of the pre-deflection optical system is denoted by ΔPC(Δγ), ΔPC(Δγ) at the scanning center caused by displacement of any one of the second and third optical elements having larger refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCM(Δγ), and ΔPC(Δγ) at the scanning center caused by displacement of the other optical element having smaller refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCm(Δγ), the following inequality is satisfied:
• ΔPCM(Δγ)/ΔPCm(Δγ)≧2.5.

An optical scanning method, comprising:

• deflecting and scanning a plurality of beams emitted from a light source having a plurality of luminous points;
• adjusting, via a pre-deflection optical system, the shape of the plurality of the beams emitted from the light source, the pre-deflection optical system being arranged anterior to the light deflecting unit along a beam path;
• focusing, via the pre-deflection optical system, the plurality of the beams roughly to a deflecting surface of the light deflecting unit as a linear image extending in a deflection scanning direction;
• focusing, via a scanning optical system, the plurality of the beams deflected and scanned by the deflecting surface of the light deflecting unit onto a scanned area, wherein
• the pre-deflection optical system includes:
• a first optical element that adjusts the shape of a plurality of beams emitted from the light source;
• and
• a second and third optical elements that are arranged such that the second optical element is arranged closer to the light source than the third optical element is, both of the second and third optical elements having no refracting power in the deflection scanning direction and having positive refracting power only in a direction perpendicular to the deflection scanning direction,
• an interval between scanning lines formed on the scanned area and a deviation of the scanning-line interval between scanning positions are adjusted by displacement of the second and third optical elements in a direction of an optical axis of the pre-deflection optical system and displacement of at least one of the second and third optical elements in the direction perpendicular to the deflection scanning direction,
• the second and third optical elements form a zoom lens and are adjacent to one another along the beam path, without any optical element disposed between the second and third optical elements, so that the beams emitted from the light source pass through the second optical element and subsequently pass through the third optical element, and
• when an interval between scanning lines at a scanning center of scanning lines formed on the scanned area from beams emitted from two most alienated luminous points in the deflection scanning direction out of the plurality of luminous points of the light source is denoted by PC, a variation in the scanning-line interval PC caused by displacement of any one of the second and third optical elements due to rotation by an angle Δγ around the optical axis of the pre-deflection optical system is denoted by ΔPC(Δγ), ΔPC(Δγ) at the scanning center caused by displacement of any one of the second and third optical elements having larger refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCM(Δγ), and ΔPC(Δγ) at the scanning center caused by displacement of the other optical element having smaller refracting power in the direction perpendicular to the deflection scanning direction due to rotation by the angle Δγ around the optical axis is denoted by ΔPCm(Δγ), the following inequality is satisfied:
• ΔPCM(Δγ)/ΔPCm(Δγ)≧2.5.