Multi-beam scanning optical system and image forming apparatus using the same

A multi-beam scanning optical system comprising:

• n light sources;
• deflecting means which deflects n laser beams emitted from the n light sources;
• and
• scanning optical means which focuses the n laser beams deflected by the deflecting means onto the surface of a photosensitive member having a cylindrical shape and forms n scanning lines,
• wherein the scanning optical means is set such that lateral chromatic aberration is overcorrected, and
• wherein, when the first scanning line is formed on the surface of the photosensitive member at a position displaced from an end point of the photosensitive member by a distance S in a sub-scanning direction and the mth (1<m≦n) scanning line is formed on the surface of the photosensitive member at a position displaced from the end point by a distance (S+d) in the sub-scanning direction, and when the oscillation wavelength of the light source which emits the light beam forming the first scanning line is defined as λ1 and the oscillation wavelength of the light source which emits the light beam forming the mth scanning line is defined as λm, the following expressions are satisfied:
• [LeftBracketingBar] Δ ⁢ ⁢ Y 1 - Δ ⁢ ⁢ Y 2 ⁡ ( λ m - λ 1 ) [RightBracketingBar] ≤ D 4 Δ ⁢ ⁢ Y 1 = Δ ⁢ ⁢ L ⁢ ⁢ tan ⁢ ⁢ α Δ ⁢ ⁢ L = R 2 - ( S + d ) 2 - R 2 - S 2
• wherein,
• ΔY2 is a dot displacement in the main-scanning direction at the end in the main-scanning direction caused per unit wavelength,
• D is the size of a single pixel,
• α is the maximum angle among angles formed between the normal at the surface of the photosensitive member and the laser beams in the main-scanning direction, and
• R is the radius of the photosensitive member.

A multi-beam scanning optical system comprising:

• a plurality of light sources;
• deflecting means which deflects a plurality of laser beams emitted from the light sources;
• and
• scanning optical means which focuses the laser beams deflected by the deflecting means onto the surface of a photosensitive member;
• wherein the scanning optical means is set such that lateral chromatic aberration is undercorrected, and
• wherein, among angles formed between each laser beam incident on the surface of the photosensitive member and the normal at the surface of the photosensitive member in a sub-scanning direction, the oscillation wavelength of the light source that emits a laser beam forming the minimum angle is set to a value larger than the oscillation wavelength of the light source that emits a laser beam forming the maximum angle.

A multi-beam scanning optical system comprising:

• n light sources;
• deflecting means which deflects n laser beams emitted from the n light sources;
• and
• scanning optical means which focuses the n laser beams deflected by the deflecting means onto the surface of a photosensitive member having a cylindrical shape and forms n scanning lines,
• wherein the scanning optical means is set such that lateral chromatic aberration is undercorrected, and
• wherein, when the first scanning line is formed on the surface of the photosensitive member at a position displaced from an end point of the photosensitive member by a distance S in a sub-scanning direction and the mth (1<m≦n) scanning line is formed on the surface of the photosensitive member at a position displaced from the end point by a distance (S+d) in the sub-scanning direction, and when the oscillation wavelength of the light source which emits the light beam forming the first scanning line is defined as λ1 and the oscillation wavelength of the light source which emits the light beam forming the mth scanning line is defined as λm, the following expressions are satisfied:
• [LeftBracketingBar] Δ ⁢ ⁢ Y 1 - Δ ⁢ ⁢ Y 2 ⁡ ( λ 1 - λ m ) [RightBracketingBar] ≤ D 4 ⁢ ⁢ Δ ⁢ ⁢ Y 1 = Δ ⁢ ⁢ L ⁢ ⁢ tan ⁢ ⁢ α ⁢ ⁢ Δ ⁢ ⁢ L = R 2 - ( S + d ) 2 - R 2 - S 2
• wherein,
• ΔY2 is a dot displacement in the main-scanning direction at the end in the main-scanning direction caused per unit wavelength,
• D is the size of a single pixel,
• α is the maximum angle among angles formed between the normal at the surface of the photosensitive member and the laser beams in the main-scanning direction, and
• R is the radius of the photosensitive member.

A multi-beam scanning optical system comprising:

• a plurality of light sources;
• deflecting means which deflects a plurality of laser beams emitted from the light sources;
• and
• scanning optical means which focuses the laser beams deflected by the deflecting means onto the surface of a photosensitive member;
• wherein a dot displacement direction in a main-scanning direction due to a difference in light path lengths of the laser beams is opposite to a dot displacement direction in the main-scanning direction due to a difference in wavelengths of the laser beams.