F.theta. lens and image apparatus using the same

Filing Information

• Patent Number: US5111219
• Application Number: US7712184
• Filing date: 06/07/1991
• Issue date: 05/05/1992
• Predicted expiration date: 06/07/2011

• U.S. Classifications: 346/108  · 359/662  ·
• International Classifications: G01D 942 ·
• International Classifications: 346108 ·
• Foreign Priority: JP2157013 - 06/15/1990 ·

Patent Family

Abstract

An f.theta. lens acting to effect uniform speed scanning while causing a light beam deflected at a uniform angular velocity by the deflecting surface of a deflector to be imaged on a plane to be scanned, wherein the shape of the curved surface of at least the lens surface of the lens which is adjacent to the deflection point side of the deflecting surface in the main scanning plane is an aspherical shape and particularly, near the optical axis, in at least the main scanning plane, this aspherical shape is convex toward the deflection point, and when the radius of curvature of this convex shape near the optical axis in the main scanning plane is r.sub.1 and the focal length of the f.theta. lens near the optical axis in the main scanning plane is fm, 0.ltoreq.r.sub.1 <.vertline.fm.vertline. and yet, when with the point of intersection between the lens surface adjacent to the deflection point side and the optical axis as the origin and with the coordinates system of the x-axis plotted in the direction of the optical axis and the coordinates system of the y-axis plotted in the main scanning plane perpendicularly thereto, the surface shape in the main scanning plane is expressed as a function S.sub.1 (y) in which y is a variable, and when the maximum effective diameter of this surface in the main scanning plane is Ymax, S.sub.1 (y) is defined between 0 and Ymax, and when r.sub.1 <Ymax, -1<S.sub.1 (r.sub.1)/r.sub.1 <0.5 and when r.sub.1 .gtoreq.Ymax, -1.times.Ymax/r.sub.1 <S.sub.1 (Ymax)/Ymax<0.5.times.Ymax/r.sub.1.

Independent Claims | See all claims (24)

1. 1. An f.theta. lens acting to effect uniform speed scanning while causing a light beam deflected at a uniform angular velocity by the deflecting surface of a deflector to be imaged on a plane to be scanned, wherein the shape of the curved surface of at least the lens surface of the lens which is adjacent to the deflection point side of the deflecting surface in the main scanning plane is an aspherical shape and particularly, near the optical axis, in at least the main scanning plane, this aspherical shape is convex toward said deflection point, and when the radius of curvature of this convex shape near the optical axis in the main scanning plane is r.sub.1 and the focal length of the f.theta. lens near the optical axis in the main scanning plane is fm,0.gtoreq.r.sub.1 <.vertline.fm.vertline.and yet, when with the point of intersection between the lens surface adjacent to the deflection point side and the optical axis as the origin and with the coordinates system of the x-axis plotted in the direction of the optical axis and the coordinates system of the y-axis plotted in the main scanning plane perpendicularly thereto, the surface shape in the main scanning plane is expressed as a function S.sub.1 (y) in which y is a variable, and when the maximum effective diameter of this surface in the main scanning plane is Ymax, S.sub.1 (y) is defined between 0 and Ymax, and when r.sub.1 <Ymax,-1<S.sub.1 (r.sub.1)/r.sub.1 <0.5and when r.sub.1 24 Ymax,-1.times.Ymax/r.sub.1 <S.sub.1 (Ymax)/Ymax<0.5.times.Ymax/r.sub.1.
2. 12. A scanning optical system comprising a light source, a first optical system for condensing light from the light source and causing the light to emerge toward the deflection point of the deflecting surface of a deflector, a deflector for deflecting the light beam from the first optical system at a uniform angular velocity on the deflection point of the deflecting surface thereof, a second optical system comprising an f.theta. lens acting to effect uniform speed scanning while causing the light beam deflected at a uniform angular velocity from the deflection point of the deflector to be imaged on a plane to be scanned, and a plane to be scanned on which a recording material such as a photosensitive member is placed, characterized in that the shape of the curved surface in the main scanning plane of at least the lens surface of the f.theta. lens of the second optical system which is adjacent to the deflection point is an aspherical shape and particularly near the optical axis, in at least the main scanning plane, this aspherical shape is convex toward the deflection point, and when the radius of curvature of this convex shape near the optical axis in the main scanning plane is r.sub.1 and the focal length of said f.theta. lens near the optical axis in the main scanning plane is fm,0.ltoreq.r.sub.1 <.vertline.fm.vertline.and yet, when with the point of intersection between the lens surface adjacent to said deflection point side and the optical axis as the origin, the coordinates system of the x-axis is plotted in the direction of the optical axis and the coordinates system of the y-axis is plotted in the main scanning plane perpendicularly thereto and when the surface shape in the main scanning plane is expressed as a function S.sub.1 (y) in which y is a variable and when the maximum effective diameter of said surface in the main scanning plane is Ymax, S.sub.1 (y) is defined between 0 and 5 Ymax, and when r.sub.1 <Ymax,-1<S.sub.1 (r.sub.1)/r.sub.1 <0.5and when r.sub.1 >Ymax,-1.times.Ymax/r.sub.1 <S.sub.1 (Ymax)/Ymax<0.5.times.Ymax/r.sub.1.