Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-05-29
2002-02-19
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S207110, C359S216100, C347S259000
Reexamination Certificate
active
06348988
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning image forming lens and an optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus for optically scanning a surface to be scanned at a constant velocity by deflecting a luminous flux emitted from a light source via an optical deflector so as to transmit the luminous flux through a scanning image forming lens to be condensed into an optical beam spot on the surface to the scanned is well known in an image forming apparatus such as a laser printer, a digital copier, a facsimile machine and other such devices. The luminous flux is deflected in a direction corresponding to a main scanning direction for the surface to be scanned at equiangular velocity by rotation of the optical deflector such that the optical beam spot formed by the scanning image forming lens scans the surface to be scanned at a constant velocity in the main scanning direction. Typically, the surface to be scanned includes a photoconductor and the optical beam spot forms picture elements which constitute an image to be formed on the surface to be scanned.
The above-mentioned main scanning direction refers to the direction corresponding to the main scanning direction for a surface to be scanned along a light path from a light source to the surface to be scanned. A direction corresponding to a sub scanning direction refers to the direction corresponding to a sub scanning direction for the surface to be scanned along the light path. The sub scanning direction is substantially perpendicular to the direction corresponding to the main scanning direction.
In such an optical scanning apparatus, when the optical beam spot scans the surface to be scanned in the main scanning direction, if the diameter of the optical beam spot changes depending upon the position of the optical beam spot in the main scanning direction on the surface to be scanned, the size of each picture element, which is formed by the optical beam spot on the surface to be scanned so as to form an image on the surface to be scanned, changes depending upon the position where each picture element is written on the surface to be scanned in the main scanning direction. Consequently, the resolution of the formed image changes along the main scanning direction, resulting in deteriorating of image quality. A position on the surface to be scanned in the main scanning direction is sometimes referred to herein “image height”.
A change in the diameter of the optical beam spot in the main scanning direction on the surface to be scanned according to the position of the optical beam spot in the main scanning direction can be corrected to a certain degree, for example, by adjusting the time for writing each picture element on the surface to be scanned with the optical beam spot. However, a change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction cannot be corrected by adjusting the writing time for the optical beam spot. The change in the diameter of the optical beam spot in the sub scanning direction according to the position of the optical beam spot in the main scanning direction is typically corrected through adjustment of the optical performance of an optical lens system used for transmitting the light flux, which is deflected by the optical deflector in the direction corresponding to the main scanning direction, such that the luminous flux is condensed into an optical beam spot on the surface to be scanned. The optical lens system for transmitting the deflected light flux so as to form an optical beam spot and to scan the surface to be scanned with the optical beam spot is herein called a scanning image forming lens.
It is known that a change of an optical beam spot diameter in the sub scanning direction on a surface to be scanned according to the image height can be suppressed by correcting the curvature of field of the scanning image forming lens in the sub scanning direction. Typically, correction of the curvature of field in the sub scanning direction is made only relative to a paraxial luminous flux of the scanning image forming lens system.
However, correction of the curvature of field of the scanning image forming lens must be performed while keeping the other optical characteristics of the scanning image forming lens at a satisfactory level. For example, the constant velocity characteristics must be kept at a satisfactory level for enabling the optical beam spot to be moved at a constant velocity along the surface to be scanned. Typically, when the curvature of field is corrected, the other optical characteristics deteriorate.
In addition, when, for example, a photoconductor is used in an optical scanning apparatus as the surface to be scanned for forming an image thereupon, an assembly tolerance of the photoconductor in relation to the scanning image forming lens must be carefully considered in the design of the apparatus for accomplishing a desired level of the curvature of field in the scanning image forming lens. More specifically, even when the curvature of field of the scanning image forming lens is accurately corrected in the design of the scanning image forming lens, the curvature of field of the scanning image forming lens as designed may not necessarily be realized if the actual position of the surface to be scanned in relation to the scanning image forming lens is deviated from the designed position due to, for example, an assembling error relating to the position of the surface to be scanned relative to the scanning image forming lens.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a scanning image forming lens and an optical scanning apparatus that accurately correct any changes in diameter of an optical beam spot in a sub scanning direction according to the image height.
The preferred embodiments of the present invention also provide a scanning image forming lens and an optical scanning apparatus that limit any changes of the diameter of an optical beam spot according to the image height within a predetermined range in a sub scanning direction when a surface to be scanned is positioned within a predetermined range of assembling tolerance relative to the scanning image forming lens.
A scanning image forming lens system according to a specific preferred embodiment of the present invention is preferably used in an optical scanning apparatus for optically scanning a surface to be scanned by deflecting a luminous flux emitted from a light source in a direction corresponding to a main scanning direction via an optical deflector at equiangular velocity. The scanning image forming lens system transmits the luminous flux deflected by the optical deflector so as to condense the luminous flux into an optical beam spot on the surface to be scanned and to scan the surface to be scanned with the optical beam spot.
The scanning image forming lens system according to preferred embodiments of the present invention includes one or more image forming optical elements. The one or more image forming elements preferably includes at least one lens having at least one lens surface that preferably has a non-arc shape in a sub scanning cross section. The non-arc shape changes according to a position in a direction that is substantially perpendicular to the sub scanning cross section (i.e., a position in a direction corresponding to a main scanning direction, which is referred to as an image height) such that a positional deviation of the optical beam waist of the deflected luminous flux from the surface to the scanned at each position in the direction that is substantially perpendicular to the sub scanning cross section, which is caused by a paraxial curvature of field of the scanning image forming lens system in a sub scanning direction, is corrected.
The above-mentioned sub scanning cross section herein refers to a flat cross section which is substantia
Aoki Magane
Sakai Kohji
Greenberg & Traurig, LLP
Phan James
LandOfFree
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