Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-08-29
2002-09-10
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S244000
Reexamination Certificate
active
06448998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a scanning and imaging lens, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices are widely used in ‘image forming apparatus’ such as a digital copier, an optical printer, an optical plate-making machine, a facsimile machine and so forth. A writing density of optical scanning devices has been increased to 1200 dpi, 1600 dpi, and is intended to be increased, further higher.
In order to achieve such high-density writing, it is necessary to form a beam spot having a small diameter, and, also, quality and stability of a beam spot is needed to be improved. Stability of a beam spot is determined from determining whether or not ‘a variation in beam-spot diameter on a surface to be scanned due to a variation in image height’ is very small and stable. Quality of a beam spot is determined from determining whether or not ‘the light-intensity distribution of the beam spot has a simple mountain shape and does not have a complicated lower slope shape’.
In order to achieve a beam spot having high-quality and stability, it is necessary for a scanning and imaging optical system of an optical scanning device to have a high performance for forming a beam spot on a surface to be scanned using a deflected light flux. A factor causing a beam-spot diameter to fluctuate is, as is well known, ‘curvature of field in a scanning and imaging optical system’, and many scanning and imaging optical systems in which curvature of field is well corrected have been proposed. Further, it is important for an optical magnification in a scanning and imaging optical system to be fixed when an image height of a beam spot changes.
However, in order to form a beam spot having stability and high quality, not only it is necessary to correct a geometrical optical performance such as curvature of field and an optical magnification but also it is important to ‘set a wave-optical wavefront aberration to be fixed between respective image heights’.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve high-density, satisfactory optical scanning with a stable and high-quality beam spot, by well correcting not only curvature of field and optical magnification but also ‘wavefront aberration on pupil’ in a scanning and imaging optical system.
A scanning and imaging lens according to the present invention is a ‘scanning and imaging lens which condenses a light flux deflected by a deflector onto a surface to be scanned as a beam spot’.
In the following description, for each of particular lenses constituting the scanning and imaging lens, a surface on the incidence side (surface on the side of deflector) is referred to as a first surface, and a surface on the exit side (surface on the side of surface to be scanned) is referred to as a second surface. However, when lens surfaces of the scanning and imaging lens are referred to in series from the side of deflector to the side of surface to be scanned in surface number, they will be expressed as ‘first surface, second surface, third surface, . . . ’.
A scanning and imaging lens according to a first aspect of the present invention has the following features:
The scanning and imaging lens comprises ‘a plurality of lenses’.
These lenses have ‘shapes in main scanning cross section (imaginary plane cross section including an optical axis of a lens and in parallel to main scanning direction) such that a lens on the side of deflector (lens nearest to the deflector when the scanning and imaging lens comprises more than two lenses) has a convex shape in main scanning cross section in each of both surfaces, and has ‘at least one surface’ having a non-arc shape in main scanning cross section, and a lens on the side of surface to be scanned (lens nearest to the surface to be scanned when the scanning and imaging lens comprises more than two lenses) has a convex shape in main scanning cross section in the first surface, and has ‘at least one surface’ having a non-arc shape in main scanning cross section.
Further, the scanning and imaging lens has ‘at least two special toroidal surfaces’.
The ‘special toroidal surface’ is a toroidal surface such that a curvature in sub-scanning cross section (imaginary plane cross section perpendicular to main scanning direction) varies in main scanning direction.
As will be described, there are lenses included in a scanning and imaging lens according to the present invention having a ‘surface shape not having a symmetry axis’. Therefore, an optical axis of a lens in the specification and claims denotes a reference axis in direction corresponding to an ordinary optical axis in an analytic expression determining the lens surface.
The scanning and imaging lens according to the first aspect of the present invention preferably has a lateral magnification &bgr;o in sub-scanning direction on a light path from the origin of deflection by the deflector to the surface to be scanned when a chief ray passes through a point on the surface to be scanned at which an image height is 0 satisfying the following condition:
0.5<|&bgr;
o
|<1.5 (1)
Further, the scanning and imaging lens preferably has the above-mentioned lateral magnification &bgr;o and a lateral magnification &bgr;h in sub-scanning direction when a chief ray passes through the surface to be scanned at which an image height is an arbitrary amount satisfying the following conditon:
0.9<|&bgr;
h/&bgr;o
|<1.1 (2)
Further, ‘at least one surface of the special toroidal surfaces’ in the scanning and imaging lens may have a non-arc shape in main scanning cross section, and, also, a non-arc shape in sub-scanning cross section, wherein the non-arc shape in sub-scanning cross section may vary according to a coordinate Y in main scanning direction, and the non-arc shape in sub-scanning cross section in each coordinate Y may be determined so as to correct wavefront aberration on the surface to be scanned, and wherein at least one surface of the special toroidal surfaces may be such that ‘a curvature in sub-scanning cross section varies asymmetrically with respect to the optical axis in main scanning direction’.
Further, the scanning and imaging lens may comprise two single lenses L
1
and L
2
disposed from the side of deflector to the side of surface to be scanned.
Each of the lenses L
1
and L
2
of the scanning and imaging lens may have a ‘meniscus shape such that the side of deflector is concave and the side of surface to be scanned is convex’ in sub-scanning cross section on the optical axis and in the proximity thereof.
Each of the lenses L
1
and L
2
preferably has a positive power in main scanning cross section, and a radius Rm
2
of curvature of the second surface of the lens L
1
on the optical axis in main scanning cross section and a radius Rm
3
of curvature of the first surface of the lens L
2
on the optical axis in main scanning cross section preferably satisfy the following condition:
0.1<|
Rm
2
/
Rm
3
|<0.5 (3)
Further, a radius Rs
2
of curvature of the second surface of the lens L
1
on the optical axis in sub-scanning cross section and a radius Rs
4
of curvature of the second surface of the lens L
2
on the optical axis in sub-scanning cross section preferably satisfy the following condition:
0.05<|
Rs
4
/
Rs
2
|<0.5 (4)
The scanning and imaging lens may have at least two special toroidal surfaces in each of which ‘a curvature in sub-scanning cross section varies asymmetrically with respect to the optical axis in main scanning direction’, and each of the lenses L
1
and L
2
may have ‘at least one of these special toroidal surfaces’.
An optical scanning device according to the present invention is an ‘optical scanning device deflecting a light flux from a light source by a deflector, condensing the deflected light flux onto a surface to be scanned as a beam spot by a scanning and imaging lens, and, thus, scan
Aoki Magane
Atsuumi Hiromichi
Sakai Kohji
Suzuki Seizo
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pham Hai
Ricoh & Company, Ltd.
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