Optical: systems and elements – Deflection using a moving element
Reexamination Certificate
2002-10-25
2003-08-12
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
C359S212100, C359S216100, C347S259000
Reexamination Certificate
active
06606179
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical scanning device used in a writing optical system of an image forming apparatus such as a laser printer, a digital copier, a facsimile machine or the like, a measurement apparatus, an inspection apparatus or the like, and to a line-image forming optical system used in an optical scanning device, an imaging adjustment method used in the optical scanning device and an image forming apparatus.
2. Description of the Related Art
An optical scanning device deflecting a light flux from a light source, condensing the light flux onto a surface to be scanned as a beam spot through a scanning optical system and scanning the surface to be scanned has been well-known in connection with a laser printer, a digital copier, a facsimile machine, and so forth. In such an optical scanning device, in order to reduce a lens cost and/or in order to achieve a special lens surface shape, a resin-made lens (meaning a lens made of resin, through the specification and claims) is used. Especially, various shapes of lens surface have been proposed for a scanning and imaging lens of a scanning and imaging optical system causing a deflected light flux to form an image on a surface to be scanned, in order to well correct curvature of field and uniform-velocity characteristics such as a linearity, and, a resin-made lens is suitable for achieving such a special lens surface shape.
However, as well-known, a resin-made lens involves a problem in that, a change in volume due to a change in temperature causes a curvature and/or refractive index of the lens to change, and a lens performance, in particular, a position of focus on a surface to be scanned to change. Such a change in position of focus results in increase of a spot diameter of a beam spot on a surface to be scanned, and degradation of resolution in optical scanning.
In this regard, because a change in position of focus due to a change in temperature of resin-made lenses occurs in a positive lens and a negative lens reversely to one another, it has been proposed to cancel a change in position of focus of a resin-made scanning and imaging lens due to a change in temperature by disposing a resin-made lens having a power reverse to that of the resin-made scanning and imaging lens on a light path from a light source to a light deflector (see Japanese Laid-Open Patent Application No. 8-160330 and Japanese Laid-Open Patent Application No. 8-292388).
An optical scanning device disclosed in Japanese Laid-Open Patent Application No. 8-160330 includes a light source, a deflector, an entrance optical system, a scanning optical system, and a medium to be scanned. The entrance optical system includes a first optical system (collimator lens) transforming a divergent light flux from the light source into a parallel light flux, and a second optical system condensing the light flux from the first optical system in sub-scanning direction so as to cause the light flux to form an image on or in the proximity of the deflector. Either one of the first and second optical system includes a resin-made optical component (lens) having a negative power in sub-scanning direction.
A scanning optical device disclosed in Japanese Laid-Open Patent Application No. 8-292388 has a negative lens having a negative refracting power only in sub-scanning direction and made of resin in a first imaging part forming an image on or in the proximity of a deflection position of a deflector, and performs temperature compensation.
However, in such an optical scanning device, a resin-made lens disposed between the light source and deflector for the correction has a negative power only in sub-scanning direction and no power in main scanning direction. Accordingly, it is not possible to correct a shift in focus position (shift in imaging position of a beam spot) in main scanning direction due to a change in temperature of a scanning and imaging lens.
Further, a lens for the correction in the related art has an ordinary arc-shape lens section. Accordingly, wavefront aberration is increased by the lens for the correction, and, thereby, achievement of a small-diameter beam spot is obstructed.
An optical scanning device ‘transforms a beam (denoting an ‘optical beam’ through the specification and claims) from a light source into a beam having a predetermined beam style by a coupling optical system, causes this beam to form a line image long in main scanning direction by a line-image forming optical system, deflects the beam by a light deflector having a deflection reflective surface at or in the proximity of the imaging position of this line image, condenses the deflected beam toward a surface to be scanned by a scanning and imaging optical system, forms a beam spot on the surface to be scanned, and scans the surface to be scanned’. By thus causing a beam from the light source to form a line image long in main scanning direction on or in the proximity of the deflection reflective surface of the light deflector’, it is possible to correct so-called ‘surface inclination’ of the light deflector.
Recently, increase in image density has been demanded in a digital copier and/or a laser printer, and reduction in diameter of beam spot formed on a surface to be scanned is demanded. Further, as mentioned above, making a lens from resin in the scanning and imaging optical system has been performed progressively in order to reduce costs and/or to achieve ‘special surface shape’ needed to reduce diameter of beam spot.
A beam spot is formed as a result of a deflected beam deflected by the light deflector being condensed toward the surface to be scanned by the scanning and imaging optical system. Ideally, the beam spot is formed by a beam waist of the condensed deflected beam. In the optical scanning device having the above-described configuration, the scanning and imaging optical system is an ‘anamorphic optical system such that power thereof is different between main scanning direction and sub-scanning direction’. Accordingly, generally, the position of beam waist of the deflected beam in main scanning direction does not completely coincide with the position of beam waist of the deflected beam in sub-scanning direction.
An image surface in main scanning direction and sub-scanning direction is obtained from a collection the beam-waist positions in main scanning direction and sub-scanning direction. The image surface is curved in accordance with curvature of field of the optical system.
In order to achieve satisfactory optical scanning with ‘a beam spot having a diameter small in both main scanning direction and sub-scanning direction’, it is necessary that the beam-waist positions in main scanning direction and sub-scanning direction substantially coincide with the surface to be scanned ‘whatever the position in main scanning direction (image height)’, that is, the image surface in main scanning direction and sub-scanning direction substantially coincides with the surface to be scanned. In order to satisfy this condition, optical design is made so as to well correct curvature of field in main scanning direction and sub-scanning direction.
However, although a satisfactory result is obtained on design, optical characteristics on design are not actually achieved. For example, when an f&thgr; lens which is generally used as the scanning and imaging optical system includes working errors and/or assembly errors, an actual image-surface position of a deflected beam is different from the surface to be scanned. As a result, the diameter of the beam spot is larger than the designed amount.
It is possible to form a beam spot substantially having a spot diameter in accordance with design on the surface to be scanned by making an f&thgr; lens or the like ‘substantially in accordance with design’ and assembling it with high accuracy. However, when the f&thgr; lens includes a ‘resin-made lens’, a change in refractive index and/or a change in shape of the resin-made lens’ occurs when the temperature of the optical scanning device chan
Aoki Magane
Atsuumi Hiromichi
Sakai Kohji
Suzuki Seizo
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Phan James
Ricoh & Company, Ltd.
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