1. Field of the Invention
The present invention relates to a method and apparatus for optical scanning, and more particularly to a method and apparatus for optical scanning capable of reducing a dot position displacement and a dot diameter variation based on a variation in wavelength of a laser beam.
2. Discussion of the Background
Background optical scanning apparatuses for use in various image forming apparatuses including laserprinters, facsimile machines, and digital copiers have a drawback in which a focal image position on an imaging surface is varied and consequently an image quality on the imaging surface or on an output recording sheet is degraded. This variation in focal image position on the imaging surface is caused when a wavelength of a laser beam generated by a laser light source is varied and a refractive index of optical devices are accordingly changed.
A technique described in Japanese Laid-Open patent publication, No. 09-021944 attempts to avoid the effects from the variations in wavelength using a dispersion nature of a glass lens to improve an accuracy of output images.
On the other hand, an optical scanning apparatus capable of simultaneous multi-beam scanning using a plurality of laser light sources has been developed in response to an increasing trend for a high speed image reproduction. In the multi-beam method, an image quality may be degraded when the multiple beams are not precisely synchronized which is caused in many cases by variations in wavelength of the laser beams.
A technique described in Japanese Lain-Open patent publication, No. 09-076562, relates to an optical scanning lens to attempt to avoid the above-mentioned problem by reducing a difference in wavelength between multiple beams below 12 nm.
Also, in recent years, a demand for a high image density in the image reproduction using an optical scanning apparatus is rapidly increased and, to implement this demand, requirements for reduction of a dot size and a dot position displacement have grown.
There is a further problem in that an image quality is degraded due to variations in wavelength of a laser beam caused by changed in temperature. That is, when the wavelength of a laser beam is changed, an optical scan-imaging device such as fθ (ef-theta) lens changes its refractive index and, as a consequence, a start position in the scanning line on an imaging surface is changed which causes a displacement of dot position.
The above-mentioned Japanese Lain-Open patent publication, No. 09-021944 proposes a glass-made scanning lens which prevents a deterioration of an image forming accuracy by a reduction of chromatic aberration of the scan-imaging device. However, it is difficult to achieve a similar effect using a plastic lens, which falls into a complex lens structure.