Optical: systems and elements – Lens – Telecentric system
Reexamination Certificate
1999-08-30
2001-05-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Telecentric system
C359S649000, C359S651000, C359S662000, C359S740000, C355S068000, C355S063000
Reexamination Certificate
active
06226129
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging optical system suitable for an image forming apparatus such as a copier and a printer that employ a semiconductor laser array having a plurality of independently controllable laser elements, and an image forming apparatus using the imaging optical system, and more particularly to an imaging optical system and an image forming apparatus that provide a small-size design and high resolution.
2. Description of the Prior Art
In an image forming apparatus such as a laser printer and a copier that scan a laser beam, a method of scanning by a polygon mirror is commonly used. However, image forming apparatuses employing the polygon mirror have a problem in that they cause mechanical vibration and noise because of the mechanical rotation of the mirror and a high scanning speed is difficult to achieve because of the low reliability of a motor for rotating the mirror and the limitation of rotation speed. Also, different scanning line densities, bent scanning lines, different scanning densities, or the like arise because of the surface inclination of each mirror, and the precision of manufacturing f&thgr; lens, causing degradation of image quality. Accordingly, there is a demand for a method of scanning a laser beam without using such a polygon mirror.
A conventional image forming apparatus employing a method of scanning without using a polygon mirror is disclosed in Japanese Published Unexamined Patent Application No. Hei 10-16297, for example.
FIG. 15
shows the image forming apparatus. The image forming apparatus comprises a semiconductor laser array
1
as a light source having a plurality of laser elements
1
a
arranged linearly, and an imaging optical system
4
as an imaging optical system of a magnification optical system for imaging laser beams
2
from the laser elements
1
a
on a photosensitive surface
3
a
. The imaging optical system
4
, which constitutes a telecentric optical system between the composite focus of the photosensitive surface
3
a
side and the light source, comprises a group of imaging lenses. By this construction, printing can be performed over a short side (an image width of 210 mm) A4 size at a rate of 200 dots per inch.
However, the conventional image forming apparatus has a problem in that, since the incidence surface of the telecentric optical system, located nearest to the light source, is distant from the light source, a laser beam incident on the telecentric optical system is spread, with the result that an effective diameter of lens must be increased, resulting in lenses being enlarged.
Also, there is a problem in that the short distance between the composite focus of the photosensitive surface side and the incidence surface of the telecentric optical system increases the bend of a principal light beam of the laser beam
2
, with the result that high resolution cannot be obtained because of an increased aberration. Therefore, recently demanded resolutions of 600 dots or more per inch for A3 size cannot be achieved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an imaging optical system and an image forming apparatus that provide a small-size design and high resolution.
To achieve the above described object, the present invention offers an imaging optical system for imaging incident light coming vertically from a light source on a imaging surface, comprising: a first lens group disposed at the imaging surface side; a second lens group disposed with a stop sandwiched between the first lens group and the light source; and a third lens group disposed with a first distance provided between it and the second lens group and with a second distance smaller than the first distance provided between it and the light source, for forming the composite focus of the imaging surface side at the center of the stop in conjunction with the second lens group.
According to the above described construction, an optical system (telecentric optical system) between the composite focus of the imaging surface side and the light source is divided into the second lens group and the third lens group and the distance between the third lens group and the light source is shortened, whereby the spread of incident light into the third lens group is reduced and the aperture of the third lens is reduced. Also, the principal light beam of the incident light is bent at two stages, the second lens group and the third lens group, and the distance between the second lens group and the third lens group is increased, whereby the bend of the principal light beam of the incident light in the second lens group and the third lens group is reduced, with the result that small aberration is obtained.
To achieve the above object, the present invention provides an imaging optical system for imaging incident light coming vertically from a light source on an imaging surface by using a telecentric optical system comprising a plurality of lenses, wherein the telecentric optical system is constructed so that a relation of 1<2·S/L<2 is satisfied, where S is the distance between a principal point of one of the plurality of lenses, located nearest to the light source, and the composite focus of the imaging surface side formed by the plurality of lenses, and L is the distance between the light source and the composite focus of the imaging surface side.
According the above described construction, increasing S/L shortens the distance between the incidence surface of the telecentric optical system, located nearest to the light source, and the light source, and reduces the spread of incident light into the incidence surface, with the result that the aperture of the telecentric optical system is reduced. Also, increasing S/L increases the distance between the incidence surface of the telecentric optical system, located nearest to the light source, and the composite focus of the imaging surface side, and reduces the bend of the principal light beam in the telecentric optical system, with the result that small aberration is obtained.
To achieve the above object, the present invention provides an imaging optical system for imaging incident light coming vertically from a light source on an imaging surface by using a telecentric optical system comprising a plurality of lenses, wherein the telecentric optical system is constructed so that the deflection angle of the principal light beam of the incident light in each of the plurality of lenses is 13 degrees or less.
According to the above described construction, aberration is reduced by setting the deflection angle of the principal light beam of the incident light in each of the plurality of lenses making up the telecentric optical system to 13 degrees or less. At the same time, since the distance between the incidence surface of the telecentric optical system, located nearest to the light source, and the light source is shortened, the spread of the incident light into the incidence surface is reduced and the aperture of the telecentric optical system is reduced.
To achieve the above object, the present invention provides an image forming apparatus comprising: a semiconductor laser array having a plurality of laser elements, for emitting a plurality of laser beams in parallel from the plurality of laser elements based on an image signal; an imaging optical system for imaging the plurality of laser beams from the plurality of laser elements on an imaging surface; and a photosensitive material, exposed by the plurality of laser beams imaged on the imaging surface, on which an electrostatic latent image is formed in accordance with the image signal, wherein the imaging optical system comprises: a first lens group disposed at the imaging surface side; a second lens group disposed with a stop sandwiched between the first lens group and the semiconductor laser array; and a third lens group disposed with a first distance provided between it and the second lens group and with a second distance smaller than the first distance provided between it and t
Epps Georgia
Fuji 'Xerox Co., Ltd.
Lester Evelyn A.
Oliff & Berridg,e PLC
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