Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-04-25
2003-12-23
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S243000, C347S244000, C347S258000
Reexamination Certificate
active
06667757
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-133335, filed Apr. 27, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light scanning unit used in a laser printer, a digital copying machine and the like, and more particularly, to a light scanning unit of an over illumination type in which a width of luminous flux incident to a polygon mirror in a main scanning direction (a direction along a direction of rotation of the polygon mirror) is wider than a width of a surface of a reflection surface in the polygon mirror in the main scanning direction.
2. Description of the Related Art
In the laser printer, the digital copying machine and the like corresponding to an image forming apparatus of an electrostatic process copying method in which an electrostatic latent image is formed by using an optical beam and a visible (developer) image is obtained by developing the electrostatic latent image, there can be employed an optical scanning unit which decomposes an image to be output (an original image) into a first direction and a second direction perpendicular to the first direction, and repeatedly outputs, that is, scans an optical beam having a light intensity changed on the basis of an image data in any one of the decomposed first direction and second direction in a substantially linear manner at a predetermined time interval. In this case, an image corresponding to the original image can be obtained by moving a recording medium and a latent image holding body at a predetermined speed in a direction perpendicular to the scanned light beam during a time when the light beam corresponding to one line and the light beam corresponding to the next one line are scanned or during a time when one line is scanned.
In the light scanning unit mentioned above, the direction in which the light beam is scanned, that is, the first direction is generally called as a main scanning direction. Further, the second direction perpendicular to the first direction is generally called as a sub-scanning direction.
The light scanning unit includes a semiconductor laser device corresponding to a light source for emitting the light beam (the laser beam), a first lens group for adjusting cross sectional beam diameters and cross sectional shaped of the light beams emitted from the semiconductor laser device to a predetermined magnitude and shape, a deflecting unit for continuously reflecting the light beams adjusted to the predetermined magnitude and shape by the first lens group in a direction perpendicular to a direction in which the recording medium or the latent image holding body is moved, thereby deflecting (scanning) the light beams, a second lens group for forming an image of the light beam deflected by the deflecting unit at a predetermined position of the recording medium or the latent image holding body, and the like.
The image forming apparatus shifts the recording medium or the latent image holding member for holding the image (latent image) at only one line every time when the image data corresponding to one line is irradiated by the light scanning unit so as to form the aligned image in sequence in the direction in which the image corresponding to one line is orthogonal to the image corresponding to one line. In this case, the image forming apparatus can be classified into a wet type (a liquid type) and a dry type, and a direct type and a copy type (an indirect type) in correspondence to a method of making the latent image visible and whether or not the latent image is directly formed on the recording medium.
In the image forming apparatus and the light scanning unit as mentioned above, a relation expressed by the following formula (1) is established between a process speed (a speed at which the recording medium or the latent holding body is moved), an image resolution (a number of dots per unit length, generally a number of dots per 1 inch), a rotational speed (a rotational number) of a polygon mirror motor of a deflecting unit and a number of surfaces of the polygon mirror of the deflecting unit, when the process speed is set to P (mm/s), the image resolution (the number of the dots per 1 inch) is set to R (dpi), the rotational number of the polygon mirror motor is set to Vr (rpm) and the number of the surfaces of the polygon mirror is set to N.
P×R=
25.4
×Vr×N/
60 (1)
On the basis of the formula (1), it is deemed that the image forming speed and the image resolution are in proportion to the rotational number of the polygon mirror (being generally the same as the rotational number of the polygon mirror motor) and the number of the surfaces of the polygon mirror. Accordingly, in order to make the speed of the image forming apparatus high (increase the image forming speed per the unit time) and make the resolution high (improve the image resolution), it is necessary to increase the number of the surfaces of the polygon mirror or increase the rotational number of the polygon mirror.
In these days, in the light scanning unit of an under illumination type (a generic term at a time of comparing with an over illumination type) utilized in a lot of image forming apparatuses, the width in the main scanning direction of the light beam (the light flux) incident to the polygon mirror (corresponding to the cross sectional beam diameter, and the beam diameter in the main scanning direction in the case that the main scanning direction is different from the sub scanning direction) is limited to be small in comparison with the width in the main scanning direction of an optional reflective surface of the polygon mirror. Accordingly, the light beam guided to each of the reflective surfaces of the polygon mirror is all reflected by the reflective surface.
On the contrary, the cross sectional beam diameter of the light beam guided to the recording medium or the latent image holding body (the image surface) (the beam diameter in the main scanning direction in the case that the main scanning direction is different from the sub scanning direction) is in proportion to an F number Fn of the second lens group (an image forming optical system). The F number Fn is expressed by the formula Fn=f/D in the case that a focal distance of the image forming optical system is set to f, and the diameter in the main scanning direction of the light beam on the optional reflective surface in the polygon mirror is set to D. Accordingly, in order to increase the resolution, when it is intended to reduce the cross sectional beam diameter of the light beam on the subject to be scanned (the image surface), that is, the recording medium or the latent image holding body (a photo conductor), it is necessary to increase the cross sectional beam diameter in the main scanning direction on each of the reflective surfaces of the polygon mirror. Accordingly, when increasing the surface width of each of the reflective surfaces of the polygon mirror and further increasing the surface number of the reflective surfaces, the size of the polygon mirror is made large.
However, in order to rotate the polygon mirror having a large size at a high speed, a large-size motor having a large torque is required. In this case, of course, the motor cost is increased. At the same time, since a sound and a vibration are increased, and a lot of heat is generated, it is necessary to device a countermeasure against them.
As the countermeasure, there is a light scanning unit of an over illumination type. A principle of the light scanning unit using the over illumination is disclosed, for example, in Laser Scanning Notebook (attributed to Leo Beiser, SPIE OPTICAL ENGINEERING PRESS) and the like.
In the light scanning unit of the over illumination type, since the width of the main scanning direction of the optical beam irradiated onto each of the reflective surfaces of the polygon mirror is set to be larger than the
Shiraishi Takashi
Yamaguchi Masao
Foley & Lardner
Tran Huan
LandOfFree
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