Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2001-03-05
2002-10-08
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S619000
Reexamination Certificate
active
06462879
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical writing device for use in an image forming apparatus such as a digital copier, a digital printer or a digital facsimile. More particularly, the present invention relates to an optical writing device in which multiple light beams, emitted by a plurality of light sources of a light source array, are focused onto a surface of a photosensitive medium without deflection. Further, the present invention relates to an image forming apparatus and method in which the optical writing device is used as an exposure unit that exposes the photosensitive medium surface to an imaging light pattern.
2. Description of the Related Art
With the widespread use of image forming systems, such as digital copiers, digital printers and digital facsimiles, there is an increasing demand for a small-size optical writing device for use in image forming systems.
There are two major types of optical writing device: a deflection type and a non-deflection type. In the deflection type, a rotary deflector or the like is provided to deflect the multiple light beams, emitted by a plurality of light sources of a light source array (for example, a semiconductor laser array), and the deflected light beams are focused onto the surface of the photosensitive medium. In the non-deflection type, the light beams, emitted by the light source array, are focused onto the surface of the photosensitive medium without deflection.
The disadvantage of the deflection type is that a total length of the optical path in the optical writing device becomes large because of the use of the rotary deflector, which is not suitable to provide a small-size optical writing device. On the other hand, the non-deflection type does not use a rotary deflector and can shorten the total length of the optical path, and, therefore, it is more suitable to provide a small-size optical writing device. Moreover, the non-deflection type optical writing device does not require mechanical drive parts that move a rotary deflector, and it can provide a low-cost optical writing device.
A conventional optical writing device is known, which is of the non-deflection type and uses a rod lens array as the means for focusing the light beams, emitted by the light source array, onto the photosensitive medium surface.
FIG. 31
shows a distribution of light amount of a rod lens array in the conventional optical writing device.
FIG. 32
shows a relationship between the rod lens diameter and the visual field radius in the conventional rod lens array of FIG.
31
.
As shown in FIG.
31
and
FIG. 32
, the rod lens array
110
in the conventional optical writing device includes a plurality of rod lens elements
101
a
that focuses the light beams from a light source array onto an image plane. These rod lens elements
101
a
are arrayed in a row in an array direction. Each of the rod lens elements
101
a
has a distribution of light amount due to a distributed refractive index of each rod lens element. Respective images, which are formed on the image plane by the light beams passed through the rod lens elements
101
a
are overlapped each other in the array direction so as to form a line-shaped image. As the light amount distributions of the respective lens elements are superimposed, the distribution of light amount of the conventional rod lens array
101
in the array direction of the rod lens elements
101
a
is as shown in FIG.
31
. For this reason, the light amount distribution of the conventional rod lens array is liable to the periodic variations of light amount which depend on the visual field radius of each rod lens element and the pitch of the rod lens elements in the array direction. The magnitude &Dgr;E of the periodic variations of the light amount, caused by the conventional rod lens array
101
, is represented by the following formula:
&Dgr;
E
=(
E
max−
E
min)/
E
max×100 (%) (1)
where Emin is the minimum light amount in the superimposed distribution and Emax is the maximum light amount in the superimposed distribution. In
FIG. 32
, “D” indicates the rod lens diameter and “X” indicates the visual field radius in the conventional rod lens array
101
of FIG.
31
.
Generally, the periodic variations of light amount in the conventional optical writing device depend on the visual field radius of each rod lens element and the pitch of the rod lens elements in the array direction. Herein, it is assumed that the conventional rod lens array is constituted by identical rod lens elements which are arrayed in a row in the array direction, and that all of respective pitches of two adjacent ones of the individual rod lens elements in the array direction are nearly equal to the diameter of each rod lens element in the array direction. Further, it is assumed that the conventional rod lens array includes only the rod lens elements and does not include shading portions between the rod lens elements. Typically, in the conventional optical writing devices, the visual field radius X of each rod lens element in the array direction is on the order of 1 to 2 mm, and the diameter D of each rod lens element in the array direction is approximately 1 mm. Specifically, in the example of the conventional rod lens array
101
of
FIG. 32
, D=1 mm, X=1.5 mm.
Japanese Laid-Open Patent Application No.10-309826 discloses an optical writing device that uses a semiconductor laser array as a light source array for emitting multiple light beams. The semiconductor laser array used by this conventional device is, for example, an array of light emitting diodes (LED).
In the conventional device of the above document, a rod lens array is provided for focusing the light beams, emitted by the LED array, onto the photosensitive medium surface. In the rod lens array, the rod lens elements are arrayed in two rows in the array direction, and the lens elements of one row are spaced apart from the lens elements of the other row by a given pitch.
Hereinafter, throughout the specification, in order to represent a configuration of a focusing lens array, such as a rod lens array, an overlap ratio m is used, which is defined by the equation m=X/D where X indicates the visual field radius of each focusing lens element in the array direction and D indicates the diameter of each focusing lens element in the array direction.
In the conventional optical writing device of the above document, an overlap ratio m of each rod lens element of the rod lens array is defined by the equation m=X
0
/D where X
0
indicates the visual field radius of each of the rod lens elements in the array direction and D indicates the diameter of each of the rod lens elements in the array direction. The conventional device of the above document is characterized in that the rod lens array is configured such that the overlap ratio m of the rod lens array satisfies the conditions 1.85<m<2.00. This configuration is selected by the conventional device in order to eliminate the undesired variations of the sub-scanning direction alignment of the light source array and the rod lens array.
However, the conventional device of the above document is liable to having the periodic variations of light amount of the rod lens array due to the configuration of the rod lens elements having a small overlap ratio. It is known from practical experience that the magnitude &Dgr;E of the periodic variations of the light amount is in a range from 10% to 20%. An image forming apparatus using such optical writing device will produce the periodic variations of photographic density in a reproduced image due to the periodic light amount variations of the rod lens array, and it is difficult for the image forming apparatus to provide the reproduced image with good quality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved optical writing device in which the above-described problems are eliminated.
Another object of the present invention to provide an improved optical writ
Hindi Omar
Mack Ricky
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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