Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-11-27
2002-11-05
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S256000
Reexamination Certificate
active
06476846
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam scanner having a plurality of light emitting points, and to an image forming apparatus, such as a laser-beam printer, copier, and a facsimile machine, incorporated with the multi-beam scanner.
2. Description of Related Art
A conventional multi-beam scanner is known to have a light source unit with a laser diode array that includes a plurality of light sources. The multi-beam scanner uses the light source unit to simultaneously scan a plurality of light beams on the scan surface of a photosensitive body, in order to form an image on the scan surface. Such a multi-beam scanner increases the scanning speed and recording speed of an image forming apparatus.
The multi-beam scanner with the plurality of light sources has to produce, on the scan surface, the plurality of laser beam spots with the same, desired diameter, similarly to a scanner having a single light source. If the plurality of laser beams produce different spot diameters on the scan surface, then pixels will also become different sizes when recorded. As a result, images will become deformed or curved, or light and shade portions will be developed on the image, so that high quality images cannot be obtained.
Japanese Patent-Application Publication No. 62-161117 discloses a scanner which is provided with a mechanism for adjusting the position of the light source in a focus depth direction. In this scanner, the position of the laser array is adjusted in the focus depth direction using a screw and is fixed using a spring. As a result, the position of a collimator lens and the plurality of light sources can be adjusted so that the laser beams emitted from all the light sources have the same spot diameter along the focal depth direction.
However, this scanner requires a great number of components, which makes the scanner expensive. Further, because the position of the laser array is adjusted and fixed in the manner described above, the laser array will possibly be displaced out of position when the scanner is bumped. When the position is displaced, the desired spot diameter cannot be obtained on the scan surface.
Japanese Patent-Application Publication No. 9-26550 discloses a multi-beam scanner that reduces variations in the spot diameter that are caused by variations in image positions of the different light beams in the depth direction of the light beams. According to this multi-beam scanner, the position where the image should be formed in terms of design (desired spot position) exists in dispersion in the depth direction of the image formation positions (spot positions) of plural light beams, so that variation in spot diameter can be reduced to a minimum.
SUMMARY OF THE INVENTION
However, in the multi-beam scanner disclosed in Japanese Patent-Application Publication No. 9-26550, the optical system has a certain amount of curvature of field so that as the scanning position changes, the focal points of the laser beams will move in forward and backward (depth) directions. Therefore, depending on the scanning position, the focal points may not be properly located on the scan surface, so that printing quality suffers.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved multi-beam scanner having a plurality of light emitting points that are all properly focused on an object to be scanned at all the scanning positions, regardless of the curvature of field of the optical system employed therein.
In order to attain the above and other objects, the present invention provides a multi-beam scanner comprising: a light-emitting unit which has a plurality of light emitting portions for emitting a plurality of light beams, the plurality of light emitting portions being separated from one another in a depth direction of the light beams; a scanning unit that deflects the light beams in a main scanning direction in a plurality of lines across a surface of an object to be scanned, the plurality of lines being arranged along an auxiliary scanning direction that is substantially perpendicular to the main scanning direction; and a light converging unit converging the plurality of light beams onto the surface of the object to be scanned, the light converging unit allowing depths of focus of the light beams from all the light emitting portions to overlap on the surface of the object.
Thus, according to the present invention, the multi-beam scanner includes: a light-emitting unit that emits light beams from a plurality of light emitting points, a scanning unit that deflects the light beams to scan the light beams on an object to be irradiated, and light converging unit which is disposed between the light-emitting unit and the object to be irradiated. An optical system is constructed from the light-emitting unit, the scanning unit, and the light converging unit.
The light-emitting unit has light emitting points that are shifted from one another in the depth direction of the light beams. The optical system has a characteristic value, such as a longitudinal magnification or an image side numerical aperture, which is set to a value so that the depths of focus of light beams from all the light emitting points overlap at least partly with one another along the depth direction, and the object to be irradiated is located within the range where the depths of focus overlap.
With this configuration, because the light emitting points are separated by a distance in the depth direction, the depths of focus in the light beams from the respective light emitting points are shifted from one another at each scanning position on the object to be irradiated. To insure that the light beams from all the light emitting points be properly focused on the object at all the scanning positions, the scan surface of the object to be scanned has to be located within the range where the depths of focus of all the light beams overlap.
In order for the depths of focus of all the beams to overlap, the value derived by subtracting the shift (&Dgr;S) in the depths of focus from the range (d
e
) of the depths of focus has to be positive. In other words, it is necessary to satisfy the following inequality:
d
e
−&Dgr;S>
0.
It is noted that the shift amount (&Dgr;S) can be obtained by multiplying the longitudinal magnification &agr; of the optical system by the distance &Dgr;Z that separates the light emitting points from one another in the depth direction. That is, &Dgr;S=&agr;·&Dgr;Z. Therefore, by setting the longitudinal magnification a to an appropriate value, the value, which is derived by subtracting the shift (&Dgr;S) from the range (d
e
), will become positive, so that the depths of focus in all the light beams will overlap.
By positioning the surface of the object to be scanned within the range where the depths of focus overlap, the light beams from all the light emitting points will be properly focused on the object surface.
Especially, according to the multi-beam scanner of the present invention, the longitudinal magnification &agr; of the optical system is preferably set in association with depth of focus d
e
according to the following inequality:
d
e
−(&agr;·&Dgr;
Z+C
f
)>0 (A)
wherein
&Dgr;Z is the distance separating the light emitting points in the depth direction; and
C
f
is the amount of the curvature of field caused by the optical system.
By setting the longitudinal magnification &agr; to satisfy the inequality (A) described above, it can be ensured that the light beams from the all light emitting points will be properly focused on the surface of the object to be scanned, regardless of the scanning position and regardless of the amount of the curvature of field.
It is noted that according to the multi-beam scanner of the present invention, even when the light emitting surface of the light-emitting unit is not located on a plane normal to the optical axis, the optical system can be easily designed by satisfying the above-described relationship (A) while substituting the v
Hattori Yutaka
Kashima Hiroyuki
Matsumoto Yumio
Brother Kogyo Kabushiki Kaisha
Pham Hai
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