Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-03-04
2004-08-31
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06784914
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a clock-generating circuit and image-forming apparatus employing a polygon mirror in its optical writing section, and specifically relates to a combination of an image-forming apparatus and a clock-generating circuit, which makes it possible to reduce influence to the image quality caused by variations of transmittances and reflectances in the optical-writing section when the polygon mirror scans a laser beam.
In conventional image-forming apparatus, an image is formed on an image-bearing member rotating in a sub-scanning direction by scanning a laser beam, modulated in response to image data, in a main-scanning direction. In this operation, the laser beam is modulated in response to the image data in reference to reference signals called as dot clock pulses.
Accordingly, in accordance with a predetermined number of dot clock pulses, it is necessary to generate the dot clock pulses in order that the length of the image, formed on the image-bearing member, is always kept constant in the main-scanning direction.
Further, in recent years it has been developed a color image forming apparatus equipped with a plurality of units comprising means for charging, exposure, and development respectively in the vicinity of the image-bearing member, on which a full color toner image is formed during one revolution of it, and transferring the full color toner image onto a recording paper at a time. On the other hand, it has been also developed a color image forming apparatus equipped with a plurality of image-bearing members in the vicinity of an intermediate transfer member and equipped with means for charging, exposure, development and transfer around each of the image-bearing members respectively, transferring the toner images having been formed on the respective image-bearing members sequentially onto the intermediate transfer member, and further transferring the full color toner image born on the intermediate transfer member onto the recording paper at a time.
Incidentally,
FIG. 8
shows perspective view of a configuration of optical writing section
490
employing a laser beam in the image-forming apparatus as described in the above. As shown in
FIG. 8
, laser diode
470
(LD) emits the laser beam based on the laser driving signals generated by circuit section
400
.
Further, the laser beam, emitted by laser diode
470
and having been passed through collimator lens
491
and cylindrical lens
492
, is deflected for scanning by polygon mirror
493
, and scans image-bearing member
1
for writing, after passing through f&thgr; lens
494
so as to adjust the scanning velocity at constant and cylindrical lens
495
. In addition, a part of the laser beam deflected by polygon mirror
493
is led to index sensor
412
for detecting the timing.
[First Problem]
As shown in
FIG. 9
, the laser beam, deflected for scanning by polygon mirror
493
, is led to image-bearing member
1
after passing through the optical elements, including f&thgr; lens
494
, cylindrical lens
465
and dustproof filter
496
.
During the abovementioned scanning operation, since the laser beam enters into each of the optical elements from a direction substantially orthogonal to them at a low incident angle when the laser beam scans in the vicinity of the scanning center, the reflectances become low values while the transmittances become high values. On the contrary, since the laser beam enters into each of the optical elements from a direction inclined to them at a high incident angle when the laser beam scans in both end portions of the scanning region, the reflectances become high values while the transmittances become low values. Incidentally, the length of the arrow in
FIG. 9
illustratively indicates the intensity of the reflected light.
The variation of the light intensity to be irradiated onto image-bearing member
1
, caused by the above-mentioned changes of reflectances and transmittances, results in a quality deterioration of the image formed on image-bearing member
1
, which is called as an unevenness of scanning-light amount (a shading). Although the scanning-light amount is generally apt to decrease at the both end portions of the scanning region, the shading property would vary depending on the shapes and arrangements of various kinds of lenses and filters included in the optical system, as shown in FIG.
10
.
Further, in a certain manner of disposing dustproof filter
496
, the scanning-light amount would be possibly lowered at only one end portion of the main-scanning region. In the example shown in
FIG. 9
, since an amount of light reflected by the dustproof filter
496
increases at the right end portion of the main-scanning region, the unevenness of the scanning-light amount would be apt to also increase. Accordingly, as for conventional image-forming apparatus, not only the mechanically optimum disposition, but also the unevenness of the scanning-light amount has restricted the design flexibility of disposing dustproof filter
496
.
Still further, to suppress the above-mentioned unevenness of the scanning-light amount as lower as possible, it has been effective to apply an expensive multi-coating treatment onto each of the optical elements concerned. It has been a problem, however, that such the treatment surely raises the total cost of the optical elements.
[Second Problem]
With respect to the first problem mentioned above, in an image-forming apparatus in which multi-valued pulses for driving the laser diode are generated in the PWM (Pulse Width Modulation) process, it would be an applicable countermeasure that the PWM values, generated at a portion where the scanning-light amount does not decrease, are lowered to a level equivalent to the light amount to be reduced by the shading effect, so as to cancel the unevenness of the image caused by the shading effect. For instance, when the scanning-light amount decreases by 15% of it at the end portion of the main-scanning region due to the shading effect, it is possible to keep the resulted scanning-light amount constant over the whole region of the image by lowering the PWM values for the portion where the scanning-light amount does not decrease.
The abovementioned countermeasure, however, restricts the dynamic-range of the scanning-light intensity at a maximum light amount side. In the above example, the dynamic-range of the scanning-light intensity is restricted within a range of 0-85%. In other words, although the abovementioned countermeasure can suppress the shading effect, there would arise another problem that such the countermeasure adversely affects the gradation characteristic of the image.
SUMMARY OF THE INVENTION
To overcome the abovementioned drawbacks in conventional image-forming apparatus, it is an object of the present invention to provide a clock-generating circuit and an image-forming apparatus employing the polygon mirror in its optical writing system, which make it possible to reduce the influence to the image quality, caused by the unevenness of the scanning-light amount (the shading effect).
Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by clock-generating circuits and image-forming apparatus described as follow.
(1) A circuit for generating dot clock pulses for driving a light-emitting element employed in an optical-writing section of an image-forming apparatus, comprising: a digital-delay dot clock adjusting section to adjust timings of rising-edges or falling-edges of the dot clock pulses generated by changing a selection for a plurality of delayed-clock pulses, which are generated by delaying clock-pulses, outputted from a reference oscillator, in slightly different delay times; and a controlling section to control a selecting operation for the plurality of delayed clock pulses, performed in the digital-delay dot clock adjusting section, so as to compensate for unevenness of scanning-light amount caused by an optical element employed in the optical-writing section.
(2) A
Morita Shinji
Takaki Kouichi
Frishauf Holtz Goodman & Chick P.C.
Konica Corporation
Tran Huan
LandOfFree
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