Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-03-14
2004-01-20
Tran, Huan (Department: 2863)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S235000
Reexamination Certificate
active
06680744
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-082611, filed Mar. 22, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a laser scanner that can suitably be applied to an electrophotography type image forming device in the recording section of a digital copying machine. The present invention also relates to a copying machine realized by using such a laser beam scanner.
2. Description of the Related Art
A digital copying machine is adapted to read a document by means of a document reading device and copying the document on a sheet of paper by means of an image forming device on the basis of the image data obtained by reading the document.
The original reading device and the image forming device are configured and regulated in such a way that the document reading characteristics of the former and the printing characteristics of the latter match each other so that the obtained image may be an exact copy of the document. However, the two sets of characteristics may be mismatching due to the variance in the make of those devices and other reasons. Then, it will no longer be able to obtain an exact copy of the document.
More specifically, when a document as shown in
FIG. 6
that shows straight lines drawn in the sub-scanning direction and arranged at a pitch of 58.6 mm in the main-scanning is copied, the pitch of arrangement of straight lines may be narrowed in the copy as shown in
FIG. 7
because of the mismatching of the reading characteristics of the document reading device and the printing characteristics of the image forming device. Or, the pitch of arrangement of straight lines may be enlarged in the copy as shown in FIG.
8
.
In the case where the image forming device is an electrophotographic device comprising a laser scanner A as shown in
FIG. 12
, the above problem can be dissolved by regulating the laser scanner A. Referring to
FIG. 12
, reference symbol
7
a
denotes an operation board to be used for inputting data relating to the rotary speed of polygon mirror
2
and reference symbol
8
a
denotes a selected value output section for outputting a value selected on the basis of the data input through the operation board
7
a
, while reference symbol
9
a
denotes a conversion table for storing numerical values corresponding to the selected value and a frequency divider circuit
13
divides the frequency of the master clock to a ratio given from the conversion table
9
a
and outputting the obtained signal to a polygon motor drive circuit
14
as polygon drive clock. The polygon motor drive circuit
14
drives the polygon motor
3
so as to make it turn by a predetermined angle per unit time in synchronism with the polygon drive clock applied to it from frequency divider circuit
13
.
A laser scanner A of the above described type is generally designed to cause a laser beam emitted from a stationary laser diode
1
to scan the photosensitive surface of a photosensitive drum
100
by making the revolving polygon mirror
2
reflect the emitted laser beam. Therefore, the scanning cycle time, that is the time necessary for the laser beam to scan a line, is determined by the rotary speed of the polygon mirror
2
. The laser scanner A modulates the laser beam according to the image data given to it at a rate that is determined on the basis of the relationship between the scanning cycle time and the amount of image data on the line to be scanned.
In the laser scanner A, the scanning speed of the laser beam can be modified by regulating the rotary speed of the polygon mirror
2
. Therefore, the pitch of arrangement of the lines drawn in the sub-scanning direction in
FIG. 6
can be modified by utilizing the modifiable scanning speed.
Referring to
FIG. 9
, S
1
, S
6
and S
9
denote respective timings at which the laser beam passes through a photodetector
4
. S
1
shows the timing that is obtained when the polygon mirror
2
is driven to rotate at a standard rotary speed. S
6
shows the timing that is obtained when the polygon mirror
2
is driven to rotate at a rotary speed higher than that of S
1
. S
9
shows the timing that is obtained when the polygon mirror
2
is driven to rotate at a speed lower than that of S
1
. While the photodetector
4
is stationary, the time interval necessary for the laser beam to get to the photodetector repetitively can be modified by modifying the rotary speed of the polygon mirror
2
.
S
2
, S
7
and S
10
in
FIG. 9
denote output signals of the photodetector
4
that correspond to S
1
, S
6
and S
9
respectively. Any of them may be used as horizontal synchronizing signal (HSYNC signal).
S
3
in
FIG. 9
denotes an HSYNC start signal for causing the photodetector
4
to emit a laser beam. Counter circuit
6
a
starts counting by referring to the rising edge of the output of the photodetector
4
. The HSYNC start signal S
3
is brought up to level “H” at the timing when the counter circuit
6
a
has counted a predetermined number (e.g., 9726 clocks). Then, the HSYNC start signal S
3
is brought down to level “L” at the rising edge of the output of the photodetector
4
. In
FIG. 9
, the HSYNC start signal S
3
corresponds to the timing of S
1
.
S
4
in
FIG. 9
denotes an image data signal containing the data of the image to be printed in a printing area. The image data signal is always transferred at a predetermined rate in synchronism with the master clock or an arbitrarily selected dot clock.
S
5
, S
8
and S
11
in
FIG. 9
denote light modulation signals for actually controlling the emission of a laser beam. They correspond to S
1
, S
6
and S
9
respectively. These light modulation signals are formed by adding the HSYNC signal and the image data signal by means of OR circuit
11
. Since these light modulation signals S
5
, S
8
and S
11
are defined in a manner independent from the rotary motion of the polygon mirror
2
except the timing of the falling edge of the HSYNC start signal, they show same timings.
Thus,
FIG. 9
shows the timings of each of the above described signals when the rotary speed of the polygon mirror
2
is changed. In other words, the timing of the printing operation in a scanning period does not change if the rotary speed of the polygon mirror
2
is changed. However, the time necessary for the laser beam to scan a physical printing area on the photosensitive member
100
across the entire width thereof varies, although the width of the printing area is invariably fixed. Therefore, the width of the printing area as indicated by a, b and c in
FIG. 9
appears to vary as a function of the rotary speed of the polygon mirror
2
because it is expressed in terms of time.
FIG. 10
illustrates the relationship of the light modulation signals S
5
, S
8
and S
11
in terms of a printing area or a single scanning period. As seen from
FIG. 10
, when the laser beam is emitted by means of the light modulation signal S
8
that corresponds to a high rotary speed, the intervals of laser beam emissions is shorter than those of laser beam emissions by means of the light modulation signal S
5
that corresponds to a standard rotary speed of the polygon mirror
2
. On the other hand, when the laser beam is emitted by means of the light modulation signal S
11
that corresponds to a low rotary speed, the intervals of laser beam emissions is longer than those of laser beam emissions by means of the light modulation signal S
5
. In this way, the pitch of forming pixels on the photosensitive member
100
is modified. In other words, the pitch of arrangement of pixels on the photosensitive member
100
can be regulated by regulating the rotary speed of the polygon mirror
2
.
More specifically, when the pitch of arrangement along the main-scanning direction of the drawn lines is reduced as shown in
FIG. 7
, the problem is dissolved by raising the rotary speed of the polygon mirror. If the pitch of arrangement of the drawn l
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