Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-06-02
2001-04-24
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S237000, C347S132000
Reexamination Certificate
active
06222580
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophotographic image forming method and apparatus, in which a driving current to be supplied to a laser output means is pulse-width-modulated in correspondence with image data.
2. Related Background Art
Conventionally, an electrophotographic image forming apparatus such as a laser printer or the like comprises a semiconductor laser as a laser output means, and supplies a driving current to this semiconductor laser to make it output a laser beam of a corresponding amount. At this time, the laser beam output from the semiconductor laser is modulated in correspondence with image data, and is deflected and scanned in the main scanning direction by a rotating polygonal mirror or the like. At the same time, a photosensitive drum as a latent image carrier means is rotated to move its circumferential surface which is an exposure area in the sub-scanning direction, and the circumferential surface of the photosensitive drum, which is moving in the sub-scanning direction is charged by a charger.
The charged circumferential surface of the photosensitive drum, which is rotating in the sub-scanning direction, is scanned and exposed by the deflected and scanned laser beam to form an electrostatic latent image thereon, which is developed using toner by a developer as a latent image developing means. The developed toner image on the circumferential surface of the photosensitive drum is transferred onto a recording medium by a transfer charger as a toner transfer means, and the toner image transferred onto the recording medium is fixed by a fixing device.
In the above-mentioned image forming apparatus, since a print image is formed by a large number of dots in a matrix, a gradation image can be formed even by binary dots using a dither method or the like. However, since such method lowers the resolution of the print image, an image forming apparatus which forms a gradation image using multi-valued dots is now available.
In such image forming apparatus, for example, when a driving current supplied to the semiconductor laser is pulse-width-modulated in correspondence with image data, the emission time of the semiconductor laser is varied in units of dots. In this case, since each of a large number of dots that form the print image is formed to have a size corresponding to the image resolution, a high-resolution image which is graduated in units of dots can be formed. Furthermore, in such image forming apparatus, since the driving current is adjusted by monitoring the amount of light output from the semiconductor laser, the laser beam amount can be maintained constant.
One prior art of a laser printer as the above-mentioned image forming apparatus will be explained below with reference to
FIGS. 1
to
4
. Note that
FIG. 1
is a schematic block diagram showing principal part of the laser printer,
FIG. 2
is a schematic plan view showing the positional relationship among optical parts of the laser printer,
FIG. 3
is a circuit diagram showing the internal arrangement of a laser driver and the like, and
FIG. 4
is a timing chart showing the relationship among various signals.
As shown in FIG.
1
and the like, a laser printer
1
of this prior art comprises a laser device
2
, which integrates a semiconductor laser
3
serving as a laser output means, and a photodiode
4
serving as a laser monitor means. The semiconductor laser
3
outputs a laser beam in correspondence with an input driving current, and the photodiode
4
monitors the laser beam output from the semiconductor laser
3
to output a current signal corresponding to the light amount.
A main body power supply
5
and laser driver
6
are connected to the laser device
2
, and they form a current supply means for generating a driving current. A current modulation circuit
7
corresponding to a current modulation means is connected to the laser driver
6
, and a data generation means
8
corresponding to a data input means is connected to the current modulation circuit
7
.
The data generation circuit
8
comprises a communication I/F (Interface) to which an external apparatus such as a host computer or the like is connected, and externally receives image data defined by a large number of main scanning lines, which are continuous in the sub-scanning direction. The current modulation circuit
7
pulse-width-modulates a driving current supplied to the semiconductor laser
3
in correspondence with image data externally input to the data generation circuit
8
in cooperation with the main body power supply
5
and laser driver
6
.
The reflection surface of a polygonal mirror
11
corresponding to a beam deflection means is located on the optical axis of the semiconductor laser
3
of the laser device
2
, as shown in
FIG. 2
, and the circumferential surface of a photosensitive drum
12
serving as a latent image carrier means is located on the reflection optical path of this polygonal mirror
11
via, e.g., a correction optical system (not shown) such as an f-&thgr; lens and the like.
The polygonal mirror
11
is rotatably axially supported by a scanner motor (not shown), and deflects and scans a laser beam output from the semiconductor laser
3
in the main scanning direction. The photosensitive drum
12
is rotatably axially supported by a drum driving mechanism (not shown) as a sub-scanning means, to relatively move its circumferential surface, which is scanned and exposed by the laser beam, in the sub-scanning direction.
A BD (Beam Detect) sensor
13
as a beam detection means is placed at a position that leads the photosensitive drum
12
in the main scanning direction within the scan range of the polygonal mirror
11
. The BD sensor
13
detects the laser beam deflected and scanned by the polygonal mirror
11
immediately before the laser beam reaches the photosensitive drum
12
.
As shown in
FIG. 3
, an operation control circuit
15
is connected to the BD sensor
13
via an amplifier
14
, and is connected to the current modulation circuit
7
and laser driver
6
. The operation control circuit
15
comprises, e.g., a microcomputer, and serves as various means when an appropriate control program is installed as its software.
For example, the operation control circuit
15
serves as an exposure control means and output control means when it controls the operation of the current modulation circuit
7
in correspondence with the laser beam detection timing of the BD sensor
13
. In this case, as shown in
FIG. 4
, the operation control circuit
15
controls the semiconductor laser
3
to continuously output a laser beam at a timing the laser beam deflected and scanned by the polygonal mirror is detected by the BD sensor
13
, and controls the current modulation circuit
7
to start pulse width modulation of the driving current units of main scanning lines of image data a predetermined period of time after the beam detection timing of the BD sensor
13
.
Also, the photodiode
4
of the laser device
2
is connected to the operation control circuit
15
via an amplifier
16
. In this case, the operation control circuit
15
serves as a detection control means, and controls the photodiode
4
to detect the laser beam continuously output from the semiconductor laser
3
.
The laser driver
6
comprises a laser driving circuit
17
and an APC (Automatic Power Control) circuit
18
corresponding to a current adjustment means. The laser driving circuit
17
is connected to the semiconductor laser
3
in the laser device
2
and to the current modulation circuit
7
, and supplies a driving current, which is modulated in correspondence with image data, to the semiconductor laser
3
.
The laser driving circuit
17
comprises an analog switch
21
, current buffer transistor
22
, resistor
23
, operational amplifier
24
, and the like. The analog switch
21
comprises, e.g., a CMOS (Complementary Metal Oxide Semiconductor) or the like, that is capable of high-speed operation, and turns on/off the semiconductor laser
3
in response to a control si
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le N.
Nguyen Lamson D.
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