Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2002-05-31
2003-10-21
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S132000, C347S233000, C358S300000
Reexamination Certificate
active
06636251
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus using multiple beams, and more particularly, to stabilization of the density of a formed image.
2. Description of the Related Art
In a conventional image forming apparatus using an electrophotographic method in which a visual image is formed according to charging, exposure and development, an approach has been adopted widely in which, after performing primary charging of an electrophotographic photosensitive member, which serves as an image bearing member, exposure is performed, using a semiconductor laser as means for forming an electrostatic latent image on the photosensitive member. (To be more exact, a laser chip comprising a laser diode and a photodiode sensor is used as the means for forming an electrostatic latent image.) By feeding back an output signal from the photodiode sensor to a bias power supply for the laser diode, and automatically controlling the amount of the bias current, a laser beam is stabilized.
Recently, in order to realize high-speed printing with an image forming apparatus, means for forming an electrostatic latent image using a multi-laser device has been practically used, in which a plurality of laser beams are simultaneously emitted at one main scanning operation. For example, in a multilaser method using two lasers, the above-described configuration is adopted: that is, each laser comprises a pair of a laser diode and a photodiode sensor, in order to stabilize the obtained laser beam.
Various image-signal processing techniques for improving the quality of an image are being used. For example, a method has been proposed in which, when forming an image by binary coding a digital image signal, the digital image signal is first converted into an analog signal, and a binary signal subjected to pulse-width modulation (PWM) is generated by comparing the analog signal with a periodic pattern signal, such as a triangular-wave signal. An invention in which the above-described PWM method is applied to a multibeam laser printer is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 8-317157 (1996). In this invention, in order to prevent variations in the image density due to individual differences among multiple laser beams, a pattern signal for each laser is corrected by PWM. That is, each laser beam is sometimes subjected to peculiar PWM in accordance with the characteristics of the beam, so that variations in the image density are suppressed by providing a uniform light-portion potential by laser scanning, by reducing variations in the amounts of light output by the respective laser beams.
However, in a multibeam laser printer, there is the problem that the halftone image density differs even if variations in the characteristics of the laser beams are not present. This is a new problem such that the halftone image density differs if at which the position to start image writing shifts even just by one line in the sub-scanning direction. It is considered that this phenomenon is caused by nonlinearity of the curve of the amount of light E of the photosensitive member versus the potential V (E−V curve). For example, the amount of light E is expressed by E=I×t, where I is the intensity of light, and t is the exposure time. The above-described variations in the density are produced because, even if the same amount of light E is provided for the photosensitive member, the sensitivity differs and the potential may change if the intensity of light I changes or the exposure time t changes. This phenomenon is called reciprocity. With respect to reciprocity, an example in which the sensitivity increases by projecting weak light beams onto a photosensitive member a plurality of times is reported in Japanese Patent Application Laid-Open (Kokai) No. 4-51043 (1992).
An example of differences in the halftone density caused by reciprocity in multiple beams will now be illustrated.
FIG. 15
is a schematic diagram illustrating a halftone image with two dots and two spaces obtained by simultaneously projecting beams A and B. A pair of laser beams are defined as beams A and B. The beam A corresponds to the first line of writing positions on paper, and the beam B corresponds to the second line. Thereafter, the beams A and B are alternately projected on odd lines and even lines, respectively. After the beams A and B simultaneously are in an on-state to scan two-dot lines at the first scanning by a polygonal mirror, the beams A and B simultaneously assume an off-state turned of at the next scanning by the polygonal mirror, to provide two spaces. A halftone image with two dots and two spaces is obtained by sequentially repeating simultaneous on-state and off-state of the beams A and B. In
FIG. 15
, the pairing of the laser beams is indicated by being surrounded by broken lines.
FIG. 16
is a schematic diagram illustrating a halftone image with two dots and two spaces obtained by simultaneously (sequentially) projecting the beams A and B. At the first scanning by the polygonal mirror, the beam A is in an off-state and the beam B is in an on-state, to provide a one-dot line and one space. At the next scanning by the polygonal mirror, the beam A is in an on-state and the beam B is in an off-state, to provide a one-dot line and a one-dot space. A halftone image with two dots and two spaces shifted by one line is obtained by sequentially repeating the above-described one space and one dot, and one dot and one space.
The densities of the images with two dots and two spaces shown in
FIGS. 15 and 16
were compared with each other. The density of the image with two dots and two spaces shown in
FIG. 15
in which the two laser beams were simultaneously projected in the main scanning direction, was 1.15. The density of the image with two dots and two spaces shown in
FIG. 16
in which the laser beams were alternately projected, was 1.21. Accordingly, the density in simultaneous irradiation is lower than the density in alternate irradiation.
In order to study the reason for this difference, first, it was checked if a difference was present in the amount of light. It can be considered that the amount of light may decrease during simultaneous irradiation due to mutual influence between the laser beams caused by thermal and electrical crosstalk between the laser beams. Accordingly, the amount of light of laser beams when a pair of laser beams were simultaneously projected and the amount of light of a laser beam when a single laser beam was projected were measured and compared with each other.
FIG. 17
is a graph illustrating the value of the amount of light measured by a pin-photodiode when only the beam A performed scanning.
FIG. 18
is a graph illustrating the value of the amount of light measured by the pin-photodiode when only the beam B performed scanning.
FIG. 19
is a graph illustrating the value of the amount of light measured by the pin-photodiode when the beams A and B were simultaneously emitted to perform scanning. The sum of the amounts of light of the beams A and B shown in
FIGS. 17 and 18
, respectively, coincides with the amount of light of simultaneous emission of the beams A and B shown in FIG.
19
. This result indicates that the amounts of light of multiple beams are not reduced and stable even if the two beams are simultaneously emitted.
Next, it was studied if there is a difference in the potential of the photosensitive member. Since the diameter of the used laser spot is not small, it is estimated that superposition of spots of a pair of laser beams occurs, and the potential differs at a superposed portion. Scanning was performed in the conditions that the beams A and B had the same spot diameter with the size of 70 &mgr;m both in the main scanning and sub-scanning directions. The size of one pixel of an image with a resolution of 1,200 dpi (dots per inch) was 21 &mgr;m.
FIG. 20
is a schematic diagram illustrating a state in which a light-amount distribution at simultaneous exposure is converted into a potential-distributio
Inami Satoru
Namiki Takayuki
Saitou Masanobu
Shinohara Seiichi
Fitzpatrick ,Cella, Harper & Scinto
Pendegrass Joan
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