Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-06-09
2001-05-29
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S253000, C358S001900
Reexamination Certificate
active
06239824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image forming apparatuses and methods for forming an image based on a digital image. In particular, the invention relates to an image forming apparatus and an image forming method for forming a high-quality image by controlling exposure energy density.
2. Description of the Background Art
Image forming apparatuses represented by copiers and printers are requested these days to be more excellent in the image quality and reproducibility. In order to meet this request, regarding a digital image forming apparatus of the electrophotography system, a method has been proposed according to which the diameter of a spot formed on a photoreceptor is optimized for a dot pitch determined by resolution.
For example, Japanese Patent Laying-Open No. 58-152269 discloses a technique of optimizing exposure energy with respect to a relational expression of dot pitch, diameter of exposure spot, and exposure energy density. Japanese Patent Laying-Open No. 7-61036 discloses a technique of not only optimizing a spot diameter for a dot pitch but additionally considering photoreceptor characteristics to obtain a high-quality image.
In addition to the technique of optimizing a spot diameter, a method is proposed for improving the quality of an image by image processing.
Japanese Patent Laying-Open No. 63-64763 discloses a technique of achieving reproducibility for an isolated dot by detecting isolated one-dot print data from print data and correcting one bit preceding or following the detected data as the one-dot print data so as to improve the image quality.
However, if these techniques are applied to an image forming apparatus with a high resolution such as the one which exceeds 600 dpi, it is difficult to achieve a superior reproducibility for images having different dot densities.
Specifically, a problem in the techniques disclosed respectively in Japanese Patent Laying-Open Nos. 58-152269 and 7-61036 is that, if the resolution is ensured of a line pattern with a relatively high dot density, for example, having lines on every second lines (hereinafter referred to as “periodic line pattern”), the diameter of a dot in an isolated one-dot pattern having a low dot density (hereinafter referred to as “isolated dot pattern”) is smaller than a desired value or the dot itself is not formed in some cases.
On the contrary, if the dot diameter of the isolated dot pattern is set at a value appropriate for resolution, the line width of the periodic line pattern increases and a contrast corresponding to the resolution cannot be ensured.
Therefore, it is highly difficult for an image forming apparatus with a high resolution to obtain an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern merely by adjusting the diameter of a spot formed on a photoreceptor, and accordingly to obtain an output image with a superior reproducibility.
According to the technique disclosed in Japanese Patent Laying-Open No. 63-64763, print data is changed (corrected) so that the isolated dot pattern itself can be reproduced. However, the reproduced image is not exactly the original image. Further, the reproduced image could have an unnatural portion depending on cases. Therefore, this technique does not provide a fundamental solution to the issue of ensuring a superior reproducibility for both of the periodic line pattern and the isolated dot pattern as described above.
It is noted that the issue of ensuring an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern relates not only to the resolution but to a difference of sensitivity, i.e., photoconductivity of the photoreceptor. Considering this, it could be possible to achieve such an excellent reproducibility for both of the periodic line pattern and the isolated dot pattern by decreasing the film thickness of the photoreceptor, for example.
However, the decrease in the film thickness of the photoreceptor results in deterioration of dark decay and sensitivity characteristics due to deterioration by wear caused by repeated use. A problem then arises concerning durability. A further problem regarding gradation which is not negligible is that a photoreceptor having a small film thickness cannot provide a satisfactory gradation.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an image forming apparatus and an image forming method which can enhance an image reproducibility for both of the periodic line pattern and the isolated dot pattern without decreasing the film thickness of a photoreceptor, even if the resolution is high.
This object of the invention is achieved by an image forming apparatus having following components. Specifically, the image forming apparatus according to one aspect of the invention includes an exposure unit for exposing a photoreceptor charged to a predetermined potential to light, a control unit for controlling exposure energy density of the exposure unit based on image information, and a development unit for visualizing an electrostatic latent image formed on the photoreceptor by the exposure unit. The control unit controls exposure energy density E
dot
(&mgr;J/cm
2
) to be applied to an isolated one-dot pattern of the image information to satisfy the following relation,
−0.11
P+
6.4≦
E
dot
/E
line
≦−0.2
P+
10.7
where E
line
(&mgr;J/cm
2
) represents exposure energy density to be applied to a line pattern of the image information having lines on every second lines and P (&mgr;m) represents dot pitch.
According to the present invention, exposure energies to be applied respectively to the periodic line pattern and the isolated dot pattern are optimized. Consequently, it is possible to provide an image forming apparatus capable of improving an image reproducibility for both of the periodic line pattern and the isolated dot pattern without decreasing the film thickness of the photoreceptor even if the resolution is high.
Preferably, the image forming apparatus further includes a detection unit for detecting an isolated one-dot pattern from the image information. The control unit applies the exposure energy density E
line
(&mgr;J/cm
2
) when no isolated one-dot pattern is detected, and applies the exposure energy density E
dot
(&mgr;J/cm
2
) when the isolated one-dot pattern is detected.
In this way, a predetermined exposure energy density E
line
(&mgr;J/cm
2
) is applied when no isolated dot pattern is detected and an exposure energy density E
dot
(&mgr;J/cm
2
) which satisfies a certain condition is applied only when an isolated dot pattern is detected. The reproducibility of the isolated dot pattern can thus be improved by a relatively simple control.
Preferably, the control unit includes a power modulation unit capable of controlling exposure energy density for each dot.
As a result, the exposure energy density can be controlled for each dot according to image information so as to form an image with a superior reproducibility.
Preferably, the image forming apparatus sets an exposure spot diameter D (&mgr;m) of the exposure unit to satisfy the following relation,
D≦
2
P
where P (&mgr;m) represents dot pitch.
The reproducibility can accordingly be ensured for the periodic line pattern.
Still preferably, the photoreceptor has a film thickness of 15 to 25 &mgr;m.
Durability as well as a satisfactory gradation can thus be ensured.
According to another aspect of the invention, a method of forming an image includes the steps of exposing a photoreceptor charged to a predetermined potential to light, controlling exposure energy density of an exposure unit based on image information, and developing an electrostatic latent image formed in the exposing step on the photoreceptor so as to visualize the latent image. In the controlling step, exposure energy density E
dot
(&mgr;J/cm
2
) to be applied to an isolated one-dot pattern of the image formation is controlled to satisfy the following relation,
−0.11
P+
6.4≦
E
dot
/E
line
≦&minu
Azuma Nobuyuki
Iwamatsu Tadashi
Mutou Yoshinori
Nakajima Yoshinori
Toyoshima Tetsuro
Conlin David G.
Dike Bronstein Roberts & Cushman
Pendegrass Joan
Sharp Kabushiki Kaisha
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