Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-04-04
2002-08-13
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S132000, C347S252000, C358S296000
Reexamination Certificate
active
06433803
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus for image formation that is applied to an image forming apparatus such as color copying machine and printer for forming an image by means of electrophotography.
2. Description of the Background Art
An image forming apparatus forming an image through electrophotography converts a latent electrostatic image formed on the surface of a photoreceptor in an exposure process into a visible developer image in a subsequent development process. With recent progress in the enhancement of resolution, strong requests have been made to this type of image forming apparatus for a higher image quality as well as a higher reproducibility. Accordingly, various studies and developments as well as a great number of technological innovations have been made regarding the image formation technique over a wide range, for example, retrieval and quantification of parameters contributing to enhancement of the image quality, improvement of image processing technique, improvements of energy modulation and control technique, and the like.
Regarding a technique of forming a latent electrostatic image on the surface of a photoreceptor, an improvement is accomplished by optimizing the spot diameter of an exposure spot on the photoreceptor surface with respect to a dot pitch determined by resolution, and image formation is thus achieved in an improved state.
For example, Japanese Patent No. 2,668,440 discloses a technique employed in an image forming apparatus that uses pulse-width modulation, according to which a halftone image is formed by full-image exposure of pixels and, in order to prevent a drastic change in density due to cross talk of adjacent spots, the maximum spot diameter per pixel is made smaller than the size 0.7 times as large as one pixel size. In this way, halftone is accurately reproduced.
According to a technique disclosed in Japanese Patent Laying-Open No. 7-61036, the spot diameter of an exposure spot is set in a range from 1.0 to 1.3 times a dot pitch and the maximum exposure amount of the exposure spot is set to at least 11 times a luminous sensitivity coefficient on the surface of a photoreceptor. A black image of a large area having a high density and evenness can thus be generated and exposed isolated dots and non-exposed isolated dots are suitably reproduced.
According to a technique disclosed in Japanese Patent Laying-Open No. 58-152269, all pixels are exposed with the spot diameter of an exposure spot that is set to 1.3 to 1.9 times the distance between scan lines. The difference in exposure energy density between the area on a scan line and the area between scan lines is thus decreased to lessen nonuniformity of exposure. A properly reproduced image is accordingly achieved.
According to a technique of forming a latent electrostatic image on the surface of a photoreceptor, the image consisting of a collection of dots, the exposure energy is modulated according to the number of adjacent dots to enable enhancement of image reproducibility with respect to various image data and accordingly a high-quality image can be produced regardless of the type of image data.
For example, according to a technique disclosed in Japanese Patent Laying-Open No. 9-85982 employing a photoreceptor with a high &ggr;-value, an exposure amount for a dot of interest is decreased as the number of dots adjacent horizontally and vertically to the dot of interest increases.
In addition, according to a technique disclosed in Japanese Patent No. 2,839,006 regarding control of binary image data, a pulse width of image data is increased with respect to character data and isolated-dot data in a pseudo halftone region, and the pulse width is decreased with respect to continuous-dot data in the pseudo halftone region so as to achieve clear and smooth characters as well as an excellent tone reproduction.
In order to simultaneously accomplish image reproducibility of an isolated dot and an isolated line in view of enhancement of image quality, the technique disclosed in Japanese Patent No. 2,668,440 could be used to achieve an adequate image reproducibility by performing exposure with an exposure spot diameter smaller than a pixel size without modulating energy. However, as the image density increases, a lens of an expensive material that is precisely processed is required. Further, the lens must be positioned precisely because the depth of focus of an exposure spot is smaller. Resultant problems are a considerable difficulty of the manufacturing technique and an increase of cost caused by a decreased yield.
The techniques disclosed in Japanese Patent Laying-Open No. 7-61036 and Japanese Patent Laying-Open No. 58-152269 could produce an image excellent in reproducibility by means of optimum dot-energy control according to image pattern. However, when the resolution exceeds 1200 dpi, the exposure spot diameter is less than 40 &mgr;m. Then, in order to achieve such an exposure spot diameter, a lens must be processed and positioned with a high precision, resulting in increase of cost due to decrease in yield as the problem of the technique disclosed in Japanese Patent No. 2,668,440.
Additionally, the technique disclosed in Japanese Patent Laying-Open No. 9-85982 could adjust the dot diameter with a slight change in light quantity by employing a photoreceptor having a high &ggr;-value. However, in formation of minute dots that is necessary for forming a high-resolution image, this technique is likely to be affected by variation of the dot diameter caused by change in the light quantity, resulting in a problem of difficulty in control of the light quantity.
Further, an image forming apparatus of the electrophotography system generally uses a laser beam with an exposure spot diameter of 60 to 80 &mgr;m in an exposure process for forming a latent electrostatic image. This size is three to four times the dot pitch (about 20 &mgr;m) for a resolution of 1200 dpi. Then, cross talk between peripheral dots becomes conspicuous resulting in deterioration in image quality.
For example, regarding a periodic line pattern consisting of paired image lines (black lines) and non-image lines (white lines) per pixel, the non-image lines between image lines intended to be formed are not reproduced due to interference of adjacent lines with respect to each other, and a resultant image is solid black as a whole. If the exposure energy of the laser beam is reduced for avoiding this, latent image potential does not attain exposure energy density potential and thus no image is formed.
Similarly, regarding image data having an isolated-dot pattern where no dot is present around a dot of interest and an isolated-line pattern where a plurality of dots continue in one direction, there could occur situations in which the isolated-line pattern can be formed while the isolated-dot pattern cannot be formed and in which the isolated-dot pattern can be formed while the isolated-line pattern has an increased line width. Accordingly, adjustment of exposure energy does not accomplish image reproducibility for both of the isolated-dot pattern and the isolated-line pattern at the same time.
In particular, techniques which have actually been employed frequently reach a result where the isolated-line pattern can be formed while the isolated-dot pattern cannot be produced. Then, in order to achieve an image of a high resolution exceeding 1200 dpi and a high quality by adjustment of exposure spot diameter, an optimum exposure energy should be defined for each pattern. Even if an exposure spot diameter can be obtained that is close to the dot pitch for the resolution of 1200 dpi or higher by reducing the exposure spot diameter, it would be difficult to achieve image reproducibility for both of the isolated-line pattern and the isolated-dot pattern by means of binary exposure energy control.
When a thick photosensitive layer is employed at the surface of a photoreceptor, image reproducibility conspicuously deteriorates. If the photoreceptor uses a thin photosensi
Azuma Nobuyuki
Iwamatsu Tadashi
Mutou Yoshinori
Nakajima Yoshinori
Toyoshima Tetsuro
Conlin David G.
Dike Bronstein, Roberts & Cushman IP Group, Edwards & Angell, LL
Lee Susan S. Y.
Pledger Timothy Carter
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