JPH0574066B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical scanning type electrophotographic printer that expresses print information including halftones by a multi-value dither method.

Optical scanning electrophotographic printers use laser beam scanning using a polygon mirror or LED
A dot latent image consisting of a recorded portion and a non-recorded portion is formed on a charged electrophotographic photoreceptor by scanning a light beam using an array or LCD array, and this is developed to obtain an image composed of dots. ing.

Here, the black-and-white binary halftone can be expressed by changing the dot density within one pixel by the dither method or the density pattern method (hereinafter simply referred to as the dither method). However, recently there has been a growing demand for improved image quality, and it has become necessary to improve gradation reproducibility by using a three-value or four-value multi-value dither method instead of the conventional binary dither method. This multilevel dither method is useful when removing false contours that tend to occur in highlight areas, or when simultaneously reproducing an image containing a mixture of halftones and line images.
This technology is also essential when reducing the pixel matrix size to improve resolution.

The concept of a dither matrix in the multilevel dither method is shown in FIGS. 1a and 1b. Figure 1a shows a 2x2 ternary dither matrix, with areas S1, S2,
S3 represents three density levels of white, gray, and black, respectively. Also, Figure 1b shows 2×2 4
is a dither matrix of values, with areas S1, S2,
S3 and S4 are white, light gray, dark gray, respectively.
It represents the four-value density level of black. The dot size is, for example, 16 dots/mm.

Figures 2a and 3a and 3a and 3b show the exposure intensity distributions in an optical scanning type electrophotographic printer when performing ternary recording, and the corresponding electrostatic Potential distribution of latent image Fig. 2b, Fig. 3b
It represents. The broken lines in Figures 2a and 3a represent the signal output for outputting a light beam to form a multivalued latent image. Figure 1a
In this method, a gray level (hereinafter referred to as M level) corresponding to S ₂ and a black level (hereinafter referred to as H level) corresponding to S ₃ are obtained. For example, the M level can be obtained with a laser output that is 1/2 that of the H level.
FIG. 3a shows a method for obtaining the M level and H level by pulse width modulation that controls the laser output time. This can be obtained, for example, by setting the pulse width of the M level to 1/2 that of the H level. The potential distribution of a latent image created by a light beam having the exposure intensity distribution shown in FIGS. 2a and 3a is as shown in FIGS. 2b and 3b. The latent image contrast of the M level tends to be smaller than that of the H level due to a decrease in the MTF of the latent image. Therefore, the image density of this M level after development becomes almost the same gray as the image density of the M level shown in FIG. 2b due to brightness modulation.

Figure 4 shows the conventional development characteristics (V _S -D _P characteristics) when developing a multivalued latent image, and Figure 2 b
In order to reproduce the M level and H level latent images (respective potential contrast is represented by development characteristics (shown by the solid line) are required. Figure 3b
In order to develop an M-level latent image, a development characteristic with a large gamma (indicated by a broken line) may be used, but if variations in the latent image characteristic or development characteristic cause image skipping, that is, undeveloped portions may occur. Therefore, the characteristics shown by the solid line are preferable.

However, in order to reproduce the level in any of the development characteristics shown in FIG. 4, the rising portion of the development characteristics is used, so there is a problem in that the developed density varies greatly with changes in the latent image potential. Moreover, when the development characteristics themselves fluctuate, there is of course a problem in that the development density is greatly affected. For this reason, there is a problem in that the image quality of images obtained by the multilevel dither method is unstable.

The present invention eliminates the drawbacks of the conventional example, and stabilizes the image density of the halftone dots of the dither matrix when expressing a halftone image using the multi-value dither method. It is an object of the present invention to provide an electrophotographic printer that can obtain image quality stability equivalent to that of the dither method.

The present invention achieves the above objects by optically scanning an electrophotographic photoreceptor based on image information, forming a multivalued dot-shaped electrostatic latent image, and developing this latent image by a reversal development method. A plurality of developing means having different development saturation densities are disposed in the moving direction of the photoreceptor, and the electrostatic latent image formed on the photoreceptor is
By sequentially developing with each developing means, among the multivalued dot-shaped electrostatic latent images formed, the latent images with low contrast are developed with the developing means having characteristics of low development saturation density, and the latent images with high contrast are developed. Developing is carried out using a developing means having a characteristic of high development saturation density.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 5 shows an embodiment of an electrophotographic printer to which the present invention can be applied. The electrical signal corresponding to the image information input to the laser modulation unit 1 is output as a modulated laser beam, and is sent to the scanner mirror 2.
The longitudinal direction of the photosensitive drum 4 is scanned by the f/θ lens 3. The photosensitive drum 4 rotates in the direction of the arrow, making it possible to scan the laser beam two-dimensionally. An organic photoconductor is used for the photosensitive drum, and after the potential on the surface of the photosensitive drum is leveled by an AC static eliminator 5, it is charged to -600V by a charger 6. after that,
Images are created on the photoreceptor by laser beam exposure.
A scan method forms a dot latent image using a three-value dither method. The ternary M level is formed by brightness modulation of laser light as shown in FIG. 2a.
The latent image potential is -80V for H level and -80V for M level.
It was 340V.

Among these dot latent images, the H level latent image is the 6th dot latent image.
Development characteristics with high toner density (saturation density: 1.4), which is required when the gamma shown in A in the figure is large and the saturation density, that is, the amount of toner adhesion after developing the electrostatic latent image is saturated. The first developing unit 9 (fifth developing unit) contains negative polarity toner having a
(Fig. 7a). The developing bias at this time was -400V. Therefore,
The actual H level contrast is 320V. Note that the optimum value of the bias voltage applied to the first developing device 9 is selected within the range between the charging potential of the photoreceptor and the latent image potential of the M level.

Next, the M-level latent image among the dot latent images has a large gamma as shown in B in FIG. The developing device 10 (FIG. 5) performs reversal development (FIG. 7b). The developing bias at this time was -520V. Therefore,
The actual M level contrast is 180V.

The substantial development characteristics in the two sequential development steps described above are as shown in FIG. The characteristics shown in FIG. 8 are characteristics synthesized by taking into account the development characteristics A and B shown in FIG. 6, the developer used, and the bias voltage. With the characteristics shown in FIG. 8, the saturated image densities D _M and D _H can be kept constant even if the latent image contrast varies somewhat. Further, fluctuations in the development characteristics are often large at the rising edge of the characteristics, and fluctuations in the saturation densities D _M and D _H are small.

Note that the developer having the development characteristics as shown in FIGS. 6A and B used in the present invention is prepared by adjusting the toner colorant content, selection of binder resin, toner particle size, charge control agent, toner particle size distribution, etc. It is known that it can be produced by

Insulating 1 as a developer used in the second developing device 10
When a component developer is applied and development is performed by the jumping development method described in JP-A-55-18656-9, the previously developed H level image is not disturbed by the second development. Therefore, it is convenient.

The image developed by the first developing device 9 and the second developing device 10 in FIG. be done. Further, toner remaining on the photosensitive drum 4 without being transferred is collected by a cleaner 14. In this way, the gradation is good on the transfer paper,
An image with excellent image quality stability is formed.

In addition, in the present invention, the development of three image density levels S ₂ , S ₃ , and S ₄ in FIG. Needless to say, this can be achieved by sequentially developing with three developing devices.

As detailed above, in the present invention, in an optical scanning type electrophotographic printer that expresses halftones of print information by a multi-value dither method, multi-value dots are produced using a plurality of developing means with different development saturation densities. Since the latent images are developed sequentially, an image with good gradation and excellent image quality stability can be obtained.

[Brief explanation of the drawing]

FIGS. 1a and 1b are diagrams showing the concept of a multivalued dither matrix. FIGS. 2a, b and 3 a, b are characteristic graphs showing the exposure intensity distribution and the potential distribution of an electrostatic latent image when performing ternary recording. FIG. 4 is a graph showing the development characteristics of a conventional device. FIG. 5 is a diagram showing an embodiment of an electrophotographic printer to which the present invention is applied. FIG. 6 is a graph showing the development characteristics of two types of developers having different saturation densities used in the present invention. FIGS. 7a and 7b are diagrams explaining the development process of the present invention. FIG. 8 is a graph of the substantial development characteristics obtained in the apparatus of the present invention. In the figure, 1 is a laser modulation unit, 2 is a scanner mirror, 3 is an f/θ lens, 4 is a photosensitive drum, 5 and 6 are corona dischargers, 9 is a first developer,
10 represents a second developing device.

Claims (1)

[Scope of Claims] 1. An image forming apparatus that performs light scanning on an electrophotographic photoreceptor based on image information to form a multivalued dot-shaped electrostatic latent image, and develops this latent image by a reversal development method, comprising: A plurality of developing means having different development saturation densities are disposed in the moving direction of the image forming apparatus, and the electrostatic latent image formed on the photoreceptor is sequentially developed by each developing means, thereby forming a multivalued dot-like static image. Among the electric latent images, a latent image with a low contrast is developed with a developing means having a characteristic of a low developing saturation density, and a latent image with a high contrast is developed with a developing means having a characteristic with a high developing saturation density. image forming device.