JPH0462383B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a color electrophotographic method in which an electrostatic latent image formed on a photoreceptor by imagewise exposure is developed using multicolor toner, and in particular, it relates to a color electrophotography method in which an electrostatic latent image formed on a photoconductor by imagewise exposure is developed using multicolor toner. This paper relates to a color electrophotographic method that provides excellent gradation reproducibility for original images with a wide range.

[Prior art]

Conventional color electrophotographic photosensitive materials (hereinafter simply referred to as photosensitive materials) have bright decay characteristics similar to those for black and white use.
Since the relationship between the original image density and the sensitive material potential is not linear as shown in Figure 1, the relationship between the original image density (DIN) and the copy density (DOUT) is also nonlinear, resulting in the so-called dynamic range as shown in Figure 2. It became narrow.

For this reason, in color electrophotography, when inputting gradations of the same hue, there may be hue discrepancies between low and high density areas, and for original images with a wide contrast range like photographs, there may be a difference in hue between low and high density areas. Poor gradation occurred in the area.

[Purpose of the invention]

It is an object of the present invention to eliminate the above-mentioned drawbacks and to provide a color electrophotographic method capable of producing excellent continuous tone reproduction even for original images with a wide contrast range and images without hue shift.

[Structure of the invention]

In other words, the object of the present invention is to repeat the combination of charging and exposure of the same hue multiple times on the same document, to uniformly charge it with a constant current power supply before development, and to process the above-mentioned steps. The process includes a step of developing the obtained electrostatic latent image with a developer of a specific color, and a step of transferring and fixing the developed image onto transfer paper, and the series of steps is repeated in sequence according to the number of colors to be reproduced. The images formed for each reproduced color are superimposed and transferred, and the second and subsequent charging of the same original is performed with a constant current power source, and the images are transferred multiple times with the same hue. This is achieved by color electrophotography, which is characterized in that the amount of exposure is changed for each exposure.

〔Example〕

Hereinafter, the method of the present invention will be explained using the drawings.
FIG. 3 is a schematic diagram of a color electrophotographic copying machine used in an embodiment of the present invention.

In FIG. 3, this color electrophotographic copying machine includes a first corona charging device 2 that is located close to the surface of a drum-shaped photoreceptor 1 and uniformly charges the surface of the photoreceptor, and a first corona charging device 2 that uniformly charges the surface of the photoreceptor. A first exposure light source 3 that exposes the same document to the same color as the charged amount by increasing the amount by 30% (0.5 in terms of density) than the set amount, and a second constant current type corona that uniformly charges the photoreceptor 1 again. A charging device 4 and a second exposure light source 5 having the same hue as the first exposure light source 3 and having a normal exposure amount.
Then, a third constant current corona charging device 6 uniformly charges the photosensitive surface again, and first, second, and third developing devices 7, 8, and 9 perform pre-transfer processing for multicolor development. A transfer charging device 10, a transfer drum 11, and a cleaner device 12 are sequentially arranged along the photosensitive surface.

Further, a transfer paper 1 is provided adjacent to the transfer drum 11.
A fixing roll 14 for fixing the image 3 under pressure is provided.

Using the copying machine configured as described above,
An embodiment of the color electrophotographic method of the present invention will be explained along with the copying process.

The photoreceptor 1 uniformly charged by the first corona charging device 2 is imagewise exposed by the first exposure light source 3 with a higher exposure amount than usual to form an electrostatic latent image,
Furthermore, the photosensitive surface is uniformly charged by a second constant current type corona charging device 4. This uniformly charged photosensitive surface is then exposed to the same image by the second exposure light source 5 with the normal exposure value, and then the third exposure light source 5 is exposed to the same image.
It is uniformly charged by a constant current type charging device 6.

In the third charging device 6, the charging potential is lowered than the previous two times, for example, from about 900V to 100V.
Charging takes place at around 600V.

By performing the above steps, the relationship between the original image density (DIN) and the sensitive material potential in the brightness decay characteristic becomes the fourth
The result will be as shown in the figure.

That is, the bright attenuation characteristic created by the first charging and exposure step with increased exposure amount (shown by curve A in the figure) is different from the bright attenuation characteristic created by the conventional normal exposure amount (shown by curve B in the figure; (same as Figure 1), compared to
The high-density area image of the original image is emphasized and extracted. Therefore, repeating the second charging and exposing process using the normal exposure amount on the same original after the first charging and exposing process can improve the bright decay characteristics (indicated by the broken line C in the figure). As I get older,
An image with a wider density range is extracted.

Furthermore, by performing a third charging step in which the charging potential is lowered, the linearity of the bright decay characteristic (broken line C) created by the second charging and exposure step is corrected, especially in the low concentration region side. The image was emphasized, and in the end, Dynamite Cleanse became 1.0.
A bright attenuation characteristic (curve D in the figure) that is broadened to a certain extent is obtained.

The electrostatic latent image obtained through these steps is developed, for example, in yellow, by the first developing device 7, and then the developed image is neutralized in charge by the transfer charging device 10, and then transferred to the photoreceptor. The image is transferred onto a transfer paper 13 that is conveyed between the photoreceptor 1 and the transfer drum 11 as the photoreceptor 1 rotates.

The transfer paper 13 on which the image has been transferred is fixed under pressure by the fixing roll 14, and then, in preparation for the next transfer, it is wound around the transfer drum 11, rotates once, and returns to the original transfer start position where it is held. That is, the transfer drum 11 is provided so that it can hold the transfer paper 13 onto which the first color image has been transferred, and can superimpose and transfer the second and subsequent color images.

On the other hand, when the residual toner on the photoreceptor 1 is removed by the cleaner device 12, an electrostatic latent image of the same image is formed in the same manner as the previous time. This latent image is then developed in magenta, for example, by the second developing device 8 to form a second color image, and then the transfer paper 1 holding the first color image is
3 and is superimposed and transferred and fixed. In the same manner, the third color image is developed by the third developing device 9 by performing the series of steps described above, and is superimposed on the first and second color images, and when the fixing is completed, the image is transferred to the transfer paper. 13 is conveyed outside the machine and the transfer is completed.

In the above embodiment, by using the constant current type charging devices 4 and 6 for the second and subsequent charging, the potential difference between the latent image area and the non-latent image area formed on the photosensitive material is as shown in FIG. 4P. , the linearity of the bright attenuation characteristic is increased, and the dynamic range is expanded.

Further, in this embodiment, the combination of charging and exposure process is performed twice, but it may be performed more times.

That is, the present invention changes the exposure amount as a means of matching the density range of the original image, which has a wide contrast range, to the highest sensitivity region of the photosensitive material (the part where the brightness decay characteristic has a large slope). Therefore, by performing this combination of charging and exposure processes multiple times, the density regions of the original image with a wide range are extracted step by step by changing the exposure amount, and the images obtained for each extracted region are superimposed. The aim is to improve reproducibility.

In the above embodiment, the exposure amount is gradually decreased, but the exposure amount may be gradually increased.

Further, although pressure fixing is used in the above embodiment, various other conventionally known fixing methods can be used, but it is necessary to consider a fixing method that does not affect the transfer process that is repeatedly performed.

〔Effect of the invention〕

As described above, according to the present invention, in order to improve the reproducibility of original images with a wide contrast range, the dynamic range is expanded by superimposing images obtained by changing the exposure amount for each density region. An image with excellent continuous tone shading can be obtained in the same hue. Further, by performing the above steps for each hue, a clear image without muddiness or color shift can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between original image density and photosensitive potential in the bright decay characteristics of electrophotographic photosensitive materials, FIG. 2 is a diagram showing the relationship between original image density and copy density, and FIG. 3 is a diagram showing an example of the present invention. FIG. 4 is a schematic diagram of the color electrophotographic copying machine used, and is a diagram illustrating the brightness attenuation characteristic formed by the present invention. DESCRIPTION OF SYMBOLS 1...Photoreceptor, 2,4,6...Corona charging device, 3,5...Exposure light source, 7,8,9...Developing device, 10...Charging device for transfer, 11...Transfer drum, 12 ... Cleaner device, 13 ... Transfer paper, 1
4... Fixing roll.

Claims (1)

[Claims] 1 In color electrophotography, which involves the processes of charging, exposure, development, transfer, and fixing, and develops an electrostatic latent image formed on a photoconductor by image exposure in color, the same original is charged and has the same hue. A step of repeating the combination with exposure multiple times, a step of uniformly charging with a constant current power source before development, a step of developing the electrostatic latent image obtained through each of the above steps with a developer of a specific color, The process includes a step of transferring and fixing the developed image onto transfer paper, and the series of steps is repeated in sequence according to the number of reproduced colors, and each image formed for each reproduced color is superimposed and transferred. The method is characterized in that the second and subsequent charging of the same document is performed by a constant current power supply, and that the amount of exposure is changed for each exposure when the same hue is exposed multiple times. Color electrophotography.