JPS6289073A - Image forming device - Google Patents

Abstract

PURPOSE:To obtain a device that can reproduce halftone such as a photograph original and black original part such as characters as well at high fidelity and to obtain satisfactory density by incorporating the developers having the same hue but different tone to two developing devices respectively and providing an equally exposing device between the two devices. CONSTITUTION:When forming an image, a photosensitive body 1 is charged uniformly and forms a latent image, and the latent image potential level of white parts is raised by lowering the quantity of light of the latent image. The latent image is developed by the sleeve 3 of the first developing device 4, made to a visible image at a potential according to a character image, a photographic image and other image tone. Then, the photosensitive body 1 is exposed uniformly by a uniform exposure light source 7. By this exposure, the lowering of latent image potential of halftone near white becomes large, and potential difference due to difference of gradation becomes distinct. Then, the potential difference depending on gradational difference is emphasized by the second developing device 6, and the second developing bias set to conform to white level is added to the latent image, and developed by the sleeve 5. Since the quantity of sticking of gray toner to the intermediate gradation part caused by the developing device 6 becomes large, the influences of black toner having small sticking quantity is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus that develops a latent image formed by charging and exposing a photoreceptor to light.

In an electrophotographic copying machine, which is one of the conventional image forming devices, in order to faithfully reproduce the halftones of photographic originals,
Currently, a method is used in which the amount of carbon black added to the toner contained in the cross-linking agent is reduced to make the toner gray.

In such a conventional apparatus, although the halftone gradation is improved, there is a problem in that the top part of the document no longer appears as black, resulting in an overall image with poor contrast. Black toner alone has a limit to the reproducibility of halftones, and gray toner alone has a limit to the reproducibility of black images, making it impossible to faithfully reproduce original images.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems and to provide an image forming apparatus that can faithfully reproduce both halftones of photographic originals and black parts of originals such as characters, and can obtain satisfactory density. It is said that

Composition In order to achieve the above object, the present invention provides two developing devices each containing developers having the same hue but different tones, and a uniform exposure device being provided between the two developing devices. It is characterized by

The structure and operation of the present invention will be explained in detail with reference to embodiments shown in the drawings.

FIG. 1 shows an example of an electronic copying machine as an image forming apparatus. The photoreceptor 1 is charged by a charging charger (not shown), exposed and imaged by light reflected from a document by an optical system (not shown) to form a zero-electrode latent image, which is developed by a developing device 2 to form a visible image, and then transferred to a copying machine. In this case, the image is transferred to the copy paper fed from the paper feed section under the action of the transfer charger.

The developing device 2 has two developing devices, a first developing device 4 having a first developing sleeve 3 and a second developing device 6 having a second developing sleeve 5. In the illustrated example, the casing of the first developing device 4 and the casing of the second developing device 60 are integrally formed, but they may be formed separately. A uniform exposure light source 7 is provided between the first developing sleeve 3 and the second developing sleeve 5.

During image formation, the photoreceptor 1 is uniformly charged and exposed to image light using a normal optical system to form a latent image. In this case, as shown in FIG. 2(a), the latent image formed is made by lowering the amount of light and increasing the potential level of the latent image in the white part. In FIG. 2, the horizontal axis shows image type and the vertical axis shows latent image potential.

The formed latent image is developed by the first developing sleeve 3 of the racking device 4.The first developing device 4 stores a developer containing black toner (including the case of only toner), and 2
A first developing bias is applied that is set in accordance with the potential level of the white portion (hereinafter abbreviated as white level) in FIG.

An example of the difference in toner adhesion state due to development is shown in FIG. 2(b). In this way, the latent image is visualized in accordance with the potential determined according to the type of color tone of a character image, a photographic image, and other various images.

A black image has the highest potential, and the closer to black in an intermediate tone such as gray, the higher the potential.

After development by the first developing device 4, the photoreceptor is uniformly exposed to light by a uniform exposure light source 7. Due to this exposure, a potential difference of the latent image is generated in accordance with the difference in the amount of toner adhered after development by the first developing device 4, and the closer to white the intermediate tone is, the greater the decrease in fIa image potential is, and the potential difference due to the gradation difference becomes larger. This becomes clear as shown in Figure 2(c).

Thereafter, the second developing sleeve 5 of the second developing device 6 performs development. The second developing device 6 stores a developer (base material, for example, a gray toner) having the same hue as the developer in the first developing device 4 but with a lower brightness.

As shown in FIG. 2C, the potential difference is emphasized by the gradation difference, and the latent image is developed by the second developing sleeve 5 to which a second developing bias set in accordance with the white level is applied. In the high potential area, the amount of black toner deposited by the first developing device 4 is large, so even if gray toner is deposited, the change in image density is small. However, in the intermediate gradation areas, the second developing device 6 applies a large amount of gray toner, so the amount of adhesion is small! The influence of toner is reduced.

According to the old book, the high density area has an overwhelming amount of black toner attached, and is therefore dominated by the density of black toner. Since black toner and gray toner are mixed in the halftone density area, the color gradation changes depending on the degree of mixing.

As an example, the image density after development is as shown in Figure 3. In the case of curve A using only black toner and curve B using only gray toner, reproducibility of halftones is poor with black toner only, and gray When using only toner, the reproducibility of intermediate tones was good, but the development of black was not good, but with the present invention, as shown by curve C, the reproducibility of both intermediate tones and black was good. It was done.

Image exposure light amount, first development bias, uniform exposure light amount, second
By appropriately controlling the developing bias, the range of image quality options can be expanded depending on the combination.

If the reproducibility of only black density is a problem, such as in text originals, the image exposure amount and development bias should be the same as in a normal copying process, and the first developing device should be in an active state and the first developing device should be in an inactive state. It's good to do that.

If intermediate tone reproducibility is important, the first developing device can be left inactive! According to the present invention, it has become possible to reproduce both dark tones and intermediate tones, which could not be reproduced by the conventional developing method using a single-tone developer.

The present invention not only improves the image reproducibility of originals containing a wide range of color tones, but also provides good reproducibility of originals containing only dark and intermediate tones.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of an apparatus according to the present invention, and FIG. 2 is a diagram showing changes in latent image potential and toner adhesion during a part of the process by the apparatus according to the present invention. (
b) is the first development, (C) is the uniform exposure, and (d) is the second development.
FIG. 3, which is a diagram showing the time of development, is a diagram showing the relationship between document density and image density. 1... Photoreceptor 2... Developing device 3... First developing sleeve 4... First developing device 5... Second developing sleeve 6... Second developing device 7... - Side Light source Figure 1 Figure 3 I don't like it

Claims (1)

[Claims] In an image forming apparatus having two developing devices that develop a latent image formed by charging and exposing a photoreceptor, each of the two developing devices accommodates a developer having the same hue but different tones; An image forming apparatus characterized in that a uniform exposure device is provided between two developing devices.