JPS6227754A - Method for adjusting contrast of electrophotographic picture - Google Patents

Abstract

PURPOSE:To extend a density reproducing range to reproduce gradations of an original picture faithfully and well by charging the surface of an image carrier with a certain potential lower than the potential of uniform charging before development after irradiation of an image light. CONSTITUTION:The surface of a photosensitive body 1 consisting fundamentally of a conductive substrate 11 and a photoconductive layer 12 is charged uniformly by a corona charger 2. When light I of an original image is irradiated on a photosensitive body 1, carrier pairs 13 proportional to the quantity of light are generated in the photoconductive layer 12, and plus carriers out of them are migrated to the photoconductive layer 12 and are flowed to the conductive substrate 11, and discharging is caused between minus carriers and the plus electric charge on the surface of the photosensitive body 1. The next process is started just after exposure of the light I. That is, the surface of the photosensitive body 1 is charged by a corona charger 3 having a grid G to which a bias V1(<V0) is applied. By this charging, a certain corona electric charge is given to only the part where the surface potential of the photosensitive body 1 is lower than V1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image contrast adjustment method for faithfully reproducing and copying the gradation of an original image using electrophotography.

[Conventional technology]

Conventionally, the Carlson method has been known as a basic electrophotographic method. In this method, an electrophotographic photoreceptor is used, in which a photoconductive layer of amorphous selenium or the like is provided on the surface of a conductive substrate.
The surface of the document is uniformly charged in a dark place, and then the document image is exposed to light to discharge the surface charge of the photoreceptor at the light irradiation part and form an electrostatic latent image. This electrostatic latent image is developed with toner particles and transferred to a transfer material.

Generally, the density reproduction range for originals using the Carlson method is about 0.6 to 0.8. For example, when amorphous selenium is used, the relationship between the latent image potential and the original density is shown by the solid line a in Figure 8. That's it. And this manuscript 1
The reality is that it is not possible to reproduce the copy density completely with respect to the M degree, as shown by the solid line in FIG.

1. As another method of electrophotography, Japanese Patent Publication No. 42-23910 filed by the present applicant or Japanese Patent Publication No. 43-2
It is disclosed in Japanese Patent No. 4748.

This electrophotographic method uses a photoreceptor in which a photoconductor layer and a transparent insulating layer are laminated on a conductive substrate. PFI for original density! The relationship between the potentials is also as shown by the chain line in FIG.

The reproduction covey formed by the electrophotographic method as described above is
It was fully satisfactory for uses such as poor fields. However, it is not possible to faithfully reproduce a document containing a wide range of grays and hues, and therefore it cannot be used for precision printing or fine art printing that requires faithful reproduction of the document image.

That is, in the case of conventional electrophotography, light gray in the original image becomes lighter, and dark gray in the original image becomes darker in the reproduced image. In the case of color copying, if a subtle color change is attempted, the color of the copy becomes muddy, and there is also the risk that the color change with respect to the original document will be large. Although these have been improved somewhat by improving toner properties, it is difficult to faithfully reproduce original images.

[Problem that the invention seeks to solve]

In view of the above-mentioned points, the present invention provides an image contrast adjustment method for electrophotography that can faithfully reproduce the gradation of a document image.

[Means for solving problems]

In order to solve the above problems, the present invention provides electrophotography in which the surface of an image carrier having at least a conductive layer and a photoconductive layer is uniformly charged and then an electrostatic latent image obtained by irradiating image light is developed. In this method, the surface of the image carrier is charged to a constant potential lower than the uniform charging potential at the same time as the image light is irradiated and before development.

[For production]

By doing so, the density reproduction range is expanded and the gradation of the original image is faithfully and satisfactorily reproduced.

〔Example〕

Embodiments of the present invention will be described below with reference to FIG.

In the first step (a), a corona charger 2 uniformly charges the surface of the photoreceptor 1, which basically consists of a conductive substrate 11 and a photoconductive layer 12, when the photoconductive layer 12 has a P-type semiconductor characteristic. teeth,
It is positively charged (surface potential is ■).

Next, in a second step (b), the photoreceptor l is irradiated with the light I of the original image. At this time, carrier pairs 13 proportional to the amount of light are generated in the photoconductive layer 12, positive carriers among them move through the photoconductive layer 12 and flow to the conductive layer 11, and negative carriers are formed on the surface of the photoreceptor 1. discharges between the positive charge and the positive charge. The efficiency of this discharge depends on the electric field applied to the photoconductive layer 12, and varies depending on the photoconductor, but it roughly shows the log E-V characteristic (electrostatic latent image potential characteristic with respect to exposure amount) as shown by the dotted line α in FIG.
Its dynamic range is 0.6 to 0.8.

The third step (c) is performed immediately after the light exposure.

A grid G is applied with a bias voltage V+ (<VO).
The high-voltage power source 5 of the corona charger 3, which charges the surface of the photoreceptor 1, is preferably a constant current power source so that the corona current does not change depending on the ring tomb.

Due to this charging, a certain corona charge can be applied only to the portion of the photoreceptor 1 where the surface potential is approximately 1 or less. As a result, as shown by the solid line β in FIG. 2, the area where the surface potential of the log E-V characteristic is about 1 or less is extended toward the direction where the amount of light is greater, and the gradation can be controlled. In other words, the dynamic range will be expanded.

Note that the third step (c) is exposure for original image exposure (second step).
In this case, since the electrostatic latent image is corona charged even during formation, its log E-V characteristic will be as shown by the solid line β, and the peculiar change near l will be reduced, and it will be as shown by the chain line γ. It shows certain characteristics.

FIGS. 3 and 4 are diagrams explaining another embodiment of the present invention,
In both cases, finer gradation control than in the previous embodiment is possible.

FIG. 3 shows that the grid of the corona charger 3 used in the third step is divided into two parts, 01 and G2, and each grid is divided into 01 and G2.
A bias voltage of 150V is applied. The discharge wire 6 is connected to a constant current high voltage power source 5. Figure 4 is the second
The document image exposure I, which is a process, and the first half of controlled charging by the corona charger 3, which is a third process, are superimposed. vl=÷250V and V2=+150 for grids G1 and G2, respectively
A bias voltage of V was applied.

FIG. 5 shows the relationship between the latent image potential and the original density. The solid line C indicates the potential of the electrostatic latent image formed in the embodiment shown in FIG. The potential is dotted line d
The luminance reproduction range has been expanded to about 1.2.

Further, when applying the document image exposure I in the second step, it is effective to further expand the brightness reproduction range of dark areas by uniformly applying weak light-bias light to the entire surface. The dashed line e in FIG. 5 shows the electrostatic potential when such bias light is irradiated, and at this time the luminance reproduction range has expanded to about 1.4.

FIG. 6 is a schematic diagram showing a copying apparatus suitable for carrying out the uninvented method. The photoreceptor 1 is a rotating drum whose basic structure is a conductive substrate and a photoconductive layer. A metal drum made of aluminum or the like is used as a conductive substrate, and about 50p of amorphous selenium alloy is deposited thereon as a photoconductive layer. While rotating, the drum photoreceptor 1 is first uniformly charged to approximately +500V by the positive corona charger 2. The manuscript is on manuscript table 1
The photoreceptor 1, which is uniformly charged, is exposed to light by an exposure means placed on the photoreceptor 5 and comprising a light source 16, movable mirrors 17 and 18, a lens 19, and mirrors 20-21. After being irradiated with the original image light I, the photoreceptor 1 is charged by a positive corona charger 3 having a grid G. Corona charges are then added partially depending on the latent image potential. As a result, the potential of the electrostatic latent image becomes as described above. A sleeve-type magnetic brush developer 22 is provided to apply developer to the photoreceptor surface after the latent image is formed.
A suitable bias voltage is applied to the developing sleeve on which the developing sleeve is placed to prevent fogging caused by adhesion of developer to bright areas, and development is performed. The developed images formed on the photoreceptor are transferred onto the transfer material P by the transfer corona discharger 23 of the paper feeding transfer means. The transferred image on the transfer material P is melted and fixed onto the transfer material P by the heat fixing device 24, and then discharged onto the paper discharge tray. On the other hand, the developer remaining on the photoreceptor 1 after the transfer is cleaned by the cleaning means 25, and the photoreceptor 1 is prepared for reuse.

The photoconductor of the photoconductor l described so far is composed of one photoconductor layer, but the photoconductor used in the present invention can be any photoconductor that can be used in the so-called Carlson method. For example, it is effective for all photoreceptors, such as the one shown in FIG. 7, which consists of a charge generation layer 12a and a charge transfer layer L2b on a conductive substrate 11.

〔Effect of the invention〕

As described in detail in the second specific example, the method of the present invention expands the density reproduction range and makes it possible to faithfully and satisfactorily reproduce the tonality of an original image.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating an embodiment of a method of applying the present invention;
Figure 2 shows the electrostatic latent image formed by the method to which the present invention is applied.
Potential characteristic diagrams, FIGS. 3 and 4 are diagrams explaining another embodiment, FIG. 5 is a potential characteristic diagram of an electrostatic latent image formed in that embodiment, and FIG. 6 is a diagram showing the implementation of the method of the present invention. 7 is a diagram showing an example of the configuration of a photoreceptor, FIG. 8 is a potential characteristic diagram of an electrostatic latent image formed by a conventional method, and FIG. 9 is also a copy density characteristic diagram. It is a diagram. 1 is a photoreceptor, 3 is a corona charger, G1101.02 is a grid, V-V, V2 is a power source, and 5 is a constant current high voltage power source.

Claims (1)

[Claims] (1) In an electrophotographic method in which an electrostatic latent image obtained by uniformly charging the surface of an image carrier having at least a conductive layer and a photoconductive layer and then irradiating it with image light, when the image light is irradiated, An image contrast adjustment method characterized in that the surface of the image carrier is charged to a constant potential lower than the uniform charging potential before development is performed.