JPS5950064B2 - Electrostatic latent image forming method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming an electrostatic latent image on a photoreceptor so that an original having continuous gradation can be accurately reproduced in a wide gradation range by an electrophotographic copying machine.

Conventionally, in order to accurately reproduce continuous tone originals by electrophotography using a photoreceptor with a photoconductive layer on the surface, as typified by xerography, there has been a method using a developing electrode, a dot-like or a mesh-like pattern, etc. A method is known in which a light image is divided and exposed.

However, in order for these methods to be effective in continuous tone reproduction, the condition is that the electrostatic latent image is formed in accordance with the gradation of the original.

That is, if the photoreceptor is charged in accordance with continuous gradations as shown in FIG. 1, it is possible to reproduce the maximum density determined by the type of door using the developing electrode.

However, as shown in FIG. 2, the linearity of the relationship between the amount of exposure and the potential on the photoreceptor deteriorates on the brighter (white) side of the document, that is, on the side where the amount of exposure is greater, and the relationship deviates from the ideal straight line a.

For this reason, in the past, gradations were only reproduced within the range where linearity was close to ideal, in the range of , as shown in Figure 2, so the brighter side of the original could not be reproduced, resulting in a narrow gradation range. It has the problem of getting old.

The present invention was made in view of the above circumstances, and its purpose is to idealize the relationship between the exposure amount and the potential on the photoreceptor in the brighter areas of the original so that the brighter areas of the original can also be faithfully reproduced. An object of the present invention is to provide a method for forming an electrostatic latent image that can be approximated to a straight line.

That is, in the present invention, when performing secondary charging of the same polarity as the primary charging while irradiating a photoreceptor that has been primarily charged in the dark with a light image, the secondary charging is controlled by a screen wire to which a voltage is applied. This increases the potential of the portion on the photoreceptor, which should be approximately lower than the voltage applied to the wire, to bring it closer to the ideal straight line a.

Specific examples will be described below.

In FIG. 3, the photoreceptor 1 is positively charged by the primary charger 2, and at the same time when the optical system 4 irradiates the photoreceptor with a light image, the photoreceptor 1 is charged by the secondary charger 3.
becomes positively secondary charged.

The secondary charger 3 has a corona discharge wire 6 to which a voltage of 6000V is applied and a screen wire 7 to which a voltage of 350V is applied, and the upper part is optically open. As a result, as shown in Fig. 4, the relationship between the photoreceptor surface potential and the exposure amount is the same as in the conventional case where only exposure is performed after charging (shown by the solid line), and the relationship where the exposure amount is larger is shown by the broken line. It can be corrected as follows.

This is because when only exposure is performed on the primarily charged photoreceptor 1, the areas exposed to more light are discharged and the surface potential decreases.
This is because, since secondary charging is performed at the same time, more current flows to a portion of the photoreceptor 1 whose potential is lower than the voltage applied to the screen wire 7.

Even if 350V is applied to the screen wire 7, the correction in Figure 4 starts from around 600V, which is why some current flows from between the screen wires 7 to the part on the photoreceptor 1 where the potential is high. It's for a reason.

For this reason, the relationship between the exposure amount and the potential on the photoreceptor in the brighter part of the original can be brought closer to the ideal straight line a, and the range b in which continuous gradation can be reproduced is widened. But it can be reproduced.

5, 6 and 7 show different embodiments of the invention.

In FIG. 5a, a single layer of screen wire 7 is divided into two parts 71 and 72 in the forward and backward directions of the photoreceptor, and a voltage is applied so that the screen wire 72 at the rear, that is, the rear screen wire 72 in terms of process, is lower. Voltage VSC of screen wire 71
Voltage Vse2 of screen wire 72 after 350V
The correction curve when the voltage is set to 50V is shown in FIG. 5b.

Here, the solid line shows the relationship when only exposure is performed after primary charging, and the broken line 1 is the curve corrected by the starne wire 7 before, and the screen wire 7 after. The broken line 2 shows the correction. In FIG. 6a, two layers of screen wires 7 are provided, and the voltage applied to the screen wire 73 closer to the photoreceptors 73 and 74 is lowered.

In this case, as shown in FIG. 6b, the broken line 2 is the curve 1 corrected by the screen wire 73 of only one layer, and the curve 1 corrected by the screen wire 74 of the other layer. At this time, a voltage of 440 V was applied to the first screen wire 73 and a voltage of 500 V was applied to the second screen wire 74. Figure 7a shows a secondary charger that combines Figures 5a and 6a, in which two layers of screen wires 75 and 76 are divided into two parts 751 and
752, 761, 762, the voltage of the latter screen wires 752, 762 is lowered, and the voltage of the screen wire 75 of the two layers, which is closer to the photoreceptor, is lowered.

Then, the voltage applied to each screen wire was set to 761 to 400.
100V for V762, 350V for 751, 50V for 752
The correction curve when V is set is A, and 761 is 450■,
150V for 762, 350V for 751, 50V for 752
The correction curve when this is done is B.

Furthermore, by varying the voltage applied to the Starne wire in these embodiments, the gradation can be changed arbitrarily.

In other words, the range of reproducible gradations can be changed. If an electrostatic latent image is formed by the electrostatic latent image forming method according to the present invention,
Since more current flows to the part of the photoreceptor 1 whose potential is lower than the voltage applied to the screen wire 7, the relationship between the exposure amount on the bright side of the original and the potential on the photoreceptor is ideal. It can be made closer to a straight line, the range in which continuous tone can be reproduced is widened, and even bright areas can be reproduced.

In other words, a surface potential suitable for continuous tone reproduction can be obtained,
By using development methods that are relatively easy to obtain continuous gradation, such as magnetic brush development, liquid development, and cascade development with electrodes, it is possible to reproduce even the brightest parts of the original, widening the possible gradation range. can do.

Further, as shown in the embodiment, by changing the way the screen wires 7 are provided, the possible range of gradation can be changed.

[Brief explanation of the drawing]

Figure 1 is a table showing the relationship between the potential on the photoreceptor and the density after development, Figure 2 is a table showing the relationship between the exposure amount and the potential on the photoreceptor on a logarithmic scale, and Figure 3 is a table showing the relationship between the potential on the photoreceptor and the density after development. A schematic explanatory diagram of a copying machine using the invention, FIG. 4 is a table showing a correction curve when the screen wire is one layer, and FIGS. 5, 6, and 7 are other embodiments of the second charger. a and a table b showing correction curves based on them, 1 is a photoreceptor, 6 is a corona discharge wire for secondary charging, and 7 is a screen wire.

Claims (1)

[Scope of Claims] 1. When the photoreceptor 1 having a photoconductive layer on its surface is primarily charged in the dark and then subjected to secondary charging with the same polarity as the primary charging while irradiating a light image, An electrostatic latent image forming method characterized by controlling secondary charging by applying a voltage of the same polarity as the primary charging voltage to a screen wire 7 provided between a charging corona discharge wire 6 and a photoreceptor 1. . 2. The electrostatic latent image forming method according to claim 1, wherein one layer of screen wires 7 is provided. 3. The electrostatic latent image forming method according to claim 1, wherein two or more layers of screen wires 7 are provided, and a voltage is applied such that the voltage decreases as it approaches the photoreceptor 1. 4. Electrostatic latent image formation according to claim 2 or 3, in which the screen wire 7 is divided into two parts, front and back in the direction of movement of the photoreceptor 1, and a voltage is applied so that the rear screen wire 7 is lower. Method.