JPH0347506B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic method, and more particularly, to an electrophotographic method that enables image formation with good gradation reproduction.

The present applicant has previously proposed a technique disclosed in Japanese Patent Application Laid-open No. 14237/1983 as an electrophotographic method for reproducing gradation well. According to the electrophotographic method described in the above publication, it is possible to expand the brightness reproduction range (original density reproduction range) for originals to 1.0 to 1.3, whereas the brightness reproduction range of general electrophotography is around 0.7. In comparison, the gradation of the original can be reproduced better.

However, in the above method, it is necessary to repeat the photoimage exposure and to increase the light intensity of the photoimage exposure stepwise.

Therefore, a large amount of exposure is required compared to normal optical image exposure, and there is a problem in that at least one of the control process or the implementation device becomes complicated during implementation.

An object of the present invention is to provide a reproduced image with rich gradation. Another object of the present invention is to faithfully reproduce halftones of an original. Another object of the present invention is to enable reproduction of high quality color images.

The present invention, which achieves the above object, is an electrophotographic method for forming an electrostatic latent image on a photoreceptor having a conductive layer, a photoconductive layer, and a surface insulating layer, the method comprising: (a) forming a surface insulating layer on the photoreceptor; (b) applying a corona discharge having a magnetic component opposite to the uniform charging polarity of step (a) while irradiating the photoreceptor with a light image; (c) ) A step of irradiating this photoreceptor with the same light image as in step (b) above to increase the surface potential of the halftone region of the light image of the photoreceptor, and (d) depending on the influence of the light image irradiation, the above ( A corona discharge having a polarity component opposite to the uniform charging polarity of the step (a) is applied, and the potential in the area above a certain potential among the surface potentials in the halftone area raised in the step (c) above is removed. (e) irradiating the photoreceptor with uniform light.

Hereinafter, specific examples will be explained with reference to the drawings.

First, the conventional electrophotographic process described in the above publication on which the improvement of the present invention is based will be explained.

Each figure in the horizontal row indicated by symbol 1 in Fig. 1 is a photoconductor A.
Each figure in the horizontal column indicated by symbol 2 schematically shows the change in the surface potential of photoreceptor A, and each figure in the horizontal column indicated by symbol 3 shows the change in the surface potential of photoreceptor A corresponding to the original density. This figure schematically shows the change in surface charge density of .

That is, in the first step, the surface of the photoreceptor A, which basically consists of a conductive substrate a, a photoconductive layer b, and a transparent insulating layer c, is primary charged with a specific polarity by a corona discharger _D1 . This charging polarity is positive (+) when the photoconductive layer b is of N type, and negative (-) when it is of P type, but the following explanation will be given as an example of the case where N type is applied.

Next, in the first step, the optical image of the original O whose image density changes continuously is exposed to the primary charged photoreceptor, and at the same time the discharge wire of the corona discharger D ₂ is charged with a polarity opposite to that of the primary charge (- ), or apply AC or a high voltage of negative (-) biased AC to remove static electricity. A grid g ₁ is provided at the opening of this corona discharger D ₂ .
Control the voltage applied to _g1 . When removing static electricity in the above-mentioned step, the surface potential of the photoreceptor after static elimination V ₀
is controlled to be slightly negative or 0V.
At this time, even though the charges trapped near the interface between the photoconductive layer and the insulating layer are not released, in the area where some of the surface charges have been removed, the interface will respond to the removed charges. Holes of opposite polarity to the trapped charges of the layer are induced in the conductive layer.

Next, the following gradation adjustment process is performed.
That is, in step _a1 , by exposing the original image to a suitable light intensity, the trapped charges on the bright side are released, combine with holes induced in the conductive layer, and disappear. Therefore, the surface potential on the bright side is a ₁
- Changes as shown in Figure 2. On the other hand, on the dark side,
No change in surface potential occurs because the trapped charges are not released. Or even if it occurs, it is only slight.

Next, in the _second step a, a corona discharger is
The voltage applied to grid g ₂ provided at the opening of D ₃ is
The photoreceptor surface potential is adjusted to the potential V ₁ with V ₁ higher than V ₀ . Apply a neutralizing voltage of the same polarity as in the above process to corona discharger D ₃ to reduce the potential on the surface of the photoreceptor.
Control to V ₁ .

At this time, by the photoimage exposure in step _a1 , the potential of the portion exceeding the changed surface potential _V1 is controlled to approximately _V1 , and a part of the surface charge is removed.

Next, in step b ₁ , the original image is further exposed to light at an appropriate light intensity (for example, with a stronger light intensity than in step a ₁ ) to remove the dark areas captured near the interface layer. The charge on the side closer to is released. On the other hand, near the darkest part, naturally the photoreceptor A
Since no light reaches the , such release of trapped charges does not occur. Therefore, a change in surface potential as shown in Fig. _b1-2 occurs.

Furthermore, in step _b2 , the voltage applied to the grid _g3 of the corona discharger _D4 is maintained at _V2 , which is higher than the previous control potential _V1 , to perform corona static elimination. Through this static elimination process, static electricity is removed only from the surface potential portion of the photoreceptor A that is higher than approximately _V2 , and the surface charge of that portion is removed, resulting in the potential polar curve shown in the second column of _b2 . .

In the first step, the entire surface of the photoreceptor is uniformly exposed to release all trapped charges at the interface that are not restrained by charges on the surface of the insulating layer. This makes it possible to form an electrostatic latent image on the surface of the photoreceptor with a surface potential that faithfully reproduces the density contrast of the original image.

In addition, following the above steps a ₁ and a ₂ and steps b ₁ and b _2, the same exposure and static elimination steps are performed while increasing the amount of exposure light and increasing the set potential to make the surface potential uniform. May be repeated.

Further, the a ₁ and a ₂ steps and the b ₁ and b ₂ steps may be performed simultaneously.

That is, in the above technology, the first step, a _first step,
Increase the light image exposure intensity in the order of step b ₁ , and use a corona discharger in the order of step b 1, step a ₂ , and b ₂ .
By increasing the voltage applied to the grids D ₂ , D ₃ , and D ₄ , it is possible to reproduce even better halftones.

The method of the present invention realizes even more excellent faithful reproduction of halftones. First, an electrophotographic apparatus, which is a specific example of implementing the method of the present invention, will be explained as an example.

FIG. 2 is a sectional view of essential parts of a specific example electrophotographic apparatus based on the present invention.

The photoreceptor 1 has the basic structure of a conductive substrate a, a photoconductive layer b, and a transparent insulating layer c, similar to the photoreceptor shown in FIG. 1, and is shaped like a rotatable drum. A metal plate such as aluminum was used as the conductive substrate, and the photoconductive layer was made of steel-activated cadmium sulfide dispersed in a transparent resin binder, and was coated to a thickness of about 50 μm. Additionally, a Mylar film of approximately 25 μm is adhered to the surface using an adhesive. First, a latent image forming means 2 is arranged around the drum-shaped photoreceptor, and the latent image forming means has an AC charger 21 and a pre-exposure lamp ₂₂ , erases the previous history of the photoreceptor 1, and performs continuous copying. Excluding variations in image quality. Next, the surface of the photoreceptor 1 is uniformly charged by the primary charger 2 ₃ so that the surface potential is approximately +1500V. To form a light image on the uniformly charged photoreceptor 1, the original is placed on the document table 3, and the original image is exposed onto the photoreceptor 1 by an exposure means.

The exposure means includes an illumination light source 4 ₁ that illuminates the original document table 3, movable mirrors 4 ₂ and 4 ₃ that move together with the illumination light source and scan the document surface, an optical system lens 4 ₄ that forms a light image thereof, and further It has fixed reflection mirrors 4 ₅ and 4 ₆ that guide the optical image to the surface of the photoreceptor. Further, a bias light source ₂₆ of white light or near-infrared light is provided on the optical path, and is turned on at the same time as the original exposure, if necessary.

Furthermore, a latent image is formed using a negative corona static eliminator ₂₄ that operates simultaneously with the photoimage exposure. and,
At the opening of the corona static eliminator, a group of control grids to which a plurality of potentials are set is arranged, as will be described in detail later.
A predetermined voltage is applied, and gradation control, which will be described later in the present invention, is performed.

Next, the surface of the photoreceptor is uniformly exposed using the provided entire surface exposure lamp ₂₅ to form an electrostatic latent image. The surface potential at this time is -50 to - in the bright area.
100V, +300 to +500V in dark areas.

Next, a sleeve type magnetic brush developing device 5 is arranged to apply developer to the surface of the photoreceptor after the latent image has been formed. The developed images formed on the photoreceptor are transferred onto the transfer material P by the transfer corona discharger 6 ₃ of the paper feeding transfer means 6 . The transferred image on the transfer material P is melted and fixed onto the transfer material by the heat fixing device 7, and then discharged onto the paper discharge tray T. On the other hand, the developer remaining on the photoreceptor after the transfer is cleaned by a cleaning device 8, and the photoreceptor 1 is prepared for the next image forming process. Next, the corona charger ₂₄ , which is used simultaneously with the photoimage exposure, will be described in more detail.

FIG. 3a shows a partially enlarged cross-sectional view. A large number of grid wires were stretched perpendicularly to the rotational direction of the drum 1 at predetermined intervals (for example, about 1 mm). Further, the distance between the grid wire group and the surface of the photosensitive drum 1 was maintained at an appropriate distance (for example, approximately 1 mm).

The grid wire group is divided into three groups, grids G ₁ , G ₂ , and G ₃ , and grid G ₁ on the primary charger side is
A bias voltage of about -100V was applied to grid _G2 , a bias voltage of about +100V was applied to grid G2, and grid _G3 was set to ground potential. Further, 2 ₄₁ is a discharge wire of the corona discharger, and 2 ₄₂ is a shield plate of the discharger.

The photosensitive drum 1, which has been uniformly charged to a predetermined polarity by the primary charger ₂₃ , is neutralized by the reverse polarity corona charger ₂₄ at the same time as the photoimage is exposed.
As mentioned above, the static electricity removal effect of the discharger ₂₄ first passes through the grid _G1 to which -100V is applied to eliminate static electricity in the corona, then passes through the grid _G2 to which +100V is applied to eliminate the corona static electricity, and then It passes through the grounded grid _G3 and is neutralized by the corona. Thereafter, the entire surface of the photosensitive drum is uniformly exposed by the lamp ₂₅ to form a desired electrostatic latent image.

FIG. 4a shows the relationship between the original density D ₀ and the surface potential Vs of the photoreceptor 1 at this density, and the solid line is based on the electrostatic latent image formed by the apparatus of the above embodiment. Compared to other curves, the brightness reproduction range is
It was found that the number has spread to about 1.6.

The characteristic curves shown by dashed lines, dashed-dotted lines, dashed-double-dot lines, and dashed-three-dot lines in the same figure correspond to the grid in Figure 3a.
The bias voltages of G ₁ and G ₂ were fixed at −100 V and +100 V, and the bias voltage of grid G ₃ was varied as +50 V, +100 V, +200 V, and −50 V, respectively.
It has been found that the case where +100V or +200V is applied to grid _G3 corresponds to the above-mentioned conventional tone control method, and does not lead to tone reproduction according to the present invention.

Comparing the curve with other curves, Vs=60
The clear inflection point seen around ~70V is a curve,
It became less clear in the order of , and no curved line could be seen. Further, no inflection point of the curve was observed.

The luminance reproduction ranges of curves, , and are 1.5, 1.3, 1.2, and 1.3, respectively. Also, the characteristics,,
, and the slope when the original density D ₀ is around 0.6 (shown as the change in potential with respect to the unit of original density 0.1)
are 20V, 25V, 40V, 60V, and 35V, respectively, and the larger these values are, the higher the contrast becomes, and the harder it is to reproduce halftones.

Therefore, when evaluating the effect of gradation control from two points: the brightness reproduction range and the slope of the latent image characteristics around the original density of 0.6, the curve is the best, and the following:
, in order.

That is, when the bias voltages applied to the grids G ₁ , G ₂ , and G ₃ are respectively VG ₁ , VG ₂ , and VG ₃ in the charger configuration for static elimination shown in FIG. 3a, [VG ₁ <VG ₃
With the relationship <VG ₂ ], a high gradation control effect was obtained, and no fogging phenomenon occurred in the background part of the original in the reproduced image.

In Fig _. 4a, +
Although the case where 100V was applied was described, 0 to +300V was selectively used for grid _G2 .
In either case, the above-mentioned relationships VG ₄ to _{VG 3} hold true. The corona discharge current for static elimination is reduced or substantially stopped by the voltage applied to the grid _G2 .

The corona discharge current distribution and illuminance distribution in the case of the charger ₂₄ shown in FIG. 3a are shown in FIGS. 3b and 3c, respectively.

Figure 3b shows the corona discharge flow distribution on the photosensitive drum surface by the charger ₂₄ (distribution when the drum surface is at ground potential), and the discharge wire ₂₄₁ has a voltage of -8.5KV.
The solid line shows the distribution when -100V, +100V, and 0V are applied to grids _G1 , _G2 , and _G3, respectively. Further, the distribution when −100V, +100V, and +100V are applied to the same grids G ₁ , G ₂ , and G ₃ respectively is shown by a broken line, and the distribution when −100V, 0V, and 0V are applied is shown by a dashed-dotted line. +100V to Grid G ₂ here
It can be seen that when is applied, the corona discharge flow is rapidly interrupted as shown by the solid line or the broken line. Furthermore, when grid _G3 was at ground potential, a small distribution of passing corona discharge currents was generated in the corresponding part as shown by the solid line. One point on the surface of the photosensitive drum is a grid.
The time taken to pass through G ₁ , G ₂ , and G ₃ was, for example, 0.12 Sec, 0.10 Sec, and 0.08 Sec, respectively.

The distribution shown by the solid line in FIG. 3c is the illuminance distribution exposed to the photosensitive drum through the charger ₂₄ in FIG. 3a (when the original is a white background). The broken line in c in the figure is the distribution of bias exposure, which is uniformly exposed at the same time as image exposure, if necessary. In the case of white light, the illuminance of this bias exposure is less than one-tenth of the illuminance of image exposure, and several tenths of the illuminance of full-surface exposure in Figure 1.
It was as follows. This also applies when the bias light is near-infrared light.

In Figure 4b, the bias voltages of grids _G1 and _G3 in Figure 3a are fixed at -100V and 0V, and the grid
The relationship between the original density and the surface potential of the photosensitive drum is shown when only the bias voltage of _G2 is changed to 0V and -50V. The solid line indicates the characteristic according to the present invention shown in FIG. 4a above. The dashed line shows the case when grid _G2 is set to 0V, and the dashed-dot line shows the case when grid _G2 is set to -50V.

As is clear from the graph in FIG. 4b, neither of the curves produced the inflection point that appears in the curve for preventing fogging in bright areas. Furthermore, the curve,
The brightness reproduction ranges of the curves were 1.3 and 1.2, respectively, which were lower than the curve's 1.6. Furthermore, the slope when the original density _D0 , which is one of the criteria for the gradation reproduction effect, is around 0.6 (amount of potential change [V] with respect to the unit of original density 0.1)
were 20V, 30V, and 35V, respectively. Therefore,
The curve has the best gradation reproduction effect, and the following:
It can be said that the curves are inferior in this order.

Next, we will discuss the causes of the difference in gradation reproduction effects shown in FIGS. 4a and 4b. First, when the entire surface exposure as shown in Fig. 1 is performed without performing the simultaneous static electricity removal in the third exposure through grid _G3 after the second exposure through grid _G2 of the static eliminator ₂₄ in Fig. 3. The gradation of the latent image characteristics was even worse than that of the curve shown in FIG. 4a, but this will be explained below.

Appropriate bright area potential on the photosensitive drum (around -80V)
In order to obtain this, approximately 3 lux.sec of light is required to be exposed to the drum through grid _G1 of FIG. 3a.

If the exposure amount corresponding to grid G ₁ is 3 lux.sec, the exposure amount corresponding to grid G ₂ is 3 lux.sec x 60% = 1.8 lux.sec from the illuminance distribution ratio in Figure 3 c.
It is. Therefore, in the second exposure carried out through grid _G2 , which corresponds to step a1 in Fig. ₁ , the exposure amount at the part corresponding to the original density of 0.8 is 1.8 lux.
sec×1/6.3=0.29lux.sec.

FIG. 5 is a characteristic diagram showing the change in surface potential with respect to elapsed time after the first exposure is applied to the portion corresponding to the original density of 0.8 in FIG. It shows the characteristics in the dark after the first exposure (Dark) and the characteristics when uniform exposure (0.3, 1.0, 2.0 lux.sec) is applied to the photoreceptor after the first exposure.

As the uniform exposure amount increases, the surface potential rises faster, but when the uniform exposure amount was in the range of 0.3 to 1.0 lux.sec, it took about 0.1 second for the surface potential to reach an equilibrium state. Of course, this time depends on the mobility of the carrier and varies depending on the characteristics of the photoreceptor, but it does not deviate significantly from the above value for photoreceptors used in ordinary copying machines.

Therefore, as mentioned above, when the second exposure through grid _G2 in FIG. 3a is about 0.3lux.sec,
For example, the static electricity removal time by grid G ₂ is 0.1 seconds (process speed is 120 mm/s, width of grid G ₂ is 12 seconds).
mm), then from the characteristics shown in Figure 5, the first
The local rise in surface potential corresponding to a ₁ in the figure is
It is considered that the voltage is less than 100V, and the static electricity removal through the grid _G2 is hardly performed or the static electricity removal effect is small, resulting in poor gradation faithful reproduction effect.

Therefore, at approximately 0.1 seconds after the first exposure,
It was extremely effective in terms of gradation control to perform the static electricity removal simultaneously with the third exposure through grid _G3 in FIG. 3a.

However, in this third exposure simultaneous static elimination, the grid
The bias voltage applied to grid _G3 had to be lower than that of grid _G2 , otherwise the gradation reproduction effect would be weak. This may be due to the small illuminance distribution corresponding to _G3 . Therefore, it is thought that this is because it is necessary to lower the bias voltage applied to the grid _G3 and increase the amount of corona discharge flowing through it to enhance the static elimination effect corresponding to Figure 1 _a2 .

However, when the bias voltage of grid G ₃ is brought closer to the voltage of grid G ₂ on the upstream side, selective charge removal of a specific density of the original image is no longer performed, and fogging occurs as shown by the curve in Figure 4a. It is thought that the inflection point that prevents the occurrence of this phenomenon will no longer be observed.

Note that when grid G ₂ is brought closer to less than 0V, that is, to a negative bright area potential, even if the local rise in the potential shown in Figure 1 a ₁ is low, the _second
Sufficient static electricity removal is carried out in simultaneous exposure static electricity removal, but
In this case, as in the case of the curve in Figure 4a,
Static charge removal is performed excessively from the low density area to the high density area of the original, making it difficult to selectively remove static electricity at a specific density of the original, making it impossible to see a clear inflection point in the latent image characteristics, and causing the latent image to disappear. The entire characteristic has shifted in the negative polarity direction.

FIG. 6 is an explanatory diagram of a charger 9 of another shape for showing the effect of the present invention, and is a diagram showing the grid of FIG. 3.
Instead of _G2 , a transparent plastic plate ₉₁ was stretched over the entire length of the grid with a distance of about 1 mm from the photosensitive drum surface.

In addition, the grid G ₁ a and G ₃ a bias voltages are −
It was set to 100V and 0V. A second exposure corresponding to a ₁ in FIG. 1 is made on the photosensitive drum surface through the transparent plate 9 ₁ . The latent image characteristics at this time are shown by the broken line in FIG. For comparison, the solid line in FIG. 4 is also shown.
As is clear from the graph, the broken line showed almost the same tendency as the solid line. However, in the case of FIG. 6, the potential is high throughout, and the sensitivity is relatively low. Furthermore, the inflection point seen in the solid line in FIG. 4 is not clear. However, the brightness reproduction range is good at 1.45.
By the way, when this transparent plate ₉₁ is installed in a static eliminator, it is necessary to take into account the effects of charge-up or dirt on the transparent plate. For comparison, the latent image characteristics when the transparent plastic plate ₉₁ of FIG. 6 is made opaque are shown by the dashed-dotted line in FIG. Compared to the broken line when the line is transparent, the increase in potential from near the original density around the inflection point of the solid line is particularly large.
The linear change in properties was reduced. In this case, the brightness reproduction range decreased to 1.3. In FIG. 6, 9 ₂ is a shield plate, and 9 ₃ is a corona wire to which the same voltage as in FIG. 3 is applied.

FIG. 8a is a sectional view of a charger 10 for neutralizing static electricity, explaining another embodiment. 10 ₂ in the diagram is the discharge wire,
10 ₁ is a shield case. There are 5 groups of grids.
It is divided into G ₁ b, G ₂ b, G ₃ b, G ₄ b, G ₅ b,
From the first grid group to the fifth grid group -
100V, 0V, -50V, +50V, and 0V were applied. When -8.0KV is applied to corona discharge wire ₁₀₂ ,
The corona discharge flow distribution on the photosensitive drum surface through these grid groups (when the drum surface is at ground potential) is shown in FIG. 8b. The luminance reproduction range of the latent image characteristics obtained when this static eliminator is used in the second illustrated device is 1.8
reached close. (When bias exposure is used at the time of image exposure described above.) FIG. 9a is a sectional view illustrating a charger 11 of a different embodiment. 11 ₁ is a shield plate, 2 corona discharge wires 11 ₂ are arranged, and the grid is in 3 groups.
Divided into G ₁ c, G ₂ c, and G ₃ c. This charger for static elimination
The bias voltage applied to G ₃ c needed to be slightly higher than in the case of the charger shown in FIG. The reason for this is thought to be that there is a discharge electrode near grid _G3 , and the negative polarity corona discharge is strong. _In Figure _9b , -100V _and +
This figure shows the distribution of the corona discharge flow that has passed through the grid group and reached the photosensitive drum surface when 100V and +50V were applied. At this time, the brightness reproduction range of the latent image characteristics is the third
The value was about 1.5, which is slightly lower than when using a charger.

In this way, the present invention controls the grid bias voltage of the secondary charger that performs static elimination after primary charging, so that the corona discharge current distribution passing through the grid group corresponds to Figure 1 a ₁ or b ₁ . During image exposure, the surface potential is sharply attenuated to enhance the reproduction effect of sufficient gradation.

Note that when the gradation control method of the present invention is applied to a color electrophotographic copying apparatus, the gradation control effect for each color separation exposure of blue, green, and red may be different. For example, the clarity of the inflection point of the latent image characteristics or the original density at which the inflection point occurs were slightly different. This is due to the presence of space charges in the photoconductive layer, which causes blue, green,
This seems to be because the effect of each exposure of red is different, and for this reason,
The grid shown in Figure 3 is used to adjust the latent image characteristics of each color separation.
It was preferable to adjust the bias voltage of _G2 for each color.

FIG. 10 is an explanatory diagram showing the latent image forming process of the present invention. As in the case of FIG. The horizontal row indicated by symbol 3 indicates the change in the surface potential of the photoconductor A, and the column indicated by the symbol 3 indicates the change in the surface charge density of the photoconductor A. The photoreceptor A used here has the same characteristics as described in FIG. 1 above.

In particular, the process shown in Fig. 10 above is an explanation using the five-stage grid described in Fig. 8a, and the chargers _D5 to _D8 correspond to the dischargers of the corona wire ₁₀₂ in Fig. 8a. However, grids g ₁ to g ₇ correspond to grids G ₁ b to G ₅ b of the charger of FIG. 8a. Therefore, when using the charger shown in FIG. 3a, the entire surface exposure step is performed after steps a and b.

As is clear from the figure, in the step of reducing this uniform charging following the uniform primary charging step (step), following the corona discharge step (step) having a polarity component opposite to that of the uniform primary charging, The method includes a step (step a) of irradiating a light image while reducing or even stopping the discharge action of corona having the same opposite polarity component on the photoreceptor. For this reason, the next step (step b) of neutralizing only the halftone part using corona having an opposite polarity component is performed with the potential of the halftone part of the document being sufficiently increased, so that the sensitivity of the photoconductive layer can be reduced. It was possible to make the potential curve of the halftone part linear without being restricted by the exposure amount.

That is, since step b is carried out after the potential of the halftone portion is reliably raised due to the existence of step a, the potential curve drawn by the solid line in FIG. 4a can be stably obtained. Here, as shown in FIG. 6 of the present invention, the grid
If the corona ions do not reach the photoreceptor at the position G ₂ (FIG. 3a), the static elimination by the grid g ₄ in step a will not work, so that static elimination in the halftone area will be somewhat insufficient. As a result, as shown by the chain line in FIG. 7, although sufficient gradation was reproduced, the intermediate tones became slightly higher.

In addition, the grid of the charger shown in Figure 3a and Figure 6
Irradiation with a photoimage in G ₃ or G ₃ a is not necessarily necessary if the photoconductive layer of the photoreceptor has high optical memory properties. That is, the light emitted in the previous step is stored in the photoconductive layer in substantially the same state as when it was irradiated with the light image.
Static electricity could be removed to a predetermined potential at the grid _G3 or _G3a position.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a conventional latent image formation process for determining gradation. FIG. 2 is a sectional view of essential parts of an electrophotographic copying apparatus to which the present invention is applied. Figure 3a is a cross-sectional view of a charger for simultaneous exposure and neutralization, Figure 3b is a graph showing the amount of discharge on the photoreceptor surface within the width of this charger, and Figure 3c is a graph showing the amount of exposure within the width of this charger. Graph showing. Fourth
Figures a and 4b are graphs showing the relationship between document density and surface potential, showing the effects of the present invention. FIG. 5 is a graph showing the rise characteristics of the surface potential after exposure. FIG. 6 is a sectional view of another secondary charger of the present invention. FIG. 7 is a graph showing the relationship between document density and surface potential, showing the effects of the present invention. FIG. 8a and FIG. 9a are sectional views showing other embodiments of the charger of the present invention. Figures 8b and 9b are
3 is a graph showing the amount of corona discharge reaching the photoreceptor within the width of the charger. FIG. 10 is an explanatory diagram showing the latent image forming process for determining gradation according to the present invention, and in particular, FIG.
This corresponds to the case when the charger shown in the figure is used. In the figure, 1 is a photoreceptor, 2 ₁ is a primary charger,
2 ₃ is a secondary charger, 2 ₅ is a lamp for full-surface irradiation, 2
₄ , 9, 10, 11 are secondary chargers, 9 ₁ is a transparent plate,
a is a conductive layer, b is a photoconductive layer, c is a surface insulating layer,
G ₁ to G ₃ are grids, G ₁ a, G ₃ a are grids, G ₁ b
~ _G5b represents a grid, and _G1c ~ _G3c represent a grid.

Claims (1)

[Scope of Claims] 1. In the step of forming an electrostatic latent image on a photoreceptor having a conductive layer, a photoconductive layer, and a surface insulating layer, (a) uniformly charging the surface insulating layer of the photoreceptor with a predetermined polarity; (b) applying a corona discharge having a polarity component opposite to the uniform charging polarity of step (a) above while irradiating the photoreceptor with a light image; (c) applying the above to the photoreceptor. (b) A step in which the same light image as in step (a) is irradiated to increase the surface potential of the halftone region of the light image on the photoreceptor; Charging polarity refers to applying a corona discharge having a reverse magnetic component to eliminate the potential in the area above a certain potential among the surface potentials in the halftone area raised in step (c) above, and setting the photoreceptor to a predetermined potential. (e) irradiating the photoreceptor with uniform light.