US3928033A

US3928033A - Colour electrophotographic method in which the recording sheet is charged to its saturation voltage

Info

Publication number: US3928033A
Application number: US438610A
Authority: US
Inventors: Masayasu Anzai
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1973-02-05
Filing date: 1974-02-01
Publication date: 1975-12-23
Anticipated expiration: 1992-12-23
Also published as: DE2405287A1; GB1423793A; JPS49104637A; FR2216609A1; JPS5518901B2; FR2216609B1

Abstract

An electrophotographic method for reproducing a colour picture on a recording paper sheet of an original picture by repeating an image-forming process a plurality of times in which in imparting an electric charge to the entire surface of the recording paper sheet in each image-forming process the quantity of the electric charge imparted to the recording paper sheet in an image-forming process is made higher than that in the preceding image-forming process to bring the charged voltage of the recording paper sheet around its saturated value.

Description

United States Patent 1191 Anzai Dec. 23, 1975 COLOUR ELECTROPHOTOGRAPHIC METHOD IN WHICH THE RECORDING SHEET IS CHARGED TO ITS SATURATION VOLTAGE [75] Inventor: Masayasu Anzai, Hitachi, Japan [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Feb. 1, 1974 [21] Appl. No.: 438,610

[30] Foreign Application Priority Data Feb. 5, 1973 Japan 48-13742 521 US. (:1. 96/1.2; 96/1 LY 51 Int. (:1. G03G 13/01 [58] Field of Search 96/12, 1 LY [56] References Cited UNITED STATES PATENTS 3,729,311 4/1973 Langdon 96/1.2 3,806,340 4/1974 Sato et al. 96/l.2

OTHER PUBLICATIONS Electrofax Direct Electrophotographic Printing on CHARGED VOLTAGE Paper, RCA Review, Young et al., Dec., 1954, pp. 469-484.

Characteristics of Electrophotographic Papers-Part l, Jour. Photo. Science, Axford et al., Vol. 12, 1964, pp. 20-27.

Primary Examiner-David Klein Assistant ExaminerJuds0n R. Hightower Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT An electrophotographic method for reproducing a colour picture on a recording paper sheet of an original picture by repeating an image-forming process a plurality of times in which in imparting an electric charge to the entire surface of the recording paper sheet in each image-forming process the quantity of the electric charge imparted to the recording paper sheet in an image-forming process is made higher than that in the preceding image-forming process to bring the charged voltage of the recording paper sheet around its saturated value.

25 Claims, 5 Drawing Figures V2 V3 v4 VOLTAGE US. Patent Dec. 23, 1975 Sheet10f3 3,928,033

F l G.

U.S. Patent Dec. 23, 1975 Sheet20f3 3,928,033

ll mQSUO QMQWEIU VOLTAGE US. Patent Dec. 23, 1975 Sheet 3 of 3 3,928,033

CHARGED VOLTAGE HIGH VOLTAGE DIRECT CURRENT SOURCE COLOUR ELECTROPHOTOGRAPHIC METHOD IN WHICH THE RECORDING SHEET IS CHARGED TO ITS SATURATION VOLTAGE The present invention relates to a colour electrophotographic method, and more particularly to an improvement in a method of imparting an electric charge to an electrophotographic sensitive paper sheet (hereinafter referred simply to as sensitive paper or recording paper) in a colour electrophotographic method.

In colour electrophotography, a reproduced colour picture is obtained by superimposing images of, for example, three colours, yellow, magenta and cyan, or four colours, yellow, magenta, cyan and black of an original colour picture on a sensitive paper. In this method, for developing a charged latent image on a sensitive paper either a powder developer or a liquid developer is utilized. Generally, the latter developer provides a better reproduced picture. The colour electrophotography utilizing a liquid developer (wet type electrophotography) is carried out as follows:

First, the entire surface of a sensitive paper is charged uniformly through an electric charge generator or an electrostatic generator, and then the charged sensitive paper is exposed to the original picture through a first colour separation filter, for example a blue filter. Thus, a charged latent image corresponding to the first colour separation filter is formed on the sensitive paper. In other words, only the charge corresponding to the yellow part of the original picture which is the complementary colour of the first colour separation filter remains on the sensitive paper proportionally to the gradation or tone of the original picture. Then, the sensitive paper carrying the charged latent image is immersed in a first developer solution of the complementary colour, yellow, of the first colour separation filter to allow the colour toner to adhere the charged latent image part of the sensitive paper. Next,

the colour toner is fixed to the sensitive paper to com plete a first image formation process. To provide a highly reproduced colour picture of the original picture the above image-forming process is repeated a number of times of the necessary number of colours while replacing the colour separation filters and the developers to superimpose colour images on each other on the sensitive paper.

In the above image-forming process a fixing process is performed after each developing process by passing the sensitive paper in a rinsing liquid and a drying liquid and by drying the sensitive paper, and then the next image-forming process is performed. However, since such a fixing process requires a long time for providing a reproduced colour picture, it is widely practised that a non-volatile liquid developer having a high viscosity of 2 to 3 cm stokes is utilized to simplify the fixing process, and the next colour image-forming process is started in the state that a layer of the liquid developer remains on the sensitive paper.

On the other hand, as stated above, a uniform charge impartation by the charge generator to the entire surface of the sensitive paper is performed as the first step of each image-forming process. The quantity of the charge to be imparted to the sensitive paper by the image-forming process is quite different from that in the preceding image-forming process. In particular, the

difference between those in the first image-forming process and the second and the subsequent imageforming processes is larger. On the surface of the sensitive paper subjected to the second or the subsequent image-forming process a liquid layer of the dispersing agent (electrically insulating liquid) containing the developer supplied in the developing step in the preceding image-forming process remains.

As described above, the quantity of the electric charge imparted to the sensitive paper is maintained substantially constant. Consequently, the sensitive paper is subjected to the exposure step in an insufficiently charged state in one image-forming process, while in another image-forming process the sensitive paper is subjected to the exposure step in an overcharged state. As a result, the image formation is uneven from process to process, so that there is the disadvantage that the gradation or tone reproduction and the colour balance of the reproduced colour picture are poor.

Since the sensitive paper generally consists of a base paper sheet and an overlying zinc oxide (ZnO) film to be endowed with an electrophotographic sensitive characteristic, the sensitive paper has a semiconductor characteristic. Consequently, if the sensitive paper is over-charged, the decay or extinction characteristic ,of the charge becomes marked, so that a great difference occurs locally in the charged voltage on the sensitive paper depending on the local time intervals between the charging and the exposure, resulting in the disadvantage that unevenness occurs in a reproduced colour image.

Further, if the over-charging is too excessive, the sensitive paper partly breaks down to produce specks in a reproduced colour image.

An object of the present invention is to provide an electrophotographic method of reproducing a colour picture from an original picture with a high reproducibility.

Another object of the present invention is to provide an electrophotographic method of reproducing a colour picture improved in the uniformity of image formation in a plurality of image forming processes and improved in colour balance.

Another object of the present invention is to provide an electrophotographic method of reproducing a colour picture with highly faithfully reproduced gradation or tone reproduction.

A further object of the present invention is to provide an electrophotographic method of reproducing a colour picture with a low degree of colour unevenness.

A still further object of the present invention is to provide an electrophotographic method of reproducing a colour picture with no specks resulting from the overcharging of the recording or sensitive paper.

An electrophotographic method of reproducing a colour picture of the present invention, which is performed by successively superimposing colour images on a recording paper sheet formed through plural image-forming processes controls the quantity of imparted electric charges over an entire surface of the recording paper sheet in an electric charge imparting step in each image-forming process in accordance with the electric charging characteristic or property of-the recording paper sheet in each image-forming process.

3 Further, an electrophotographic method of reproducing a colour picture of the present invention, which is performed by successively superimposing colour images on a recording paper sheet formed through plural image-forming processes, controls charged volt-' age of the recording paper sheet over an entire surface of the recording paper sheet in an electric charge imparting step in each image forming process so that the charged voltage is rendered around its saturation value in the condition of the recording paper sheet in each image-forming process.

In the above statement and hereinafter the term around the saturation value" refers to the saturated point and points slightly over it.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention made by referring to the preferred embodiments when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of the colour electrophotographic apparatus for explaining the colour electrophotographic method according to the present invention;

FIG. 2a is quantitative charged voltage vs. applied voltage characteristics of a sensitive paper when a dispersing agent (including no colour toner components) of a bulk resistance of IO ohms or more is used in a developer in the colour electrophotographic method according to the present invention and here distance and relative moving speed between a charge generator and a sensitive paper are kept constant;

FIG. 2b is characteristics similar to those in FIG. 20 when a dispersing agent (excluding colour toner components) of a bulk resistance of below ohms is used in a developer in the colour electrophotographic method according to the present invention;

FIG. 3 is characteristics similar to that of FIG. 2a except that the abscissa represents the relative travelling speed between the charge generator and the sensitive paper instead of the voltage applied to the charge generator in FIG. 2a and here distance between a charge generator and a sensitivepaper and quantity of electric charge generated from a charge generator per unit time are kept constant; and

FIG. 4 is a circuit diagram of a charge generator used in the colour electrophotographic method according to the present invention.

Referring to FIG. 1, a sensitive paper 12 wound around a drum 10 rotates along with the drum 10 at a constant speed by the signal from a controller during which the entire surface of the sensitive paper 12 is charged uniformly by being supplied by an electric charge generator or corona generator with electrostatic charges. The charge generator 30 consists of a core 32 connected to a high DC voltage source 100 and a grounded shield 34.

The colour image of an original picuture 52 placed on a transparent support 50 is projected on the uniformly charged sensitive paper 12 through an exposing device 60. The rotation of the drum 10 and the travel of the transparent support50 in the direction of the arrow are synchronized by the controller 20. The transparent support 50 is moved by a motor 54 based on the synchronizing signal from the controller 20.

The exposing device 60 consists of a light source 62 for illuminating the original picture 52, a lens 64, a reflector 66 for reflecting a reflected light image from the original picture 52 to the lens 64, and a colour separating filter device 68 for colour-separating the light from the lens 64.

The colour separating filter device 68 consists of three kinds of colour seprating filters of, for example,

blue, green and red, or four kinds of colour separating filters of, for example, blue, green, red and white. The colour sepration filters are successively charged by the controller 20 for image-forming processes.

When, in the first image-forming process, a colour sepration filter of blue is positioned on the optical path of the reflected light from the original picture 52, the charges remain only on the part of the uniformly charged sensitive paper 12 corresponding to the yellow part of the original picture 52 which is in complementary relationship with the blue of the filter correspondingly, to the gradation of the yellow part of the original picture 52 to form a first charge latent image on the sensitive paper 12.

A developing device 40 is formed of afirst developer reservoir 41 filled with a developer for developing yellow which is complementary to the blue of the first colour separation filter on the sensitive paper 12, a second developer reservoir 42 filled with a developer for developing magenta which is complementary to green of a second colour separation filter on the sensitive paper 12, a third developer reservoir 43 filled with a developer for developing cyan which is complementary to red of a third colour separation filter on the sensitive paper 12, and a fourth developer reservoir 44 filled with a developer for developing black which is complementary to white of a fourth colour separation filter on the sensitive paper 12. In the developer reservoirs 41 to 44 are provided rollers 45 to 48, respectively, for well immersing the latent image carrying sensitive paper 12 in the developers. The change in the position of the developing device 40 relative to the drum 10 is effected by a motor 49 based on the signal from the controller 20. The position of the developing device 40 shown in FIG. 1 is for the state that the developing step in the second image-forming process has been completed. In other words, the state that the magenta image based on the latent image on the sensitive paper 12 corresponding to the magenta part of the original picture 52 which is complementary to the green of the second colour separation filter is superimposed on the yellow image formed in the first imageforming process.

Since the insulating liquid contained in the developer is carried on the surface of the sensitive paper 12 subjected to the developing step in each image-forming process, the carried insulating liquid is squeezed out and simultaneously therewith a colour toner is fixed to the sensitive paper 12 by a squeezing roller (fixing device) 70. Dirt or fouling on the squeezing roller is removed by a cleaning pad 72. Then the sensitive paper 12 is returned to the initial position to start the next image-forming process.

The high DC voltage source in FIG. 1 is shown in a practical form. The primary winding 101 of a step-up I transformer 106 is connected with an AC source 102 through a inulti-contact switch 80, while the secondary winding 103 thereof is connected with a series circuit of a capacitor 104 and a rectifier 105. The supply of a high DC voltage to the core 32 of the electric charge generator 30 is made from the junction point 107 between the capacitor 104 and the rectifier 105.

Curves I in FIGS. 2a and 2b are charged characteristics of the sensitive paper in the first image-forming process but not yet subjected to the developing step. Since the sensitive paper is not yet exposed to the developer, it is saturated in the charged voltage with a low voltage applied to the electric charge generator. Consequently, in the first image-forming process the movable contact 85 of the switch 80 is connected to the first fixed contact 81 by a signal from the controller 20 to apply a DC voltage V or V between the core 32 and the shield 34 of the charge generator 30 for imparting charges uniformly to the sensitive paper 12. Points (a) and (e) on the curves I in FIGS. 2a and 2b indicate the charged voltages of the sensitive paper 12 corresponding to the applied voltages V and V respectively, and defined as points slightly exceeding the saturated charged voltages of the sensitive paper 12 in the first image-forming process.

Curves n, m and IV in FIGS. 2a and 2b represent charged characteristics of the sensitive paper 12 in the second, third and fourth image-forming processes, respectively.

The voltage to be applied to the charge generator 30 is set at V or V by changing the movable contact 85 of the switch 80 to the second fixed contact 82 by a signal from the controller 20 in the second image-forming process, at V or V by changing the movable contact 85 of the switch 80 to the third fixed contact 83 in the third image-forming process, and at V., or V by changing the movable contact 85 of the switch 80 to the fourth fixed contact 84 in the fourth image-forming process depending on the property of the dispersing agent in the developer. In this manner, the quantity of the charge imparted to the sensitive paper 12 per unit time is controlled to set the charged voltage of the sensitive paper 12 around the saturated value of the charged voltage in each image-forming process. Similarly in the first image-forming process, points (b), (f), (c), (g), (d) and (h) on the curves II, III, and IV in FIGS. 2a and 2b corresponding to the applied voltages V V V V V and V,,, respectively, are set at points slightly over the saturated charged voltages of the sensitive paper 12 in individual charge-forming processes.

An example of the voltages V V V and V to be applied to the charge generator 30 is 5.5 KV, 5.8 KV, 5.9 KV and 6.0 KV, respectively. In this case, the distance between the charge generator 30 and the sensitive paper 12 is 15 mm, and the relative travelling speed therebetween is 50 mm/sec.

As can be seen from the curves II, III and IV in FIGS. 2a and 2b, the charged characteristics of the sensitive paper 12 in the second and subsequent image-forming processes are similar to one another. Consequently, the present invention can even be achieved by only a twostep change-over of the quantity of the imparted charge per unit time instead of the four-step change-over described above. That is, it is possible that the voltage applied to the charge generator is taken to be V or V in the first image-forming process, and the voltages are taken to be V or V in the second and subsequent image-forming processes.

As another expedient for controlling the quantity of the charge to be imparted to the sensitive paper 12 per unit time the distance between the charge generator'30 and the sensitive paper 12 may be adjusted in each image-forming process.

FIG. 3 shows charged characteristics of the sensitive paper 12 when the travelling speed of the sensitive paper 12 relative to the electric charge generator 30 is varied. Curves I, II, III and IV are for the charged characteristics of the sensitive paper 12 in the first, second, third and fourth image-forming processes, respectively. By setting the speed of the drum 10 at S S S and S in the first, second, third and fourth image-forming processes, respectively, by the controller 20, the charged voltage of the sensitive paper 12 is set at the points (i), (j), (k) and (l) for individual image-forming processes which are slightly over the saturated charged voltages of the sensitive paper 12 in those processes. Thus, the absolute quantity of the charge imparted to the sensitive paper 12 is properly controlled depending on the state of the sensitive paper 12 in each imageforming process. Similarly to what is described above, even a two-step speed change-over from S to S, can achieve the present invention in place of the four-step speed change-over.

FIG. 4 is another example of the electric charge generator 30. Those parts of the charge generator 30 in FIG. 4 which perform similar functions to those in FIG. 1 are designated by the same reference numerals. The core 32 of the charge generator 30 is connected to a high DC voltage source 110 similarly to the system in FIG. 1. The charge generator 30 is provided with a grid 36 and a resistor group connected in series therewith consisting of first, second, third and

fourth resistors

91, 92, 93 and 94. The resistances R R R and R of these resistors are in a relation R R R R In the first image-forming process a first resistor 91 is grounded through a switch 95. Similarly, in the second, third and fourth image-forming processes second, third and

fourth resistors

92, 93 and 94 are grounded, respectively, through the switch 95. The changeover of the switch 95 is performed by the signal from the controller 20. Thus, the charge flowing through the grid 36 is controlled. In other words, the quantity of the charge flowing into the ground through the grid 36 in the first image-forming process is highest, and successively decreases in the second, third and fourth image-forming processes. In this manner, the quantity of the charge imparted to the sensitive paper 12 per unit time in each image-forming process is properly controlled.

in the fourth image-forming process the switch 95 may be in an open state instead of a closed state.

For a similar reason to those described above, the control of the quantity of the charge imparted to the sensitive paper per unit time can be made by a two-step change-over instead of the four-step change-over.

Though in the above embodiments the original picture is moved relative to the exposing device and the sensitive paper is moved relative to the charge generator, the exposing device and the charge generator may be moved contrary thereto.

I claim:

1. An electrophotographic method of reproducing a colour picture on a recording paper sheet from an original picture while successively changing a plurality of colour filters corresponding to the number of required colours and the same number of developers individually producing the complementary colours of the corresponding colour filters and repeating a number of times corresponding to the number of the filters the image-forming process comprising the steps of imparting an electric charge to the entire surface of the recording paper sheet, forming a first charge latent image on the recording paper sheet corresponding to the complementary colour of the first colour filter of the original picture through the first colour filter, developing a first colour image on the recording paper sheet on which the first latent image has been formed by a first developer producing the complementary colour of the first colour filter, and fixing the first colour image on the recording paper sheet, characterized in that in the electric charge imparting step in each imageforming process the charged voltage of the recording paper sheet is selected around its saturation value.

2. A colour electrophotographic method according to claim 1, wherein quantity of the electric charge imparted to the recording paper sheet per unit time in each of second and the subsequent image-forming processes is higher than that in the first image-forming process.

3. A colour electrophotographic method according to claim 2, wherein quantity of the electric charge imparted to the recording paper sheet per unit time in at least one image-forming process is higher than that in the preceding imaage-forming process.

4. A colour electrophotographic method according to claim 1, wherein quantity of the electric charge imparted to the recording paper sheet in each of the second and the subsequent image-forming processes is higher than that in the first image-forming process.

5. A colour electrophotographic method according to claim 4, wherein quantity of the electric charge imparted to the recording paper sheet in at least one image-forming process is higher than that in the preceding image-forming process.

6. A colour electrophotographic method according to claim 2, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generator means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the voltage applied to the electric charge generating means.

7. A colour electrophotographic method according to claim 2, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the distance between the electric charge generating means and the recording paper sheet.

8. A colour electrophotographic method according to claim 2, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and control of the quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the grid resistance of the electric charge generating means.

9. A colour electrophotographic method according to claim 3, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the con trol of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the voltage applied to the electric charge generating means.

10. A colour electrophotographic method according to claim 3, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the distance between the electric charge generat ing means and the recording paper sheet.

11. A colour electrophotographic method according to claim 3, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the grid resistance of the electric charge generating means.

12. A colour electrophotographic method according to claim 4, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet is made by controlling the time period during which the electric charge is imparted by the electric charge generating means to the recording paper sheet.

13. A colour electrophotographic method according to claim 5, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet is made by controlling the time period during which the electric charge is imparted by the electric charge generating means to the recording paper sheet.

14. In an electrophotographic method for reproducing a color picture on a recording sheet from an original picture in which a plurality of color images are formed in superposed relationship on said recording sheet, each color image being formed by an imageforming process comprising imparting an electric charge to the entire surface of said recording sheet, forming a charge latent image on said recording sheet corresponding to the respective color image to be formed on said recording sheet, developing the respective color image on the recording sheet on which the latent image has been formed by a liquid developer containing a developing agent and an electrically insulating liquid, and fixing the respective color image on the recording sheet, the recording sheet when subjected to second and subsequent image-forming processes being wet with said electrically insulating liquid, the improvement wherein sufficient electric charge is imparted to said recording sheet during each respective image-forming process so that the charged ,voltage of the recording sheet in each respective image-forming process is selected at or slightly above its saturation value.

15. A color electrophotographic method according to claim 14, wherein the value of the saturation charged voltage of said recording sheet in the first image-forming process is different from the values of the saturation charged voltages of the recording sheet in second and subsequent image-forming processes.

16. A color electrophotographic method according to claim 15, wherein the values of the saturated charged voltages of the recording sheet in respective image-forming processes are different from one another.

17. A color electrophotographic method according to claim 16, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generator means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the voltage applied to the electric charge generating means.

18. A color electrophotographic method according to claim 16, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generating means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the distance between the electric charge generating means and the recording sheet.

19. A color electrophotographic method according to claim 16, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generating means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the grid resistance of the electric charge generating means.

20. A color electrophotographic method according to claim 16, wherein said liquid developer is applied to said recording sheet by means of a roller.

21. A color electrophotographic method according to claim 15, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generator means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the voltage applied to the electric charge generating means.

22. A color electrophotographic method according to claim 15, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generating means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the distance between the electric charge generating means and the recording sheet.

23. A color electrophotographic method according to claim 15, wherein charge impartation to the recording sheet in each image-forming process is made through electric charge generating means and control of the quantity of the electric charge imparted to the recording sheet per unit time is made by controlling the grid resistance of the electric charge generating means.

24. A color electrophotographic method according to claim 15, wherein said liquid developer is applied to said recording sheet by means of a roller.

25. A color electrophotographic method according to claim 14, wherein said liquid developer is applied to said recording sheet by means of a roller.

Claims

1. AN ELECTROPHOTOGRAPHIC METHOD OF REPRODUCING A COLOUR PICTURE ON A RECORDING PAPER SHEET FROM AN ORIGINAL PICTURE WHILE SUCESSIVELY CHANGING A PLURALITY OF COLOUR FILTERS CORRESPONDING TO THE NUMBER OF REQUIRED COLOURS AND THE SAME NUMBER OF DEVELOPERS INDIVIDUALLY PRODUCING THE COMPLEMENTARY COLOURS OF THE CORRESPONDING COLOUR FILTERS AND REPEATING A NUMBER OF TIMES CORRESPONDING THE NUMBER OF THE FILTERS THE IMAGE-FORMING PROCESS COMPRISING THE STEPS OF IMPARTING AN ELECTRIC CHARGE TO THE ENTIRE SURFACE OF THE RECORDING PAPER SHEET, FORMING A FIRST CHARGE LATENT IMAGE ON THE RECORDING PAPER SHEET CORRESPONDING TO THE COMPLEMENTARY COLOUR OF THE FIRST COLOUR FILTER OF THE ORIGINAL PICTURE THROUGH THE FIRST COLOUR FILTER, DEVELOPING A FIRST COLOUR IMAGE ON THE RECORDING PAPER SHEET ON WHICH THE FIRST LATENT IMAGE HAS BEEN FORMED BY A FIRST DEVELOPER PRODUCING THE COMPLEMENTARY COLOUR OF THE FIRST COLOUR FILTER, AND FIXING THE FIRST COLOUR IMAGE ON THE RECORDING PAPER SHEET, CHARACTERIZED IN THAT IN THE ELECTRIC

9. A colour electrophotographic method according to claim 3, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the voltage applied to the electric charge generating means.

10. A colour electrophotographic method according to claim 3, wherein charge impartation to the recording paper sheet in each image-forming process is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording paper sheet per unit time is made by controlling the distance between the electric charge generating means and the recording paper sheet.

14. In an electrophotographic method for reproducing a color picture on a recording sheet from an original picture in which a plurality of color images are formed in superposed relationship on said recording sheet, each color image being formed by an image-forming process comprising imparting an electric charge to the entire surface of said recording sheet, forming a charge latent image on said recording sheet corresponding to the respective color image to be formed on said recording sheet, developing the respective color image on the recording sheet on which the latent image has been formed by a liquid developer containing a developing agent and an electrically insulating liquid, and fixiNg the respective color image on the recording sheet, the recording sheet when subjected to second and subsequent image-forming processes being wet with said electrically insulating liquid, the improvement wherein sufficient electric charge is imparted to said recording sheet during each respective image-forming process so that the charged voltage of the recording sheet in each respective image-forming process is selected at or slightly above its saturation value.