US3722992A

US3722992A - Apparatus for creating an electrostatic latent image by charge modulation

Info

Publication number: US3722992A
Application number: US00105432A
Authority: US
Inventors: G Zweig
Original assignee: Pitney Bowes Inc
Current assignee: Pitney Bowes Inc
Priority date: 1971-01-11
Filing date: 1971-01-11
Publication date: 1973-03-27
Anticipated expiration: 1990-03-27

Abstract

A transparent dielectric film is juxtaposed with the electrostatically charged surface of a photoconductive member. An optical image is projected onto the photoconductive member to create a corresponding first electrostatic latent image on the photoconductive member surface. The exposed dielectric film surface is charged by an AC scorotron; the first electrostatic latent image acting to modulate the charge deposition such as to produce a zero potential at the exposed film surface. The photoconductive member is flooded with light to dissipate the first electrostatic latent image, rendering the charge deposition on the exposed film surface effective as a second corresponding electrostatic latent image for ultimate development and image transfer to plain paper.

Description

United States Patent Zweig 1 Mar. 27, 1973 [54] APPARATUS FOR CREATING AN Primary ExaminerSamuel S. Matthews ELECTROSTATIC LATENT IMAGE BY Assistant Examiner-Richard A. Wintercorn CHARGE MODULATION Attorney-William D. Soltow, Jr., Albert W. Scribne Martin D. Wittstein and Louis A. Tirelli 1 [75] Inventor: Gilbert Zwelg, Stamford, Conn. I [73] Assignee: Pitney-Bowes lnc., Stamford, ABSTRACT l A transparent dielectric film is juxtaposed with the [22] Filed; Jam 1 19 1 electrostatically charged surface of a photoconductive member. An optical image is projected onto the [21] PP photoconductive member to create a corresponding first electrostatic latent image on the photoconductive 52 us. Cl. .355/3 355/8 355/14, member surface- The exlwsed dielectric film surface is 3 5 5/16 charged by an AC scorotron; the first electrostatic [51] Int Cl 603g 15/00 latent image acting to modulate the charge deposition Such as to produce a Zero potential at the exposed fi 1m [58] held of Search 96/1'4 surface. The photoconductive member is flooded with light to dissipate the first electrostatic latent image, [56] References Cited rendering the charge deposition on the exposed film UNITED STATES PATENTS surface effective as a second corresponding electrostatic latent image for ultimate development and Gundlach ..355/3 image transfer to plain paper Burdige ..355/3 4 Claims, 7 Drawing Figures PATENTEDHAR271975 3 2, 9

SHEET 20F 2 16 3g 40 v U I [NVE/VIWR. Gdherz" Zwelg BY Mai 12 mm BACKGROUND AND OBJECTS OF THE INVENTION The present invention relates to an improved method for creating electrostatic images useful in electrostatic photocopiers and to apparatus for carrying out the improved method.

In presently available electrostatic photocopiers, it is the usual practice to create an electrostatic latent image corresponding to the image to be copied on the surface of a photoconductive insulating member. This electrostatic latent image is then developed using electroscopic developer or toner particles. The developed image is then either fixed on the photoconductive member surface, typically zinc oxide coated paper, as in the direct imaging process, or the toner in image formation is transferred or offset printed onto plain paper and fixed, as in the indirect imaging process. In either the director indirect imaging processes, the electrostatic latent image carried by the photoconductive member surface must be preserved in absolute darkness until it is visibly developed, as it can be readily erased or dissipated by incident light radiation.

In the case of the indirect imaging process, the photoconductive member is typically formed of a relatively expensive xerographic plate or drum which is reused during each copy cycle. Thus the surface of this photoconductive member is constantly being subjected to the abrading action of the developing materials as they are cascaded or otherwise brought into contact with the electrostatic latent image borne by the photoconductive member. Moreover, the surface of the photoconductive member must be brought into 'contact with the surface of a paper sheet to which the developer particles in image formation are transfer-red. This too creates wear and errosion'of the photoconductive member surface. Eventually the photoconductive member wears out and must be replaced at considerable cost to the user.

In addition, since the photoconductive member surface is subjected directly to the developer materials, the residue left after image transfer to the plain paper sheet should be scrupulously removed if the quality of subsequent copies is to be maintained. Unfortunately despite repeated automated cleaning of the photocon ductive member surface, a film of contamination gradually builds up on the surface, ultimately requiring removal of the photoconductive member from the photocopy machine and manual cleaning of the drum with a suitable solvent. This produces more wear of the photoconductive member surface and thus further decreases its operating life.

dielectric film surface or the developer particles in image formation can be transferred to plain paper and fixed. Thus, by this technique the photoconductive member surface is not subjected to the developer materials and is not contacted against a transfer sheet with the result that its operating life is materially increased. Moreover, the photoconductive member surface need not be mechanically cleaned, also contributing to longer operating life.

A number of such techniques for creating electrostatic latent images on non-photoconductive dielectric films are disclosed in Schafferts reference book entitled, Electrophotography (1965), Chapter VI. Other disclosures of such methods may be found in Hall U.S. Pat. Nos. 3,084,061 and 3,234,019 and Walkup U.S. Pat. Nos. 2,825,814 and 2,833,648.

It is an object of the present invention to provide an adapted to reproduce-photocopy images on plain bond paper.

Yet another. object of the inventionis to provide apparatus for practicing the improved method of the Various methods have been advanced for translating the second electrostatic latent image can be preserved on its surfacedespite incident light radiation, and thus the image can be visibly developed under ambient light conditions even apart from the photoconductive member. The developed image can be fixed on the above-noted character.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

SUMMARY OF THE INVENTION In accordance with the present invention, a surface of a photoconductive insulating member, such as vitreous selenium, is uniformly electrostatically charged to a predetermined, suitable placed over the photoconductive member in virtual physical contact with the charged surface thereof. As

an importantfeature of the invention, the overall.

uniform electrostatic charge on the photoconductive member surface is effective to create an attractive force on the dielectric film, holding it in intimate and uniform relation with the photoconductive member surface.

The photoconductive member is then exposed to an optical image effective to create a corresponding electrostatic latent image on the photoconductive member .surface. Preferably, the dielectric film is transparent such as to permit the photoconductive member to be exposed to the optical image to be copied through the dielectric film. The exposed surface of the dielectric film is subjected to an electrostatic charge of a character to establish a substantially uniform electric potential plane adjacent the exposed film surface. Specifically, the influence of the underlying electrostatic latent image on the photoconductive member surface, acting through the dielectric film, serves to modulate the charge'depositionon the exposed film surface such as to create thereon a charge pattern of a character to effectively cancel out the electrostatic electrostatic DC potential. A non photoconductive insulating or dielectric film is then,

latent image, leaving the exposed film surface at zero electrical potential.

In practice, the exposed film surface is subjected to an Ac electrostatic charging source which is effective to establish the zero electric potential plane. As a result, the charge deposition pattern on the exposed film surface conforms but relatively oppositely charged, to the electrostatic latent image, thus, in effect, cancelling each other to produce the uniform zero electric potential plane at the film surface.

Due to the joint presence and close proximity of the charge pattern on the exposed film surface and the electrostatic latent image on the photoconductive member surface, neither can be visibly developed using electroscopic toner particles. Thus prior to development, the influence or effect of the electrostatic latent image at the exposed surface of the film must be eliminated. Preferably, in accordance with the invention, this is achieved by flooding the photoconductive member with light to dissipate the electrostatic latent image, rendering the charge deposition pattern on the exposed film surface effectiveas a second corresponding electrostatic latent image which is then capable of visible development.

. After the second electrostatic latent image has been developed,.the dielectric film and the photoconductive member are separated from each other, and the toner particles held by the second electrostatic latent image in image formation are transferred or offset printed onto plain paper. It is to be noted that the second electrostatic latent image is developed prior to the separation of the dielectric film and the photoconductive member, thus insuring that any charges created on the adjacent surfaces of these'two dissimilar materials upon separation will have no effect on image development.

While the dielectric film and the photoconductive memberv are separated, residual toner and charges remaining on the dielectric film are removed and the exposed photoconductive member surface is recharged pursuant to repeating the process.

The invention accordingly comprises several steps and the relation of one or more such steps with respect to each of the others and apparatus embodyingfeatures of construction, combination of elements and'arrangement of parts which are adapted to effect such steps, all

as "exemplified in. the following detailed description,

and the scope of the invention will be indicated in theclaims. i.

FIG. 4 is a diagrammatic illustration of charging the exposed surface of the dielectric film with an AC scorotron to create a charge pattern on the dielectric film surface corresponding to the electrostatic latent image on the photoconductive member surface;

FIG. 5 is a diagrammatic illustration of flooding the photoconductive member through the dielectric film with light to remove the electrostatic latent image from the surface of the photoconductive member and render the charge pattern on the exposed surface of the dielectric film effective as a second corresponding electrostatic latent image;

FIG. 6 is a diagrammatic illustration of visibly developing the second electrostatic latent image on the exposed dielectric film surface; and

FIG. 7 is a schematic illustration of apparatus for practicing the successive method steps illustrated in FIGS. 1 through 6.

DETAILED DESCRIPTION In accordance with the embodiment of the invention illustrated in FIGS. 1 through 6, the upper surface of a photoconductive insulating member 10 is charged to an overall uniform DC potential. While there are known ploy a corona charging unit such as generally indicated Fora fuller understanding of the nature and objects 7 of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:- I

' FIG. -I is a diagrammatic illustration of charging a surface of a photoconductive member to a uniform DC potential in accordance with the initial step in the process of'the present invention;

FIG. '2is a diagrammatic illustration of placing a nonphotoconductive dielectric film in juxtaposition with the charged surface of the photoconductive member in accordance with the method of the present invention;

FIG. 3 is a diagrammatic illustration of imaging the photoconductive member through the dielectric film to create a corresponding electrostatic latent image on the photoconductive member surface;

at 14 in FIG. 1. The corona charging unit 14 is passed over the photoconductive member 10 or the photoconductive member is moved under the corona charging unit in order to uniformly deposit electrostatic surface charges. The polarity of these charges is indicated as being positive in FIG. 1, however, it will be understood that a photoconductive member surface may be charged to a negative potential. The polarity of the electrostatic charges is determined by the type of photoconductor material used in member 10. g

It will be appreciated thatthe photoconductive member 10 may take a variety of forms, but typically in practice it comprises a photoconductive insulating layer coated on a conductive backing member. In FIG.

1, the photoconductive member is illustratedas having a conductive. substrate or electrode material 10a applied to its lower surface. The photoconductive insulating material used in member 10 may be, for example,

vitreous selenium, anthracene, sulphur, tellurium or various organic compounds which areknown to have photoconductive insulating properties. i

As illustrated in FIG. 2 an insulating or dielectric film 12 is placed over the photoconductive member 10 with its lower surface in virtual physical contact with the charged photoconductive member surface. The p're-existing charge on the surface of the photoconductive member 10 serves to attract the dielectric film 12 and' It will be understood that the charging of the photoconductive member 10, as illustrated in FIG. 1,

and the placement of the dielectric film 12 over the charged photoconductive member surface are carried out in the dark in order that the photoconductive member be insulative to preserve the electrostatic charges deposited by the corona charging unit 14.

The next step in the process is to expose the photoconductive member to an optical image to be reproduced as indicated in FIG. 3. The optical image, illustrated as having dark image areas a and a light background area 15b borne by a document 15, is projected by a lens system, schematically indicated at 16, through the dielectric film 12 onto the photoconduc tive member 10. Thus, the dielectric film is optically transparent. Alternatively, the electrode 10a applied to the bottom surface of the photoconductive member 10 may be made optically transparent and the optical image may then be projected onto the photoconductive member from below.

As is well understood in the electrophotography art, the areas of the photoconductive member struck by light lose their insulative properties and become conductive. The surface charges overlying these light struck areas are conducted through the semi-conductive member body to the electrode 10a, which is illustrated as being electrically connected to ground in FIG. 3. TI-lose areas of the photoconductive member which are not illuminated by the optical image retain their insulative properties, thus preserving the surface charges overlying these dark areas. In FIG. 3, the non-illuminated areas of the photoconductive member 10, in-

dicated at l7a, receive little or no reflected light from.

dark image areas 15a, and the light struck area is indicated at 17b, which receives reflected light from light background area 15b of document 15. Thus, the surface charges overlying area 17b of the photoconductive member 10 have been conducted away, leaving the surface charges overlying the areas 170 on either side, which are not conducted away. There is thus created on the surface of the photoconductive member an electrostatic latent image corresponding to the optical image projected by lens system 16.

The next step in the inventive process, illustrated in FIG. 4, involves subjecting the upper or exposed surface of the dielectric film 12 to an AC electrostatic field. Preferably in accordance with my invention, the corona charging device, generally indicated at 18, is a scorotron charging device such as disclosed in Walkup US. Pat. No. 2,777,957. The corona wires 20 are excited from an AC voltage source while the grid 22 is connected to a suitable grid bias potential, which in practice may be and preferably is ground potential. As the corona charging unit 18 passes over the upper surface of the dielectric film 12 or the dielectric film is passed under the corona charging unit, positive and negative electrostatic charges are deposited on the dielectric film surface in a manner such as to, in effect, cancel out the electrostatic latent image retained on the surface of the photoconductive member 10. Stated another way, the electrostatic charges'are deposited on the upper surface on the dielectric film by the corona charging unit-18 so as to establish an equal potential plane at the upper dielectric film surface. Specifically, the equal potential plane preferably has a zero electric potential. Thus, the charges carried by the surface of the photoconductive member 10 as a first electrostatic latent image act as a template to modulate the charge deposition on the upper surface of the dielectric film 12 so as to create a charge deposition pattern corresponding to the first electrostatic latent image and thus also to the optical image projected by lens system 16.

Specifically as seen in FIG. 4, if the first electrostatic latent image consists of positive charges, the corona charging unit 18 deposits negative charges on the dielectric film surface of the regions overlying the positive charges on the photoconductive member surface. Positive charges are not deposited, but instead are repelled by the underlying positive charges of the first electrostatic latent image. Over area 17b where the positive charges were conducted away through the photoconductive member body, as illustrated in the step of FIG. 3, the corona charging unit deposits mutually cancelling positive and negative electric charges on the overlying dielectric film surface. Thus the resulting charge deposition pattern on the dielectric film surface consists of negatively charged areas over image areas 17a and an uncharged area over background area 17b, as is illustrated in FIG. 4. This results in the creation of a zero electric potential plane at the exposed film surface.

At this point it will be appreciated that any nonuniformities in the contact relationship between the photoconductive member and the dielectric film will create inaccuracies in the formation of the charge deposition pattern since the influence of the first electrostatic latent image will not be accurately represented at the exposed film surface.

It will be understood that if the corona charging unit 14 in FIG. 1 deposits negative charges on the photoconductive member surface, then the corona charging unit 18 of FIG. 4 will deposit overlying positive charges in order to achieve a zero electric potential plane at the upper surface of the dielectric film 12.

The next step in the process, illustrated in FIG. 5, is to flood the photoconductive member 10 with light from a lamp 24, rendering it overall electrically conductive, thus dissipating the first electrostatic latent image through the photoconductive member body to electrode 10a and ground. If the dielectric film 12 is transparent, the photoconductive member 10 is preferably flooded with light through the dielectric film, as illustrated in FIG. 5. Alternatively, if the electrode 10a is transparent, the photoconductive member 10 may be flooded with light from below to dissipate the first electrostatic latent image borne on the upper surface thereof. At the conclusion of the method step illustrated in FIG. 5, the charge deposition pattern on the film surface remains and is now rendered effective as a second, developable electrostatic latent image corresponding to the first.

Turning to FIG. 6, the second electrostatic latent image is visibly developed, as schematically indicated at 28, using any conventional development technique. Basically the development technique involves depositing electroscopic particles on the upper surface of the dielectric film 12; the electric charges making up the second electrostatic latent image being effective to attract and hold the electroscopic particles or pigmented toner particles thus, in effect rendering the electrostatic image visible. Of course, those areas of the second electrostatic latent image which are uncharged do not attract and hold the toner particles.

The dielectric film 12 illustrated in FIGS. 2 through 6 is preferably quite thin, typically on the order of l to 5 mils thick, with 2 mils being a representative figure. Using a thin dielectric film serves to maximize the modulating effect of the first electrostatic latent image during deposition of the charge pattern on the film surface and also serves to insure good image resolution by minimizing the effects of fringing electrostatic fields produced by the first electrostatic latent image during the process step of FIG. 4 and also any problems of optical image distortion during the imaging step of FIG. 3.

Apparatus for carrying out the method steps illustrated in FIGS. 1 through 6 is disclosed in FIG. 7. The photoconductive member consists of a layer 30 of photoconductive insulating material coated on a drum 32 formed ofelectrically conductive material. The drum 32 is mounted for rotation in the clockwise direction as indicated by the arrow 32a. The transparent dielectric film is in the form of a belt which passes around drum 32 and an upper roller 36. Thusthe dielectric belt 34 moves around in the clockwise direction with rotation of the drum 32 coming in contact with the photoconductive coating 30 on the drum at point A and separating from the drum at point B. Going from point B in the clockwise direction around to point A, the surface of the photoconductive coating 30 on drum 32 is conveniently exposed in order that it may be charged by the corona charging unit 14 in the manner described in connection with FIG. 1. As the drum 32 rotates around to point A, the uniformly charged surface of the photoconductive coating 30 converges with the dielectric belt 34, achieving intimate contacting engagement between the charged surface of the photoconductive coating and the inner surface of the dielectric belt. The pre-existing overall uniform electrostatic charge on the photoconductive coating insures an overall uniform and intimate relation between the opposing surfaces of the drum coating and the belt.

Continuing in the clockwise direction beyond point A, the photoconductive coating 30 is imaged by an optical image derived from an original document 38 moving in synchronism with rotation of drum 32. The image is projected by the lens system 16 through the dielectric belt 34 onto the charged surface of the photoconductive coating 30, thereby producing the electrostatic latent image corresponding to the projected optical image. It will be observed that this operation corresponds to the method step illustrated in FIG. 3.

Still referring to FIG. 7, after formation of the first electrostatic latent image on the surface of the photoconductive coating 30, the dielectric belt 34 moves passed the corona charging unit 18 to deposit a charge pattern on the dielectric belt surface effectively cancelling the underlying first electrostatic latent image on the photoconductive coating surface. As was described in connection with FIG. 4, the corona charging unit 18 is an AC scorotron whose corona charging wires are excited from an AC source and its grid is grounded.

Upon continued rotation of the drum 32, the first electrostatic latent image and the corresponding charge deposition pattern are moved to lamp 24 which acts to flood the photoconductive coating 30 with light transmitted through the dielectric belt 34 to discharge the first electrostatic latent image on the photoconductive coating surface. This corresponds to the method step illustrated in FIG. 5. It will be appreciated that the lamp 24 in practice would be suitably enclosed such that its light output is supplied to a limited transverse segment of the drum coating, and thus the first electrostatic latent image is progressively erased as it moves passed the lamp.

As the dielectric belt 34 continues beyond lamp 24 the charge deposition pattern, now effective as a second corresponding electrostatic latent image, is presented at a development station, generally indicated at 40. As was noted in connection with FIG. 6, a variety of development techniques may be employed to visibly develop the second electrostatic latent image. For ex.- ample, a cascade development technique may be employed wherein toner particles carried by developer beads are cascaded over the surface of a dielectric belt; the charges making up the second electrostatic latent image being effective to attract the electroscopic toner particles from their carrier beads. Alternatively, the development station 40 may employ a magnetic brush technique wherein the second electrostatic latent image is effectively brushed with the toner particles pursuant to visibly developing it. It is also contemplated that a liquid developer may be applied to the second electrostatic latent image carried on the surface of the dielectric belt 34. I

Regardless of which development technique is employed, it is preferable that development of the second electrostatic latent image take place before separation of the dielectric belt 34 from the photoconductive coating 30. It has been found that, when the belt 34 is separated from the drum, electrostatic charges may be developed on the inner surface of the belt due to the dissimilarities of the belt material and the photoconductive coating material and the triboelectric relationship therebetween. The presence of these electrostatic charges during the development of the second electrostatic latent image degrades the quality of the developed image.

After development of the second electrostatic latent image, the dielectric belt 34 passes upwardly between transfer rollers 42, which act to press the toner clinging to the dielectric belt surface in image formation against the surface of a plain paper sheet, indicated at 44 in FIG. 7. The toner in image formation is transferred or offset printed onto the paper sheet 44 as both the dielectric belt and the paper sheet pass between the transfer rollers 42. As is known in the art, suitable transfer electrodes may be positioned adjacent the transfer rollers to assist in toner transfer to the paper sheet. The image transferred to the paper sheet 44 is then fixed, typically by the application of heat, as is well known in the art.

The dielectric belt 34 continues upwardly around the upper roller 36 to a mechanical cleaning station, indicated at 48, which typically consists of a rotating brush operating to remove any residue toner remaining on the dielectric belt surface. The dielectric belt 34, as it leaves the upper roller 36, passes between opposed scorotron units 50 which serve to remove any residue charges on the outer and inner belt surfaces. In the meantime, the surface of the photoconductive coating 30 on drum 32 is recharged by the corona charging unit 14. The dielectric belt, completely devoid of any surface charges, arrives at point A for juxtaposition with the uniformly charged surface of the photoconductive layer and the operation repeats for ensuing copy cycles.

It will be appreciated that after repeated operation, the dielectric belt will become dirty and possibly have its outer surface marred or scratched, which degrades image reproduction quality. When this occurs the relatively inexpensive dielectric belt is replaced, a far less costly proposition than having to replace the photoconductive member.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

l. Electrostatic photocopy apparatus comprising, in combination:

A. a photoconductive insulating member;

B. a non-photoconductive insulating film;

C. means mounting said member and said film for movement into and out of juxtaposition;

D. a first corona charging unit for depositing a uniform electrostatic charge on the surface of said photoconductive member prior to its movement into juxtaposition with said film;

E. an imaging station for projecting an optical image on said photoconductive member while in juxtaposition with said insulating film, whereby to produce a first electrostatic latent image on the surface of said photoconductive member;

F. a second corona charging unit stationed beyond said imaging station in the direction of movement of said juxtaposed photoconductive member and insulating film, said second corona charging unit having l a grounded grid, and

2 corona charging wires excited from an AC source,

3 the deposition of charges being modulated by said first electrostatic latent image so as to deposit on the insulating film surface a charge pattern effective to create a zero electrostatic potential plane at the insulating film surface, and G. means beyond said second corona charging unit in the direction of movement of said juxtaposed photoconductive member and insulating film for rendering said charge pattern effective as a developable second electrostatic latent image corresponding to said first electrostatic latent image,

said means including l a light source for flooding said photoconductive member, and

2 a development station located beyond said light source in the direction of movement of said juxtaposed photoconductive member and insulating film for depositing electroscopic toner particles on said exposed film surface to visibly develop said second electrostatic latent image. 2. The apparatus defined in claim 1, wherein said photoconductive member is in the form of a coated layer on the surface of a drum and said insulating film is in the form of a continuous belt wrapped around a portion of the peripheries of said rotatable drum and a roller rotatably mounted in spaced relation to said drum, said apparatus further including:

A. transfer means stationed beyond said development station in the direction of movement of saidbelt adjacent its path of movement between said drum and said roller, said transfer means operating to transfer the toner particles in image formation carried by said belt to a plain paper sheet.

3. The apparatus defined in claim 2, which further includes:

A. means stationed adjacent said insulating belt in advance of its movement into juxtaposition with the photoconductive coated layer on said drum for removing residue toner particles and electrostatic charges from said belt.

4. The apparatus defined in claim 3, wherein said first corona charging unit is stationed adjacent said photoconductive coatedlayer on said drum at a location where said belt is removed from juxtaposition with said drum.

Claims

1. Electrostatic photocopy apparatus comprising, in combination: A. a photoconductive insulating member; B. a non-photoconductive insulating film; C. means mounting said member and said film for movement into and out of juxtaposition; D. a first corona charging unit for depositing a uniform electrostatic charge on the surface of said photoconductive member prior to its movement into juxtaposition with said film; E. an imaging station for projecting an optical image on said photoconductive member while in juxtaposition with said insulating film, whereby to produce a first electrostatic latent image on the surface of said photoconductive member; F. a second corona charging unit stationed beyond said imaging station in the direction of movement of said juxtaposed photoconductive member and insulating film, said second corona charging unit having 1 a grounded grid, and 2 corona charging wires excited from an AC source, 3 the deposition of charges being modulated by said first electrostatic latent image so as to deposit on the insulating film surface a charge pattern effective to create a zero electrostatic potential plane at the insulating film surface, and G. means beyond said second corona charging unit in the direction of movement of said juxtaposed photoconductive member and insulating film for rendering said charge pattern effective as a developable second electrostatic latent image corresponding to said first electrostatic latent image, said means including 1 a light source for flooding said photoconductive member, and 2 a development station located beyond said light source in the direction of movement of said juxtaposed photoconductive member and insulating film for depositing electroscopic toner particles on said exposed film surface to visibly develop said second electrostatic latent image.

2. The apparatus defined in claim 1, wherein said photoconductive member is in the form of a coated layer on the surface of a drum and said insulating film is in the form of a continuous belt wrapped around a portion of the peripheries of said rotatable drum and a roller rotatably mounted in spaced relation to said drum, said apparatus further including: A. transfer means stationed beyond said development station in the direction of movement of said belt adjacent its path of movement between said drum and said roller, said transfer means operating to transfer the toner particles in image formation carried by said belt to a plain paper sheet.

3. The apparatus defined in claim 2, which further includes: A. means stationed adjacent said insulating belt in advance of its movement into juxtaposition with the photoconductive coated layer on said drum for removing residue toner particles and electrostatic charges from said belt.

4. The Apparatus defined in claim 3, wherein said first corona charging unit is stationed adjacent said photoconductive coated layer on said drum at a location where said belt is removed from juxtaposition with said drum.