DE2255132A1 - METHOD AND DEVICE FOR CONTROLLING THE FLOW OF ELECTROSTATICALLY CHARGED PARTICLES - Google Patents

Description

Patent attorneys

DIPL ING. R. SPIANEMANN "..,"

Pr.B. REtTZNER 2255132

DIPt-ING, J. RICHTER

«Oo monks 2

Valley 13

1913-111-7917

Electro-Print Ina.

IOO6I Bubb Road

Cupertino, Califoralen (V «8t« A.)

Method and device for controlling the flow of electrostatically charged particles

The invention relates to a method and a device for controlling the flow of electrostatically charged Particles as well as for the formation of a planar pattern of electrostatic charge areas which are arranged in such a way that they correspond to an image · The invention finds particular application in a system for reproducing or reproducing a copy of a visible image on a suitable medium «such as Paper, Use.

Image reproduction systems of the type described above are, for example, in the US patent applications: 3.Ν « && A9 & of October 6, 1967, SN 776 X46 of November 15, 1968 and StK. 85070 of 29 October 1970 besahrieben include a support for a Aufzeiohnungsm _"f dium such as 2, B, paper," on which a visible image is to be formed · Derartig® systems.

The system can continue to release a source of particles, the one that contrasts with the color of the recording medium. Have color (such particles are typiaebemtelea

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referred to as "toner particles", and this designation becomes to be used in the following). One type of this system is between the toner component source and the recording medium an apertured screen grid or mesh is arranged. Between the toner particle source and the Recording medium, an electrostatic field is created to move the particles through the openings in the screen grid hinduroh forward toward the recording medium move. Kit With the help of the screen grid, a large number of electrostatic charge areas are created in a pattern that corresponds to the image to be reproduced, so that electrostatic fields in the openings selectively the Control the passage of toner particles. From the one with openings provided screen grid outgoing Tonertellohen therefore correspond to the image pattern to be reproduced, after the hit the toner te highs on the recording medium can all be This can be fixed or fused according to known techniques to produce a permanent image.

In another type of system "as for example in the UtS." Patent applications 9, N. 709 578 of March 1, 1968, S.N. 864 022 of October 6, 1969 and 3, n. 101 681 of Described December 28, 1970 1st, are ions or similar charged heights are aligned in a suitable pattern and passed through a cloud of toner particles. The inventive Screen grid can be used in both types of systems or in any other system * of those the formation of a pattern of charged particles is required «

In the systems described above, the invention results an improved screen grid «, since« many advantages over has the various known screen grids. The invention wall Structure of the screen grid · enables operation

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at relatively low voltages, thereby reducing the manufacture and the operation of the element is simplified.

An exemplary embodiment of the invention explained in more detail below includes a composite screen grid provided with openings or a screen which is formed from four layers. On the d in the movement direction ■ ^": ir% bneh ^'i or front toner particles or front side of the screen, on the side which is directed to the Tonerteilchenquelle to, comprises the composite screen a conductive layer on at which an insulating layer is A second conductive layer is arranged on the rear end face of the insulating layer in the direction of movement, under which in turn an outer insulating layer lies. The exposed surface of the outer insulating layer forms the rear outer surface of the screen grid in the direction of movement of the toner particles or ions.

The improved screen grid is inserted into the system in such a way that that the two conductive layers are biased so that an electrostatic blocking field is formed in the openings.

/ releasing

det that has a polarity and size around the passage to block toner particles through the openings. The outer insulating layer is selectively applied electrostatically a polarity and charged with a magnitude sufficient to form a field that counteracts dividing the blocking field and overrides them to allow the passage of toner particles in a pattern resembling an image is equivalent to.

In a satisfactory embodiment of the invention is the outer insulating layer made of photoconductive material, a material that has a high and in the dark state

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Ια illuminated state has a low resistance. In the dark or unexposed state, the photoconductive layer is charged with a polarity and to a size which is sufficient for the formation of fields in the openings which remove the above-mentioned blocking field or counteract it. Therefore, in the most unpopular screen, all openings are biased so that a passage of toner particles moving from the toner source to the recording medium is possible. The image to be recorded on the recording medium is then projected onto the photoconductive target light in such a way that parts of this layer, which correspond to the bright parts of the image, assume a conductive state * In this state, the charge previously formed on the photoconductive layer is discharged, thereby creating the counteracting field Discharge accordingly and throw the blocking field effectively. Therefore, openings located in such illuminated areas block the flow of toner highs, while openings associated with dark or unexposed parts of the sole still allow a passage of toner cells. If the screen is in this state, a supply of charged toner is thrown through directed the sole na in the direction au, f the recording medium, so that the ^{particles emerging from the screen 1} are arranged in a pattern which corresponds directly or positively to the image.

Further advantageous refinements and developments of Invention result from the subclaims *

The invention is explained below with reference to In the drawing illustrated embodiments explained in more detail.

In the drawing show:

Vi g. 1 a schematic view of an embodiment

d "* u using Snliinngitierß" !! Systems;

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Pig 2 is an enlarged fragmentary view of a Execution form of the screen grid in which the The position of the electrostatic fields in an opening is shown;

Pig · 3 a bruohstüok-like view of a larger one Part of a screen grid; Pig. 4 one of the pig. j? similar breakfast-like view *

which shows the charged state of the screen prior to exposure to an image;

Pig "5 looks like Pig" 3 and 4, which has the State of charge of the screen after a visible image has been impressed on the photoconductive one Shift »βigt;

Pig 6 one of the Pig * 1 Similar similar view,

which shows an alternative embodiment;

W§ «T an» ohematic view of a further alternative version «beieplela«

In the drawings and in particular in Pig. 1 denotes the reference number 12 ech "matleoh an air" and liohtunclurchiUaeige UMechlieflimg or an oefciuse in which "a system is included" because an implementation of the improved screen grid "is used, in the" housing 12 is a toner partohenque ^ lxk, a xusawengesetstes ^ screen grid provided with openings i6 _f a Biieaufseiohnungsmedium l8 and "ine conductive holder SO 'for the medium attached

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of the toner source towards the conductive plate. The screen grid 16 has a large number of openings 24 on. In which suitable fields for blocking or transferring toner particles according to the shape of an image I to be imaged on the recording medium 18 are formed.

Furthermore, in the housing 12 there is a charge level source, such as a corona source 26, which is used to form a uniform charge on the rear surface 28 of the screen grid «24 is used. Upon formation of such a uniform charge, an image 30 becomes over a one Reflector 32 and a spanking mechanism 34 insole optlaones system on the rear upper level of the screen grid 24 projected. The illumination of the rear surface 28 of the screen grid 24 changes the charge pattern in the openings of the Screen 24 in such a way that toner particles from the screen openings into to emerge from a pattern «which corresponds to Figure 30, so that Image I is formed on the recording medium 18

An advantageous embodiment of the screen grid x 16 is bruohattiokhaft with sterile enlarged dimensions tab shown in flg. 2. The screen grid 16 has a front surface 36 "in" dl · is directed to dl · toner part source. Dl front surface Is formed by a conductive tube 38, under which an insulating layer 40 is arranged. On that of the conductive layer 36 facing away from the surface of the insulating layer 40, a second or inner conductive layer 42 is arranged. The structure de · Sohirmei 16 is made of photoconductive by a «Sohioht 44 Completed material, the outer surface of which dl · Btiokfläohe In a practically designed screen grid, the insulating layer 40 and the photoconductive layer 44 each had a thickness of approximately 0.02 mm (0.001 inch) with conductive pads 38 and 4fi as

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int iutLJ *. tdm ϊ iei »«. &

thin film © on these insulating layers ** removed with this design at EPL®! vtiemm oils openings 2 a. diameter of mjgafito 0.15 mm (Q ₀ OOS inches) extended to the openings see rci © this is, dure "all S
in which the

In the openings 24 weräan Bpeprf © ldeF Bias or potential

and 42 trained, namely with © Ines ³ size "

did sufficient © iae Lacta layer ^ O aussubilden "di ©« Smsr- Sp © rr "fsld Zweok wnd © *, vorgeepamit under the AnnaTas so ISFC _S is having the conducting © ^ -Sohiufot layer is 2% positively pre-spasmto A d @ £ wfelg ©

by connecting a tap # β € © r L © litimgsqii © ll® 2

the leading Schicht JB wnö the power source © with dsr l®ife @ Eänen Sehiaht

Effect of a d @ rarfcig © xs ¥ orgpgisrasßg teateht dariis _fl

against the conductive layer JB are in contact with the surface

At the end of the layer 40, there is a multitude of positive Ladtagon imö on the © barfläcsli ©, ö ©! ⁵ isolating Sohlobt © ine a variety of negative * Laoymgen is formed. The interferesas of such charges to each opening forms an electrostatic field in the opening that is denoted by field lines 50 ", the © in © gum locking de®

■ The passage of toner particles through the opening has sufficient polarization * In the practical example explained above! A potential of 200 to 300 volts between the conductive layers> ^f 3 and 42 is to be regarded as sufficient to pin Feiü in the Of Fining 2k auagubii-, the one that completely blocks the passage of toner particles through (you opening;

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In order to allow toner heights to pass through the opening 24, the present invention provides for the generation of an electrostatic field which counteracts or exceeds the field indicated by the field lines 50. In FIG. 2, the field lines of the countercurrent field are denoted by the reference number 52. This counter-effect field is formed by the fact that charges with a suitable polarity are impressed on the surface of the photoconductive soliioht the dark or non-conductive state and, in the execution example shown in the drawings, positive charges can be applied by any suitable means, for example through white Bombarding the surface 28 with positive ions from a corona emitter "radioactive source" or the like can be formed In the embodiment shown in the drawings, the charges formed on the back surface are more positive than those due to the conductive soles at JB and 42 connected bias voltage »sources resulting charges ·

Those assigned to the field lines 50 and 52 in the drawings Directional arrows only show that the fields are oppositely polarized. Obviously, the direction of the Field or the force resulting therefrom depends on the polarity of the charge on the toner cells and not on any Vorseteung from the adopted to explain the invention became. Furthermore, blocking and counteracting fields can be used one for a given screen grid or viewed in plurality will.

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The field formed between the charges on the surface 28 and the charges on the surfaces of the inner insulating 8d & ± Qht 40 causes edge or interferers in the opening 24 which have such a shape and KiofetaBg having "sufficient so counteract the lock field 50 'that the toner particles can duroh fen the opening 2h hinctaotila.u ·», with this state so ergibtj lang® as the conductive layer wiri kept in the dark state 44th

A projection of the image I on the Btickfläoh® 28 illuminates all parts of the conductive Sohiohfc with the exception of the B © r © ioh ©, which correspond to the location of dark lines and / or areas of the image «The illuminated or exposed parts of the photoconductive layer 44 become locally conductive, so that a current can flow through the layer. Such a current flow discharges the counteracting field 52 in all openings 24 "the bright parts of the image I are assigned" so that only the blocking field 50 4n is present in these openings. In openings corresponding to dark areas of the image, however, no current flows through the photoconductive layer 44 so that counteracting fields 52 are present in these openings. Thus, even after the image has been removed from the rear surface 28, an electrostatic image remains on the screen l6, so that only the openings corresponding to the dark parts of the image I when the acceleration or projection field is switched on to move the toner particles from the source 14 to the conductive plate 20 enable the access and transfer of toner particles to the recording medium 18. All other openings, that is to say openings corresponding to illuminated or exposed areas of the screen 16, block the passage of toner particles, since only the blocking field 50 is effective in these openings. Therefore, a positive image is formed by the toner particles on the recording medium 18, and this image can pass through

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subsequent fixing or melting of the toner particles can be made permanent on the medium 18.

A typical image projected onto the back surface of the screen grid 16 would include ttBere almost without contrast information, areas _f that are completely black (or of a different contrasting color) and areas that lie between these two & Ktr4 "n" n (ie have different shades of gray). The screen grid according to the invention enables an accurate reproduction of all areas of the image. When the image is projected onto the photoconductive layer 44, areas of this layer become conductive in proportion to the intensity of the incident light. Areas of the photoconductive layer corresponding to light or brightest parts of the image are maximally illuminated and therefore essentially completely discharged. Areas of the photoconductive layer which correspond to the gray parts of the image are only partially exposed and thus only partially discharged. Areas of the conductive layer corresponding to black parts of the image are not exposed and therefore not discharged. Correspondingly, the passage of toner particles through various openings 24 in the screen grid I6 takes place in direct proportion to the position and relative intensity in the image »

The above-mentioned control functions are carried out according to conventional techniques and circuits and the like Usual circuit is sohematisoh in Fig. 1 by the Beaugsziffer 53 designated. To repeat the operation of the screen grid according to the invention is described below on the Pig. 3 to 5 referred to. Fig. 3 shows the screen grid 16 in the state that is present after the Power source 22 has been turned on. In this state, a potential difference forms between the inner electrode 42 and the outer electrode 38 blocking fields 50 in each opening 24 from «so that any negatively charged particles

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those of the front «at lower end face nMhera «gosparct the screen grid shows aaeh through the openings in FIG from the Tellchenquell © while the

will this state be counter-armed forces?

24 formed »whereby ütem © constantly or partially overflowed opening overdrive approaching toner particles ta ° oh öas

In Fig. 5 the marked and by 31 © Bejgugssiffer free and bright parts of a part of the screen corresponds to parts that are black or any and advertise DL® DA® maintaining charges ALSG © l®ii5S5fe _a the openings in the 'of the opening 24a' ausschlle @ ll @ b- at .Sperrf © ld ^ O ^^ wirlssi so that toner particles ", for example, l% a" push γοη and not through the öffmmg 24a In the not illuminated by the picture or openings remains, however the 0 «@@ nwirleung © field wiite« un.1 Overrides the blocking field »so that the © Ton @ rt @ lloh @ B» pioleweise l4b »Is5m®% Di® pass through the openings in partially exposed areas of the screen. Correspondingly, the toner particles behind »ie in Flg. 5 An image is formed over the screen grid by the toner particles which were picked up with the Bessugselffer 14c.

■ ■ .A

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Various alternative techniques for forming scroll fields are shown in Figures 6 and 7. In These figures show a modified screen grid 16 *. This modified screen grid has conductive elements Layers 38 and 42 and an inner insulating layer 40 on «identical to the structure described above and therefore have identical reference digits. In this modified base grid, however, the conductive base is replaced by an insulating or dielectric base 60 » which is insensitive to Lioht. Because the layer 60 is made of insulating material, it can carry a charge Speiohern in the same catfish in the dark or light state, choose the photoconductive layer in the dark state Charge stores «

In the embodiment shown in FIG. 6, it is assumed that counteracting fields 52 by impressing a Charge has been formed on the conductive layer 60 in a manner equivalent to that described above, for example through the impact of ions or other charged particles from a corona source or similar Discharge plate 6l on which an electrostatic image corresponding to the desired visible image has been formed, is brought into contact with the insulating sole 60, to selectively discharge the force fields 52.

The plate 61 includes a transparent carrier layer 62, on one surface of which there is a thin, thermoset layer conductive Sohloht 64 applied 1st. Over the transparent conductive layer 64 is a photoconductive layer 66 " those with the insulating layer 60 of the screen grid 16 * can be brought into contact. A suitable bias potential is applied by connection to the conductive base 64

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Plate applied. In the dark state, the plate 61 is brought into contact with the insulating layer 60, whereupon the visible image is projected through the transparent soles 62 and 64 onto the photoconductive layer 66. The exposed areas of the photoconductive layer become conductive and therefore discharge fields 52 in corresponding areas of the screen grid 16 '. Correspondingly, the charge distribution on the insulating layer 60 is selectively modified in accordance with the field projected onto the plate 61, so that after the plate 61 and acceleration of toner particles through the screen grid 16 * the toner particles are arranged according to the visible image

Flg. 7 shows a further modification in which the screen grid Ιβ ¹ is identical to that described above in connection with FIG. The embodiment of Figure 7 includes a plate 68 on which an electrostatic latent image has been formed in accordance with the methods "described in US" patent applications SN 673 499 and SN, 776146. The charges on plate 68, which form an electrostatic latent image on that plate, are transferred to the surface of insulating layer 60 of screen grid 16 'when the plate is brought into contact with the screen grid. Such charges form opposing fields 52 only in the openings through which the passage of toner particles is desired. Thus, by using the modification according to FIG.

It can thus be seen that the invention provides an improved * Screen grid for the formation of an electrostatic image **, which is compulsorily related to a visible image, g «* ohaf-

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fen is «so that with such a screen grid positive Images can be formed on a suitable resolution medium · Furthermore, the screen grid can be used at relatively operated at low voltages and has a relatively straightforward structure.

Claims t

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Claims (1)

* · ■■ '■ "- 15 ■ -, Claims; Device for controlling the flow of electrostatically charged particles along a path, identified by devices (22,; J8, 40, 42) for the formation of a first electric field (50) with such a polarization that a movement of Particles along the path is prevented, and means (52) for selective counteraction against the first electrical Box (50). 2. Apparatus according to claim 1, characterized in that the means for selective Counteraction against the first electric field (50) comprise a second electric field (52) which is spaced apart from the first electric field (50) arranged 1st. 3 · Device according to claim 1, characterized in that - indicates that the facilities for selective Counteraction against the first electric field (50) liohtabhänglg and the devices for forming the first electric field are light-independent. 4. Apparatus according to claim 1, characterized in that the means for counteraction against the first electric field (50) means (44, 26) for the formation of a second electric field (52) with a polarity opposite to the first field (50) and greater strength to counteract the first Field (50) and devices for the selective discharge of the second field (52). 309820/0972 5. Apparatus according to claim 4, characterized in that the facilities on training one of the fields depends on income and the institutions for training the other of the fields are independent of income. 6. Apparatus according to claim 5, characterized in that the devices for forming the fields eo are arranged * that the fields (52, SO) have a distance. 7. Device according to claim 4, characterized by a body (16) with an opening (24), which forms a wall surrounding the Tellohenpfad «where the first and second devices for forming the first and second fields (50, 52) are arranged in the body (16) and the respective fields (50, 52) in the opening (24) form. 8, device according to claim 7, characterized in that the body (16) first «and second conductive layers (33, 42) and a layer of the conductive Layers inserted insulating layer (40), wherein the soles (38, 40, 42) end in the wall of the opening (24) " so that a potential difference between the conductive soles can be obtained in order to sur Form training of the first field (50). 9. Device according to Anspruoh 8, characterized by a photoconductive layer (44) which over the curses facing away from the insulating layer (40) the second conductive layer (42), the photoconductive layer (44) terminating in the wall of the opening (24) 309820/0972 and together with the first and second conductive layers (38'42) the means for forming the second electric field (52) forms. 10. Device according to one of the preceding Requirements for the formation of an electrostatic image * which corresponds to an optical image «characterized by a composite screen grid (16» l6 ') with a plurality of spaced apart openings (24) and an outer conductive layer (38) forming the front surface (26) of the screen grid, an insulating layer (40) overlying the outer conductive layer (38) inner »over the insulating layer (40) lying conductive layer (42) and with a photoconductive» over the Inner conductive layer (42) arranged and the back surface (28) of the screen grid forming layer (44), between the outer and inner conductive layer (38 * 42) Connected devices (22) for forming a voltage gradient along the insulating layer (40) «, the voltage gradient being a first electrostatic field (50) that forms in the openings (24) to form In a first direction polarized blocking fields (50) extends «means (26) for the formation of a charge the Rttokflache (28), which together with the inner and outer conductive layers (38 «42) a second field (52) in the Openings (24) forms "the" ine polarity and sufficient OrOIe has "to counteract the" advised field (52) "if the photoconductive Sohioht (44) 1 "dark condition 1st" and executions (32-24) for imprinting the optical image (30) on the back surface (28) in such a way that it is relatively bright area of the image (30) cause a conductivity corresponding area of the photoconductive layer (44) and a discharge of corresponding parts of the wide field (52). Device according to one of the preceding claims for the formation of a positive image of an object on a surface, characterized by a multiplicity of particles with a color which contrasts with the color of the surface of an image recording medium. Devices for the formation of an electric field between the particles and the image recording medium to accelerate the particles on a path in red direction on the surface of the image recording medium, a screen grid (16) permeable to particles, which is arranged in the path essentially parallel to the surface of the image recording medium "Wherein the screen grid has a back surface (28) directed onto the surface of the image recording medium," a front surface (36) opposite this back surface (28) and a plurality of screen grids extending through the front surface between the back surface (28) and the front surface (36) Has openings (24) which are of sufficient size to allow the passage of particles, the screen grid (16) also having an electrically conductive outer layer (38) on the front surface (36), one under the outer conductive layer ( 38) lying electrically insulating sole (40), one under the insulating one An electrically conductive inner layer (42) lying beneath the inner layer (40) and a photoconductive layer (44) lying beneath the inner conductive layer (42) and forming the back surface (28). Devices (22) for the formation of a first electric field between the inner and outer conductive layer (38, 42), which is polarized in such a way that particles approaching the openings from the direction of the front surface (36) are repelled Formation of a "wide electric field (52) including the inner conductive sole (42) and the photoconductive sole (44), which has a polarity and orientation that is sufficient for the 309820/0972 «19 - · first field (50) counteract ₉ and devices 02 «54) for projecting an image of an object (30) on the back surface (28) in such a way that exposed parts of the photovoltaic layer (44) provide a conductive path for discharging the second field (52) in the exposed parts so that particles from the first field (50) at openings (24) Ia such exposed parts of the screen grid (16) will repel 12. Method of controlling the flow of Electrostatically charged Te Hohen through an opening «characterized by the steps of training a first field (50) in the opening (24) "which is so polarized" that the passage of particles through the opening (24) is blocked »the formation of a second field (52) in the opening (24), which has a polarity opposite to the first field (50) and a greater strength than this first field (50) in such a way "that it counteracts this first field (50)" and the selective discharge of the second field (52) * 13. The method according to claim 12, characterized by the formation of a coplanar arrangement of a first plurality of bipolar electrostatic ones Fields of a bit sufficient to block a flow of charged particles directed through the arrangement (16) Field strength and orientation of the overlay of a coplanar Disposing a second plurality of bipolar electrostatic fields (52) over the first plurality of bipolar electrostatic fields (50) »being the second plurality of bipolar electrostatic fields (52) opposite the first Multiple fields (50) oriented in opposite directions and having a «field strength that is sufficient» to to counteract these first fields (50), and the selective 30982.0 / 0972. Reduction of the field strength of the second plurality of bipolar electrostatic fields (! 32) corresponding to one image to be reproduced. 14 »Method for electrostatic printing according to one of claims 12 or 12 * by the steps of using a screen grid (16) with an arrangement of openings (24) formed therein, the formation of blocking fields (50) * effective in the openings (24) and the selective formation of counter-effect defenders (52) corresponding to an image to be reproduced in the openings and at a distance from the restricted fields (50) and passing charged particles through the Screen grid (16) in the direction of the receiving medium in accordance with the charge pattern in the openings (24) is. 15, system for carrying out the method according to any one of claims 12 to 14, marked lohne t by means (38, 40 «42) for forming a coplanar arrangement of a first plurality of bipolar electrostatic fields (50) which are uniformly aligned and have a field strength which is sufficient to prevent the flow of charged particles through the arrangement, Means for forming a coplanar arrangement of a second plurality of bipolar electrostatic fields (52), that are uniformly aligned in a direction that that of the first plurality of fields (50) is opposite, the second plurality of electrostatic fields (52) the first plurality of fields (50) is superimposed and has a field strength sufficient to the first plurality of fields to counteract, and devices (44) for the selective derivation of the second number of bipolar peI- 309820/0972 . 21 - dem (52) such that the flow of charged and through the arrangement of raw Hindu particles corresponds to a image to be reproduced is modulated. l6. Multi-layered, apertured Screen grid for use in a system according to claim 15 » marked ze i without t by a first layer (38) of conductive material »a second, the first layer (38) Adjacent layer (40) of insulating material »one third »adjacent to the insulating layer (40) and electrically separated from the first conductive layer (38) Sohioht (42) and a fourth layer of photoconductive Material (44) on the outside of the third conductive Sohioht (42) is applied. 17 · Multi-layer screen grid according to · & Ε®ρ »© & 16« I denote net by an insulating sheet ^ (40) covered on both sides with layers (28, 42) of conductive Material is coated, with a layer (44) of photoconductive material on the outside of one of the conductors. applied to the layers 18. Multi-layered, apertured A screen according to any preceding claim for controlling the flow of charged particles, not designated by four alternating layers of electrically conductive material (38, 42) and electrically insulating material (40, 60) formed Sohirmaohiohten, the alternating conductive layers (38, 42) insulated from one another • Ind. 19. The method according to any one of the preceding claims for the electrostatic modulation of the flow of 309 8 20/097 2 Charged parts, characterized by the practice of forming a coplanar arrangement of a first set of bipolar electrostatic fields (50) which are uniformly aligned and have a substantially uniform field strength such that the flow of charged "passes through the arrangement (16 *) Particle locked and the selective superposition of a second coplanar array of a second plurality of bipolar electrostatic fields (52) over the first array of fields (50), the second fields being uniformly oriented in opposite directions from the first plurality of fields (50) and being of a selected size to selectively oppose the first plurality of patches (50) in the form of an image to be reproduced. 20. The method according to claim 19, characterized in that the step of superimposing a second plurality of fields (52) the steps of forming an array of a second number of bipolar electrostatic fields which are parallel to the first fields (50), the second fields (52) having a polarity and field strength which is sufficient to counteract the first fields, and the selective discharge of at least one of the second fields (52). 309820/0972