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Publication numberUS3881921 A
Publication typeGrant
Publication dateMay 6, 1975
Filing dateJul 23, 1973
Priority dateOct 1, 1971
Publication numberUS 3881921 A, US 3881921A, US-A-3881921, US3881921 A, US3881921A
InventorsFrank Lee F
Original AssigneeEastman Kodak Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electrophotographic process employing image and control grid means
US 3881921 A
Abstract
An electrographic apparatus and process for producing, on a record medium having a layer of insulating material in contact with an electrically conductive backing member, an electrostatic charge image corresponding to an image to be recorded comprises an image grid and at least one control grid arranged between a corona discharge device and the layer of insulating material. The image grid comprises an electrically conductive core having insulating and conducting areas defining the image to be produced. The control grid is electrically conductive and arranged in spaced and generally parallel relation to the image grid and between the latter and the corona discharge device. When the core of the image grid and control grid are individually biased to a potential for establishing electrical fields of different strengths between the respective areas and the backing member and a flow of ions is directed toward the grids and the record medium, the flow of ions through the grids is modulated by the electrical fields to produce an electrostatic charge image on the layer of insulating material.
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Description  (OCR text may contain errors)

O United States Patent 11 1 1111 3,881,921

Frank May 6, 1975 ELECTROPHOTOGRAPHIC PROCESS EMPLOYXNG IMAGE AND CONTROL GRID OTtIER PUBLICATIQNS MEANS Cass1ers, P. M., Memory Effects 1n lilectrophotography, Journal of Photographic Science, March- 75 Inventor: Lee 1-. Frank, Rochester, NY. April, 1962, Vol- 1 PP- [73] Assignee: Eastman Kodak Company,

Rochester, Primary Exammer--Norman G. Torchm Assistant Examiner-John R. Miller [22] Filed: y 1973 Attorney, Agent, or Firm-L. F. Seebach [21] Appl. No.: 381,839

Related Application Data Llectrographic az fiiz process for produc sszsasxizsz-atastiest;1612;; a of Sept 27 1965, 3,680,954 material 1n contact with an electncally conductive backing member, an electrostatic charge image corre- [52] US. Cl. 96/1 R; 96/1 E sponding to an image to be comprises an [51] Int. Cl G03g 13/00 image grid and at least one control g arranged be- [58] Field of searchwm 96/] R, 1 E; 250/495 ZC tween a corona discharge device and the layer of insulating material. The image grid comprises an electri- [56] References Cited cally condu t: tive are having irlgsulatinjg an? ignductin areas de min t e image to e ro uce e con- UNlTED STATES PATENTS tr l grid is elec trically conductive: and arranged in 2,712,607 7/1955 Orlando 96/1 RX spaced and generally parallel relation to the image K12 i fig grid and between the latter and the corona discharge 2 808 328 10/1957 13561:: 96/1 2 device the the image grid and i 2:845:348 7,1958 Kallman I 0 96/1 grid are indlvidually biased to a potential for establish- 2,890,343 6,1959 Bolton 250/495 ZC ing electrical fields of different strengths between the 3,220,324 11/1965 Snelling 96/1 R respective areas and the backing member and flow 3,220,833 11/1965 McFarlane 96/14 of ions is directed toward the grids and the record me- 3.390,266 6/1968 Epping 250/496 ZC dium, the flow of ions through the grids is modulated 3,393,6 7/ 1 Gaynor R X by the electrical fields to produce an electrostatic i g charge image on the layer of insulating material. 9 0 $5 a. 3,694,200 9/1972 Pressman 96/1 1; 18 Claims. 60 Drawing Figures PATENTEBIAY 5:975

SHEET UIDF 10 LEE FFRA/VK INVENTOR. B 0'/ W y AGE/VT FIE- HIGH

VOLTAGE SOURCE LEE F FRANK INVENTOR.

AGE/VT PATENTEUMAY ems 1881-921 Sum as ur 10 FLOOD/N6 ILLUMINATION \Oflzlo I216 EEEEE IMP f FIG- 43 I266 I258 I252 FIG- 44 EQQQ LEE F FRANK INVENTOR.

AGENT ELECTROPl-IOTOGRAPl-HC PROCESS EMPLOYING IMAGE AND CONTROL GRID MEANS This is a continuation of application Ser. No. 185,647 filed Oct. 1, 197], now abandoned, which is a divisional application of Ser. No. 492,988, filed Sept. 27, l965, now US. Pat. No. 3,680,954.

This invention relates to electrographic recording and in particular to the following preferred embodiments: (1) document copying, (2) document copying in color, (3) duplicating, (4) character printing, and (5) multiple copying of documents.

With reference to the document copying" embodiment of this invention, there are at present two types of commercial electrophotographic document copying processes. One employs a single-use copy paper having a photoconductive coating and the other employs a reusable drum having a photoconductive coating. Both processes employ the image-wise discharge of a uniformly charged photoconductive insulating layer by image-wise exposure to produce an electrostatic charge image. This charge image is developed to a visible image by depositing thereon a finely divided powder or toner. The resultant powder image may be fixed to the photoconductive layer (as is done in the process which employs a single-use copy paper) or it may be transferred to another surface (as is done in the process which employs a reusable drum).

The electrophotographic process employing the reusable drum has the following disadvantages which are overcome by the present invention. The machine is very expensive for the low-volume user; it does not do a good job of copying photographs and large solid areas; the machine is relatively complex and requires more than the usual amount of maintenance; it uses a selenium coated drum which is expensive and fragile and which must be replaced periodically (normally, after about every 40,000 to 50,000 copies); the process requires a developed-image transfer step; and it has a relatively low sensitometric speed. The electrophotographic process which employs a single-use, photoconductor-coated, copy paper has the following disadvantages which are overcome by the present invention. Copies must be made on paper having a coating of photoconductive insulating material; because of this coating, the paper is relatively expensive; the paper is heavier than ordinary paper; copies can be marred by scratching with metals; the process has a relatively low sensitometric speed; and the transparency, luster, color, dullness, etc. of such paper are to a great extent dependent upon the appearance of the photoconductor.

With reference to the document copying in color embodiment of this invention, conventional-electrm photographic systems, if used for color printing would involve either (I) the transfer of developed images which transfer has inherently associated therewith problems of registration or (2) repeated development on top of the same sensitive surface, with the problems of strong inter-image effects. In addition, in the latter case there would be difficulty in obtaining good whites due to the dye sensitizer in the photoconductive coat- With reference to the duplicating embodiment of the invention, conventional duplicating systems either require large, expensive machines and long makeready times or have the problems of poor quality, limited numbers of copies, and of being messy.

With reference to the character printing embodiment of the invention, conventional printers involve the use of mechanical elements to make an impression on the page. These elements consequently become worn and are limited in writing rate by inertia. Xerographic and photographic character printers have a somewhat greater printing rate than mechanical printers but need a special cathode ray tube and the xerographic ones are relatively insensitive. Photographic recording requires a relatively long processing time, and does not provide an inexpensive, real-size (standard type size), quick-access copy. The size of the type in the photographic copy is usually substandard.

With reference to the "multiple copying of documents" this embodiment incorporates the above discussed advantages of both the document copying" and the duplicating embodiments of the invention. The subject embodiment can be used to make either a single copy or a very large number of copies from a single exposure.

It is an object of the present invention to provide an electrographic recording system.

It is a primary object of the present invention to provide an electrophotographic document copying and duplicating system which is free of all of the abovementioned disadvantages of the present systems.

It is thus an object of the present invention to provide an electrophotographic copying system which is inexpensive, capable of copying onto a large variety of materials, employs an indefinitely reusable photoconductive insulating member, does not require the deposition of toner onto a photoconductive insulating member, and which can be embodied in a small, light and inexpensive machine.

lt is a further object of the invention to provide an electrographic system which, in addition to overcoming all of the above disadvantages of the prior systems, exhibits very high sensitometric speeds and provides extreme flexibility.

It is a still further object of the invention to provide an electrographic color recording system which eliminates the above-mentioned problems of prior color systerns such as the requirement for transfer with its associated registration problems and the requirement for repeated development on top of the samesensitive surface with the associated strong interimage effects.

It is a further object of the invention to provide an electrographic color recording system which is fast, which allows dyes to be chosen primarily for stability and color and not for their chemical properties, which does not require charge transfer or toner transfer, which provides for adjustment of color balance on a single print rather than only on a series of prints therefore reducing the rejection rate, which allows the use of white light in inspecting the process, which provides for variable contrast by electronic controls and by design of the photoconductive element, which employs low cost equipment, which provides an inherently correct neutral scale rendition, which is subject to color balancing, in which sufficient, deliberately introduced interimage effects are available to accomplish color masking, and which provides greatly decreased color degradation (improved color rendition) compared to color xerography because the sensitive surface is above the previously deposited toner and is not affected by it thereby eliminating autopositive interimage effects.

lt is a still further object of the invention to provide an electrographic duplicating system in which the printing-master can be made in any of a number of different ways, in which the printing-master is simple, inexpensive and rugged, and in which the duplicating process can be carried out using the same apparatus used for document copying by simply replacing the photoconductive element with the printing-master.

It is another object of the invention to provide an electrographic character printing system which overcomes the above-mentioned disadvantages of prior systems.

It is another object of the invention to provide an electrographic alphanumeric character printing system which involves no moving parts in the printing head, which has high speed and durability, which allows the making of quick-access, stable, real-size, black-onwhite prints, in which a wide range of type sizes can be obtained, which allows selection of arbitrary type fonts, which uses less expensive copy paper than that which the photographic systems use, which provides real-time writing capability for a computer thus eliminating or at least reducing the need for a buffer or printer allocating stage, which has a wide range of applications, such as ticker-tape, radio or wired-teletype, and punched-card printer, etc., and which eliminates the need for an expensive character display cathode ray tube.

It is another object of the invention to provide a process and apparatus for making multiple prints from a single exposure.

These objects are accomplished by the following invention. l have discovered a unique electrographic recording system which comprises directing a flow of ions toward a record medium and imagewise modulating the flow of ions to produce an image on said record medium. The imagewise modulation is accomplished by interposing a grid or an array of grids in the flow of ions.

The primary difierence between the several embodiments of the invention is in the nature of the grid or grids used. in the document copying embodiment of the invention, the grid is a photoconductive grid. The color recording embodiment uses the same photoconductive grid but employs color separation filters in the exposing step and differently colored developers in the developing step from the document copying embodiment. The duplicating embodiment employs a grid similar in appearance to the photoconductive grid but different in construction in that it employs an imagewise distributed coating of insulating material on a conductive grid. One character printing embodiment employs conductive grids formed in the shape of the characters to be printed. The multiple copying of documents embodiment employs both photoconductive grids and insulator coated grids.

ln the preferred embodiments of the invention a record medium is used which has an insulating surface coating on a relatively conducting support layer. The flow of ions, as imagewise modulated by the grid or grid array, produces an electrostatic charge image on the insulating surface, which charge image can be xerographically developed to produce a visible image.

In the document copying embodiment of the invention, for example, the imagewise modulation of the flow of ions is accomplished by means of a photoconductive grid comprising a biased or grounded electrically conductive core or grid which is, in the preferred embodiments, covered with a layer of photoconductive insulating material.

It has been found extremely important for the operation of this invention that the photoconductor completely cover all of the exposed surface of the conductive grid with a uniform coating. Even microscopic cracks or holes in the photoconductive coating on the grid are detrimental to the operation of the process.

This photoconductive grid is positioned directly in the ion flow and preferably just above the record medium. The photoconductive grid is imagewise exposed to produce a conductivity image in the photoconductive material, while the flow of ions is being directed through the grid and toward the record medium, and, hence, can be designated as an image grid means. The terms electrically energize and imagewise energize" are intended to encompass, for the purpose of this specification and claims, the biased or grounded, electrically conductive areas of the grid. In the areas of the grid which are insulating or non-energized (where the grid is not exposed) the flow of ions will first produce a small surface potential (from a few to a few hundred volts) on the grid surface and will then pass through the grid to charge the underlying insulating surface of the record medium. ln the conducting, and thus imagewise energized, areas of the grid (where it is exposed) the ions are captured by the grid and are thus removed from the ion flow, whereby the areas of the record medium underlying the exposed or energized areas of the grid remain uncharged. An electrostatic charge image corresponding to the light image is thus formed on the insulating surface of the record medium. This electrostatic image is then xerographically developed and the developed image fixed to the record me dium or alternatively transferred to a final receiving sheet, in which case the record medium can be cleaned and reused.

One of the most startling effects noted with this invention is in connection with the exceptional sensitivity of the process. In xerography, because the process is electrostatic, the maximum operating gain of the system is unity. The operating gain of the system is defined as the number of stored charges removed by one absorbed photon. [n the present system, which is electrodynamic, operating gains greater than unity can be achieved. This process has exhibited speeds of up to 300 times the speed of present electrophotographic systems. An additionally startling effect in this connection is that of an increase in effective speed with increased resolution, i.e., a 200-line per inch photoconductive grid gives many times the speed of a lOO-line per inch grid. A 300-line per inch grid gives speeds over times faster than previous electrophotographic systems and previously considered unavailable. This simultaneous increase of speed and resolution is opposite to the relationship found in other systems, for example, in photography.

A great degree of flexibility is available in designing an electrophotographic document copying machine to operate on the principles of the present invention, as will be evidenced by the following discussion. Many kinds of photoconductors (both n and p types) having various levels of sensitivity and dark current, can be used. Useful photoconductors are, among others, cadmium sulfide, selenium, selenium and tellurium mix tures, zinc oxide, arsenic trisulfide, cadmium telluride, cadmium selenide, germanium (PN or NP) and organic photoconductors such as triphenylamine in an insulating organic resin vehicle (such as that sold under the trademark Vite] PE-lOl sensitized with 2,4-bis(4- ethoxyphenyl )-6-(4-n-amyloxy-styryl) pyrylium fluoroborate. Further it is possible to vary the effective sensitivity of a given photoconductor within the system. For example, electrical bias can be used to employ photoconductors that do not exhibit low resistance upon exposure; increasing the corona current changes the time constant depending on the geometry of the source and the nature of the photoconductor; and increased exposure tends to decrease time constants. It is understoood of course that other materials which exhibit a change in conductivity upon activation can be used in the present invention in place of a photoconductor. Such other materials include photoinsulators, i.e., materials which are normally conductive but which become insulating upon exposure to light, and heabsensitive materials which exhibit a change in conductivity when heated. Hence, the image grid means can be considered as being coated with a radiation responsive insulating material. A layer of photoconductive insulating material may be formed on the grid, for example, by evaporation techniques or by spray coating. It is necessary, however, to spray or evaporate from a widely diverse number of angles with respect to the grid so that all of the surface of the grid will be completely covered with a uniform layer of photoconductor, including the inside walls of the holes in the grid. Woven mesh is hardly suitable for this process, if coated by evaporation, particularly in the finer weaves, because of the difficulty of completely covering the wire surface in the region where the wires cross each other. Spray coating can be used to totally coat woven mesh for this process, provided the coating has reasonable leveling or wetting action on the mesh. A wide choice is also available in connection with the electrically conductive grid which forms the core of the photoconductive grid and to which the photoconductive material is applied. The choice as to the shape, size, material and method of manufacture is large. It has been found that an etched or electroformed mesh is superior in mechanical properties for one-to-one docment copying; window screening works well for moderate size posters and is stronger and less expensive; and hardware cloth can be used for very large prints. Neither the toner nor any part of the document copying apparatus ever needs to come into contact with the grid; the grid is thus indefinitely reusable. It is also relatively simple to construct and relatively inexpensive. An extremely wide choice of record mediais available, including ordinary paper at low humidities. The record medium can be, for example, a single layer of insulating material, a sheet of paper or other support having a thin insulating coating, a sheet of thermal-deformable plastic or an ion-sensitive silver halide emulsion layer, but is preferably an insulating surface with a relatively large capacitance per unit area. The record medium, after formation of the electrostatic image thereon, can be xerographically developed, for example, by any of the well-known methods and the developed image can be fixed thereon to form the final copy, or the toner can be transferred to a final copy sheet and fixed thereto, in which case the record medium can be cleaned and reused. In this latter case, the record medium can conveniently be in the shape of a rotatable drum; in connection with this embodiment it should be noted that the drum does not have an expensive, fragile, photosensitive coating but rather a simple, rugged, inexpensive, electrically insulating coating. Although the preferred embodiments of the invention utilize a record medium having an insulating surface whereby an electrostatic image is produced thereon which can be xerographically developed, it is noted that the invention is not limited to such record media. For example, a Berchtold layer" (a mosaic of conducting areas separated by insulating layers as described in US. Pat. No. 2,866,903) can be positioned behind the photoconductive grid with an insulating recording sheet positioned behind the Berchtold layer. Further, the record medium can be a conducting sheet which changes color in response to a flow of current therethrough, as is known in photoconductography. lf the record medium is replaced with an electroluminesoent panel, the electrographic system functions as a light amplifier. Since the electroluminescent panel glows where there is current, the image has a tonal scale reversed from that of the normal (i.e., a negative). When the gain is less than unity, the reversed tonal scale would be useful either in direct viewing of a photographic negative as a positive, or in converting a negative to a positive. In other words, the imagewise pattern of ions coming from the grid can be used to form a record (either permanent or temporary) in various ways.

Further, a wide choice of exposing methods is available, including projection and contact exposing methods with either area exposure or line scanning. The associated benefits of line scanning are usable, i.e., rightreading, wrong-reading and co and counter-current scanning. Various scanning embodiments useful in the invention include l) stationary corona charger, grid and lens with moving document and record medium for both 00- and counter-current scanning, and (2) stationary document and record medium with (a) moving corona charger, grid and lens system, (b) moving corona charger with stationary grid and lens system, (c) moving corona charger and lens system with stationary grid, and (d) moving corona charger and grid with stationary lens system. The conductivity image produced on the photoconductive grid may thus be spatial (in the case of large area exposure) or temporal (in the case of line scanning) or both in the case of small area scanning.

Many types of ion sources, including corona discharge electrodes such as needles and wires, are wellknown in the art and any of such may be used in the present invention.

Certain arrangements of the document copying embodiment of the invention require the use of a photoconductive material which will remain conductive (exhibit persistance of conductivity) for a period of time afler the illumination has been turned ofiand in the presence of a corona discharge. The persistence of conductivity of a photoconductive material is often altered in the presence of a corona discharge. For example, in zinc oxide photoconductive layers, the photoconductivity which would normally persist for many minutes is destroyed in slightly over a second in the presence of a corona discharge even at very low corona levels. Contrarywise, in certain cadmium sulfide photoconductive layers, normally the photoconductivity will decay in less than O.l second, but under a corona will exhibit persistence of conductivity for several minutes.

The above description is applicable to the color recording embodiment of the invention since it also employs a photoconductive grid. Many of the unique advantages of this color recording embodiment result directly from the fact that toner is not deposited on the photosensitive layer as it is in the prior systems. Thus there is no need to transfer toner and no associated registration problems. Further, there are no detrimental interimage effects caused by charging, exposing, and developing a photosensitive layer containing previously deposited toner. The dyes to be used can be chosen for their color and stability. Many other advantages of this embodiment are disclosed above in the objects of the invention.

The duplicating embodiment employs a grid comprising a grounded or electrically biased conductive core or grid having an imagewise distribution of insulating surface areas. The insulating and conductive areas of the grid modulate the flow of ions in the same manner as described above with respect to the photoconductive grid. The primary difference between the two embodiments is that the image is effectively permanent (permanent for the duration of the copy run) on the printing-master grid. As stated above this embodiment has many advantages over the known duplicating systems.

The character printing embodiment employs a grid comprising a grounded or biased conductive electrode formed in the shape of a character to be printed. This grid modulates the flow of ions in somewhat the same manner as do the grids of the above embodiments. The grid is energized to either attract or repel the ions in the flow of ions to produce an electrostatic charge image or shadow" of the grid. A stack of individual electrodes, made of thin wire in the shape of characters, or a grid comprising a plurality of individual electrode segments corresponding to parts of characters, can be moved across the record medium to reproduce a page of type. The corona current is transmitted to the paper only when a character is to be printed. The dc. corona current can be turned on and off by auxilliary means, the dc. supply, or extra grids. Each electrode or electrode segment is connected to ground or to a bias potential through a switch. The switch (or switches in the case of the grid employing electrode segments) corresponding to the character to be printed is closed and the remaining electrodes have practically no blocking effect and do not cast a shadow." Alternative to the above described scanning motion, a complete row of grids with a full set of electrodes for each space in the line can be used to provide printing of a line as a whole. The switches can be photocells to allow the switching to be done by light. In one embodiment, described below, the photocells are arranged in an X-Y array. The light pattern can be simultaneous, as in the case of exposure through a punched card, or sequential, as in the case of the output of a cathode ray tube.

The multiple copying of documents embodiment employs a photoconductive grid or grid array to produce an electrostatic charge image on an insulating grid (or on itself or another photoconductive insulating grid in the dark) and then this grid having the electrostatic charge image is used to modulate an ion flow to imagewise charge an insulating record medium. Many hundreds of copies can be produced from a single exposure. This embodiment employs a different principle of operation from that of the previously described embodiments. In this embodiment ions are not imagewise removed from the ion flow by means of conductive grid areas. The ion flow is modulated by electrostatic fields. The ions are prevented from flowing through the areas of the grid which have the charge image, but flow freely through the remaining areas of the grid.

in the present specification and claims the term ion flow" or flow of ions is employed in describing the step of imagewise charging the record member. Although it is true that the preferred source of charges is a corona discharge electrode and the preferred charges are air ions, it is to be recognized that electrons, other charged subatomic particles, charged particles of matter, etc., can be used as the flow of charges in the present invention which flow is directed toward the record member and imagewise modulated to produce an electrostatic charge image thereon. This charge image can be made visible by any known xerographic developing methods. Various types of charges can be used in the present invention and it is intended that the term flow of ions and ion flow be interpreted to include any of such charges and that it not be limited in meaning strictly to air ions. The grid or conductive mesh in this invention is analogous to the grid in a vacuum tube in which relationship the term grid is generally defined as an electrode having one or more openings for the passage of ions therethrough, which electrode exercises control on the passage of ions without collecting more ions than is necessary. Thus, the use of the term grid for the electrode of the invention which controls the flow of ions to the record medium is consistent with present usage of the term. The tenn grid, as used in the present specification and claims is intended to encompass any and all electrode configurations which allow for the passage of ions therethrough; the term grid thus encompasses such constructions as are also known by the terms screen, mesh, perforated plate, slot etc. Since the resolution of the ultimate image depends on the number of openings per linear inch in the grid and since this is commonly called lines per inch in halftone production, the same phrase lines per inch will be used in the present specification and claims to define the size of the grid. This term together with information about the percent of open area of the grid adequately defines the size of the grid. It is noted that the resolution of the grid system is determined by the number of holes per unit length only in the case of stationary operation without interaction between the holes. An example of this is the single grid system operated stationary, close to the record sheet. If a scanning system is used, the system in the direction of motion has a resolution equal to the reciprocal of the diameter of the holes, or slot width, if there is no interaction between the holes and the time frequency response of the system is not limiting. The relationships in the scan perpendicular direction are much more complex. There is a marked difference between the stationary and the scanning relationships for cases where only a small portion of the area of the grid is open, or single apertures are used. In these cases the slow scanning resolution is higher than the stationary exposure. The slot is a limiting case in that the resolution in the scanning direction is obtained only by virtue of the scanning operation. With respect to the embodiments of the invention which use photoconductors, the phrase imagewise exposing the grid means, of course, imagewise exposing the photoconductor to suitable radiation to which the photoconductor is sensitive. As used in the specification and claims, exposing includes exposing to visible light, x-rays, alpha, beta, and gamma rays, and particulate radiation. Any type of radiation may be employed that will render the photoconductor conductive. In the specification and claims, the term insulating" as used with respect to certain types of record media is intended to encompass any material which will hold an electrostatic charge image for a period of time long enough to allow for the development thereof. The period of time needed to develop an electrostatic image may be extremely short, as in the case of thermal-deformable or electro-deformable plastic sheet. The term potential or connected to a predetermined potential is intended to include any potential including ground potential. The imagewise modulation of the flow of ions can be either spatial or temporal or both; line scanning would be both, point scanning would be only temporal, and overall exposure would be only spatial.

These and other embodiments of the present invention will be more fully understood by reference to the following detailed description of the invention when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of one document copying embodiment of the present invention;

FIGS. 2-6 are enlarged, perspective views of various photoconductive grids useful in the invention;

FIG. 7 is an enlarged cross-sectional view through another photoconductive grid useful in the invention;

FIG. 8 is a greatly enlarged cross-sectional view through a photoconductive grid and a record medium which illustrates certain principles of the invention;

FIGS. 9A and 93 each schematically illustrate a multigrid embodiment of the invention;

FIG. 10 is a schematic diagram of an equivalent circuit of the circuit shown in FIG. 1;

FIG. 11 is a schematic diagram of an equivalent circuit of the circuit shown in FIG. 9A;

FIG. 12A is a schematic illustration of a two-grid system;

FIG. 12B is a graph showing the characteristic output current of the circuit of FIG. 12A;

FIGS. 13-15 each schematically illustrate another multigrid embodiment of the invention;

FIGS. 16-20 each schematically illustrate a document copying embodiment of the invention;

FIG. 21 shows, greatly enlarged, an alternative exposure station for use in the embodiment of FIG. 19;

FIGS. 22A-22B schematically illustrate a simple reflex printing embodiment of the invention;

FIG. 23 is a schematic diagram of a document copying apparatus according to one embodiment of the invention;

FIG. 24 illustrates a lens system useful in the embodiment of FIG. 23; FIG. 25 is a schematic illustration of an embodiment in which the grid is controlled by a photoconductor spaced from the grid;

FIGS. 26-28 schematically illustrate an embodiment of the invention which employs a photoinsulating material;

FIG. 29 schematically illustrates an embodiment employing a layer of cellular material on top of the photo conductive grid for the production of reversals;

FIG. 30 is a schematic illustration of a color reproduction embodiment of the invention;

FIG. 31 is a schematic illustration of a duplicating embodiment of the invention;

FIG. 32 shows a duplicating embodiment of the invention employing multiple grids.

FIG. 33 is a schematic illustration of a character printing embodiment of the invention;

FIG. 34 is a plan view showing a line of character printing grids for use in printing a line at a time;

FIG. 35 is a schematic illustration of a grid composed of electrode segments for use in printing alphanumeric information;

FIG. 36 is a schematic illustration of a alphanumeric character printing embodiment of the invention;

FIG. 37 is a schematic illustration of a modification of the character printing embodiment of FIG. 33;

FIG. 38A schematically illustrates certain principles of operation of the embodiment of FIG. 33 when the grid is biased to attract ions; FIG. 38B is a graph showing the charge density across an electrostatic image produced by the embodiment of FIG. 38A;

FIG. 38C is a graph showing the nature of the toner deposit on the electrostatic image of FIG. 38A;

FIG. 39A schematically illustrates certain principles of operation of the embodiment of FIG. 28 when the grid is biased to repel ions;

FIG. 39B is a graph showing the charge density across an electrostatic image produced by the embodiment of FIG. 39A;

FIG. 39C is a graph showing the nature of the toner deposit on the electrostatic image of FIG. 39A;

FIGS. 40A and 40B schematically illustrate a twostep process using three grids to produce multiple copies from a single exposure;

FIGS. 41A and 41B schematically illustrate a variation of the embodiment shown in FIGS. 40A and 40B employing an integral array of grids using a foraminous insulating spaces;

FIG. 42 schematically illustrates another integral grid construction useful in the process of FIGS. 41A and 41B;

FIGS. 43, 44, and 45 schematically illustrate more complex grid arrays containing larger numbers of grids for use in the multiple copying of documents embodiment of the invention;

FIGS. 46A, 46B, and 46C show a preferred threestep process for making multiple copies from a single exposure;

FIG. 47 schematically illustrates the exposure step of another grid structure useful for reflex printing in the general process of FIGS. 46A, 46B, and 46C; and

FIG. 48 is a schematic illustration of an apparatus employing, for example, the grid of FIG. 47 for making single or multiple copies of documents by reflex exposure.

I DOCUMENT COPYING FIG. 1 illustrates one embodiment of the present invention. In FIG. 1 a transparency 10, having an image to be reproduced, is illuminated by a light source 12. The image is focused by a lens 14 onto a photoconductive grid 16. The grid 16 consists of a grounded, electrically conductive electrode core or grid 18, for example of metal, completely covered with a layer 20 of photoconductive insulating material. Positioned immediately below the grid 16, is a record medium 22 consisting of an electrically insulating layer 24 on a support 26, such as paper. It should be noted that the insulating layer 24 is not, or at least need not be, a photoconductive insulating material. The record medium 22 can therefore be quite inexpensive. The support 26 is positioned in overlying contact with a grounded field electrode 28 during the step of imagewise exposure of the grid 16, a corona discharge is produced adjacent the grid 16 but on the opposite side thereof from the record medium 22. The corona discharge is produced, for example, by connecting a corona discharge electrode 32 to one terminal of a voltage source 34 by means of a switch 36. The other terminal of the voltage source 34 is connected to ground 38. The corona discharge provides a source of ions, and the electric field produced between the corona discharge electrode 32 and the field electrode 28, directs a flow of ions toward the record medium 22. In the imagewise exposed areas of the grid 16, the photoconductive layer 20 is conducting, and the ions which come into proximity thereto are attracted to the photoconductive layer 20 and pass directly to ground. In the remaining (dark) areas of the grid 16 the photoconductive layer 20 is insulating and the ions, after building up a small potential (from a few to a few hundred volts) on the surface of the grid, pass through the openings in the grid 16 to charge the underlying surface of the insulating layer 24. This modulation or control of the flow of ions from the corona discharge electrode 32 to the record medium 22 by the grid 16 is described in more detail in connection with FIG. 8 below.

FIGS. 2-6 are perspective views showing alternative grid constructions which are useful in the present invention. FIG. 2 is a perspective view of the grid 16 of FIG. 1 showing the photoconductive insulating layer 20 and the core or grid 18. The grid 18 may be formed by etching or electroforming. FIG. 3 shows a grid 40 consisting of a photoconductive insulating layer 42 on an electrically conductive electrode grid which consists of a series of equi-spaced, parallel electrodes 44 connected to a common electrical line 43. FIG. 4 shows a grid 50 formed from a perforated metal plate which forms the electrode core or grid 52. The grid 52 is completely covered with a layer 54 of photoconductive insulating material. FIG. shows a photoconductive insulating layer 56 on an electrically conductive electrode core or grid 57. The core or grid 57 can be formed, for example, by electroforming in which a durable high quality stainless steel plate is covered with a photoresist, exposed to the desired pattern (to harden the exposed areas of the photoresist the photoresist in the background being subsequently washed away), etched to leave posts and then electroplated with, for example, nickel, copper, gold or silver. The plating is then peeled off of the steel plate resulting in an excellent electrically conductive electrode foil which forms the core or grid 57. FIG. 6 illustrates a grid 60 in which an electrically conductive grid 61 is provided with a single, narrow slot or opening 62 having tapered faces 64. The surfaces of the grid 61 adjacent the opening 62 are covered with a layer 66 of photoconductive insulating material. The opening 62 is of the order of 0.lmm. wide. The grid 60 finds use in connection with scanning processes, as is more fully discussed below. FIGS. 2-6 show examples of the various shapes and constructions which the photoconductive grid of the present invention can take. The resolution of the ultimate image depends on the number of openings or holes in the grid per linear inch, hereinafter referred to as lines per inch. The openings or holes in the grids of FIGS. 2-5 are on the order of about 50 to 500 lines per inch. The process of the present invention has been found to operate very well with ISO, 200 and 300 lines per inch grids. It is noted that in scanning, the number of holes per inch does not determine the resolution but the size of the holes or slot, electrically and optically, does. The term lines per inch is only justified for non-scanning operations. In the scanning case resolution is more nearly the reciprocal of the hole diameter in lines per unit length.

FIG. 7 is an enlarged cross-sectional view through a photoconductive grid 70 which is identical to the grid 16 of FIGS. I and 2 except for the nature of the layer 72 of insulating material which covers the electrode core or grid 76. The grids of FIGS. 1-6 are well shown as being completely covered with a photoconductive insulating material. In FIG. 7 only a part of the insulating layer 72 is photoconductive. The insulating layer 72 of the grid 70 of FIG. 7 consists of a photoconductive insulating layer 74 on one half of the grid 70 and a nonphotoconductive, preferably opaque, insulating layer 78 covering the other half of the grid 70. Alternatively, an opaque insulating coating can be coated over the photoconductor on one side of the grid. The two layers 74 and 78 meet to provide the complete insulating layer 72. In general, the photoconductive insulating layer 74 faces both the corona discharge and the light source during exposure and imagewise charging; however certain embodiments of the invention, to be discussed below, employ different arrangements.

FIG. 8 illustrates how the flow of ions is modulated by the imagewise exposed image grid means or photoconductive grid. The ions are either attracted to the grid in the exposed areas thereof (and thus removed from the flow of ions) or repelled from the grid due to the surface charge thereon in the unexposed areas (and thus pass through the openings in the grid). For the purpose of this description, 21 grid 80 (similar, for example, to the grid 16 of FIGS. 1 and 2) is shown having a grounded electrode core or grid 82 completely covered with a layer 84 of photoconductive insulating material. The grid 80 is imagewise exposed as indicated by the small arrows 96. Ions, illustrated by the long, open arrows 86, are directed from a corona discharge (not shown) to the field electrode 88 positioned behind the insulating record sheet 90, and through the grid 80. In striking the photoconductive layer 84 in the unexposed areas 92 thereof, the ions produce a small surface charge thereon. This charge will build up to somewhere between a few volts and a few hundred volts which will prevent further charging thereof and which will force the ions which would otherwise hit the unexposed areas of the grid 80 to flow around the grid structure and through the openings in the grid 80. These ions, along with other ions which are flowing through the openings, continue through the grid and impinge upon the insulating record sheet to deposit charges thereon in the underlying areas 94. However, in the imagewise exposed areas 93 of the grid 80, the photoconductor becomes electrically conductive and any ions striking it pass through the photoconductive layer 84 to the electrode 82 and to ground and are thus removed from the flow of ions. Furthermore, the conducting areas 93 of the photoconductive layer 84 have a trapping action extending a certain distance from the photoconductive layer 84. As soon as the photoconductivity reaches a certain value, the distance at which the trapping action is effective extends to and beyond the middle of the openings or holes between the individual elements or wires of the grid 80 and thus any ions 86 directed toward such areas of the grid 80 are essentially completely trapped; that is, the ions are drawn over to the photoconductive layer 84 and pass to ground so that no ions pass through the exposed areas of the grid 80 to charge the insulating record sheet 90. There is, however, some leakage (pass-through of ions) in the exposed areas or light is wasted. This trapping action is substantially the same whether the insulating layer surrounding the electrode grid 82 is all photoconductive or partly photoconductive and partly nonphotoconductive as shown in FIG. 7. The degree of trapping depends not only on the amount of exposure and the degree of photoconductivity of the photoconductor, but also on the potential of the electrode grid 82 relative to that of the field electrode 88.

If the electrode grid 82 has a potential somewhere between that of the corona source and that of the field electrode 88, the trapping action in the illuminated areas is somewhat reduced. If the grid 82 has a potential opposite in sign to that of the corona electrode, the trapping action in the illuminated areas is somewhat increased thus tending to clean-up the background and effectively increase sensitivity. It should be noted that it is the complete coating of all the conductive surface of the grid within the picture area that allows the use of the preferred potential opposite in sign to that of the corona electrode. Even microscopic holes or cracks in the coating of photoconductor on the grid will make the system wholly inoperative with the preferred bias.

It is noted that since the developing station is preferably remote from the charging station, developer powder or toner never needs to touch the photoconductive grid; the photoconductive grid can be roused indefinitely.

FIG. 9A illustrates an embodiment of the invention which employs a control grid means which can take the form of an additional electrode grid 100 between a photoconductive grid 102, which can be, for example, any of those shown in FIGS. 2-7, and an insulating record sheet 104 carried on a field electrode 106. The conductive grid 100 forms an electrostatic shield and does not absorb a majority of the ion flow. By this means the voltage across the photoconductive grid 102 can be decoupled from the voltage across the record sheet 104. The advantage of such decoupling will be more fully understood by reference to FIG. 10 which is an approximate equivalent circuit of the system shown in FIG. 1. A current source 110 supplies current to a current divider circuit consisting of two branches. One branch represents the photoconductive grid 16 of FIG. 1 and is shown in FIG. 10 by a capacitor 112 in parallel with a variable resistor 116 (representing photoconductance). The other branch represents the air resistance between the grid 16 and the record medium 22 of FIG. 1 and is shown in FIG. 10 by a resistor 118 in series with a capacitor 114 (the series capacitance of the paper). Since no current flows from the grid 16 to the record medium 22, or vice versa, when the current source (corona) is shut off, a diode is included in each branch. The transient behavior of this circuit can be analyzed in detail. However, the most significant characteristics involve the terminal voltages on the capacitors. When the system is at equilibrium, there is no current flowing through the capacitor 1 14, so that it is charged to the potential across the photoconductor represented by variable resistor 116. All the current is flowing through the photoconductor 20 so the final potential across it and the record medium 22 is equal to the current through it times the photoconductor resistance. The record medium 22 potential is then limited in the simple grid system of FIG. I to the current times the change in photoconductor resistance. It should be noted that this equivalent circuit implies that the potential deposited on the record medium is limited to the maximum potential that the photoconductor can stand. This establishes a minimum thickness and resistivity in the dark of a given photoconductor in order to provide a developable image for any given development process. It should be noted that at maximum deposited charge on the record medium the total flow of current is through the photoconductor while it has the maximum potential across it, resulting in maximum power dissipation in the photoconductor.

An approximate incremental equivalent circuit for the circuit of FIG. 9A is shown in FIG. 11. FIG. 11 shows a current source 120 (the corona source of FIG. 9A) which supplies current to two branches of a circuit. One branch represents the photoconductive grid 102 of FIG. 9A and is shown by a resistor 126 in parallel with a capacitor 122. The other branch represents the air resistance between the two grids and is shown by a resistor 119. Since there is no capacitor in series with the resistor 119, at equilibrium current will flow through the resistor 119. A dependent current source 12] supplies current to the paper capacitance 123. The current source 12] supplies a current equal to or slightly less than that which flows through the electrical resistance of the air. This means that in the steady state the current to the paper is independent of the charge on it and that the paper can be charged to an arbitrary level extending the charging time. In practice the photoconductive grid 102 delivers its output into the additional grid 100, which grid appears to the grid 102 to be a grounded metal plate, though it is not actually grounded. The major part of the current which is delivered to the grid 100, however, is transmitted through it to the record sheet 104 provided there is a potential of about 300 volts or more (for convenient dimensions) between the record sheet 104 and the additional grid 100 to accelerate the flow of ions. Since the flow of ions is independent of voltages above about 300 volts (for convenient dimensions) it is possible to put a large potential between the additional grid 100 and the record sheet 104. I

This potential decreases the transit time of the ions between the additional grid 100 and the record sheet 104 to the point whose diffusion of the image due to kinetic motion is negligible. The main source of diffusion is between the photoconductive grid 102 and the additional grid 100. However, this is between two permanent parts of the apparatus which are stationary relative to each other and the amount of diffusion can be minimized by proper manufacture. Since the transit time is inversely proportioned to the potential, the record sheet 104 can be positioned more distant from the grid 100 by just increasing the potential. In embodiments which do not employ the additional grid 100, ions diffuse at angles of about 45 so that the record sheet 104 should be in virtual contact with the photoconductive grid 102.

FIGS. 12A and 12B give an idea of the limits of the applicability of the incremental circuit model of the double grid. A metal grid system was set up as shown schematically in FIG. 12A. The current to a metal receiving sheet 132 was measured as a function of the voltages on the two metal grids 130 and 131. In the area where the grid 131 -toreceiving sheet 132 voltage (V is above about 300 volts, the delivered current depends only on the intergrid voltage V,, and can vary between zero and about 4 microamps for the system shown. In this operating area the charging of the paper is dependent on the charging time and the intergrid surface voltage V, only, making it possible to charge the paper to hundreds of volts (limited only by breakdown or discharge through the insulator coating on the paper) with an intergrid potential of a few volts.

FIG. 12B shows the characteristic output current of the circuit shown in FIG. 9A as a function of the additional grid 100 to record sheet 104 voltage. The parameter of variation is the potential across the photoconductive grid 102. These curves were actually obtained from the all-metal circuit shown in FIG. 12A (using a 10 KV corona), in order to be able to measure the surface potential on each grid. Note that about 30 volts is all that is required to control a microampere delivered to 1,000 volts. This is approximately the charging current used to charge in 0.l second over the 3 square inches of the electrode used. The voltage gain is approximately a factor of 10, and is equal to the power gain of the device. In the embodiment shown in FIG. 1, in order to produce an acceptable image, the photoconductive grid has to stand off or hold without discharging a surface potential of about 300 volts. However, using the embodiment shown in FIG. 9A, having a second, all-metal grid 100, only a tenth of the voltage is needed at about the same current as before. Onetenth as thick a layer of photoconductor can be used on the photoconductive grid 102. The thickness of the photoconductor layer on the photoconductive grid 102 is the primary limiting factor of the device; therefore, the resolution can be theoretically increased by about a factor of 10.

If a thinner layer of photoconductive material on the photoconductive grid 102 is not desired, it is also possible to use, instead, a photoconductor with a higher dark current than was previously possible. This is particularly useful in extending the response of the system into the infrared, where most of the photoconductors are characterized by high dark currents.

By attaching the two grids I00 and 102 to each other through an occasional intermediary insulating spacer, there are some advantages for large stationary exposures. The definition is no longer strongly dependent on the spacing between the metal grid 100 and the record sheet 104 so that some bowing of the assembly due to gravity and/or electrostatic forces is allowable. The field between the field electrode 106 and the metal grid 100 exerts a force attracting the grid assembly to the record sheet 104. Due to the incremental current source characteristic of the device, this field can be increased to a high enough level to overcome the attractive force of the corona source 108 on the photoconductive grid 102. Thus, the arrangement shown in FIG. 9A, when using such spacers, is suitable for large spans without rigid support. The grid 100 is preferably metallic and will wear well even if it touches the dielectric surface of the record sheet 104. However, at these points, some contact-charged spots will occur in the image and some contour to the surface of the dielectric layer may be needed to minimize the areas of these spots.

The divorcing of the charging rate from the potential on the record sheet 104 causes an increase in the average charging rate, i.e., the charging proceeds at a uniform relatively high rate instead of tapering off. Thus, the additional grid effectively increases the current gain of the system. Any difficulty in employing papers and developers that would work well at low potentials can be eliminated by using the embodiment shown in FIG. 9A. It is possible to develop an insulating record sheet on a temporary conducting backing, such as an insulating paper on a metal sheet, and then remove the record sheet from the backing when the image is developed and fixed.

When using the embodiment of FIG. 9A with a scanning exposure step, a certain amount of difficulty is encountered. Normally, the system appears in either the one or the two grid versions (FIG. I and FIG. 9A, respectively), to have a rapid decrease in response between l0 and 100 cycles/second when the current is delivered to a metal plate. In the one grid system (FIG. I) the interaction between the charge on the record sheet and the rate of current delivery acts like a negative feedback loop and extends the frequency response at the expense of gain, producing acceptable scanned images. There is no evidence of such an occurrence in the two-grid system (FIG. 9A). The primary use of the embodiment shown in FIG. 9A is for relatively high resolution and high sensitivity stationary exposure of images. A typical use would be in making x-ray photographs or electrographs. In such cases some additional sensitivity may be secured by coating the metal grid 100 with an x-ray fluorescent electrically conducting coating. If the dimensions of the grids 100 and 102 are appropriate, they may be held together by a simple, tacky adhesive layer, preferably with an equally perforated interleaving material therebetween to hold the two grids a certain distance apart (usually about twice the distance between the openings in the grid). Moire patterns can then become a problem, but such problems are solvable by the methods used in color halftone patterns. In the visible spectrum, the two-grid device FIG. 9A is particularly adapted to stationary exposures for moderate-to-high resolution such as is required in a microfilm reader-printer or a hand-held camera.

The two-grid system of FIG. 9A can use any photoconductor that the single grid system of FIG. 1 can use. It is also possible to accomplish some compensation for the electrical properties of some photoconductors that is impossible to do in the single-grid system of FIG. I, specifically, there are some photoconductors that have sufficient resistance even when well exposed to develop a sufficient surface charge to allow some current to reach the paper, but when given an attractive bias, will attract the current and produce a clean image. Addi tional bias can compensate for resistance in the exposed areas of the photoconductors, rendering a usable iamge when at lower light levels, thus increasing the cf fective sensitivity of the system. Alternatively, photoconductors with very high impedance can be used if sufficient bias is used.

FIG. 9B shows an alternative arrangement of a twogrid system having the same electrical characteristics as the arrangement shown in FIG. 9A, but with improved image resolution. A foraminous insulating spacer 107 is coated on its two surfaces with metal electrodes 101 and 103, which thus form metal grids corresponding to grid 100 and the metal core of photoconductor coated grid 102 of FIG. 9A. It is necessary in applying metal electrodes 101 and 103 to ensure that the metal does not coat the inner walls of the foraminous insulating spacer 107. The spacer 107 may be made by drilling a regular array of small holes, typically 0.003 inches in diameter, on 0.005 inch centers, in a sheet of insulating plastic 0.006 inches thick. The thickness of the spacer should optimally be about twice the diameter of the holes. The number of holes per linear inch determines the resolution of the finished print, and the individual holes should have a diameter as large as is mechanically consistent with the center-to-center hole spacing. Metal electrode 103 is then coated completely with photoconductor 105 either by evaporation or spraying, taking care that the holes are not filled, but that the edges of electrode 103 are thoroughly covered. The use of the spacer 107 prevents any migration of charge from one hole to another in the low field region between electrodes 101 and 103 and thus improves the resolution of the finished print. There is no significant sideways migration of charge in the space between electrode 101 and receiving sheet 104 because of the high field in this region.

FIG. 13 shows a 3-grid system which provides a means for correcting frequency response at the expense of current gain and resolution. FIG. 13 shows a corona discharge electrode 140, an image grid means or a photoconductive grid 142, an insulating record sheet 144 on a grounded, conductive field electrode 146 and a metal grid 148 analogous to the metal grid 100 in FIG. 9A. The difference between the embodiment of FIG. 13 and that shown in FIG. 9A is the use of another metal grid 150, placed in front of the Z-grid system of FIG. 9A, the photoconductive grid 142 being closer to the rear grid 148 than to the front grid 150 and the grids 148 and 150 comprising the control grid means. The front grid 150 is provided with a slight repelling bias and acts to limit the current through the system. With a fixed bias on grid 148, the bias on grid 150 is set at such a value that at an input frequency of 100 cps a small change in bias in one direction will not affect the current while a small change in the other direction will affect the current. At lower frequencies, this is also the maximum current that can flow, while at higher frequencies a smaller current will flow for the same input amplitude.

In addition to the improvement in frequency response obtained with the arrangement shown in FIG. 13, this arrangement also has the advantage of limiting the excessive buildup of charge on the photoconductive grid I42. Referring to this advantage, the grid 150 can be referred to as a limiter grid. For its use as a limiter grid it can be much coarser (be of larger mesh) and have a spacing from the photoconductive grid 142 which is large compared with the spacing of the latter from the record sheet 144 or from the screen grid when the limiter grid is used in conjunction with a double-grid system. The limiter grid therefore does not interfere appreciably with the optical image falling on the photoconductive grid 142, and does not require a high degree of mechanical precision in its construction. The purposes of the limiter grid are: (l) to limit the potential on the surface of the unilluminated photoconductive grid 142 and therefore to prevent damage to the photoconductor which might result from exceeding the voltage tolerance of the photoconductor, and (2) to prevent bulging or arching of the center of the photoconductive grid by shielding it from the strong electrostatic field which is generated by the high-voltage corona wire.

The limiter grid is held at a potential, relative to the conductive core of the photoconductive grid 142, which is of the same polarity as the potential on the corona wire and which is of a magnitude roughly equal to the voltage which the photoconductor can withstand. As the surface of the photoconductor builds up potential to approach that of the limiter grid, the electrostatic field between the photoconductor surface and the limiter grid is such as to prevent further charging of the photoconductor. The potential across the photoconductor is therefore held to a safe value. The limiter grid can be used whether the photoconductive grid is followed directly by the record sheet or by other grids.

In some of the other embodiments of the subject invention the high voltage impressed between the photoconductive grid and the corona wire produces a strong electrostatic attraction which tends to bulge or arch the center of the photoconductive grid relative to its supports. This changes the spacing between the photoconductive grid and the record sheet or the following screen grid, which in turn alters the electrical characteristics and tends to degrade the definition in the electrostatic image, differentially across the field.

In this embodiment the limiter grid also acts as a shield between the control grid and the corona wire. The electrostatic attractive force is transferred from the control grid to the limiter grid, and the coarse limiter grid can be allowed to arch under this attraction, without appreciably affecting the control characteristics of the semiconductor grid or the definition in the electrostatic image on the record sheet.

In the above description of the 3-grid embodiment of FIG. 13, the system is primarily intended to operate with the photoconductive grid 142 potential approximately that of the rear grid 148 potential. When the system is operated in this manner, there is a significant current through the photoconductive grid 142, approximately equal to or up to twice the current being delivered at a maximum to the record sheet 144. The system can also be operated with the potential on the photoconductive grid 142 more nearly at the potential of the front grid 150. In this case, increasing the potential on the photoconductive grid 142 decreases the current arriving at the record sheet 144 giving a condition which is contrary to the mode of operation described above with respect to FIG. 13. In this mode of operation the current through the photoconductive grid 142 was much less than that of the plate current (the plate current is the current to the record sheet 144; this current has very nearly the same functional relationship to the grid voltages that the plate current does in a vacuum tube). Thus, there is an equivalent current gain between the photoconductor and the controlled ion flow. The current gain is high enough so that there is insufficient time for the photoconductor on the photoconductive grid 142 to become fully charged during a combined exposing and charging period. Therefore, a charging period preceding the exposure is desirable in this embodiment. It is also desirable to expose without having the ion flow transmitted to either the photoconductive grid 142 or the record sheet 144. Several alter-

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Classifications
U.S. Classification430/53, 430/68
International ClassificationG03G15/05
Cooperative ClassificationG03G15/051
European ClassificationG03G15/05A