United States Patent 3761173

In the formation of visible copies of an image, the use of an ion modulating array provided with an asymmetrical photosensitive coating together with means to electrostatically develop the ion image.

Fotland, Richard A. (Warrensville, OH)
Straughan, Virgil E. (Euclid, OH)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
399/159, 430/53
International Classes:
G03G5/00; G03G15/05; (IPC1-7): G03G15/00
Field of Search:
355/3,16,17 117
View Patent Images:
US Patent References:

Foreign References:
Other References:

Defensive Publication, T879,010, L. F. Frank, Oct. 1970, "Exposure Latitude in Electrophotographic Systems"..
Primary Examiner:
Greiner, Robert P.
Parent Case Data:

This application is a continuation in part of our earlier filed application Ser. No. 178,521 filed Aug. 27, 1971.
We claim

1. In the formation of visible copies of an image, an apparatus for modulating the flow of an ion beam in conformance with an optical image, which apparatus includes:

2. The apparatus of claim 1, wherein said array comprises a mesh screen, the openings through said screen comprising said ion-permeable locations.

3. The apparatus of claim 2, wherein said mesh is formed of wire.

4. In the apparatus of claim 1, means operatively connected to said element for moving it substantially at right angles to the direction of travel of said ion beam during operation of said apparatus.

5. In the combination of claim 1, a source of ions on one side of said element, a chargeable member on the other side thereof, and means for forming an optical image on said element.

6. The apparatus of claim 5, wherein said array comprises a mesh screen, the openings through said screen comprising said ion-permeable locations.

7. The apparatus of claim 6, wherein said mesh is formed of wire.

8. In the apparatus of claim 5, means operatively connected to said element for moving it substantially at right angles to the directions of travel of said ion beam during operation of said apparatus.

9. The combination of claim 5, in which said optical image is formed on the side of said element facing said source of ions.

10. The apparatus of claim 9, wherein said array comprises a mesh screen, the openings through said screen comprising said ion-permeable locations.

11. The apparatus of claim 10, wherein said mesh is formed of wire.

12. In the apparatus of claim 9, means operatively connected to said element for moving it substantially at right angles to the direction of travel of said ion beam during operation of said apparatus.

13. The apparatus of claim 1 including, in addition:

14. The apparatus of claim 1 including, in addition:

15. The apparatus of claim 1, in which said element is a non-conductor and is provided with a conductive layer thereon in electrically conductive relation to said photosensitive layer.

16. The apparatus of claim 15, in which said element comprises an organic filament woven mesh screen.

17. The apparatus of claim 15, in which said conductive layer is on one side of said element and said photosensitive layer is on the other side thereof, said layers making electrical connection with one another.

18. The apparatus of claim 1, wherein said element comprises a plurality of strands arranged to define a plurality of openings between strands, said photosensitive layer being on said strands and having substantially a crescent shape when viewed in a cross section taken perpendicular to the axis of said strands.

19. The apparatus of claim 18, in which said photosensitive layer is disposed of a sector of the periphery of said strands which is circumferentially offset from the top-to-bottom direction of said element by between about 15° and 75°.

20. The apparatus of claim 19, in which said angle is about 45°.

21. A screen comprising a plurality of strands arranged to define a plurality of openings between strands, and a photosensitive layer on said strands arranged asymmetrically around said openings.

22. The screen of claim 21, wherein the photosensitive layer on the strands, when viewed in a section taken as a plane perpendicular to the axis of each strand, is a crescent shape.

23. The screen of claim 22, in which said photosensitive layer is disposed on a sector of the periphery of said strands which is circumferentially offset from the top-to-bottom direction of said screen by between about 15° and 75°.

24. The screen of claim 23, in which said angle is about 45°.

25. The screen of claim 21, wherein said screen is a woven screen and said strands are metallic.

26. The screen of claim 21, wherein said strands are nonmetallic and said strands are coated on a first portion of their outer surface with a layer of photosensitive material and on a second portion of their outer surface with an electrically conductive layer, said layers making electrical connection with one another.

27. The method of fabricating a screen to modulate its ions discharged from a corona source and passing to an image receptor member in a copying apparatus, which comprises:

28. The method of claim 27, in which said angle is about 45°.

29. The apparatus of claim 1 wherein said element is an apertured plate.

30. The apparatus of claim 2 including means to supply fresh screen within an exposure region in order to provide for continued and repetitive operation of said apparatus.

31. The apparatus of claim 1 including, in addition, an image receptor surface, means to develop a visible image on said surface and means to transfer the visible image developed on said image receptor surface onto a permanent record member.

32. The apparatus of claim 6 including, in addition, a second screen positioned very closely adjacent to the chargeable member, and means to maintain said second screen at a suitable potential.

33. The apparatus of claim 31 wherein said chargeable member is a conductive drum coated with an insulating layer.

34. The apparatus of claim 1 wherein said array is and endless belt.

35. The apparatus of claim 1 wherein the electrically conducting element is comprised of a grid of parallel electrically conductive strands and alternate strands are coated with a photosensitive layer.

This invention relates to image reproduction and more particularly to a method and apparatus for the efficient formation of ion patterns corresponding to an optical image.

In conventional plain paper electrostatic photography, an insulating photoconductor is charged with a corona source of ions, exposed, the charge image developed with toner, the developed image transferred to plain paper, and finally, the toned image is fixed, generally by fusing. After the transfer operation, the residual image is erased from the surface of the photoconductor and the photoconductor is cleaned in preparation of a repetition of the process. Although employing plain paper, this process is complicated by the requirement for a number of different machine operations. In addition, the photoconductor suffers wear over a period of time, since the surface of the photoconductor is repeatedly rubbed by toner particles, cleaning brushes and paper surfaces.

A related process employs a photoconductively coated conducting paper. The photoconductor, generally zinc oxide (although organic photoconductors may be employed), is first charged, then exposed, and the image toned. Here the photo-conductor is not reusable and thus the wear and tear restrictions in the aforementioned process are eliminated. In addition, the machine operation, requiring four steps, is simplified. The disadvantage of this process is associated with the requirement for coating the paper with a photoconductor. These photoconductively coated papers are significantly more expensive than plain uncoated paper. In addition, because of the heavy photoconductor coating (the coating weight generally amounting to 20 pounds per 3,000 ft2 ream), the papers are heavy and have a feel quite different from plain paper.

A principal object of the present invention is to simplify the conventional plain paper electrophotographic process and the apparatus by which it is carried out.

Another object of the invention is to provide an image reproduction method wherein there is no physical contact of the photoconductor with either developer or paper.

In addition to having the advantages of eliminating photoconductor wear and simplifying the number of machine operations, the method and apparatus of this invention do not require a photoconductively coated paper. In comparison therefore to electrostatic copy processes employing photoconductive paper, the process of this invention has the advantage of lower paper cost through the use of a dielectric coated paper which has the feel, weight and appearance of a plain bond paper.

Another object of the invention is to provide an image copying means whereby photoconductor defects, attracted dust and the like do not appear in the final copy; these defects being integrated out during an exposure.

In the present invention, a fine mesh screen or grid coated with a photoconductor is employed to spatially modulate a flow of ions in accordance with an optical image projected onto said fine mesh screen or grid.

U.S. Pat. No. 3,220,324 discloses an apparatus and method of forming an electrostatic charge pattern conforming to an optical image on a chargeable member which includes an electrically conductive screen having a photoconductive layer thereon. In following the teachings of this patent, it is found that the control ratio, i.e., the ratio of ion current passing through the screen between nonilluminated and illuminated conditions, is quite low. Under optimum conditions, this ratio is near 2.

According to the present invention, a conductive screen or apertured plate which is coated asymmetrically with a photoconductor is utilized. Asymmetry, as employed in this sense, refers to a variation in photoconductor thickness with position around the periphery of the apertures in the screen or plate. (Presence and absence of photoconductor is an extreme case of thickness variation.) As a consequence of this asymmetry, it is possible to obtain a contrast ratio of several hundred.

One preferred embodiment of the present invention employs an insulating screen woven from a Nylon or Dacron monofilament which is first coated with an electrically conductive material and then coated, in an asymmetrical fashion, with a photoconductor.

The screen structure, the method of forming the screen and the apparatus for utilizing the screen for the formation of visible images will be more fully apparent from the description which follows taken with the drawings in which:

FIG. 1 is a schematic view of an apparatus for preparing electrostatic charge images, corresponding to a projected optical image, upon an image receptive surface;

FIGS. 2, 2A and 2B are cross section views of a modulating conductive screen illustrating the asymmetric nature of the photoconductive coating thereon;

FIG. 3 is a similar cross section illustrating the geometry of a dielectric or insulating screen coated asymmetrically with both a conducting layer and a photoconductor;

FIG. 4 is a fragmentary view of a section through a metal plate having a plurality of apertures and which is asymmetrically coated with a photoconductor;

FIGS. 3A and 4A are enlarged views, in section, of portions of FIGS. 3 and 4;

FIG. 5 illustrates schematically a means for moving the photoconductively coated screen and the corona wires during an exposure;

FIG. 6 illustrates a means for continuously supplying a fresh modulating screen during the operation of a copy device;

FIG. 7 illustrates a modification including a photoconductively coated screen in copy operations wherein the final image is formed upon plain paper, that is, paper which is not capable of sustaining a charge image; and

FIG. 8 illustrates an embodiment of the invention which employs a screen grid to electrically isolate a chargeable member's surface potential from the screen potential.

Referring now to FIG. 1, illustrating an apparatus for preparing electrostatic images on an image receptor surface, the apparatus comprises an electrically conductive platen 10 upon which is supported a conducting paper 12 having a thin dielectric coating 14. A corona modulating screen, grid or aperture plate 16 controls the ion current reaching the surface of the dielectric paper in accordance with an optical image projected onto element 16. A corona source is provided, which may comprise one or more fine wires 18. The corona operating potential is supplied by power supply 20. The paper support substrate 10 is maintained at a selected potential provided by power supply 21. Electronic controls 24 provide a means for simultaneously turning on power supplies 20, 21 and an illumination source for a projector 22. Projector 22 provides the image which is to be copied; this image being focused upon screen 16.

Although in this embodiment the optical image is provided by a projector such as might be employed in the projection of microfilm images to obtain hard copy, it will be understood that projector 22 could be replaced by a cathode-ray tube display using a projection lens system or by an original document support plus a projection system for conventional office copy, or any other suitable source of optical image depending upon the application of the apparatus. It should also be understood that, although these examples herein employ an optical (light) exposure, the input to the screen may consist of alternate forms of energy to which the photoconductor employed exhibits sensitivity. These other radiations generally include X-rays, gamma rays, and alpha and beta particles.

A single corona wire 18 is shown in FIG. 1. In order to provide a uniform corona over a large area, a plurality of corona wires may be utilized -- all connected in parallel to power supply 20. In order to provide sufficient corona current, the corona wire diameter should be less than 10 mils and to simplify handling of the wire, the wire diameter should be greater than 1 mil. A preferable wire diameter for this application is 2 mils. Using a single corona wire spaced approximately 1 inch above modulating screen 16, uniform charging, in accordance with the projected optical image, of the dielectric paper occurs over an area equal to the length of the corona wire and a distance between 1 and 2 inches normal to the direction of the corona wire at the paper. In order to provide for more uniform charging, the corona wire(s) may be moved, in a plane parallel to the screen, during the exposure.

A dielectric paper 12 is shown in FIG. 1, such papers being available from a variety of paper mills and being employed widely in high speed computer printers and recorders. The dielectric coated paper may be replaced with any of a wide variety of plastic films ranging in thickness of 0.1 to 5 mils. Images have been successfully formed on both polyester and acetate films; and, indeed, any film which has a dielectric relaxation time in excess of a few seconds and which falls within the aforementioned thickness range may be employed in the apparatus of FIG. 1.

Since, under conditions of toner development to completion, the final image density is proportional to the charge density on the dielectric coating; variations in dielectric coating thickness do not lead to variations in the final image density and thus the uniformity of the dielectric coating is not critical in this application.

Means for mechanically transporting the dielectric paper or plastic film under the corona modulating screen, maintaining said paper (film) stationary during the exposure, and then removing the paper from the imaging station are not shown in FIG. 1; these mechanical features being well known to those skilled in the art. FIG. 1 shows the corona modulating screen maintained at ground potential. In this event, the potential on the corona wire 18 and backing plate 10 must be opposite in polarity. Thus, if the corona wire is maintained at a positive potential, the backing plate must be maintained at a negative potential so that positive ions emitted from the corona wire are accelerated to the dielectric paper after passing through the meshes of screen 16. Alternately, the backing plate 10 might be maintained at ground potential, screen 16 at a positive potential, and corona wire 18 at an even higher positive potential.

A single corona wire may be mounted upon a carriage and, during an exposure, the wire may be made to traverse the screen thus providing a uniform corona current exposure at the screen. In this case, a scanning imaging system, well known in the art and employed in a number of commercial copy machines, may be utilized to project a traveling section of the image at the corona wire.

The potential required between corona wire 18 and screen 16 must be at least sufficient to initiate a corona current, i.e., at least 4 to 5 kv. The higher the potential the greater the ion current and hence the more rapidly dielectric paper may be charged and the lower the required exposure time. The upper limit of corona potential is realized when sparking occurs between corona wire 18 and screen 16. This is, of course, a function of the spacing between 16 and 18. Corona potentials as high as 25 kv have been successfully employed in this invention.

The potential required between screen 16 and backing plate 10 depends upon the spacing between said members and the required resolution of the electrostatic image formed on the charge supporting member. If the potential for a given spacing is too high, sparking will occur between the chargeable member and screen 16. Furthermore, at high potentials for a given spacing, the resolution of the charge image is sufficiently high so that a screen pattern corresponding to the screen 16 is observed in the charge pattern laid down on the chargeable member. A preferred electric field, in this region, is 20 kv per inch. This corresponds to an applied potential of 10 kv at a 1/2 inch spacing or 1 kv at a 50 mil spacing. At this electric field, the corona current passing through screen 16 and onto the chargeable member follows the field line sufficiently well so that a resolution of 6 to 10 line-pairs/mm is readily obtained with screens having from 325 to 500 meshes per inch. At electric fields in the range of 50 to 100 kv per inch, sparking occasionally occurs and the screen mesh pattern appears in the image. At fields below approximately 3 kv per inch, ion spreading is observed with subsequent degradation of image resolution.

The exposure times required are a complicated function of the corona voltage, corona-to-screen spacing, light intensity at the screen, nature of the photoconductor, and also the nature of the charge receiving member and the type of development employed in converting the electrostatic image into a visible image. In general, the required screen illumination ranges from 1 to 50 ft.-candles of tungsten illumination and the exposure times range from 0.1 to 3 seconds.

FIG. 2 is a cross-sectional view of a wire mesh screen coated with a photoconductor. The wire mesh 30 may be formed of any available metal or alloy, typical materials including brass, stainless steel, aluminum or phosphor bronze. The mesh size, i.e. the numbers of wires per linear inch, may range from 100 to 1,000. A 200 mesh screen will provide a resolution of 2 to 4 line-pairs/mm while a 500 mesh screen is capable of providing 7 to 14 line-pairs/mm. The photoconductive coating 32 is shown here as being offset by an angle of 45° from the normal. Hence one side of the mesh openings will have a thicker photoconductive coating than the other side (said other side may have no such coating at all). By forming the photoconductor in an asymmetrical manner such as this, much higher contrast ratios, i.e., ion current transmissivity ratio between dark and light areas, are obtained in comparison to when the photoconductor either totally surrounds the screen or is evaporated normal to the plane of the screen. Increases in contrast ratio are observed at deposition angles from the normal between 15° and 75°, the region of 30° to 60° providing the highest contrast ratios.

One preferred way of applying the photoconductor to the screen 30 is by vacuum vapor deposition. The material to be vaporized is placed in a crucible or metal container which is electrically heated. The screen to be coated is supported above the crucible at an angle, generally 45°, with the normal. The orientation of the weave of this screen is not critical. Thus, either the warp or woof of the weave may be parallel to the ground or run at any angle to the ground without adversely affecting the contrast ratio.

In FIG. 2A, a single element of the array is shown, in section, showing the manner in which the photosensitive coating 32 is disposed on the base 30 in order to produce the asymmetry relative to the mesh aperture peripheries. FIG. 3A is a similar view showing the disposition of both the photoconductive coating 37 and the electrically conductive coating 36 on the insulating filament base 34. FIG. 4A is a similar view of the array in FIG. 4.

FIG. 2B is a diagrammatic cross section of another suitable asymmetrical grid structure. Here, a large number of fine wires 30 are stretched to form a parallel wire grid. Typical wire diameters are 1 mil and the center-to-center spacings 2.5 mils. Only alternate wires are coated with a photoconductive layer 32. These alternate wires may be precoated by dipping in a molten selenium bath, or by vacuum vapor deposition, or any of a number of techniques for providing such coatings. The grid is formed by simultaneously winding the two wires (the one photoconductively coated and the other bare) on a mandrel to form the precise required grid structure.

FIG. 3 shows another embodiment of the present invention; the screen 34 being fabricated from an insulating material. Typical insulating materials employed in this invention are woven fabrics consisting of monofilament polypropylene, polyester or polyamide. Such woven fabric screens are available in mesh sizes to over 325 mesh, are extremely strong and are much less expensive than corresponding metal woven screens. In FIG. 3A, the dielectric mesh 34 is shown having a conductive coating 36 on the bottom and a photoconductive coating 37 on the top and offset from the normal by 45°, the two coatings overlapping or otherwise touching so as to produce an electrical connection therebetween. The conductive layer is preferably formed by vacuum vapor deposition of an electrical conductor, such as aluminum, gold or Nichrome.

Satisfactory results are obtained if the woven screen is replaced by a grid of closely spaced wires; the wires being formed either from conducting material, as illustrated in FIG. 2, or from insulating monofilaments which are subsequently coated with a conductive material, as shown in FIG. 3.

An alternate photoconductor support is shown in FIG. 4, which is a cross-sectional view of an aperture plate. The supporting plate 38, having a thickness in the range of 1 to 5 mils, may be fabricated by etching a plurality of holes through the surface and then coating the material with a photoconductor at an angle from the normal as shown in FIG. 4. Alternately, the plate 38 may be fabricated from a plastic sheet which also contains a plurality of holes etched in the surface. In the case of an insulating support sheet, a conductor would be deposited on the sides of the holes and bottom of the plate in a manner similar to that shown in FIG. 3.

The mesh screens may be woven with either a plain square weave or a twill square weave. With a twill square weave, however, the resolution in one direction is degraded slightly.

In many cases, a higher contrast ratio is observed if the screen is mounted so that the photoconductor coated side faces the corona wires. A somewhat lower contrast is obtained with the photoconductor coated side facing away from the corona wires.

One of the many advantages of the present invention in comparison to conventional electrostatic photographic systems involves a relaxation in the requirements for high dark resistivity of the photoconductor. A typical selenium xerographic plate or drum has a capacity close to 100 pf/cm2. If such a plate is charged to 500 volts and the allowable voltage decay must be 100 volts or less in a period of 1 second (the minimum time interval between charging and image development), then a simple calculation will show that the dark current through the plate must be less than 10-8 amp/cm2, or the plate dark resistance must be in excess of 5 × 1010 ohm/cm2 of plate area. In a typical screen modulation apparatus, as described herein, the corona current to the screen might be in the range of 3 × 10-6 amp/cm2. In order to provide effective modulation of the screen corona current, it is estimated that a voltage drop of at least 100 volts is required across the photoconductor coating of the screen. Thus, the photoconductor resistance in the dark must be in excess of 3 × 107 ohm/cm2 of the screen area. This represents a 1,000 fold reduction in the maximum dark resistance of the photoconductor coating the screen in comparison to photoconductors employed in conventional electrostatic photography. The relaxation of this constraint permits the utilization of a much wider range of photoconductor materials, particularly those having higher sensitivity and/or extended red response. Evaporated photoconductors such as zinc cadmium sulfide, zinc cadmium selenide and cadmium sulfide, which, in the vapor deposited form, have too low a resistivity for conventional electrostatic photography are suitable for preparing screens in the manner described in this invention. In addition, the selenium alloys having extended red light response such as selenium-tellurium alloys containing more than 10 percent tellurium and selenium-arsenic alloys containing at least 50 percent arsenic may also be employed in this invention.

Although the asymmetrical photoconductor deposition onto either a conducting or nonconducting screen may be readily carried out by vacuum vapor deposition, it is also possible to prepare photoconductive coated screens using photoconductor binder layers. One preferred method of forming such a screen is to spray, using an air gun, the photoconductor binder layer material on the screen; the spray being directed onto the screen at an angle. Either suitably doped and dye sensitized zinc oxide or doped cadmium sulfide dispersed in a suitable solvent with any of a number of appropriate binders may be sprayed onto the screen or mesh at the appropriate angle to form an effective asymmetrical ion control screen. As previously indicated, because of the requirement for a relaxation of the dark resistivity requirement, the concentration of photoconductor pigment in the binder that may be employed on screens is significantly higher than that which must be utilized for the standard electro-photographic processes. This permits the fabrication of higher photosensitivity surfaces.

Another alternate approach for preparing binder layer screens involves settling the powder onto a screen held at an angle away from the horizontal; such settling operation being carried out in a liquid bath. The settling operation is carried out by first supporting a screen, at an angle of approximately 45° to the horizontal, at the bottom of a large beaker. A suspension of photoconductor particles in a fluid is poured into the beaker and the particles are allowed to settle onto the screen under the influence of gravity. The binder may be either dispersed or actually dissolved in the fluid. After the particles have all settled onto the screen, the fluid is siphoned out of the beaker and the screen allowed to dry in the beaker.

Organic photoconductors in a binder layer may also be employed as photoconductors suitable for the present invention.

Photoconductors possessing a "memory" or high fatigue, such as certain types of ZnO, may be employed with this invention if it is desirable to separate the exposure from the actual time of charging an ion receptor member.

As previously mentioned, the screen pattern on the chargeable member may be eliminated by operating at electric fields sufficiently low so that the screen is not resolved on the chargeable member. In inexpensive commercially available wire or plastic monofilament screens having very small mesh sizes or high mesh counts, the weave is found to be somewhat nonuniform, i.e., there are small random variations in mesh spacing which result in mesh irregularities appearing in the image developed upon the chargeable member. These small irregularities, which appear as mesh lines in the copy, may be eliminated by moving the screen over a very slight distance during the exposure. One manner of effecting this motion is illustrated schematically in FIG. 5. Here, the modulating screen 16 and a series of corona wires 18 are mounted together on a rigid framework 40. This framework is supported in such a manner that it may be translated transversely from left to right. The frame is also spring loaded so that it is urged against cam 42 which is driven by low speed motor 44. Motor 44 is energized during the exposure so that the corona wires and screen move relative to the dielectric receptor sheet during the exposure. Motions as small as 0.1 inch are generally sufficient to eliminate all screens nonuniformities from the developed image. It has been found that the maximum screen velocity during an exposure is approximately 1 to 2 inches per second before image smear occurs, depending upon the intensity of the corona current to the screen and the nature of the response time of the photoconductive coating. The screen may be advantageously moved in two directions as by a circular or figure eight motion. In addition to eliminating screen variations from the developed image, the technique of moving the screen during an exposure also provides the advantage of eliminating the development of other screen defects such as random dirt and dust which settle upon the screen. The screen motion, in order to eliminate irregularities in the direction of both screen wires or monofilaments, should be such that the motion is not in the direction of either wire or monofilament. A preferred direction of the motion is at an angle of 45° with each wire. Besides a linear motion, orbital motion or a zigzag motion may be employed to successfully eliminate screen nonuniformities from the image and mechanism in place of cam 42 to provide such motions is readily available.

FIG. 6 illustrates a means for moving the screen during an exposure and for simultaneously providing for the replenishment of new screen within the exposure region. As shown screen 46, having a width of between 4 and 18 inches, depending upon the copy size desired, and coated asymmetrically with a photoconductor is supplied from supply drum 45. A takeup drum 47 collects the screen after it passes through the exposure area. In operation, during each exposure, the screen advances a distance of approximately 1/16 inch. In this way for every few hundred copies that are made, the screen is completely replaced with previously unused screen from drum 45. With many vacuum coated screens, several thousands of copies have been formed from each screen with no degradation of the process.

Thus far, in the description of this invention, the use of dielectric coated paper or plastic films have been indicated. In addition to these materials, papers fabricated from plastic (the so-called plastic papers) may also be employed in this invention. Conventional plain papers fabricated from cellulose generally contain a sufficient quantity of moisture and free ions so that these papers will not support an electrostatic charge image for the time intervals required to practice the present invention. By suitably treating plain papers, however, images may be formed using this invention. Plain bond papers or plain blade-coated papers may be rendered sufficiently insulating by first heating the papers to a temperature of between 120° and 200° C for a period of a few seconds. This may be carried out in a small oven. Immediately after removal from the oven and while the paper is cooling down, the paper is wet with a hydrocarbon. A preferred material for this application is the aliphatic hydrocarbon solvent known under the tradename of Isopar, manufactured and marketed by the Humble Oil & Refining Company. Paper so treated is capable of sustaining an electrostatic charge on the surface for long periods of time. A lateral surface conductivity is still present, however, so that once a charge image has been placed on the surface of the paper, the development must be carried out within a period of 1 to 2 seconds if excessive resolution degradation is to be avoided.

The latent electrostatic image formed by the corona modulation screen may also be employed in recording an image using a deformable thermoplastic film composed of polystyrene, Staybelite, Piccolastic or other deformable synthetic polymer material. After the formation of an electrostatic image on the film surface, the latent image is developed by softening the film by exposure to either heat or solvent vapors as is well known in the art.

In addition to providing a permanent image, the corona modulating screens of this invention may also be employed with electric field sensitive cholesteric liquid crystal films in display applications. Here, the dielectric coated paper of FIG. 1 is replaced by a liquid crystal film and the support platen 10 replaced by a glass sheet having a transparent conductive coating on the side adjacent the film. Under the influence of an electric field provided by ions reaching the free surface of the liquid crystal film, the optical scattering and/or reflective properties of said film are modified, leading to the formation of a visible display on the film. Cholesteric materials suitable for this application are described in British Patents 1,123,117 and 1,167,486, and also by L. Melamed and D. Rubin, Appli. Phys. Lett. 16, 4, 149 (1970) and by J. J. Wysocki, J. Adams, and W. Haas, Phys. Rev. Lett. 20, 19, 1024 (1968).

FIG. 7 illustrates an apparatus in which the toned image is first formed on an intermediate endless belt and subsequently transferred to a plain paper sheet or web. In this drawing an endless plastic belt 62, preferably fabricated of polyester and containing a conductive coating on the inside surface is supported on rollers 64 and 70. An electrostatic image is formed on this belt using a corona modulation screen, corona wire and projection source in a manner similar to that shown in FIG. 1. After the electrostatic image has been formed, it is developed by immersion in liquid developer tank 66 In the region between roller 64 and 70, the developed image is partially dried and then offset onto a paper sheet or web 72 as the paper and plastic film are held in contact by rollers 70. The image is fixed on the paper and some residual solvent removed as the paper is heated by radiant heater 74. Cleaning brush 76 removes residual toner from the plastic endless belt.

Rather than employ the endless plastic belt (as shown in FIG. 7), a conductive drum coated with a hard insulating surface, such as a glass-based enamel, may be employed. In apparatus employing a drum, the operational steps are the same. The electrostatic image is formed using a corona wire and a corona modulating screen; the electrostatic image is toned, employing either a dry or liquid electrostatic developer; the image transferred by offset to a plain paper sheet or web; and the drum cleaned. This apparatus is rather complicated but does possess several advantages over conventional plain paper electrostatic photography. A principal advantage is the fact that the photoconductor is never in physical contact with either a developer material or paper and, hence, is not subject to the usual wear which occurs in standard electrophotographic plain paper copies. An insulating surface enameled drum possesses a hard abrasion resistant surface and hence has a lift significantly greater than a typical selenium drum. Devices employing an endless plastic belt would be subject to a higher degree of wear; however, the belt may be readily changed and is relatively inexpensive compared to a selenium drum.

It has been found that when the surface of a charge receptor member is charged to voltages high in comparison to the voltage existing between screen 16 and backing plate 10, image distortion occurs. This distortion arises from fields at the surface of the chargeable member; these fields, existing between a charged and uncharged region. This results in corona generated ion beam bending in a manner so as to reduce the width of uncharged lines. Secondly, when a high potential is built up in a significantly large area, a reduction in the local field immediately below screen 16 occurs in this region with subsequent diffusion of ions passing through the screen in this region. This effect is not serious for low charging voltages, particularly when high potentials are applied between screen 16 and backing electrode 10. In charging relatively thick plastic films, which requires high surface voltage potentials (several thousand volts, for example, in case of 3 to 5 mil polyester or acetate film) these distortions are observed.

A means for circumventing this problem is shown in FIG. 8. This apparatus is identical to that shown in FIG. 1 but includes a second find mesh screen 100 whose potential is established by power supply 102. This fine mesh conducting screen is spaced very close to the surface of the chargeable member, generally within a distance of 5 to 25 mils. The screen potential, as established by power supply 102, is maintained between the potential of backing plate 10 and screen 16. This screen serves the same function as a screen grid in a conventional tetrode electron vacuum tube; its function being to isolate the potential at the surface of a chargeable member from potentials existing in the region between screen 100 and screen 16. Thus, surface potentials may be built up without resulting in the ion beam diffusion and distortions mentioned previously. We have found that, because of the high system resolution, moire patterns are formed in the image corresponding to screen mesh overlap between screen 100 and screen 16. In order to eliminate this problem, screen 100 may be vibrated or caused to move by employing a motor and cam assembly 104 operating in a manner similar to that shown in FIG. 5.

The following examples illustrate the techniques of the method, process and apparatus described in this disclosure. These examples are not meant to be restrictive in any way, however.


A plain square weave 400 mesh phosphor bronze screen was stretched over a square brass frame whose inside dimension was 4 inches on a side and whose outside dimension was 5 inches. The phosphor bronze screen was soft soldered onto the frame. The frame was mounted in a vacuum coater an average distance of 12 inches from a quartz crucible mounted in tantalum heater. The screen was inclined 45° from the normal. A charge of 30 grams of xerographic grade selenium was placed in the evaporation crucible. The system was evacuated to a pressure of 10-5 torr and the selenium evaporated from the boat onto the screen over a period of 45 minutes. During the evaporation, the screen was heated, with an electrical heater, to a temperature of 90° C. The selenium coating thickness was found to be 25 microns.

The screen was removed from the vacuum evaporator and mounted in the apparatus shown in FIG. 1. A 6 inch corona wire comprised of a 3.5 mil thick diameter platinum wire was supported a distance of 1 inch above the screen. The screen to conducting platen spacing was 1/2 inch.

The contrast ratio, defined here as the ratio between the ion current to the conductive backing plate 10 with the photoconductive screen in the dark and ion current with the same screen illuminated, was determined by connecting a Keithley Model 600A electrometer between paper supporting electrode 10 and power supply 21. At a counterelectrode potential of -5 kv and a corona potential of +16 kv, the dark current was 24 μamperes and the current obtained when the screen was uniformly illuminated with tungsten illumination at a level of 10 foot-candles was 0.3 μamperes. The contrast ratio was thus 80. At a corona potential of +12 kv, the dark current was 11 μamperes and the light current was 0.15 μamperes; yielding a contrast ratio of 75.

It may be seen from the aforementioned measurements that a higher contrast potential is obtained at lower corona potentials. In this event, however, the corona current is lower, and longer exposure times are required to charge the dielectric paper. At a screen-paper separation of 1/2 inch, an applied potential of -3 kv is sufficient to accelerate the ions to the surface of a dielectric coated paper and still maintain a resolution of 3 line-pairs/mm in the developed image.

Copies of a projected image were obtained by placing sheets of dielectric coated paper on the counterelectrode 10. An image having a high-light brightness of 10 foot-candles was projected on the screen with a simultaneous application of corona and counterelectrode potentials; the total exposure time being 3 seconds. The paper was then removed from the counter-electrode and immersed in a beaker of liquid electrostatic toner containing positively charged particles and having a solids concentration of 1 percent. Since the paper surface was charged positively and since the liquid developer toner particles are also positively charged, a reversal image was obtained. When the same image was projected on the screen and the resulting latent electrostatic image developed in liquid toner with negatively charged particles, then the developed image was a positive image, i.e. black characters on a white background as exhibited by the positive original. After the paper was removed from the developer, excess liquid was squeegeed from the surface and the paper dried in an air stream which, in accordance with a preferred mode, was heated. The image was of high quality, having negligible background and a maximum density of 1.1. The development time was 3 seconds.


A selenium coated ion current modulating screen was prepared in a manner identical to that of Example 1 with the exception that the evaporation was carried out with the screen mounted normal to the line of evaporation. When evaluated in the apparatus of FIG. 1, it was found that the dark current was 7.3 μamperes and the current, with an illumination level of 10 foot-candles, was 4.6 μamperes; providing a contrast ratio of 1.6. A number of attempts were made to obtain satisfactory copies of the light image in a manner described in Example 1. In no case was it possible to obtain a high contrast between light and dark areas on the paper. High background levels were obtained together with low image density.

Example 2 illustrates results obtained employing the teachings of U. S. Pat. No. 3,220,324. This example is included to indicate the advantages realized when one practices the teachings of the present invention.

The following examples illustrate the diversity of photoconductor materials, deposition methods, and geometry of screen arrangements suitable for utilization in the present invention. ##SPC1## ##SPC2##


In order to further demonstrate that symmetrically coated photoconductor screens are inferior to asymmetrically coated screens, a 6 inches × 6 inches 325 mesh stainless steel screen was selenium coated on both sides by a technique designed to produce a uniform or symmetrical deposit on the wires. The screen was mounted in a motor driven rotating jig at a 45° angle with respect to the evaporating selenium direction. Average distance from boat to screen was 15 inches, and 60 grams of selenium were evaporated from the boat held at 260° C. Once the evaporations proceded to completion, the screen was reversed at the same angular orientation and coated in an identical manner on the opposite side. This reversal of the same angular orientation resulted in uniform coverage of the screen wires with photoconductor, producing a symmetrically coated screen. Test conditions were: corona voltage, -14 kv at 1 inch spacing; screen at ground potential; accelerating plate potential, -5 kv at 1/2 inch spacing; light level 140 foot candles, steady state measurement. The contrast ratio was found to be only 2.1 on one side of the screen and 2.6 on the other.


Several screens were asymmetrically coated on both sides with photoconductor, one example of which is described below. A framed 325 mesh stainless steel screen with an area 50 cm2 was coated with selenium by vacuum deposition at an angle of 45°, the boat charge being 60 g. The screen was then reversed and rotated 180° and only 5 g of selenium was evaporated onto the opposite side. This coating arrangement led to an exaggeration of the asymmetry of the deposit with respect to the opening as viewed from either side of the screen. Steady state test measurements under the conditions noted in Example 11 showed the contrast ratio of the 60 gram side of the screen to be 900 at 11 foot candles. The opposite or thinner coated side indicated a lesser though relatively high contrast ratio of 32 at 7 foot candles.


Several screens were prepared to determine the effect of angle of deposition for achieving asymmetry. Type 304 plain weave, 325 mesh stainless steel screen was stretched and cemented to aluminum frames 7 inches × 7 inches O.D. with 6 inches × inches open screen area. Five screens were cleaned by solvent vapor degreasing and subsequently selenium coated by vacuum, evaporation under identical conditions except for angle of deposition. Conditions were as follows: substrate temperature, 90° C; selenium charge, 65 grams; target-to-boat distance, 18 inches; evaporator boat temperature, 250° to 270° C; pressure 2 × 10-5 torr. Deposition angles, as measured from the normal, were 15°, 30°, 45°, 60° and 75°. The results are shown below in Table II. While the angle is not critical, the optimum value is between 30° and 45°. Test conditions were: corona voltage, +14 kv at 1 inch spacing; screen at ground potential; accelerating plate potential, -5 kv at 1/2 inch spacing; light level, 140 foot candles while chopped at a rate of 100 cycles/minute. ##SPC3##

Without intending to limit the present invention to a specific mechanism, it is believed that the asymmetrical ion current modulating screen functions through the control of ion transport through the screen apertures by a transverse electric field, i.e., a field set up in a direction of the plane of the screen which arises due to the asymmetrical nature of the photoconductive coating on the screen.

While but a limited number of embodiments of the present invention have been here disclosed, it will be apparent that many variations may be made therein without departing from the spirit of the invention as defined in the following claims.