US3531646A - Enhancement of electrostatic images - Google Patents

Enhancement of electrostatic images Download PDF

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Publication number
US3531646A
US3531646A US582858A US3531646DA US3531646A US 3531646 A US3531646 A US 3531646A US 582858 A US582858 A US 582858A US 3531646D A US3531646D A US 3531646DA US 3531646 A US3531646 A US 3531646A
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layer
image
charge
semiconductor
field
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US582858A
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Benjamin Kazan
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Xerox Corp
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Xerox Corp
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    • G—PHYSICS
    • G04—HOROLOGY
    • G04B—MECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
    • G04B21/00—Indicating the time by acoustic means
    • G04B21/02—Regular striking mechanisms giving the full hour, half hour or quarter hour
    • G04B21/027—Regular striking mechanisms giving the full hour, half hour or quarter hour with locking wheel
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G13/00—Electrographic processes using a charge pattern
    • G03G13/04—Exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material
    • G03G13/045—Charging or discharging distinct portions of the charge pattern on the recording material, e.g. discharging non-image areas, contrast enhancement
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S430/00—Radiation imagery chemistry: process, composition, or product thereof
    • Y10S430/001—Electric or magnetic imagery, e.g., xerography, electrography, magnetography, etc. Process, composition, or product
    • Y10S430/102—Electrically charging radiation-conductive surface

Definitions

  • Electrophotographic processes are invariably characterized by the presence at one point in the practice thereof, of an electrostatic latent image, which image may or may not be subsequentially developed to render a visually discernible counterpart.
  • the formation of the electrostatic latent image may thus be regarded as the sine qua non of xerographic processes, and enormous amounts of research effort have accordingly been expended in improving both electrostatic imaging systems and the methodology utilized therewith. While great progress has resulted from such efforts, in one respect at least, electrophotography has lagged far behind the more conventional silver haloid photographic technology: reference here is made to the all too well known fact that the light response achievable with Xerographic imaging systems is of a low order of magnitude in comparison to that possible with light-responsive chemical emulsions.
  • the latent electrostatic imagewhen formed upon suitable photoreceptors- may itself be utilized in a unique feed-back process to augment the charge variations present in the image.
  • the discovery derives from the observation that a latent charge pattern can be formed contiguous to semiconductor materials of a type displaying, by field-effect action, varying conductivity levels in response to the electric field associated with the charge pattern.
  • a latent charge image is formed on common zinc oxide-coated paper, as the Zinc oxide itself happens to be a semiconductor of the type alluded to and responds to a latent charge image on the surface thereof, by displaying conductivity variations of the type indicated.
  • a latent image is initially formed on a photoconductor surface contiguous to such a semiconductor.
  • the semiconductor may itself comprise an effective photoconductor in which case the photoconductor surface directly bounds the semiconductor.
  • the image-bearing surface and abounding semiconductor layer are then brought into contact with an electroluminescent layer and means are provided to establish an'electric field across portions of the electroluminescent layer and the sandwiched semiconductor.
  • the electric field is selectively intensified and in some instances deflected by the presence in the semiconductor layer of volumes of material which-in accordance with the foregoing pattern of the latent electrostatic imagedisp1ay relativley increased conductivity in the such selective field intensification and/or deflection specifically acts to induce increased electroluminescent in portions of the electroluminescent layer adjacent charged areas of the latent image.
  • the electroluminescent in turn impinges upon adjacent areas of the photoconductor surface itself to further reduce its impedance.
  • the overall reaction is a feed-back phenomena according to which partially discharged areas on the photoreceptor surface become increasingly discharged as contact with the electroluminescent layer is maintained under the influence of suitable potentials between the activating electrodes at the back side of the electroluminescent layer.
  • FIG. 1 graphically depicts the manner in Which the present invention is practiced where initial formation of the faint electrostatic image is on zinc oxide-coated paper.
  • FIG. 2 shows on a highly magnified scale a portion of the FIG. 1 depiction and illustrates the mechanism involved in the instant invention.
  • FIG. 3 is a schematic electrical equivalent circuit showing the basic electrical phenomena in FIGS. 1 and 2.
  • FIG. 4 graphically depicts application of the present method with an alternate electroding scheme.
  • FIG. 5 graphically depicts another suitable surface upon which the latent electrostatic image may initially be formed.
  • FIG. 6 shows a portion of FIG. on a magnified scale and illustrates the mechanism by which conductivity variations arise in the semiconductor layer depicted in FIG. 5.
  • FIG. 1 graphically depicts a basic technique by which the present invention may be practiced.
  • an electroluminescent panel 3 is shown having a structure peculiarly adapted for use in practice of the present invention.
  • the panel 3 includes an insulating base 5 which usually will comprise a transparent material such as, for example, glass.
  • Typical phosphors utilized include copper chloride-activated zinc sulfide.
  • a composition may be utilized containing approximately 80% zinc sulfide and zinc selenide, with copper as an activator.
  • the particular phosphor composition utilized will be chosen with some variation depending upon the wavelength response of the particular photoreceptor utilized in conjunction with the invention.
  • the thickness of electroluminescent layer 7 is made quite low, usually being of the order of 1 mil or less.
  • Electrodes 8 Deposited directly upon the base layer 5 and there fore between the base layer and the adjoining electroluminescent layer 7 are a series of conductive strip electrodes 8. As adjacent strips will ordinarily be operated at opposite potentials, simplicity of wiring may be gained by depositing the electrodes as an interdigitated pattern,
  • the electrodes may by way of example be formed upon the base 7 from evaporated gold or the like, or similarly vacuum deposition techniques may be utilized to lay down apattern of copper or the like.
  • An essential step in the practice of the present invention involves formation of a latent electrostatic image on a surface contiguous to a semiconductor layer of the type displaying conductivity variations in response to an electric field imposed thereon.
  • a layer of semiconductor material exhibiting this phenomenon will be referred to as a fieldeffect layer in consideration of the use of such semiconductor layers as the essential performing element in the so-called field effect transistors.
  • a large number of semi-conductor materials are known which exhibit the specified characteristics and an extensive list may be found, for example, at page 9 of Field Effect Transistors edited by Wallmark and Johnson, Prentice Hall, Inc., Englewood Cliifs, NI, (1966).
  • the semiconductor material comprising the layer may itself be chosen to possess substantial photoconductivity; or alternatively a highly insulating photoconductive material may be bonded to the semiconductor layer and the latter element may then constitute the situs for the latent electrostatic image.
  • a subclass of materials particularly suitable for use as the field effect layer of the present invention will hereinafter be referred to as storing semiconductors, the term serving to define semiconductor materials adapted to retain electrostatic charge on their surface, to conduct current through the central portion thereof without substantially dissipating such charge and to dissipate such charge in response to impinging radiation.
  • Zinc oxide is the best known example of the materials in the defined subclass; however in addition to zinc oxide there are other materials such as lead oxide and cadmium oxide which exhibit similar characteristics.
  • the photoreceptor 13 is a common sheet of zinc oxide-coated paper and thus includes the paper base 15 and the zinc oxide layer 17.
  • the latter as is well known in the art comprises a zinc oxide pigment dispersed in a relatively transparent binder.
  • a latent electrostatic image may be formed in the usual manner. This is to say that the photoreceptor 13 may be positioned on a conductive grounded backing 35, the zinc oxide surface may be initially charged to uniform negative potential as from a negative corona source or the like, and thereafter the charged surface may be exposed to a projected optical image.
  • the resulting latent electrostatic image is suggested by the reference numeral 19.
  • the optical input supplied to the surface of the illustration is of relatively low intensity so that the elecrostatic laten image 19 is quite faint; in more precise terms this implies that the variation in charge from point to point on the zinc oxide surface is relatively small; that is, the potential variation from point to point is minor in nature.
  • the object of the present process is now to amplify the relatively slight potential variations present from point to point in the faint latent image so that a more readily utilizable image results. This is brought about in the manner depicted in FIG. 1 which shows the photoreceptor 13 bearing latent image 19 in virtual contact with the outside face of the electroluminescent layer 7. In actual practice the contact between the two surfaces would be real but for purposes of illustration slight separation is depicted in the diagram.
  • a switch 10- may be provided for such purposes.
  • pairs of electrodes of opposite polarities such as 2 and 4 are geometrically positioned with respect to each other, such that in the absence of contact with the photoreceptor 13 the electric field therebetween would be essentially parallel to the lower face of electroluminescent layer 7. In such event little penetration of the electric field into the luminescent layer 7 would occur.
  • the interelectrode capacitance between members such as 2 and 4 is quite low because of the relatively high spacing and the electric field there between is relatively low for a given potential difference.
  • the layer 17 is a field-effect layer in the sense that this term has previously been defined, conductivity variations will be exhibited in the volumes of the layer immediately below the latent charge pattern, and these variations will occur in accord with the charge pattern itself.
  • volumes of relatively decreased conductivity will be present in the zinc oxide layer 17 below areas of relatively high negative charge since such negative charge acts by field effect action to reduce the number of conduction electrons in the body of the zinc oxide semiconductor immediately adjacent the deposited charge.
  • the areas previously struck by light will accordingly be relatively conducting in comparison to volumes of the zinc oxide layer below surface areas not struck by lightthat is to say below surface areas still holding substantial quantities of negative charge.
  • FIG. 2 depicting a small section of the FIG. 1 setup.
  • an area 23 is shown on the zinc oxide layer 17 relatively free from charge.
  • a second area 25 is also shown containing a relatively high concentration of negative charge. Adjacent the area 25 the volume of zinc oxide has become relatively non-conductive in comparison to the corresponding volume of zinc oxide below the uncharged area 23.
  • the electric field between electrodes 8 and 12 is deflected little if all and this is suggested by the arrow 21 intended to represent the electric field present between these electrodes.
  • the field between electrodes 6 and 8 being adjacent to more conductive portions of the zinc oxide layer 17 is deflected in a manner suggested by the arrows at 2,4.
  • the field between electrodes 6 and 8 is thus caused to penetrate through the electroluminescent layer 7; the field is furthermore greatly intensified by the increased capacitance now present across the layer; and electroluminescence develops in this area.
  • the apparatus equivalent circuit in FIG. 3 would include only the potential source 11, the representative electrodes 6 and 8, and the single conductive path 12 therebetween.
  • the path 12 comprises in series a fixed resistance 18 and a fixed capacitance 10, the latter being indicative of the interelectrode capacitance between electrodes 6 and 8.
  • the last named factor-the interelectrode capacitance is quite small.
  • a parallel conductive path 14 may be considered to have been established.
  • the current flowing between electrodes 6 and 8 is obviously increased immediately by provision of any parallel path; however it should be noted that since the layer 7 is very thin the new capacitance 16 is far in excess of the capacitance 10. Accordingly the current flowing between the electrode pair is not only increased but increased greatly so, and the increase in electroluminescence may be accordingly attributed principally to the capacitance 16.
  • electroluminescence develops in layer 7 principally in those portions of the layer adjacent areas on the semiconductor field-effect layer which have previously been struck by light; that is to say, areas on the zinc oxide surface showing some degree of discharge. Since the electroluminescent panel 3 and photoreceptor 13 are in virtual contact the luminescence created impinges directly upon such areas of already diminished charge. The luminescence further reduces the charge level, and thus begins a positive feedback cycle serving to reduce lower and lower the charge levels in those areas originally exhibiting diminished charge.
  • FIG. 4 a modification of the FIG. 1 setup is shown which illustrates a quite different electroding arrangement for providing the conditions under which the present invention may be practiced.
  • a latent electrostatic charge image is formed upon the surface of the photoreceptor 13, which as in FIG. 1 includes a field-effect semiconductor layer 17 of the storing semiconductor variety, which may specifically comprise a zinc oxide coating of the order of 1 mil or less thickness.
  • the zinc oxide is not upon a paper-like base but rather is directly afiixed to a conductive substrate 36.
  • the latter may be of any convenient thickness; however, it is convenient to consider conductive substrate 36 as constituting a thin conductive foil of aluminum or the like.
  • a commercial product including a zinc oxide layer backed by such a conductive foil is in fact available commercially from the 3M Company of Minneapolis, Minn. It may be assumed that the latent charge image has previously been formed upon the surface 37 of the zinc oxide layer 17, after which the member 13 is brought into contact with the electroluminescent panel 38.
  • the latter includes an insulating support base 39, which may be glass, upon which is coated a thin conductive layer 40 such as of tin oxide.
  • the combination of a glass support base with such a conductive tin oxide coating is in fact available commercially under the trade name NESA from the Corning Glass Works, Corning, NY.
  • a thin layer 45 of electroluminescent material Directly deposited upon the substrate 39 and conductive coating 40 is a thin layer 45 of electroluminescent material, usually of the order of 1 mil or so, which constitutionally is similar to the layer 7 described in connection with FIG. 1.
  • An AC potential source 11 is connected through a switch 10 to the electrodes provided by conductive base 36 and conductive coating 40.
  • the sandwich structure shown in FIG. 4 is established, after which switch 10 is closed for a time period appropriate for the degree of enhancement desired.
  • the electroding arrangement in the FIG. 4 showing is such as to at all times provide an electric field directly transverse to the sandwich structure shown.
  • the primary physical mechanism producing the image intensification to be one of selective field intensification across those portions of the sandwich which include volumes of material within the zinc oxide layer 17 which display conductivity.
  • Such selective field intensification produces selective electroluminescence in the layer 45, which in turn acts in the same feedback manner as has been previously described in connection with FIG. 1 to augment the charge variations present at surface 37.
  • the present inventive method is particularly adapted for use with the zinch oxide photoreceptor described in connection with FIG. 1.
  • This is indeed a most fortunate result in view of the fact that zinc oxide coated papers are among the commonest, cheapest, and most efiec tive xerographic materials, and in view further of the fact that these same zinc oxide materials display somewhat lower photosensitivity than other common xerographic receptors such as selenium and are therefore just those materials which are most in need of latent image intensification.
  • the present invention is no way limited to utilization with zinc oxide or similar storing semiconductors but may be utilized in other invironments, providing only that the initial step in practice of the process involves formation of the latent electrostatic image on a photoconductive surface contiguous to a field-elTect layer. To illustrate this point a depiction of another type of photoreceptor is shown in FIG. 5, which receptor may be similarly utilized in the present invention.
  • a photoreceptor 13 which now consists of two distinct layers plus a conductive support layer 30.
  • 31 is the field-effect layer corresponding partly in function to the zinc oxide layer 17 of FIG. 1.
  • this field-effect layer 31 is not a storing semiconductor as is the ease with zinc oxide but rather a more general case is considered wherein the semiconductor material comprising layer 31 is neither an excellent photoconductor nor displays the ability to retain charge on its surface directly for sustained periods of time.
  • layer 31 would by way of example comprise a vacuum-deposited layer of cadmium sulfide of the order of microns or so thickness.
  • the photoconductive surface contiguous to the semiconductor now takes the form of a separate layer 32 of vitreous selenium.
  • An additional thin insulating interface layer may also be used between layers 31 and 32 to discourage charge injection from the photoconductor to the semiconductor, however this is not always necessary.
  • the photoreceptor 13 is positioned in contact with a ground plane 35 and is initially charged by a corona source or the like to a uniform potential.
  • a uniform negative potential is utilized for reasons that will become apparent momentarily.
  • the uniformly charged surface of selenium layer 32 is therafter exposed to an optical image to form the latent electrostatic image 19.
  • the exposure is of relatively low light intensity with the resulting latent image 19 being therefore relatively faint; which is to say that the potential variations induced by the light exposure from point to point on the charged surface are of relatively small magnitude.
  • FIG. 6 depicts in detail a small section of the FIG. 5 showing and is intended to detail the mechanism by which conductivity variations are caused in the semiconductor field-effect layer 31 by exposure of the uniformly charged selenium surface to light.
  • this depiction we assume that light has struck the surface of layer 32 approximately in the area designated at 42, while the area at 41 has remined in darkness. In the latter area the charge pattern designated by the minus signs has accordingly remained essentially unaltered, whereas in the former area the selenium has been rendered conductive with the result that charge has been partially dissipated at such points.
  • the conductivity in volumes of the semiconductor below area 42 such as 44 will be increased by virtue of the decreased electric field.
  • the weak electrostatic image may be placed into contact with the electroluminescent panel 3 and the precise procedure followed as was described in connection with FIGS. 1 and 2. Since increased conductivity is present in those areas which have been partially discharged by the action of the light input image, activation of the potential source 11 (FIG. 1) will produce selective luminescence in those parts of the panel adjacent to volumes of increased conductivity-such as 44 in FIG. 4. A positive feed-back action is thus once again set up which serves to increasingly discharge those areas where initial charge dissipation has already occurred, with resulting intensification of charge differences between varying portions of the latent electrostatic image.
  • the present invention has thus far been principally described in connection with embodiments suitable for image enhancement. This is because in the most frequent caseespecially where photoconductors are involvedthe problem is more often one of low gamma characteristics, rather than of too'high gamma response.
  • the same technique may be utilized with but simple modifications to produce a degenerative or negative feedback to the latent image so that less rather than more variation is present in charge density between portions of the photoconductor struck and not struck by light. Basically all that is necessary to achieve such a result is to utilize a proper type semiconductor and charge polarity so that dissipation of charge lessens conductivity in the semiconductor rather than increases it. Such a result for example may be brought about in FIG. 4 by merely utilizing an initially positive charge on surface 37, rather than the negative charge shown.
  • a method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:
  • said semiconductor layer is a storing semiconductor whereby said photoconductor surface bounds said semiconductor layer.
  • a method according to claim 2 wherein said storing semiconductor comprises zinc oxide in a binder.
  • said photoconductive surface comprises the non-adjacent face of a selenium layer positioned adjacent said semiconductor layer.
  • a method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:
  • a method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:
  • a method for amplifying charge variations in a latent electrostatic charge image on a photoconductive surface comprising:
  • said semiconductor layer comprises zinc oxide in an insulating binder.
  • a method for amplifying charge variations in a (a) forming said latent electrostatic image on a surface of a photoconductive insulating material

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Photoreceptors In Electrophotography (AREA)
  • Printers Or Recording Devices Using Electromagnetic And Radiation Means (AREA)
  • Electrophotography Using Other Than Carlson'S Method (AREA)
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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4877699A (en) * 1988-08-25 1989-10-31 Eastman Kodak Company Electrophotographic luminescent amplification process
US10689754B2 (en) * 2017-09-05 2020-06-23 Peter C. Salmon Programmable charge storage arrays and associated manufacturing devices and systems

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2927234A (en) * 1955-11-25 1960-03-01 Rca Corp Photoconductive image intensifier
US3169192A (en) * 1960-05-14 1965-02-09 Philips Corp Negative picture radiating apparatus
US3186839A (en) * 1962-04-10 1965-06-01 Dick Co Ab Light-to-electrostatic-image converter and process for manufacturing same
US3264479A (en) * 1955-01-31 1966-08-02 Sylvania Electric Prod Electroluminescent light amplifier
US3322539A (en) * 1962-11-30 1967-05-30 Gen Electric Electrophotographic process
US3348074A (en) * 1964-07-01 1967-10-17 Philips Corp Photosensitive semiconductor device employing induced space charge generated by photosensor
US3441736A (en) * 1965-06-01 1969-04-29 Electro Optical Systems Inc Image intensifier including semiconductor amplifier layer

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3264479A (en) * 1955-01-31 1966-08-02 Sylvania Electric Prod Electroluminescent light amplifier
US2927234A (en) * 1955-11-25 1960-03-01 Rca Corp Photoconductive image intensifier
US3169192A (en) * 1960-05-14 1965-02-09 Philips Corp Negative picture radiating apparatus
US3186839A (en) * 1962-04-10 1965-06-01 Dick Co Ab Light-to-electrostatic-image converter and process for manufacturing same
US3322539A (en) * 1962-11-30 1967-05-30 Gen Electric Electrophotographic process
US3348074A (en) * 1964-07-01 1967-10-17 Philips Corp Photosensitive semiconductor device employing induced space charge generated by photosensor
US3441736A (en) * 1965-06-01 1969-04-29 Electro Optical Systems Inc Image intensifier including semiconductor amplifier layer

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4877699A (en) * 1988-08-25 1989-10-31 Eastman Kodak Company Electrophotographic luminescent amplification process
US10689754B2 (en) * 2017-09-05 2020-06-23 Peter C. Salmon Programmable charge storage arrays and associated manufacturing devices and systems

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DE1597880A1 (de) 1970-10-01
GB1199462A (en) 1970-07-22
DE1597880B2 (de) 1977-01-27

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