US2825814A - Xerographic image formation - Google Patents

Description

March 4, 1958 L.. E. WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION Filed June 4, 1954 3 Sheets-Sheet l 7...... I NA\\\\\\\\\\\\\\\\\\\\\\\E 32 `l 25@ D; E

OSO C F1642 SOURCE y Zf//l/I//l/l/I//ll/l//l//l/lll//IA 27 INVENTOR.

LEWS EA WALKUP 3 SOURCE March 4, 1958 L. E. WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION 3 Sheets-.Sheet 2 Filed June 4, 1954 D.C. 0- SOURCE FIG INVENTOR.

LEWIS E. WALKUP FIG@ ATTORNEY March 4, 1958 L, E, WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION Filed June 4, 1954 5 sheets-sheet s f ,WW/Zh FIG- INVENTOR. LEWIS E, WALKUP BY @WA ASW F l G 9 ATTORNEY United States 2,825,814 XEROGRAPHIC MAGE FORMATION Application June 4, 1954, Serial N o. 434,491 11 Claims. (Cl. Z50-49.5)

This invention relates in general to a formation of electrostatic charge patterns such as xerographic electrostatic latent images and in particular to methods and apparatus for the formation of such charge images.

According to the invention of Carlson described in U. S. 2,297,691 there is provided a process and apparatus for electrophotography or xerography wherein an electrostatic charge is applied to the surface of a photoconductive insulating layer, and this charge is selectively dissipated by exposure to a pattern of light and shadow to be recorded. This selective charge dissipation results in an electrostatic latent image corresponding in its charge pattern to the pattern of light and shadow to which the photoconductive insulating layer was exposed. Conventionally, in the art now known as Xerography, an electrostatic image may be formed in this manner and may be utilized as desired, for example, by development or deposition of finely divided material in conformity with the charge pattern, optionally, together with transfer of the developed image to a print receiving surface.

Now, in accordance with the present invention, there are provided novel methods, means and apparatus for the formation of an electrostatic charge pattern or Xerographic electrostatic latent image wherein an electric field is imposed through a photoconductive layer and to a contiguous insulating layer while the photoconductive layer is subject to the action of a pattern of light and shadow of visible light or other activating radiation. The contiguous insulating surface is positioned extremely closely adjacent to the surface of the photoconductive layer and may be spaced therefrom by an extremely minute distance such as, for example, the extremely small air space existent in a condition of Virtual contact of one surface with another. While it is not intended to limit the invention to any specific theory of operation it is presently believed that electric charge under the influence of the applied field through the photoconductive layer and through the insulating layer migrates through the photoconductive layer, preferentially at those areas exposed to activating radiation, and migrates to the insulating layer across the minute air gap which may exist between this insulating layer and the photoconductive surface, again under the inuence of the applied field. For the sake of clarity of disclosure and simplicity of explanation the invention will hereinafter be described in terms of this theory of operation although it is to be clearly understood that the theory is illustrative only and is not intended to be interpreted in limitation of the scope of the invention.

The general nature of the invention having been set forth the invention will now be described illustratively in terms of the following specification and the drawings in which:

Fig. l is a diagrammatic view of apparatus according to one embodiment of the invention;

Fig. 2 is a diagrammatic view of apparatus according to another embodiment of the invention;

Fig. 3 is a diagrammatic view of an embodiment of the invention employing penetrating radiation such as X-rays;

Fig. 4 is a diagrammatic view of a further embodiment of the invention;

Fig. 5 is a diagrammatic view of a xerographic machine operating in accordance with the present invention;

@ffice 2,825,814 Patented Mar. 4, 1958 Fig. 6 is a greatly enlarged diagrammatic View of a section of apparatus according to a modification of Fig. l;

Pig. 7 is a diagrammatic view of a further modification of the invention;

Fig. 8 is a diagrammatic cross section of an induction electrode according to a further embodiment of the invention;

Fig. 9 is a diagrammatic View of a cylindrical xerographic photosensitive plate according to another embodiment of the invention.

In Fig. 1 there is illustrated a simplified embodiment of the invention. 1n this figure, as in the others, the illustration is diagrammatic, largely because certain key members and certain important spacings are so extremely thin and small as to make descriptive presentation impractical. The apparatus in Fig. 1 includes a photosensitive or xerographic member or plate designated 10, combining for example a photoconductive insulating layer 12 overlying and in intimate contact with a conductive backing member 11. Positioned immediately over the surface of the electrophotographic member 10 is an induction electrode generally designated 14 comprising a thin insulating layer 15 coated on a transparent conductive backing electrode 16. Desirably, the electrophotographic member 10 is supported on a suitable support member 17 and the induction electrode 14 may rest on the surface of the electrophotographic member or plate. Members 10 and 14 are illustrated as being spaced apart. It is to be understood that this spacing is extremely minute and preferably is the almost immeasurably small space between two reasonably flat surfaces in normal surface contact. Likewise certain members and structures such as, typically layer 12 are too thin to be reasonably illustrated, and therefore are shown out or proportion and diagrammatically.

Means are supplied to impose on the photoconductive insulating layer 12 a pattern of light and shadow such as an optical image to be recorded. As illustrated in the figure, such means may include a lens 19 or other member designed to project on the layer an image of an original document 29 or other pattern of light and shadow to be recorded. Since the induction electrode 14 in this embodiment is transparent it is readily apparent that a suitable image may be projected through this transparent electrode onto the surface of a photoconductive insulating layer 12.

A source of electric potential 21 or other suitable electric field generating means is adapted to impose an electric field of desired polarity between insulating layer 15 and conductive backing member 11. This potential source is indicated in the diagram as being conventionally a battery or other source of D. C. voltage and it is to be understood that any suitable source of electric potential may be employed for this purpose. If desired, a conventional power supply may be made from transformer operated rectifier systems as are well known in the art. it is desired that there be applied between member 16 and member 11 an electric potential such as will cause between these members an electric field at least through the photoconductive insulating layer 12 and to the insulating surface 15 whereby electric charge will be induced to migrate through said first layer and to the surface of said second layer.

In Fig. 2 there is illustrated a modification of the embodiment of the invention shown in Fig. 1. According to this embodiment of this invention, an electrophotographic plate, generally designated 10, comprising a photoconductive insulating layer 12 disposed on a conductive backing member 11, is supported on a suitable support member 17. Positioned parallel to and closely adjacent to the surface of the electrophotographic plate .10 is a transparent conductive electrode 22 mounted on support members 23 or the like. VPlaced on top of the electrode 22, or on the sideaway from the electrophotographic plate, is a document 24 or other visibleor image matter which is to be reproduced accordingito the present invention. A light source25 or other source of activating radiation is positioned above the assembly so as toY shine visible light or other radiation through the copy 24 and onto the surface of the electrophotographic member 10.

An insulating lm 27 in sheet or web form is positioned'between electrode 22 and plate 10 and desirably passes from a feed roll 28 between the electrodes and ultimatelyto a take-up coil 2,9. The web or t1mr nay optionally pass through a suitable `xerographic processing station` 30 such as for example a developing or `fixing' station or-a plurality of such stations whereby desired xerographic operations may be performed on an electrostaticv latent' image. For example, xerographic stations may include means for developing the-imagev or making it visible by deposition of finely divided material in con- "o for-mity with the image, and may includemeans for transferring the image to a'second surface and/or fusing .or iixing the image. 4

Electric field producing means such as an electric potential source 31`is suitably connected between the electrode 22 and the backing member 11 of the electrophotographic plate optionally being connected to a switch 32 or like controlmeans to operate and control the electric lield.Y In this manner electrode 22 may be raised toa desired potential either positive or negative with respect to the'backingmember whereby a desired electric iield is imposed through the photoeonductive insulating layer 12 and Yat least to the surface of the receiving iilm27, thereby inducing electric charge to migrate to the surface of this iilrn.

Atleast during the exposure operation the induction electrode and xerographic plate are generally in normal surface contact vwith the opposite surface of iilm A27. Accordingly means, either mechanical or manual, may be supplied to `move electrode 22 into and out of contacting position. Conveniently this can be accomplished by manually placing the electrode on top of the film and resting the electrode on the xerographic plate.

With respect to Figs. 1 and 2, it is apparent that there are certain generalities and similarities which may be noted. For example, in each of these figures a light or radiation image is imposed on the photoconductive insulating layer V12 and it is therefore apparent that either the backing vmember 11v supporting `this photoconductive insulatinglayer or theinduction electrode 14 or 227must be transparent. ;Thus, as shown in Figs. l and 2 thel induction electrode -is the transparent member, but it is apparent that a transparent backing memberlll would permit exposure by projection` through the Xerographic sensitive member itself. In any case, there is a'transfparent conductive member, be it member 11 or 16 of Fig. l or both, or member AY171 or 22 of Fig. 2, or both, and this member must be sufficiently conductiverto permit application of an electric eld through layer 12 and through the air gap and must permit the liow of electric charge under the iniiuence of this field and the photoconductive action from the appropriate light or image source. `It is Y.specifically understood and recommended vthat conductive glass members or conductively coated glass members should be so employed. trode'should have a specific resistivity of less than l()m ohm cms. and preferably, less than 105 ohm cms. Desirably, this member should be what is ordinarily considered to vbe conductive and should carry a substantial flow of `current under only mild potential differences.`

ffanSPafent .Suppen bases. including .glassy tr,alista-us5.`

plastic iilms and the like, and conductive transparent In general, this conductive elecmembers coated or impregnated with conductive materials such as metals or ionic salts or the like Where sufficient moisture is present to cause the materials to be conductive. Likewise, conductive liquids or uids may be employed, such aselectrolytic salt..solutions or-other ionized liquids, or ionized air or other ionized gasesor M layerare commercially available -in the art of Xerography.l

In addition to this speciiically preferred embodiment, it is to be understood that therefmay be employed other photoconductive ymembers including anthracene, sulphur, and the like, coated on suitable backing members, A as well Aas photoconductive binder layers including photoconductive materials dispersed in a suitablebinder-.and coated on a conductive *surta-ce.V T hese materials include, for example, crystalline materials generally availableas phosphors or luminous compounds whichl frequentlyexhibit photoconductivityfand which can be yemployed in suitable organic binders and iilm forming materialsonthe surface of photoconductive members.

In the case of photoconductive layers, according to the presentinvention, as distinguished from the photoconductive insulating layers preferred in the art of Xerography in general, it is to be observed that the layers according to the present invention are not required to hold anelectrostatic charge ontheir surfaceffor similarly ,long periods of time, but are required on -the contrary to permit migration of electric charge through the thickness of the layerat a substantially diierential rate depending` on activation or nonactivation by suitable radiation. Thus, whilephotoconductive layers for xerography .in general'must `be capable of supporting an electric charge on the surface for appreciableperiods of time to permit formation of an image and development of the image, this is not true in the case of the present invention where the electrostatic image is retained for development on the insulating layer 15 of Fig. l, or the insulating film 27 of Fig. 2.4 Thus xerographie members characterized by higher dark decay or current leakage in the absence of activating radiation 'may be employed in the present invention.

In Fig. 3f there isillustrateda .further embodiment 'of `theinventon whereby .the jnew invention is particularly adaptedto the formation ofV X-ray or radiographic images. Asuillustrated intheflgure .a Xeroradiographic plate lgenerally designated Vlitik comprising a normally insulatinglayer 12land a conductive backingimember 1 1is disposed rover an induction electrode-14 with an insulating :transfer member 27 therebetween. A suitable power `supply Y21Y is connected between conductivebacking-member 1l and the induction electrode 14 to apply a field from the one member yto the other. v

A suitable test VVobject or other material or member to be examined by X-,ray methods is placed on the conductive backing member .which thereby serves the dual pnrpose- 0f Supporting the test Obiect and simulteneouslyoper.-

ating as the backingreleetrode forthevsystem. illustratedV l in the drawing is a step wedge 36 such as may be particularly designed fortesting lthe Vradiograph operations. An X-r'ay source such. als V)AC-ray tube Bul is disposed and positioned to supply penetrating radiationor X-rays indicated by dotted lines 35 which are projected ontoand through the test object to strike ltheXeroradiographic plate 10. In general the basking ,member 1 1 of the xcroraso.- graphic plate will be a support sheet or plate of a metal 4or like elecfrialrcoaduclgr Whirl; .desirably .may rbe opaque to ordinary or visible light. It is apparent, therefore, that thepenetrating radiationsulchas X-rays striking this bakasalembetzwill penetrate .thrqush it to eater inte the normally insulating layer 12 disposed thereon. This normally insulating layer -is a l'yerof material which in ,g

the absence of activatingradiation is an insulator but which, under the activating influence of X-rays or other penetrating radiation becomes substantially more conductive to electricity and it may, if desired, be a photoconductive insulator such as the photoconductive insulators employed according to the embodiments of the invention operating with visible light. Alternatively, it may be such other insulating member as is substantially insensitive to visible light but activated by penetrating radiation. Thus, for example, many insulating lms and layers become conductive under the action of penetrating radiation even though they are not generally considered to be photoconductive and it is within the scope of the present invention to employ such insulating members.

In the embodiment of the invention shown in Fig. 3 it is presently preferred to employ a photosensitive xerographic plate comprising a layer of amorphous or vitreous .selenium disposed on and coated on a conductive backing plate comprising a sheet of brass, aluminum, or the like.

Illustrated in Fig. 4 is a further embodiment of the invention wherein the time or duration of exposure is controlled by controlling the application of potential between the induction electrode of the system rather than by controlling the duration of exposure to light. In Fig. 4 is shown a xerographic plate and an induction electrode 14 exposed to a projected light image from an original 20 through a lens 19 or other means. The members and operations are in general the same as the members and operations according to Fig. 1.

Connected between the backing member 11 of the xerographic plate and the conductive electrode 16-is a power supply or potential source 21 which operates through a timing switch 37 which is constructed and adapted to apply a pulse of electric potential between the two electrodes for a predetermined time as may be selected by the timing switch. According to this embodiment ot the invention the appropriate exposure conditions are set up by projecting the desired pattern of light and shadow through the transparent electrode 14 onto the xerographic plate 10. The timing switch is then set and energized to apply the desired potential difference between the conductive members 11 and 16 for the preselected time as controlled by the timing switch.

The embodiment of the invention shown in Fig. 4 is particularly desirable for use in conjunction with photo- Iconductive insulating members characterized by what is lknown in the art as high dark decay. Members of this ltype are characterized by a somewhat higher conductivity in the absence of activating radiation than are members generally preferred for conventional methods of xerography. Specifically, according to prior methods of xerography, it is usual to apply an electric charge to the surface of a photo-conductive insulating layer and to store this charge on the surface from the time the surface is charged until the charge is selectively dissipated by exposure to light and inally developed by deposition thereon by charged electroscopic particles. The usual operations of charging, subsequently exposing, and then developing, require va period of time varying from relatively large fractions of a second up to a time as much as several minutes and, accordingly, it is necessary that the charge be retained on the photoconductive insulating surface in the absence of radiation or exposure for a time cycle sufficient to permit the carrying out of these xerographic steps. Now, however, according to the embodiment of the invention illustrated in Fig. 4 it is not necessary to place and retain a charge on the photoconductive surface for a long time as has previously been required in xerography. It is apparent that this embodiment of the invention calls for activation of the photo-conductive layer or diterential conductivity through this layer during the l time when the layer is both subjected to an electric eld and exposed to the pattern of radiation. .In the absence 6. of either of these conditions there is not the combination which leads to charge dissipation since the charge once deposited on the insulating layer 15 is held in its image configuration by the insulating characteristics of this insulating layer rather than by the insulating characteristics or absence thereof of the photoconductive layer 12.

It is apparent, therefore, that the photoconductive layer 12, according to this embodiment of this invention, may be somewhat more conductive in the absence of radiation than permissible according to prior inventions of xerography, Thus, according to this embodiment of the invention, both the electric image formation and other variations in electrostatic charge pattern on insulating layer 15 are substantially restricted to the period of time indicated by timing switch 37. An image thus formed is held on the insulating surface for a period of time sufficient to permit other xerographic steps such as development or the like, regardless of whether the photoconductive layer 12 has a suiciently low dark decay to permit its use in other variations and modifications of xerography.

In Fig. 5 is illustrated diagrammatically a machine for the production of xerographic copy of suitable original material according to one embodiment of this invention. According to this figure, a xerographic cylinder 38 is adapted to be rotated by a motor or other drive means 39 optionally acting through a drive belt 40 on a pulley 41. The xerographic cylinder in general is a drum-like or cylindrical surface having at least a portion of its surface covered with a xerographic photosensitive member as in the previous figures. At one point around the circumference of the drum is a transparent induction electrode 41 fitted very closely to the drum and allowing just sucient room for passage between the electrode and the drum of a sheet or web of an insulating lm 42 which desirably passes from feed roll 43 around guide roll 44 to a take-up roll 45. During its path of travel from feed roll 43 to take-up roll 45 the lm passes between the electrode 41 and the cylinder 38 and desirably passes through one or more xcrographic stations or positions 46 which may be developing or fixing stations or the like.

At the exposure station, this being the location of the induction electrode adjacent to the xerographic cylinder, is suitable apparatus or means for exposing the xerographic cylinder to the pattern of light and shadow to be recorded. As illustrated there is slit exposure mechanism comprising projecting means or lens 48, an image slit 49, and a projection slit 50, the lens being adapted to project through the image slit a focused image of the projection slit. Material to be copied such as for example documentary information or the like desirably in roll or web form is adapted to be passed across the projection slit 50 for example as the web 51 of image material passing from a feed roll 52 to a take-up roll 53 and adapted to be driven at a desired rate of speed by drive means 54. Desirably this drive means will include suitable drive mechanism synchronized with the rate of travel of the Xerographic cylinder. In the event that web S1 is photographic microfilm or the like the drive means may be a gear wheel 54 with teeth adapted to engage the slots of microfilm to carry the microlm past the projection slit.

The suitable D. C. voltage source is connected to the induction electrode in such manner as to maintain a potential difference between the induction electrode and the backing member of the xerographic cylinder. This will be accomplished, of course, by means and methods similar to those described in the previous gure.

In use and operation the machine of Fig. 5 operates according to the principle of Fig. 2. Motor or drive means 39 is energized to rotate the xerographic cylinder in a clockwise direction. Image web 51 then is carried across the projection slit at a rate synchronized with the rotation of the xerographic cylinder and transaannam electrode. l U elec-trode and the backing member of the -xerographtc,

cylinder causes formation of anelectrostatic image Von insulating web 42 and this image-'bearing web `is vthen carried to the further xerographic stations where `it may be ydeveloped and fixed or otherwisetreated as desired' to yield a xerographic print or Ato kyieldjother xerographic results as desired including, for example, a scanning electric signal of light. In the eventthat a xerographic print is formed the resulting print is formed into taltefuprol-lS.

According to a further. embodiment V'ofthe invention the light image to be recordedQis Vprojected'.throughn Y reversing means are `suppliec'l tomlakefpossible reversing Y the reverse side ofthe xerographic'plateand'ifurther the potentials applied between the conductive backing electrode 1l and the induction electrode 14, Infig. f6 is illustrated this embodimentV of 'the invention. .As shown here, the induction electrodev 14 comprises a conductive surface connected to one pole of a rcversingswitchS?. Desirably Vthe conductive electrode isa support surface such as, for example, a at metallic lsurface suitably supported on or comprising a support Ibed plate ofthe device. Positioned over the induction electrode 14jis a xerographic plate comprising a photoconductive insulating layer disposed -on a transparent conductive'backing electrode 11 such as, for example, va glass platewith a conductive coating 11a on the stxracet-hereOf, thiscom ductive coating beingv either of a transparent material or being sufficiently thin so that it `islargely transparent to incident light. For example, the yconductive-coating may be a thin evaporated lm of aluminum or other metal on the surface of the glass plate, or the glass may be a conductive or conductively coatedV glass as isV readily commercially available. Coated on the layer surface, or conductive surface, of the glass is aphotoconductive insulating layer 12 such as the layer 12 of Fig. l.

A sheet of an insulating material is positioned lbetween electrode 14 and plate 10 or desirably a web 42 of-in sulating material is passed between said members passing from feed roll 43 to take-up roll 44. The conductive backing member 11 and the inductionelectrode 14 are connected `to the output poles of lreversingV switch-f67, and the input poles are connected to a D. C. potential-source 21. By .throwing reversing switch 57 in either direction, the-potential or 'field between member 11 and; member 14 may be made positive in either direction asvdesired. In the event that induction Velectrode 14 is .to be raised to a potential other than ground it generallyis desirable that this electrode be supported on insulating supports `-58.`

In operation, the device shown in Fig.V l6 is energized by exposing the photoconductive insulating layer `12 to the vprojected image of object 20 while a potential -diierence is applied betweenbackingelectrode-11'and'induction electrode .14. If desired, greater intensification can be achieved .by iirst-reversing the polarity of the potential or field through reversal of switch 57 and subsequent exposure and application :of the desired potential or field. Alternatively, the yelectrostatic image placedon sheet or web 42 may be transferred yto the xerographic plate after formation of the image by reversal of the polarity such as to drive the electrostatic chargeback across the air gap to deposit the charge on 'the'surface of layer 12. In this manner the electrostatic charge image can be placed either on .insulating ;lm42 or on the photoconductive layer 12 as desired, andmay. be transferred from one 'to the yother after its formation. Thus, the electrostatic latent image may be developed or otherwiseV utilized on insulating film-42er it "may if desired be developed or otherwise utilized on the xerographic plate.

In Figj7 there is illustrated in greatlyy enlarged form a diagrammatic representation ofv theformation-ofimages according to Ione..ernbodim'critici-:this Ili-n'vention. LIt is "to be understood,ithatjthisgillustration is presented in anetort to deseribejthe,operationofjthe invention in vaccordance withv oneptheory ofmechanismwhich is believed toexplain image formation. It is to' be realizedghowever, u

that otherjnmehanisms of, image.. formationmay also be in accord withexperimentally determinedfact and/that the presentV explanation ispresented as one theoretical possibility.and'thepinvention is .not Vto be limited to this expression oftheory. In Fig. 7v is la photoconductiveinsulating layer f6.1 disposed on a conductive backingmember 62. Spacedtherefrom byja minute-ain gap i'is an insulating layer 63 disposed on a conductive backing/layer .64., This conducvtive backing llayer 64 istransparent and on the back surface thereof is an ,image layer .adaptedto. interrupt the passageof light through lthelayer and to causev a light image -to be projected onto thesurface Yof therphotoconductive insulatingl layer V61-as^indicated by arrow 66,. It is understood that 4image 65 v`-is' -adiagrammatic representation Aof any means by projection or contact tocontrol, in

image configuration, the'exposure to activating radiation.V

Thus,as illustrated, ycertain portions of the photoconductive insulating layer indicated as blocks 61-[1 are struck by light and vbecome conductive vwhereas other portions 6l-a are notstruck yby light andV remain insulating.

Asuit'able vo'ltagersourceV or voltage supply 67 is operatively connected between conductive backing member 64 and conductive backing member 62 whereby a field-or potential difference-is applied between these two members. In the regions of the insulating portions 61-a of thephotoconductive insulating. layer this 'eld Ais represented by plussigns .in the induction electrode backing member 64` and byminususigns in Vthe conductive backing member 62 of the xerographic plate, -these plus and minus signs representing .positive vpolarity and negative polarity charge or potential. vIn :theJregions-of light activation, however, the seleuium-.layer'becomes conductive and, therefore, the :charge .or potential Jmigrates through the layerand can berepresented as-being Vat Lthe surface .of the Ylayer in the-areas` 61'b -which are .conductivedueto the .action of light. .Itis to be=understood, however, that 'the air gap between layer 61.and 'layer 63sis Vextremely small Vand that the application. of .the relatively high potentialbetween the surface of layer 61 and the conductive backingV member .64 of the-(induction electrode will cause charge to migrate fromthe surface of layerz61 to the surface of layer 63. This charge, .once deposited on the surface of layer 63, becomes rtrapped there .because'the layer is an insulator either; in thepresence of. light or in its absence. The. result :is anelectrostaticlatent image on the surface of lay er,63v correspondingto the .areas which are illuminated inlayer 61.

For, simplicity -ofafexplanationgin- .connection .with the current theory of iieldvemission, the formation of a negativefpolarity Vimagehas been .illustrated in .Fig 7. In

order to -explain malformation of this .image and to analyzeit,mathematicallycertain assumptions are taken withrespect tovoltages,thicknesses, `and 4distances as well as dielectric constants. I'Forthepurposes of arnathematicalinterpretation it is therefore assumed that apotential difference of .l 00.0 volts: is; appliedV between -electrode .64 anclelectrode 62 and that the surfaces of .layer .61 and 63 arel separated:byadistanceA of.2 zmicrons. In practice a separation lof. vthis order of'distance can be achieved by placing solid particles of thedesired diameter between the surfaces. Layer'l-is assumedto be alayer of vitreous or amorphous selenium about 20 microns thick and for the purposes-.hereinitis .assumed that the dielectric Aconstant of this layer,isabout 6. It is understood, KYof course, 'thatihisis a reasonableapproximation of the dielectric constant ofseleniumybut that the `exact iigure is selected arbitrarily -because of the gfactthatthedielectric constant of selenium variesdependingp.on..fits;allotropic.form and perhaps also :depending ,upon the conditionssuchasiield or potential gradient 'andthe like. "Itris also assumed for the purposes of simplicity that layer 63 is an insulating layer of the same thickness, namely 20 microns, and also has the same arbitrarily assumed dielectric constant of 6. In this situation, then, when a 1000 volt potential difference is applied between the conductive backing electrodes in the areas of nonillumination, there will be a field of approximately 385 volts through the selenium layer 61 and through insulator layer 63, and a potential difference of about 230 volts across the air gap. In the other areas 61-b there will be substantially no field or potential difference through layer 61 and the field then will be such that about 375 volts will be across the air gap and about 625 volts through insulating layer 63. These voltages and fields are calculated in the assumption that the air gap is substantially a perfect insulator and this, of course, is not the fact.

The next approximation introduced into the figures is the arbitrary assumption that a potential gradient in the order of about l volts/cm. will cause the phenomenon known as field emission in air. It has been known for some time that a particularly high potential gradient adjacent to a surface of a solid will cause electrons to be torn from or emitted from the surface. This is the phenomenon known to the art as field emission and measurements have indicated that this phenomenon comes about at a potential which is generally in the order of 105 volts/cm. Accordingly, this value, as selected here arbitarily, is a reasonable representation of the fact. Referring now to the previous figures, it is observed that when a potential difference of 375 volts is applied across an air gap of 2 microns thickness, the potential gradient across this air gap becomes a little less than 2X 105 volts/cm., or a little less than 20 times the potential gradient required for field emission. Accordingly, it is apparent that under the conditions illustrated here, field emission or other mechanism of charge transfer will occur. To put the same facts differently, field emission or other transfer of charge must occur when the potential gradient in the air space above this surface exceeds about 105 volts/cm. and for a distance of 2 microns through air the total potential difference will not exceed about 20 volts.

lt follows that in the embodiment shown in Fig. 7 charge will migrate from conductive backing member 62, through the conductive areas 61-b of the selenium photoconductive insulating layer, and across the air gap, to be deposited on the surface of layer 63 until a deposit of charge on this layer builds up to a charge density such as to reduce the potential gradient across the air gap to a value no higher than that required for field emission or namely about 20 volts. This will result in the formation of a substantial charge density in the image areas on the insulating layer 63. It is seen that for the nonconductive area 61-a of insulating layer 61 there cannot be the formation of an equal charge density in these nonimage areas of the insulating layer 63. The migration of charge across the air gap by iield emission or other mechanism in the insulating area 61-a will build up a reverse potential gradient across this area rapidly. When charge potential to the amount of 105 volts has migrated to areas 61-a, these areas become 105 volts more negative, the background areas on layer 63 become only 105 volts more positive and the potential across the air gap here again becomes the 20 Volt threshold for eld emission. Using the figures presented herein, it is seen that the image corresponding to area 61-b will be a negative polarity image on the surface of layer 63, the potential of which will be equal to the applied potential of 355 volts minus the potential of about 20 volts needed for field emission or a negative polarity image of about 355 volts against a background potential of 105 volts. Layer 63, therefore, has on it image areas charged to a potential of about 355.volts negative polarity with background areas at a potential of about 105 volts negative polarity leaving a potential ldifterence between image and background areas of about 250 volts. An image of this'v potential difference can be developed by the methods and materials of Walkup and Wise, U. S. Patent 2,638,- 416, wherein a composition of powder particles and granular bead carrier material with the powder particles being charged by triboelectric relationship with the beads is rolled or cascaded across the electrostatic latent imagebearing surface to deposit the powder particles on the surface.

In carrying out the present invention care is to be taken to employ a good electrical insulator as the image surface or ilm on which the electrostatic latent image is formed. Similarly extreme care is to be taken to place this image surface in very close proximity to the photoconductive layer. In this connection it is to be observed that the present invention differs from certain prior art methods of treating electric charges wherein it has been attempted to induce an electrostatic charge or charge pattern to a surface which is conductive or which is temporarily made conductive. Thus, for example, it may be possible to form an electrostatic image of relatively low potential on the photoconductive insulating surface 12. According to the present invention, however, it is on the adjacent insulating surface and not on the photoconductive surface that the electrostatic latent image is to be formed and the invention includes the migration of charge from one surface to a second surface, with image formation on that second surface.

A brief examination of the mechanism of operation and the geometry of the members in terms of capacitance will illustrate the advantages that call for such charge transfer. In the rst place, in Fig. l, if the induction electrode 14 is spaced at even a moderate distance from the xerographic plate 10, there is a relatively low maximum potential which can be formed on the photoconductive surface. For example, in Fig. 8 of Carlson U. S. 2,297,691 there is an illustration of a system in which a substantial air gap exists between the members and in which there is formation of an electrostatic latent image on the photoconductive surface. However, as was done hereinbefore for the present invention, an analysis on a mathematical basis can be made of this prior disclosure. For clarity, referring to Fig. 7 herein, it is assumed that layer 63 again is a 20 micron insulating layer having a dielectric constant of 6, and in this case it is assumed that the air gap between layer 63 and layer 61 is /lo inch air gap having a dielectric constant of l. Layer 61 is again assumed to be a 20 micron layer of selenium also having a dielectric constant of 6. lf in this case a potential of 1,000 volts is placed between electrode: 64 and 62 and areas 61-b are made conductive by light to induce charge to the surface of layer 61 and then the light is cut off and electrode 64 removed, there will result a very small charge potential on these areas 61-b. Because of the air gap between layers 61 and 63 of 2%@ inch or approximately times the thickness of layer 61 itself, with the air gap having the dielectric constant of one as against the dielectric constant of 6 for the layer 61, it is seen that a potential of 1,000 volts through this air dielectric can induce to the surface of the conductive areas a total charge in coulombs which, after removal of electrode 64, will result in a potential difference of only a few volts between the surface of layer 61 and the surface of conductive electrode 62. Therefore, the prior procedure of image formation on the photoconductor can produce an electrostatic latent image on the photoconductive insulating surface, but this image will be only a few volts in magnitude. When electrode 64 is at a positive polarity, this is a negative polarity image on layer 61.

As was seen earlier, if these two members are brought closer and closer together there becomes finally a maximum potential in the order of about 20 volts for a 2 or 3 micron spacing which can be produced on the photoconductive insulating surface by inductionl methods. This potential, however, will beV comparativelyless than a potential that might be left onrthe background areas by charge migration away from said areas and it is apparent that direct induction techniques are not the major cause of image formation in that case. It is apparent therefore that the electrostatic image which may be formed on the xerographic plate will be an image of comparatively low potential and that the purpose of the present invention is to form images of substantially higher potentials and to form such images on the adjacent insulating layer rather than on a photoconductive insulating layer.

Whether the mechanism of operation of the present invention is according to field emission or other phenomena such as air ionization or other methods of charge migration across the air gap, extremely close spacing is necessary between the two surfaces. It has been found that for most materials and surfaces the methods and apparatus of the invention operate at near optimum conditions when the two surfaces such as layers 12 and 15 of Fig. l are placed in nominal contact. Thus for materials such as a selenium surface of a Xerographic plate and a polystyrene lrn disposed directly on a metallic backing member the condition of normal surface contact hasl been found to be extremely desirable. It is understood, of course, that in this condition there are a relatively few points of actual surface contact between the two surfaces, and at most points there are air gaps in the order of about a micron. A second system of somewhat preferable spacing for the surfaces described and other surfaces is a spacing system wherein a predetermined distance of several microns is maintained between the two surfaces. This distance is achieved by any of several methods and the following is recommended as a preferred method. A hard, relatively clear, transparent material such as a plastic or resin is ground to a relatively uniform particle size in the order of the size of spacingk desired between the two members. A small quantity of the powdered material is dusted onto one of the surfaces in an amount to be almost invisible on the surface. The second surface then is placed on top of the dusted surface and the charging and exposing operations carried out as hereinbefore described. The presence of the powder particles scattered over the surface maintains between the two members a spacing in the general order of the same size as the particle diameter. In this manner distances between the two surfaces in the order of about 2v to 5 `microns have been found nearly optimum and it is generally preferred that the two surfaces be spaced apart by a distance of less than about microns. Somewhat greater spacing may be usable and operable under certain conditions and it is generally understood that the two surfaces according to the present invention should be spaced apart by a distance no greater than about 20 microns. It is observed that as the spacing between the surfacesincreases two characteristics of the image result. In` the rst place the image becomes somewhat less dense and in the second place the resolution of the image decreases sharply. Such decrease is significant when the two surfaces are spaced apart by as much as 10 microns. Other impaired eects of lesser signicance are apparent as spacing is increased. As a further observation it is pointed out that extremely good surface contact is not to be sought after since there can, under conditions of actual contact, be substantial transfer of charge or potential in the background areas and this transfer by contact frequently will result in a mottled background.

In Figs. 8 and 9 are illustrated two further embodi-` ments'of induction electrodes which may be employed according, to the present invention. According to the embodiment of Fig. 8 a suitable induction 'electrode may comprisefa very finemetallic or wire screen having a plastic coating on atleast one surface thereof. Desirably,

the,worl ing surface of this electrode should be substantiallyV smooth and uniform. The electrode therefore comprises a conductive metal screen consisting of interwoven .wires 71 imbedded in insulating plastic body 72; A conductive lea-d 73 is connected to the wire screen to form a means for applying a suitable potential to the electrode. Desirably the lead 73 is connected to a suitable wire mesh or the like and the electrode is formedv by spraying, dipping, or painting the insulating film material onto the screen or mesh. It is desirable that the wires be relatively fine and the screen be substantially transparent to transmitted light, and be extremely ne and uniform. A ne screen such as a 200 mesh` or finer sieving-screen is excellent for the purpose, so, that the screen or mesh pattern does not become significant as part of; the electrostatic latent image, and so that the screen or mesh pattern does not unduly affect the transmission of a projected light image to the electrode.

Iny Fig. 9 is illustrated a cylindrical induction electrode designated 75. The electrode comprises a glass or other transparent support cylinder 76 having a coating 77 on its surface comprising a thin layer of metal such asaluminum deposited in a uniform film, for example,tby evaporation on to the glass surface. Positioned on the metal film is a layer of an insulating material 78. Suitable support means are provided such as, for example, spokes 79 connecting the cylinder to a hub 80 which is adapted to be rotated around a shaft or axle or the like. As is apparent the electrode of the type described herein is particularly adapted to be used in conjunction with a continuous machine such as is disclosed in Fig. 5. If desired suitable image sources such as a light source or a reflecting mirror, prism, lens, or the like, may be mounted within the cylindrical induction electrode so that the induction electrode operates as a combination of the electrode 41 and lenseror image member 48 of Fig. 5.

It is to be understood that variations and modifications may be made without departing from the scope of the invention, and that the disclosure herein is to be taken as being illustrative of the invention and not in limitation thereof.

This is a continuation-in-part of copending application Serial No. 368,408, filed July 16, 1953.

What is claimed is:

1; The method of forming an electrostatic image comprising varying electric charges on an insulating surface said method comprising positioning an insulating surface over a conductive electrode and in virtualtcontact with the surface of a normally insulating layer disposed on a conductive backing support, said insulating surface being positioned in face-to-face relationship with the surface of the normally insulating layer and separated therefrom by a minute gas gap, applying an electric eld above the threshold of eld discharge for the particular gap distance involved between the surfaces in virtual contact through said normally insulating layer and to the insulating surface to bring about electric breakdown of the gas gap between the surface of the normally insulating layer and the insulating surface to cause ionic movement in the gap for charge deposition, as controlled by the electric fields, on the insulating surface in conformity with a pattern of penetrating radiation being recorded while exposing the normally insulating layer to said pattern of penetrating radiation, said applied electric eld being of a sutcient intensity in the .absence of photo-emission for charge deposition on the insulating surface.

2. The method of forming an electrostatic image comprising varying electric charges on an insulating surface over a conductive electrode and spaced from the surface of an insulating layer carrying an electrostatic image, the insulatinglayer being disposedron a conductive backing support, the insulating surface and the image bearinginsulating layer being spaced apart by a minute gas gap while inV face-to-face relationship, Vsaid method comprising applying a high enough electric field through theV asaasie insulating image bearing layer and to the insulating surface for the particular spacing involved between the insulating surfaces to bring about electric breakdown of the gas gap between the surface of the insulating image bearing layer and the insulating surface to cause ionic movement in the gap for charge deposition as controlled by the electric fields and electrostatic image formation on the insulating surface in conformity with the electrostatic image of the insulating image bearing layer, said applied electric field being of a suicient intensity in the absence of photo-emission for electrostatic image formation on the insulating surface.

3. The method of forming an electric image, comprising varying electric charges, on a first insulating surface in virtual contact with an electrostatic image bearing second insulating surface and in face-to-face relationship thereto, said method comprising applying an electric field above the threshold of field discharge between the two insulating surfaces to cause electric breakdown of the gap between the surfaces in virtual contact to create ionic movement for charge deposition as controlled by the electric fields and electrostatic image formation on the first insulating surface in conformity with the electrostatic image on the second insulating surface, said applied electric eld being of sufficient intensity in the absence of photoemission for electrostatic image formation on the first insulating surface.

4. The method of forming an electrostatic latent image comprising varying electric charges on an insulating surface said method comprising positioning an insulating surface in virtual contact with the surface of a photoconductive insulating layer disposed on a conductive backing support, said surface being disposed and positioned in face-to-face relationship with the surface of the photoconductive insulating layer, applying a high enough electric field through said photoconductive insulating layer and to the insulating surface to bring about electric breakdown of the gap between the surface of the photoconductive insulating layer and the insulating surface to cause ionic movement in the gap for charge deposition, as controlled by the electric elds on the insulating surface in conformity with a pattern of activating radiation being recorded while exposing the photoconductive insulating layer to said pattern of activating radiation, said applied electric field being of a suicient intensity in the absence of photoemission for charge deposition on the insulating surface.

5. The method of forming an electrostatic latent image comprising varying electric charges on an insulating surface comprising positioning said surface over a conductive electrode and in virtual contact with the surface of a photoconductive insulating layer disposed on a conductive backing support, said surface being positioned in face-to-face relationship with the surface of the photoconductive insulating layer and spaced apart therefrom by a minute gas gap, applying an electric field above the threshold of field discharge through said photoconductive layer and to the insulating surface while exposing the photoconductive layer to a pattern of radiation to be recorded to cause electric breakdown of the gas gap between the surfaces in Virtual contact to create ionic movement 1n the gap for charge deposition on the insulating surface, as controlled by the electric fields, in conformity with the pattern being recorded, said applied electric field being of suicient intensity in the absence of photoemission for charge deposition on the insulating surface.

6. The method of claim 5 in which the insulating surface and the surface of the photoconductive layer are spaced apart by a gap distance of up to about microns.

7. The method of claim 5 in which at least one of the two paths to the photoconductive insulating layer, namely through the conductive electrode and the insulating surface or through the conductive backing support, is transparent and in which the pfattern of light and shadow to be recorded is directed thrmugh the transparent path.

8. The method of claim 5 in which the photoconductive insulating layer subjected to exposure comprises photoconductive insulating selenium.

9. The method of forming anelectrostatic latent image comprising varying electric charges on an insulating surface, said method comprising positioning the insulating surface in virtual contact with the surface of a photoconductive insulating layer and in facc-to-face relationship thereto while spaced apart therefrom by a minute gap, applying an electric field above the threshold of field discharge through said photoconductive insulating layer and to the insulating surface While exposing the photoconductive layer to a pattern of activating radiation to be recorded, said electric field being suicient in the absence of photoemission to cause electric breakdown of the gap between the surfaces in virtual contact to create ionic movement in the gap for charge deposition on the insulating surface, as controlled by the electric fields, in conformity with the pattern being recorded.

10. The method of forming an electrostatic image cornprising varying electric charges on an insulating surface overlying a conductive electrode and spaced from the surface of an insulating layer carrying an electrostatic image, the insulating layer being disposed on a conductive backing support, the insulating surface and the image bearing insulating layer being spaced apart by a minute gas gap while in face-to-face relationship, said method comprising applying an intense electric field through the insulating image bearing layer and to the insulating surface, and while the field continues to be applied separating the insulating surface from the insulating layer, said applied electric field being above the threshold of field discharge and of a sufficient intensity to cause electric breakdown of the gap between the insulating surface and the image bearing insulating layer to create ionic movement and charge deposition as controlled by the electric fields to form, in the absence of photoemission, an electrostatic image on the insulating surface in true conformity with the electrostatic image on the insulating layer.

l1. The method of forming an electrostatic latent image comprising varying electric charges yon an insulating surface said method comprising positioning an insulating surface in virtual Contact with the surface of a photoconductive insulating layer disposed on a conductive backing support, said surface being disposed and positioned in face to face relationship with the surface of the photoconductive insulating layer, applying an intense electric eld through said photoconductive insulating layer and to the insulating surface while simultaneously exposing the photoconductive insulating layer to a pattern of activating radiation, and while the field continues to be applied separating the insulating surface from the photoconductive insulating layer, said applied electric field being above the threshold of eld discharge and of a sufficient intensity to cause electric breakdown of the gap between the insulating surface and the image bearing insulating layer to create ionic movement and charge deposition as controlled by the electric fields to form, in the absence of photoemission, an electrostatic image on the insulating surface in true conformity with the radiation pattern to which the photoconductive insulating layer was exposed.

References Cited in the le of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,277,013 Carlson Mar. 17, 1942 2,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 1944 2,666,144 Schaffert et al. Jan. 12, 1954 2,693,416 Butterfield Nov. 2, 1954 2,701,764 Carlson Feb. 8, 1955 FOREIGN PATENTS 188,030 Great Britain Oct. 23, 1922

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