US 3322539 A
Description (OCR text may contain errors)
MaY 30, 1967 l R. w. REDINGTON 3,322,539
` ELECTROPHOTOGRAPHIC PROCESS Filed Nov. 30, 1962 3 Sheets-Sheet 2 `/m/emor v @o w/and W Red/'ng/Qn May 30 1967 R. w. REDINGTON 3,322,539
ELECTROPHOTOGRAPHIC PROCESS y Filed Nov. 3o, 1962 3 ysheets-sheet s Fig. 80.
' /nven/or z Fig. 6. Row/0nd W Red/hymn,
A H/'s Afforney United States Patent O f 3,322,539 ELECTROPHOTOGRAPHIC PROCESS Rowland W. Redington, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. .30, 1962, Ser. No.' 241,370
- Claims. (Cl. 961.1)
This invention -relates to electrophotography and particularly to a faster, more sensitive vmethod of electroductor under the infiuence of the electric eld whe-re it contributes to a latent electrostatic charge pattern corresponding to the original light image. This latent charge pattern is developed `by applying an electrostatically attracted powder or toner material to the photoconductor. The powder, more attracted to regions of greater charge, establishes a visible representation of the latent electrostatic image which visible representation can be fixed in place using an adhesive material and/ or heat. Alternatively, theelectrostatic image may be transferred to another material and similarly fixed in place.
Another manner of forming electrophotographic images includes the transfer of the charge pattern to a heated thermoplastic materiaLThe thermoplastic material, when subjected to the charge pattern, tends to deform and establish undulat-ions in the thermoplastic Acorresponding to the original charge pattern, as set forth and claimed in the copending application of Sterling P.`Newberry, Ser. No. 862,249, entitled Direct Image Transfer toThermoplastic Tape, filed Dec. 28, 1959 and assigned to the assignee of the present invention. The undulations formed in the thermoplastic material are capable of deflecting light in an appropriate projection apparatus where they are capable of reproducing the recorded image. Alternatively, the photoconductor itself may be formed of thermoplastic material, deformable under the infiuence'of the charge pattern, as described and'claimed in the copending application of JosephGaynor, Ser. No. 79,260, entitled -Info-rmation Storage` on Deformable Medium, filed Dec. 29, 1960, now Unite-d States `Patent No. 3,291,601 and assigned to theassignee ofthe present invention.
T-he aforementioned process of electrophotography is quite useful for duplicating documents-and the like where the speed of the process is lnot of great concern. (However, the kspeed of this process as presently known approximates the order-of-magnitude A.S.A. V where the designation yA.S.A. refers to the American Standards Association scale. Such a speed is not comparable with film' photography. The speed, of course, represents'the sensivtivity of this photoconductive process, and the sensitivity in turn can be measured by the charge energy and effectiveness of the electrostatic image. Energy is limited by several factors including the voltage developed across the photoconductor and the number of charges, e.g. the number of electrons generated per photon of light energy falling upon the photoconductor. The number of electrons per photon in conventional electrophotographic processes is generally materially less than oneAnd although the 'ductor by various means, the combination ofthese factors has limited the speed of a process to the approximate value given.
It is accordingly an object of the present invention to ICC provide an improved and more sensitive system for electrophotography whose sensitivity is such as to render it comparable to commercial `photographic film in speed.
Briefly stated in accordance with an illustrated emhodiment of my invention, a photoconductive layer is disposed in contact with a -thin layer of dielectric material to form a laminate and the laminate further includes electrodes on either side thereof, at least one of which should be transparent. The photoconductor is exposed to a light image while the electrodes are coupled in a manner for providing means for a flow of current therebetween to establish a charge pattern representative of the image.
The dielectric layer is advantageously formed to have a large capacitance compared to that of the photoconductor so that most of the latentelectrostatic image comprising the charge pattern comes to reside upon the dielectric layer. The charge in transferring from the .photoconductor to the dielectric layer is multiplied so that a cha-rge corresponding to more than one electron per photon kresults upon the dielectric layer, thus enhancing the charge image.
The dielectric layer is then removed from the photoconductor and the electrode associated with the dielectric layer is removed from the dielectric layer. In this manner physical work is done upon the charge pattern, thereby materially increasing the energy thereof. This action results in considerable enhancement of the charge image.
The dielectric layer then has applied thereto means capable of differential attraction and arrangement in respon-se to the charge pattern, for example, a toner powder. In accordance with one aspect of the present invention, the development may take place by disposing the dielectric layer in contact with thermoplastic material. The dielec- .tric layer in this instance desirably has the property of a greater softening at a given temperature t-han the thermoplastic material so that heat may be applied to a laminate of these two While the thermoplastic is freely deformed byheat establishing meaningful deformations in accord- Y particularlyV pointed out 'and distinctly claimed in the conance with the char-ge pattern. n g
The-subject matter which I regard asmy invention is cludin-g portion of this specification. The invention, however, both as to organization and method of operation,
together with further objects and advantages thereof, may .best be understood by reference to the following descrip- ,tion taken in connection with the accompanying drawings wherein. like reference characters refer to like elements and in which:
FIG. 1 is a cross-section illustrating an exposing step in accordance with the present invention, Y v `FIG. 1a is apvieW of a line pattern exposure which may be used with the present invention,
FIG. 2Aillustrates an initial layer separating step in accordance with the present invention,
FIG. 3 illustrates a second layer separating step in accordance with the present invention, FIG. 4 illustrates a developing step in accordance with the present invention,
vvoltage can be-increased somewhat across the photocon- VFIG. Slis a plan View of a first continuous electrophotographic apparatus in accordance with the present invention,
y KFIG. 6 is a plan view of a second continuous electrophotographic yapparatus in accordance with the present invention, FIG. 7-is a schematic representation of a photoconductor layer in accordance with the present invention, and FIGS. 8a, b, and c are further schematic -illustrations of dielectric layers in accordance with the present invention. f
Referring to FIG. 1, a photoconductor 1 is disposed adjacent and in contact with a materially thinner storage layer of dielectric material 2 to form a laminate. The capacitance of layer 2 is materially greater than that of layer 1. Also included in the laminate are metal electrodes 3 and 4 on either side of the laminate making ohmic contact with their adjacent layers. At least one of these electrodes, for example electrode 3\,.is desirably very thin and transparent so that light rays from an illuminated object 5 may be imaged upon the photoconductor 1 by means of lens 6.
A Wide variety of materials may be employed in accordance with the present invention, for example, the
` photoconductor 1 may be formed of cadmium sulfide,
cadmium selenide, doped germanium, or lead oxide. The photoconductor which is rather thin, e.g. ten microns, desirably has a somewhat lower resistivity than is usual in electrophotography. The thinner, e.g. 2 micron thick, dielectric material 2 may be wax or plastic insulating material, for example polyethylene terephthalate tape material such as Mylar or Cronar. The dielectric material according to one embodiment desirably softens at an elevated temperature to a greater degree than the thermoplastic developing material used according to such embodiment. In the latter instance the dielectric layer 2 in general has a lower softening temperature than the thermoplastic. Wax or low temperature thermoplastic material is suitable. The thin' metal electrodes may be copper, chromium or tin, or other suitable contact materials for the photoconductor used.
The photoconductor can be operated in either a charging or discharging mode for causing a transfer of charge in response t-o a light image. The discharging mode is more adaptable to explanation and will therefore be principally described, it being understood the charging mode, more easily envisioned in other instances, is mathematically and physically identical.
A series connection of a voltage source 7 and a switch 8 is disposed across the electrodes 3 and 4 and is operated alternately with shunting switch 9. The voltage from source 7 is on the order of 700-1000 volts D.C. According to the discharging mode, the laminate 1-2 may at first be illuminated by a lighting means (not shown) disposed at object plane 5, with switch 8 in a closed position. The light is arranged to uniformly illuminate the portion of the photoconductor which is to be used. The photons fallin-g upon the photoconductor cause the formation of positive and negative charge carriers, e.g. holes and electrons. One type of carrier, e.g. an electron, drawn by the voltage across the laminate, transfers through the photoconductor to the interface 10l between the photoconductor and the dielectric layer, establishing a uniform charge. Application of light at object plane 5 hastens the charging. After charging, switch Sis opened.
Now, the laminate is exposed to an illuminated object 5 and switch 9, shunting the laminate, is closed for a sho-rt period comparable to the photoconductors lightcurrent time constant. The lighter portions of the object form an image on the photoconductor which tends to reduce the resistance of the photoconductor in that illuminated area establishing a charge reducing current flow through switch 9. However, the charge tends toI remain in place in the less illuminated areas, to the extent of illumination absence, and there is therefore built up a differing charge pattern varying point to point across the laminate in accordance with the light in the image. The laminate including the electrodes 3 and 4 may be considered during this period as comprising a photoconductor layer in parallel with a capacitor formed of dielectric layer 2.
As indicated, the thickness of dielectric layer 2 is advantageously chosen to be small in respect to photoconductor 1. For example, layer 2 may be two microns in thickness while the photoconductor is ten microns in thickness. Because of this and in addition because of the dielectric nature of the material, the capacitance thereof is higher and more of the signal charge in the combined capacitance resides as electrical stress in the dielectric layer varying the point-by-point in accordance with the image. In addition, it has been found that greater charge variation after exposure resides across the dielectric material than the charge variation which would be predicted across the photoconductor alone. That is to say, the charge image is somewhat amplified.
The charge established across the photoconductor may be viewed as transferring to the dielectric to establish a corresponding charge area on the dielectric. The charge pattern having thus transferred, a new charge may be established across the photoconductor and again a portion of this can be viewed as transferring to the dielectric. By a succession of transfers a larger total charge differential is built up than would have been the case if this additional capacitor layer were absent. The charge on the dielectric supplies potential for continuing the charge transfer process. The reverse action takes place during ydischarge whereby charge on the dielectric aids in sustaining an enhanced charge flow, varying point-by-point according to the image, so that an enhanced charge image is established on the dielectric.
Of course the foregoing charge transfer process is not accurately described las occurring sequentially, but this initial explanation is given in order to help envision the gain in charge achieved by the combination of photoconductor and dielectric layer. The action will be hereinafter more fully analyzed. It will become apparent that a larger number of electrons per photon may be carried by the dielectric layer and therefore `a stronger image is formed than would have been possible with the photoconductor alone, in accordance with the ratio C'e-l-C'p where Cp is the capacitance of the photoconductor, and Ce is the capacitance of the dielectric layer.
Referring to FIG. 2, the dielectric layer 2 together with its electrode 4 is now separated from photoconductor 1 carrying with it a large proportion of the total charge across the laminate of photoconductor and dielectric. The proportion of the total charge removed on the dielectric,
In FIG. 3, electrode 4 is stripped or removed from dielectric layer 2. It will be appreciated that removing electrode 4 decreases the capacitance of the dielectric layer. It is found the signal image or charge image upon the dielectric layer correspondingly increases in intensity. The reason for the increase in intensity of the image is that physical work is done upon the charge pattern by removing the electrode 4. The energy of the signal is correspondingly increased, and may be multiplied several times in this manner.
As thus appears in accordance with the present process, the dielectric storage layer has been disposed in Contact with an exposed photoconductor whereby the charge image produced by the photoconductor is multiplied. A large proportion 'of this charge resides on the dielectric. The dielectric electrode is then removed to decrease the capacitance of the dielectric, thus increasing the energy l of the charge pattern.
The image bearing charge pattern may now be developed employing toner techniques. An electrostatically attractable powder such as finely divided carbon or a pulverized resin is dusted upon the dielectric layer 2, for example, upon face 15. The powder tends to be attracted to areas of maximum charge, rendering visible the electrostatic charge pattern. Excess powder is removed in a convenient manner as by agitation or placement of the dielectric layer in a stream of air. The visible representation provided by the toner powder can be fixed in place using an adhesive material or heat. For example the dielectric material may be heated to the softening point to tix the powder image in place. Alternatively, the toner image may be transferred to another material and similarly fixed in place. v
Referring to FIG. 4, the latent electrostatic image on dielectric layer 2 may be developed, in accordance with one embodiment of my invention, by disposition of dielectric layer 2 in contact with,a thermoplastic layer 11.
The thermoplastic layer 11 comparable in thickness to the photoconductor 1 in FIG. l, will receive and permanently record the electrostatic charge pattern carried by dielectric 2. The thermoplastic material may be a blend of polystyrene, m-terephenyl and a copolymer of 95 weight percent of butadiene and -5 weight percent of styrene as set forth inthe copending application of William E. Glenn, Jr., Ser. No. 8,842, tiled Feb. 15, 1960, issued on Dec. 3, 1963 as United States Patent No. 3,113,179 said application being a continuation-impart of application Ser. No. 698,167, tiled Nov. 17, 1957 (now abandoned) and of application Ser. No. 783,584, tiled Dec. 29, 1958 (now abandoned), all assigned to the assignee of thepresent invention. The dielectric layer 2 should in some instances exhibit Igreater softening than the thermoplastic layer 11 at the temperatures to which the thermoplastic layer is to be raised. Layer 2 may be formed 4of wax or may be substantially similar in composition to thermoplastic layer 11, but should preferably be compounded to have greater fluidity, as for example a compound having lower molecular weight. As hereinafter set forth, the layer 2 material may in some instances be of the same composition as layer 11.
As also set forth in the aforementioned patent of William E. Glenn, Jr., the thermoplastic, 11, may be provided with a support layer of optical grade polyethylene terephthalate `which may be disposed on the opposite side of a grounded electrode backing 12. The dielectric layer` 2 and the thermoplastic layer together form a second laminate including electrode 12 which is thin and transparent. The electrode `12 may be formed of chromium metal.
To develop the electrostatic image the laminate is heated (-byfmeans not shown) to -a temperature suicient to soften thermoplastic material 11. As previously indicated, dielectric layer 2 also softens at this temperature. It has been found the thermoplastic layer will deform in accordance with the charge pattern placed thereon at the dielectric-thermoplastic interface 15, producing thickness deformations in the thermoplastic corresponding to the charge pattern. In the FIG. 4 embodiment, charge at a lpoint on the surface 15 will be drawn towards grounded electrode 12 and will yat the same time physically distort the thermoplastic-,at such point resulting in an effectively compressed thermoplastic layer at the same point. Be-
fore cooling, the thermoplastic material 11 may -be separated from the dielectric layer 2, and separated from electrode 12 if electrode 12 is not suiciently transparent. However, if the dielectriclayer 2 is chosen to be a transparent material of adequate optical properties, it may be left in place. yIn such instance it may be formed of material having substantially the same composition as thermoplastic 11, but a different index of refraction. A1- ternatively the side opposite side 15 of layer 2 may be oriented toward the thermoplastic, and the identical composition used. The thermoplastic layer 11, when it cools, permanently records the electrostatically derived deformations therein and may be employed for the deection of light in yan appropriate projection apparatus, for example, as set forth in the aforementioned` patent of William E. Glenn, J r., or in William E. Glenn, Jr., Patent Re. 25,169, assigned to the assignee of the present invention. Briefly, the -thermoplastic material, including its backing, is placed in a projecting device called a Schlieren projection system for transmitting light through the thermoplastic layer. An
optical system, including certain illuminated slits and corresponding bars used to block light from the slits, is arranged to block all transmittedlight, in the absence of deformation in the intervening thermoplastic material. However ifa deformation is recorded in the thermoplastic layer, light is deflected by the deformations aroundithe' bars of the optical system and thereby reaches thel nrojection screen where the image is reproduced.
In recording optical information in this manner it is frequently desirable to dissect the light image and record it as a series of small elements rather than in a continuous fashion. That is, the image is dissected and recorded in a manner analagous to `that in which a television picture is produced by breaking the image into smallr segments. The charge pattern representing the light image is therefore fragmented into a large plurality of charge bearing areas which cause small separate thickness deformations in the thermoplastic material. Each small area 'becomes a picture element in the thermoplastic recorded image capable of deflecting light to produce a light response'in a projection system. If the image is not thus dissected, a high intensity area yof the recording merely records as a large shallow groove or depression having an edge irregularity capable of dellecting light. By dissecting the image into a large number of elements, the true nature of thefrecorded information may be projected on a screen in full rather than simply the outline thereof. Such dissection can be accomplished in various ways as described in the aforementioned application of Sterling P. Newberry, Ser. No. 862,249. As a particular example, the image to be recorded on the thermoplastic may be fir-st presented to the photoconductor of FIG. 1 through a screen or reticle for breaking up the image into a plurality of segments. Alternatively, a reticle or iine line pattern in FIG. la may ybe rst photographed by the photoconductor prior to the exposure to the desired image.
A first continuous apparatus for producing electrophoto- Y graphic images in accordance with the present invention which may be 5 sides thereof,
is illustratedA in FIG. 5. In FIG. 5 a continuous web or tape of electrophotographic laminate is wound on reel 16. This laminate comprises a exible 10 micron photoconductive layer 17 adjacent a dielectric layer 18 formed of flexible, thermoplastic material, for example, Mylar, mic-rons thick. Transparent conducting laye-r 19, which may be formed as a thin transparent layer vof copper, chromium, or tin, provides an electrode on the photoconductor side of the laminate and is grounded by roller 60. A second conducting layer 20 is disposed in contact with the side of the dielectric layer opposite the photoconductor. This conducting layer has discontinuities as at 21, 22 .and 23 separating individual frames 24 and 25 for exposure. The laye-rs of the tape laminate are pressed securely together as contained on reel 16 so as to be tightly adherent -t-o one another. Several of the layers are made separable from adjacent layers as by subsequent stripping apart. For example, layers 17 and 18 are made separably adherent. Adhesive material of selectably graded adherency may be employed if desired.
The ldiscontinuous'conducting layer 20 is carried on a continuous plastic backing layer 33 which may be later stripped away taking conducting layer 20 with it. The plastic backing layer 33, which may be formed by Mylar, may be narrower than the tape width leaving exposed a narrow edge of conducting layer2 permitting electrical contact with layer 20. Guide rollers desirably do n-ot contact conducting layer 20 unless otherwise indicated. Alternatively, discontinuous metal layer 20 may be formed around the edge of backing layer 33, partially on both providing an edge contact on the back of layer 33. Again, rollers used for guiding the tape, desirably do not come in contact with the conducting layer. i
The photoconductor and its adherent electrode are wound toward a reel 27 driven by motor 28. Intermediate the reels 16 and 17, an individual frame, for example, frame 24, is exposed to a lamp 29 while at the same time a vvoltage source, similar to voltage source 7 in FIG. 1 is 29, through the photoconductor. The illumination and the application of voltage are discontinued after a charging period.
The remainder of the apparatus subsequent to lamp 29 along the web, is preferably housed in darkened charnbers 35 where exposure of a particular frame to the desired information takes place. Along the tape laminate between lamp 29 and reel 37 is disposed an exposure station 36 including a darkened hood 37 to permit exposure of only a desired frame, and a lens 38 for imaging an object 39 upon laminate frame 25. A shutter means 40 limits the time of exposure. At the same time the laminate is exposed to an object 39 illuminated by lamps 41 and 42, a switch 43 is closed for shunting conducting layer 2t) to ground via edge roller 44. The laminate is exposed for a short period for optimally discharging the photoconductive layer at portions thereof determined according to the light image. As will be appreciated by those skilled in the art, expo-sure is discontinued while the difference between the light current and dark current discharge flow is substantial.
After exposure the laminate is carried between guide rollers 45 and 46 and a portion of the laminate comprising dielectric 18 `as well as conducting layer 20 with backing layer 33 is stripped from the laminate as it passes around guide roller 45.
Now conducting layer 20 with backing layer 33 is taken up on a reel 47 while dielectric 18 passes around guide roller 48 into a developing chamber 49. In this manner the capacity of dielectric 18 is decreased and the energy of its latent electrostatic image is increased. Developing l chamber 49 is provided with an inlet conduit 50 and outlet conduit 51. Powdered material, for example, carbon dust or pulverized resin, is blown into the chamber (by means not shown) through conduit 50 wherein a portion of this material is attracted to the electrostatic image on dielectric 18. Excess powder is exhausted through cond-uit 51. After a brief period, the excess powder in chamber 49 and upon dielectric 18 is removed by blowing air through conduit 50 and exhausting the same at conduit 51. The dielectric now carries a visible representation of the latent electrostatic image in the form of attracted powdered material. The dielectric passes over guide roller 52 upon leaving the chamber 49 and is taken up on reel 53. The developed image, after leaving the developing chamber, is fixed through application of heat directed upon the dielectric material from an electrically operated heater 54. This heat is sucient for softening the surface of the dielectric material whereby to firmly adhere the developed image in place. The heater 54 is turned off and this area of the dielectric tape is allowed to cool somewhat before passing to takeup reel 53.
As will be appreciated by those skilled in the art, the operation of takeup reels 27, 47 and 53 is synchronized as indicated by dashed lines 55 and 56 so the various webs or tapes do not slip relative to one another. A synchronized arrangement similar to that described in the aforementioned application of Sterling P. Newberry may be utilized. Although development by powder atmosphere is illustrated in the FIG. embodiment, it is appreciated that the latent image on the dielectric material may be developed employing other conventional techniques for example magnetic brush development.
FIG. 6 illustrates a second continuous electrophotographic apparatus in accordance with the present invention. In this apparatus, development takes place upon a thermoplastic tape material in a manner similar to that described in connection with the FIG. 4 embodiment, rather than by utilizing toner development techniques. The apparatus of FIG. 6 is identical in construction and operation with the apparatus of FIG. l in the respects indicated by like reference numerals and with those changes and additions hereinafter set forth.
In the FIG. 6 apparatus, after passing guide roller 45, the conducting layer 20, with backing layer 33 is stripped 6i from the dielectric layer 18, the conducting layer being taken up on reel 57 driven by motor 58. In this manner the capacitance of the dielectric material 18 is decreased whereby the energy of the latent electrostatic image is correspondingly increased. The dielectric layer 18 passes to a takeup reel 59 driven by motor 60, but in so doing Y it passes over a large grounded metal roller 61. Also passing over roller 61 between dielectric 18 and the roller is a thermoplastic tape material 62 provided with a transparent metallized backing 63. The thermoplastic tape material, which may be one of the thermoplastic development materials hereinbefore described in connection with an embodiment of FIG. 4, is drawn off supply reel 64 over roller 61 and onto takeup reel 65 driven by motor 66. Roller 61 is heated preferably by electrical means (not shown) to a temperature for softening the thermoplastic material 62 as well as the dielectric material 18 over the space of one frame. The charge pattern upon dielectric material 18 thereby acts to deform the thermoplastic into a pattern of undulations corresponding to the charge pattern. These undulations provide a permanent record when the thermoplastic material is allowed to cool, and are rendered visible in a Schlieren optical system.
It is desirable, when employing the FIG. 6 apparatus, to rst photograph the line pattern illustrated in FIG. la for dissecting the image into appropriately sized elements capable of eflicient light deiiection in the Schlieren optical system. It will also be appreciated the operation of motors 28, 58, 68 and 66 is synchronized in the FIG. 6 apparatus so the various webs or tapes do not slip relative to one another.
The theoretical explanation for a portion of the sensitivity increase achieved according to the present invention will be considered with reference to FIGS. 7 and 8. In electrophotographic processes, a photoconductor is used with transient voltages applied thereacross where the resistance of the photoconductor helps determine the time constant of the transient. A cross-section of such a photoconductor layer is indicated schematically in FIG. 7.
Initially, the layer is given a charge Q which decays slowly in the ldark and more rapidly in the light. Suppose a charge q of electrons and of holes has been liberated by the light. If both the electrons and holes are mobile, they will be drawn to the electrodes, terminating the photoconduction process. The capacitance of the photoconductor will be discharged by just the photogenerated charge, q. In order to have the possibility of a larger effect than this, suppose that one of the carriers is trapped,
or much less mobile than the other. The mobile carriers will be drawn to the appropriate electrode, reducing its charge, but the charge on the other electrode will not be changed, land thus the field there will not be changed. Since this electrode at which the field does not change is the one which would have to supply additional mobile carriers, again the photoconduction process terminates with the arrival of the mobile carriers at the electrode. In this case the signal is less than the photogenerated charge divided by the capacitance of the photoconductor. However when a capacitor is connected in parallel with the photoconductor, it helps maintain the potential across the photoconductor, and the iield at the electrode which would supply more carriers is increased, and more charge is transported through the photoconductor.
Three cases are illustrated in FIGS. 8a, b and c. For each case, a cross-section of the layer with the appropreate charges is Shown. In FIG. 8a, both carries are mobile and in b and c only the electrons are mobile. In FIG. 8c, an external capacitor, Ce, has been added across the layer. :In this case the iield at the cathode electrode is increased so that more charge can be transported through the layer.
The addition of the capacitor permits more charge to be transported through the photoconductor and so it is of interest to calculate the maximum voltage signal for this charged to a voltage V and the photoconductor was exposed to a flash of light which generated a charge q of mobile carriers of one sign. In some instances, the dark current maybe assumed to vary as ,the square 0f the voltage. In the dark,
@f LV2 dt C' for a space limited dark currentftrap free insulator. In this expression,
L3 n In the latter equation A equals-the photoconductor area, L equals photoconductor thickness, e equals dielectric Y number of electrons permitivity and ,n equals mobility of carriers not'trapped. Y
lo.. Vdark` 1+t/tl Where t=time, V=initial voltage, and ts--C/ot Vg. For hyperbolic decay nf; t V
di C C(1+t/T) In this expression equals the dark current divided by C;
In the case of FIG. 8c let it be assumed that one electron per photon is generated, in the absence of external capacitor, Ce. That is,
V 1 electron/photon The charge Qe on n t t Cie=VO6=1 elec regi/pho on Total charge on the photoconductor and external capacitor,
1 electron/photon `(CD -l- Ce) (Fw In actual practice the rst term in the fraction is found to be slightly less than 1 electron per photon. Four-tenths in FIG. l larger. But a IO tor or dielectric layer is proportional `to the capacitance ratio Theminimum thickness of the dielectric layer should be larger than the desired deformation amplitude in the thermoplastic material. This is about l micron, peak-topeak. Thus a dielectric layer- 2 microns thick is about the minimum that could be used where it is desirable to use minimum thickness to increase the term `Ce above.
As also appears from the above ratio,
C'e-l-C'p C the attainable gain can be made larger by making a photoconductor capacitance small. This implies a thick photoconductor, at least thick incomparison to the thickness of the dielectric layer. However vthe thickness ofthe photoconductor layer is in general limited by several factors to a certain range, if we are to really achieve the per photon we would calculate with the ratio Ce-l-Cp C Stating it another way, the amplification available using a dielectric layer is limited by the ultimate availability of electrons.- The ultmte availability of electrons decreases as thickness is increased. Such availability can be increased by using a higher voltage when exposing the photoconductor, as by making the voltage from voltage source 7 substantially higher voltage tends to increase the dark'current excessively. Moreover, excessively high voltages can cause breakdown of the photoconductor. The thickness is limited to Values at which the more fundamental parameters of the photoconductor (c g. lifetime, mobility and electric breakdown eld strength) coupled With the thickness still permit desired gain to occur. The thickness of the photoconductor is also limited by the resolution required` For 50 lines per mm. of resolution, the thickness of the photoconductor should not be greater than 110 microns. The voltage of source r 7 in FIG. l is on the order of 700-'1000 rvolts for the4 films used.
In accordance with an important aspect of the present invention, the sensitivity of the electrophotography procn ess is further greatly enhanced by decreasing the capacior one-half electron per photon is more realistic. However it is seen that the total charge generated in response to a photographic image, when using the external capaciiIt Vappears energy is tance of the dielectric storage layer after the storage layer has received the charge pattern constituting the latent image. This decrease in capacityris conveniently accomplished by removing the eletrode associated with the dielectric storage layer during exposure. In removing the electrode, work is ydone upon the charge pattern to increase the energy of the charge pattern which is availa-ble for development.
The energy on the capacitive layer equals Since Q=CV, then trgy Therefore the energy associated with the charge equals proportional to the square of the charge on the dielectric, Q, and inversely proportional to its capaictance, C. Since we reduce the capacity and do work on the charge, an increased pattern energy results.
The greatly increased sensitivity achieved in accordance with the present invention results in photographic speeds as great as 10 times that heretofore attained by conventional processes. For example, with panchromatic response, employing the usual photoconductor materials, a speed of ASA. 100 can be obtained. To review, these desirable results are obtained by transferring a large .portion of a charge pattern, representative of more than one electron per photon, from a photoconductor onto a conductively backed, relatively high capacity dielectric layer. The dielectric layer is removed from the photoconductor and Work is performed on the charge pattern by removing the conductive backing to substantially raise the energy level of the charge pattern. This charge pattern is then transferred to a development material capable of differential attraction and arrangment in response to the charge pattern.
While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes `and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.
What I claim `as new and desire to secure by Letters Patent of the United States is:
1. A process of electrophotography comprising the steps of:
forming a laminate of a thin dielectric layer and a photoconductive layer to provide a common interface,
p-roviding the exposed surfaces of said laminate with electrodes, at least one of which is optically transparent,
uniformly exposing said photoconductive layer to light while simultaneously applying a voltage source across said electrodes, terminating said uniform light exposure and disconnecting said voltage source from said electrodes,
exposing said photoconductive layer to a light image while simultaneously inte-rcoupling said electrodes to cause a differential transfer of charge through said photoconductive layer to establish a latent electrostatic image corresponding to said light image whereby said charge distributes itself between said photoconductive layer and said dielectric layer such that said dielectric layer receives a larger portion of charge than said photoconductive layer,
separating said layers at said common interface,
reducing the capacity of said dielectric to increase the energy of the charge pattern thereon, 4and applying to said dielectric layer further means capable of differential attraction and varrangement in response to the latent electrostatic image carried by said dielectric layer todevelop said electrostatic image.
2. The process set forth in claim 1 wherein the capacity of said dielectric layer is reduced by removing its electrode.
3. The process 4set forth in claim 1 wherein said further means applied to said dielectric layer comprises an electroscopic toner powder.
4. The process set forth in claim 1 wherein said further means applied to said dielectric layer comprises disposing said dielectric layer in contact with a layer of thermoplastic dielectric material to form a second laminate and heating said thermoplastic material to a softened condition to cause selective deformation of said thermoplastic material in lresponse to said latent electrostatic unage.
5. The process set forth in claim 4 wherein the exposed surface of said thermoplastic material comprising said second laminate is provided with a ground plane.
References Cited UNITED STATES PATENTS OTHER REFERENCES 'Crossz Deformation Image Processing, IBM Technical Disclosure Bulletin, vol. 4, No. 7, December 1961, -pages 35-6 (copy in Scientific Library).
NORMAN G. TORCHIN, Primary Examiner. A. LIBERMAN, C. E. VAN YHORN, Assistant Examiners.