CA1129481A - Method of electrophotography and apparatus for practicing the same - Google Patents
Method of electrophotography and apparatus for practicing the sameInfo
- Publication number
- CA1129481A CA1129481A CA324,587A CA324587A CA1129481A CA 1129481 A CA1129481 A CA 1129481A CA 324587 A CA324587 A CA 324587A CA 1129481 A CA1129481 A CA 1129481A
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- Prior art keywords
- image
- photoconductor
- electrostatic image
- areas
- light
- Prior art date
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
- G03G15/00—Apparatus for electrographic processes using a charge pattern
- G03G15/04—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material
- G03G15/045—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material with means for charging or discharging distinct portions of the charge pattern on the recording material, e.g. for contrast enhancement or discharging non-image areas
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Electrophotography Using Other Than Carlson'S Method (AREA)
- Exposure Or Original Feeding In Electrophotography (AREA)
- Wet Developing In Electrophotography (AREA)
- Photoreceptors In Electrophotography (AREA)
Abstract
Title of the Invention IMPROVED METHOD OF ELECTROPHOTOGRAPHY
AND APPARATUS FOR PRACTICING THE SAME
Abstract of the Disclosure My invention comprises an improved method of electrophotography which enables me to increase the effective speed of a photoconductor. The distinguishing feature of xerography is the creation of an electrostatic image at some point in the process. Most conveniently, a photoconductor is charged in the dark and then exposed to a light and shade image of the original to produce a latent electrostatic image of the original. In order to make a satisfactory copy, the exposure time must be such that sufficient contrast is achieved to enable the latent image to be made visible by the application of a developer formed of inherently colored or pigmented minute charged particles.
The speed at which copies may be made is the function of the quantum of light falling on the photoconductive surface and the conductivity of the photoconductor under illumination.
Since the rate at which a given photoconductor discharges applied surface potential through the action of light is limited, speed can be increased only by increasing the illumination. This requires energy and produces heat. My process deliberately underexposes a charged photoconductor to a light and shade image of the original to produce a weak latent image of low contrast which is insufficient to make a satisfactory copy. The latent image, however, is sufficient in contrast to enable me to provide an optical mask or shield over the image by toning it. I then discharge the background or non-image areas of the photo-conductor by a blanket illumination of low intensity.
The optical mask prevents the image areas from discharging while enhancing the contrast of the weak latent image.
The enhanced image is easily developed by any known de-veloping method for making latent electrostatic images visible at a development station.
The apparatus can be any of the electrophoto-graphic machines known to the art, which I provide with means for producing an optical shield over the weak latent electrostatic image and a background discharging floodlight lamp interposed between the shield-forming station and the image-development station of the electrophotographic machine.
The means for producing the optical shield or mask is development station using usual or special development toners, which may be liquid-carried or be in dry powdered form. A salient feature of the apparatus is the means for applying developer to the weak latent image. This must be biased to a voltage above that of the background areas and below the potential of the image areas. This ensures that the background areas are unmasked or unshielded while the image areas are masked.
AND APPARATUS FOR PRACTICING THE SAME
Abstract of the Disclosure My invention comprises an improved method of electrophotography which enables me to increase the effective speed of a photoconductor. The distinguishing feature of xerography is the creation of an electrostatic image at some point in the process. Most conveniently, a photoconductor is charged in the dark and then exposed to a light and shade image of the original to produce a latent electrostatic image of the original. In order to make a satisfactory copy, the exposure time must be such that sufficient contrast is achieved to enable the latent image to be made visible by the application of a developer formed of inherently colored or pigmented minute charged particles.
The speed at which copies may be made is the function of the quantum of light falling on the photoconductive surface and the conductivity of the photoconductor under illumination.
Since the rate at which a given photoconductor discharges applied surface potential through the action of light is limited, speed can be increased only by increasing the illumination. This requires energy and produces heat. My process deliberately underexposes a charged photoconductor to a light and shade image of the original to produce a weak latent image of low contrast which is insufficient to make a satisfactory copy. The latent image, however, is sufficient in contrast to enable me to provide an optical mask or shield over the image by toning it. I then discharge the background or non-image areas of the photo-conductor by a blanket illumination of low intensity.
The optical mask prevents the image areas from discharging while enhancing the contrast of the weak latent image.
The enhanced image is easily developed by any known de-veloping method for making latent electrostatic images visible at a development station.
The apparatus can be any of the electrophoto-graphic machines known to the art, which I provide with means for producing an optical shield over the weak latent electrostatic image and a background discharging floodlight lamp interposed between the shield-forming station and the image-development station of the electrophotographic machine.
The means for producing the optical shield or mask is development station using usual or special development toners, which may be liquid-carried or be in dry powdered form. A salient feature of the apparatus is the means for applying developer to the weak latent image. This must be biased to a voltage above that of the background areas and below the potential of the image areas. This ensures that the background areas are unmasked or unshielded while the image areas are masked.
Description
l~Z~481 Backqround of the Invention One of the main problems with electrophotographic copiers arises from thc desire for speed in copy production.
After a photoconductor has been charged, the energy required to produce a latent image in light and shade of the oriqinal of sufficient contrast to produce an acceptable copy is a ~unction of the quantum of light falling upon the photo-conductor and the light sensitivity of the photoconductor.
Ideally, the illumination of the photoconductor should be such that the brightest part of the image will be fully discharged while the darke~t part of the imag~a will leave the photoconductor fully charged. In practice, th~s ls never achieved, owing to the limits of the light response of known photoconductors. In the current state of the art lS of photocopying machlnes, when the speed of producing copies exceeds about 30 copies per minute, the energy required to operate the copier approaches 1500 watts. Since the ordinary potential in office and house wiring is 110 volts, the power from a given outlet is limited to 1500 watts. Accordinqly, to produce satisfactory coples at a higher rate, a ~peclal electrical installatlon will be required. This mean~ that the copying machine cannot be decentralized, but mu~t be located in the region of the hlgher voltage outlet. Purther-more, the high energy will produce thermal pro~lems, both ln respect of the photoconductor and in the environment, aslda from the expense of energy consumption. Becau~e of the~e .
ilZ~
problems, many effort~ are being made to increa~o the light sensit~vity of photoconductor~.
Field of the Invention My invention relstes to a novel method of increasing the effective sensitivity of photoconductors, thus enabling me to increase the speed of electrophoto-graphic reproduction of documents.
Description of the Prior Art The following art is of interest in respect of or i8 referred to in this specification:
Steinhilper ......... Patent 2,756,676 Schaefer et al ...... Patent 3,892,481 Hayashi et al ....... Patent 3,907,423 BrooXe .............. Patent 3,912,3~7 Brooke .......... ~... Patent 3,994,723 Steinhilper, which will be discus~ed more fully hereinafter, propose~ to maXe multiple copie~ of an image produced from a 3ingle light exposuro of an original. He recharges the photoconductor after each transor of n de-veloped image and cnhances the recharged image by sub~ectlng it to illumination. There is no teaching o lncreasing the speed o the xerographic reproduction proces~. The appa-ratus shown by Steinhilper has only one development ~tatlon.
rl~he.re is no o~tica:l maskinc~ station. There is no sho~7illc~
of a biasc~cl toncr app:licator at ~ toni.nc3 station where optical shie:ld.in(l is achievc(~.
Schaefer et al show an aut:omatic control system for bias:incJ a deve,~opment elec-trode. 'l'his system can be used both for the mask-formirlg step, which is a salient feature of my inventi.on, and for the development step as taught by Schaefer et al.
}~ayashi _ al show a revcrse roller desicJIled to remove excess liqui,d from thc pho-tocollcluctor after the latent image has been developed. I employ a roller oE
this type, insulatcd from ground arld biased to a voltage of the same po:l.arity as -the charcJe on the ph(-tc)conductc)r, bu-t: at a potentill hiCJtlC`l- than the backc3roul~cl potential, in order to ellsure -that no -toner is deposited on the bc~ck-q~ound areas oE the i.mac~e when the mat;k~fol-millcl st.ep is performecl.
]3rooke Patent 3,9:L2,387 ancl i.ts cl:ivisional pclterlt 3,99~,72~ show det:c~c-t,:intJ l~ackcJround al-etls wh:i.cil arc~, un(lc~-cxposccl an('l discli~ ;.tl~l t:llcrll b~ :I.iclht: bcro~-c dc~vel.opmellt o~ the :L~LcnL elc~cl:rost,at:i.c imat~e~.
Sulnmary o:l~ t:lle Illvc3nl i.OII
. _ .. ... . , _ _ .. .. _ .... .... .
13.r:o~ld:l~ ~;E)(~(I}~ CJ tllc~ r~ t;(~ lvc~n~ l rc~tlt,~!C; to met:llocl oE elocctro~)llol:ocll-al~lly W]IC.l-C.ill a ~.at,ellt, c~loc~trost:ati.c ,im~ e oE a clocurncrlt: havinc~ l;.qht ancl slladc ~.ll'CclS :iS .fOrlllC'd Orl a photoconcluctor alld thc laterlt :ima~Jc i.s developed to produce a vlsible :ilna~Je. In accordclnce with the metllod of -the inven-tion the ~atellt elcctr.ostatic isll.lc~e it.; toned so as to for.m an .. - . i .
~ pCJ/"; - 5 -llZ9481 Optical sl~i~ld over -the shacle ar~as o:E t,he e]ectl-osta-tic image. ~l~he pl~otoconc~uctor i.s then ~ubject,ccl to lic~ht -Lo dischar-Je tlle l:ic3ht al^~as o:E the imac~e on the phoL()(.onductor b~Eore practicincJ ~he d~velopi.nc~ step~
The appara-tus ~or practicing the above Inethod includes a photoconductor, a station for el.ectrostatically charc3illy tlle photoconduc-tor, an ~xposin~ s-ta-tion for sub-jectinc3 -the charge~ photoconductor to a lic3ht and shade image o:E the document to b~ copied -to ~orm a lat~nt electrostclt.ic imacJ~ on the pllot.ocon~uc-tor and a develop-ing station to ma]{~ the laterl-t ima~ optically visible.
Betwe~n the charging stati.on and th~ dev~lopinc~ station i.s a s-tcltion Eor optica],]y maskirlcJ -the Sllc-l~tO are(ls oE thc latent elc~ctrostcat:i.c i.macJe anc~ a Stclt:iOIl :Eor l.icJht-dischclrc3-ing the unlllasked ar~as o:E tllc pl-lotocoIlcluctor. In addition, means are providcd ~or e:EI-ecti~ re:lat:ivc movemellt oF thC!
photoconcluctox Wi.tll L~Sp(~Ct: to all. o.E l-he c~bc)~o-lllentic)ned ~tation~.
Otll~r c-lIId :~ur~.ll~l~ c-l<.l~(lcl:~; o~ i.nv~ t.ion w~ b~
~l~p~ nt C~ tll~.~ f~ t~ c~ c~ i.ol~.
Br.ie:E l)esc~ .?ti.on o:E the ]~ lWi n(;JS
_ _ ___ . .. .......... . . . __. . .. .. . .. . .. ~ . .. ..... .Ill t~ ccol~l}?~ yil)(~ ill(J~; ~ wh-i.(.~ .toJ-m l~cl.rl: c)I' the i.n~tallt sE)cc~l'.:i.cat:i.on and Wll;.C]I CnIe t.o b-` reC~ j.n COn~
junctioll thcrew:i.l~
FIGUR13 ]. is a :E.~ow cliacJJ-clm showi.ncJ tllc~ steps oE
my improved method o-E el.ectrophotc)(~raplly, in which the P~/ - 6 -full-line arrows indicate necessary steps and the broken-line arrows indicate optional steps of my process.
FIGURE 2 is a diagrammatic view showing apparatus capable of carrying out my invention.
FIGURE 3 is an idealized curve in which tho ordinates are logarithmic and the abscis~ae are linear, showing the potential on the surface of the photoconductor plotted against quantum of light in foot-candle second~
to which the photoconductor has been exposed.
FIGURE 4 i9 a chart plotted with logarithmic ordinates, showing the voltages on the surface of the photoconductor, charged as shown in FIGURE 3, after an exposure of 0.25 foot-candle seconds.
FIGURE S i9 a view similar to FIGURE 4, showing the voltages on the photoconductor after the image areas oS
FIGURE 4 have been masked and the non-image or background areas have been discharged by light in accordance with my invention.
~ Descri~tion of_the Preferred Embodiment In general, my invention contemplntes charging a photoconductor in the dark. The charged photoconductor i-then exposed to a light and shade image of the originnl.
This will form a latent electrostatic image on the photo-conductor. The oriqinal exposure is only a fraction, such as 5% or 10~, of the quantum of light normally required to produce a sati~factory image -- that i~, one having 3uffi-cient contrast so the image areas are dark and the back-ground areas are white. The latent image thus form~d has sufficient contrast, however, 80 that it can be toned --that i~, developed -- with any appropriate toner such a~
resinous powder or, more preferably, by a toner di~persed in an insulating liquid, as i3 well known to the art. This masking step produces a shield substantially OpaquQ to light over the image areas of the original being copied.
A critical feature of the masking step is that a shield will be produced only over the image areas, and not over any of the background areas. This is accomplished by en-suring that the development electrode, or means for applying the toner, during the masking ~tep is biased to a potential above that existing on the background area~ and below that present on the image areas of the latent electrostatic image.
I then expose the photoconductor to a blankot of light. The electrostatic charge of the masked image on the photoconductor will decay marglnally or not at all, while the electrostatic charge of thc background areas will bo discharged to a very low voltage, such as 50 volts or thQ
like. This has the effect of enhancing the charge of tho latent electrostatic image by a very largo percentage with the requirement of about one-tenth of the energy which would normally be neces~ary to produce a latent image having tho 1129~B~
strong contrast now achieved by my method. The cnhanced electro~tatic image can then be developed in any appro-priate manner known to the art and, if de~ired, readily transferred to a carrier sheet.
More particularly, referring now to FIGURE 2 in which apparatus for carrying out my invention i9 ~hown, a metal drum 10 carries a photoconductive layer 12 which may be selenium. The metal drum 10 is supp~rted by apertured di~s 14 which are mounted or. a shaft 16 and keyed theroto for rotation therewith. The shaft 16, which may be grounded, i9 driven by any appropriate means Xnown to the art to ro-tate the drum 10 in the direction of the arrow. A charging corona 18 is adapted to charge the 3urface of the selenium photoconductor 12 to a voltage of between 800 and 1000 volts.
To accomplish this, the charging corona is energized to a positive potential of 5000 or 6000 volts. The element~ of the corona discharge unit cause ionization of the circum-ambient atmosphere and place a uniform positive chnrge over the surface of the selenium. If my proccss were belng practiced with a zinc oxide-coated paper, the corona would be powered to produce a negative charge, a9 will be readily understood by those skilled in the art. ~he photoconductor 12 is then carried past the exposure station indicntod generally by the reference numeral 20. Projection optic~, indicated diagrammntically by the lenu 22, pro~ect an imnge of the original to be copied upon the photoconductor 12.
llZ9481 In my method, the exposure time i8 extremely ~hort. I have made copies with an expo~ure of a~ little as 5X of normal and have been able to achieve completely satisfactory copie~ of the originals.
A selenium photoconductor will generally discharge to about one-fifth of its original charge in about three foot-candle second3. This can be readily seen by reference to FIGURE 3, which shows a surface potential on a selenium photoconductor of 800 volts b~ing discharged to 160 volts in between two to thrce foot-candle seconds. Normally, sufficient energy is employed in the light source of photo-copying machine~ so that the bacXground areas of the photo-conductor will be discharged to about 50 or 60 volts. Thi~
will require about five foot-candle seconds. Five percent of this quantum of light is about 0.25 foot-candle second~.
By referring to FIGU~ES 3 and 4, it will be noted that, after this short exposure, the background area~ (B) will have dropped in voltage about 100 volts from the 1mage areas ~I). If this image were toned, a very low-contrast image would be achieved. If thi~ imago, after boing toned, were to be transferred onto papor from the drum, thc density of the toned image would be so small that a poor transfer or a failure to transfer would re~ult and only a faint image would appear. The low-contrast image, however, when de-veloped, has sufficient optical density so that it provides a ma~k or shield for the latent electrostatic image which is sufficiently dense for the practice of my proce~.
~lZ948~
Referring again to FIGURE 2, I show apparatu~ for providing a mask or shield for the latent electrostatlc image. It comprises a tank 24, from which a developlng liquid 26 containing dispersed toner particles, whlch may be S charged, is drawn through pipe 28 and pumped by pump 30 through pipe 32 to nozzle 34, adapted to discharge the de-veloper between the photoconductor and a reverse roller 36.
If the toner particles are conductive, they may acquire a charge by induction, owing to their passage ad~acent the latent image under the action of its electric field. In this case, the clectrostatic charge pattern serves first to charge the particles and then to trap them. In the case of a liquid-carried toner particle, the continuous phase is an insulating liq~id -- such as a hydrocarbon liquid, a fluorinated hydro-carbon liquid, or the li~e -- having low vapor pressures at room temperature, and the disperse phase is composed of the minute particles of toner adapted to make the latent electro-static image visible. As iB known in the art, the polarity of the charged particles may be controlled by materlals added to the developing liquid. Thcse act by adsorption onto the surface of the particlcs and alter the magnitude and polarity of the charge acquircd by the particles, de-pending on the environment of the particles at the tlmc of their formation and tho method of their preparatlon. Tho toner particles must bc applicd by a dcvelopment electrode biased to a potential of the same polarity a~ that of the latent electrostatic image and to a potentlal greater thnn l~Z9481 that o~ the background areas and below that of the image are~. Preferably, I employ a reverse ro~ler a~ the de-velopment electrode when a liquid developer i~ used. Thlo reverse roller i~ made of metal and is mounted on ~haft 38 for rotation in a direction oppoRite to the rotation of the photoconductor. The reverse roller i8 insulated from ground and is positioned closely adjacent the ~urface of the photoconductor to provide a gap ranging from 0.05 to 0.1 millimeter. The reverse roller is driven by a prime mover and is controlled in speed so as to remove excess developing fluid from the photoconductor. The biasing of the development electrode i~ critical to my process, ~ince there can be no masking of the background areas in my process, as will be pointed out more fully hereinafter.
It will be appreciated that the close proximity of the insulated metal reverse roller to the ~urface of the charged photoconductor is such that it will float to a~sumo tha average potential of the photoconductor and thus be auto-biased. Since the average potential on the rever~e applicator roller 36 will be above the background potentlal on the photoconductor, toner particles will migrate to the applicator roller in~tead of to the background area~ on the photoconductor. If desired, in~tead of pcrmitting the reverse applicator or metering roller 36 to float electrl-cally, it may be biased to a potential from any appropriate D.C. voltage ~ource to above the potential of tho bac~ground areas but below the potcntial of the image areas. The ~ia~
112948~
on the toner applicator roller will substantially elimlnate the deposition of ma~king toner on the background areas of the photoconductor. In the usual developing liquid, how-ever, the carrier liquid has a low boiling point, ~o that it is easily vaporized to ensure that the developed image, when transferred to a carrier sheet such as paper, will pro-duce a copy dry to the touch. In the maskinq step, however, a hydrocarbon carrier liquid having a higher boiling point which will not vaporize may be employed. Thi~, of cour~e, will reduce the danger of atmospheric pollution during the masking step. If desired, instead of a liquid-carried toner, dry toner may be used for the masking step. Such dry toners are well known to the art. One example of a reverse roller which can be u3ed in my invention is shown in Hayashi et al Patent 3,907,423. The excess toner from the masking step will be caught in the tank 24 for recycling. A wiper blade 40 keeps the metal reverse roller clean.
After the optical shield is produced in any appropriate manner such as described, the photoconductor i~
subjected to a blanket of light. This may be accompli~hod by an elongated incandescent lamp 42 placcd adjacent the photoconductor bearing the masked image and extending thore-across. In an office copier, a ~uartz-halogen lnmp h~ving an output of about 500 watts i3 u3ually employed. In my process, a much lower-energy exposurQ lamp to project the image may be employed. Furthermore, the illuminAtion of l~Z9481 'I
the background areas to discharge them to n re~ldual v~ltago of about 50 volts requires comparatively ~mall energy. A8 will be readily appreciated by those skilled ~n the nrt, the exposure step subject~ the photoconductor to an image of the original by reflected light. The illuminated original i~
focused by the optical system upon the charged photoconductor.
Since the light gathered by the optical system i9 a small fraction of the light which illuminates the original, a bright illumination of the original is required. In the background discharging step, after the optical 3hield is in place, the illumination of the photoconductor i8 by direct light, which accounts for the small energy required to discharge the background areas. The image areas ~I) will not be discharged owing to the mask or shield which I have lS provided by my proces~ as jUQt described. The effect of discharging the potential of the background areas (~) whilQ
leaving the image areas (I) substantially undischarged t~
shown in FIGURE 5.
It will be seen that my process has achieved a contrast of substantially 750 volts between the bacXground areas and the image areas and accomplishod tha creation of this strong field nt approximately a tenfold reduction of the energy required to expose thc original. This mean~, as will be readily apparent, that a photocopying machine which prosently uses a 500-watt lamp of the quartz-halogen typo with a tungsten filament could use a 50-watt lamp or, ~2948~
alternatively, employ a fluore~cent light. It will aloo be appreciated that, where a photocopying machine presently i8 able to make only about 25 copies per minute, theoretically, I can easily make a photocopying machine capable of pro-ducing 150 copie~ per minute. A~ a practical matter, however, owing to the inertia of the parts of the photo-copying machine and in order to avoid marginal operation to provide a factor of safety, a photocopying machine which will produce 75 to 100 copies per minute can be made embody-ing my invention. Furthermore, this can be done without having to increase the energy expended significantly, since the only additional energy required will be that employed in the first developing or masking step and that in the light discharging of the photoconductor after the image areas have been masked. It will be further understood that, instead of an incandescent lamp, I may u~e any appropriata light source adapted to flood the photoconductor.
The use of a floodlight to enhance a faint latent image on a photoconductor i8 not new in and of it~elf.
Steinhilper Patent 2,756,676 dcscribes a method of making a plurality of xcrographic reproductions from a single expo-sure of an origin~l. In Stcinhilper, however, the effective speed of the photoconductor is not increased, owing to the fact that Steinhilper must go through a first development step which produce3 a fully-toned image. There i9 no m~king step a~ described in my process. After the first image i~
l~Z9481 developed, it is transferred to a carrier sheet ~uch as paper. The faint image which is left on the photoconductor is of a potential too low to be enhanced by light or to be redeveloped. Steinhilper does not erase this image on the - 5 photoconductor, but recharges the photoconductor. He then discharges the background areas by light. Since the faint image does produce a shield, an enhanced latent image will be produccd. The salient feature of my proces~, however, is absent from Steinhilper. He does not form a lcw-contrast latent e1ectrostatic image in such a manner a~ to leave the bacXground areas free of developer, owing to the fact that his development electrode is never biased but always at ground. Steinhilper must carry out his proce~s to form the residual image from the first transfer of the developed image at the normal ~low rate. Thus the unobv~ou~ re~ult which I achieve -- namely, the increasing of the effective sensitivity of the photoconductor -- is not taught, nor can it be achieved, by Steinhilper.
Owing to the tremendous range through which I am enabled to obtain sufficient optical density to produce a mask, I can employ a single exposure and a biased ~etting in the developer system and obtain a sharp, clear copy from any orlginal, whether the bacXground is ultra white or dingy gray.
It will be readily apparent to tho~e ~killed in the art that, with the contra~t potential shown in FIGURB 5, , ~lZ9~81 there is no problem in obtaining a ~harp, clear im~ge.
After the background potential has been di~chargod by tho lamp 42, the optical shield may be wiped from the enhanced latent image thus formed by a cleaning roller 44 made of sponge rubber or the like, if desired. This wiping action can take place with either a liquid-toned mask or a dry developer-toned ma~k. The enhanced latent electro~tatic image may then be toned by any usual method known to the art.
In FIGURE 2, I have shown the toning system described in Schaefer et al Patent 3,892,481, employing a tank 46 from which a liquid toner 48 circulates from pipe 50 to a toner supply tank ~not shown) and back through pipe 52 to the tank 46. A development electrode 54 i9 controlled to bias any residual voltage left on the background of the photoconductor. It will be readily appreciated, however, that since I have discharged the background potential by my method, I can use a fixed bia~ slightly nbove the average residual bias of the background. This will produce a cluar white background and enable me to eliminate, if deaired, the sensing and biasing method shown in the Shaefer et al patent.
After development with a liquid-carried toner, a rever~e roller 56, sucll ag shown in ~}ayashi et al Patent 3,907,~23, i9 positioned to remove excess developer from the developed image, The reverse roller 56 i8 provided with a ... .
wiper 5~. The reverse roller 56 i9 positioned and rotate~
at ~peeds as described in the llayashi et al patent.
The image is now ready to be transferred to a carrier sheet such as plain paper. A plain paper sheet 60 i9 fed by rollers 62 to a roller 64, past a transfer-charging corona 66. It will be recalled that the toned image still comprises a visible image over a high positive charge on the surface of the selenium drum in the image areas which haye not been discharged by light or by the bia~ applied to remove the residual background potential.
To transfer the developed image from the drum to the paper carrier sheet, a high positive charge is applied to the back of the copy paper. As a result of the application of the high positive charge to the sheet, the toner particles are pulled from the drum surface onto the paper. A pic~-off 68 ensures that the paper leaves the drum, and the end of the paper 70, now carrying the image, may be dried and passed to a receiving tray tnot shown). A cleaning roller 72 wipes the drum clean of any particle~ of toner which have not been removed from the drum, and a wiper blade 74 complates the drum-cleaning operation.
It will be understood, of course, that lf my process i9 applied to a ~inc oxide-coated paper, the im~ge will remain on the coated paper. It will al80 be under-stood that a dry toner, made of fusible resinous powder nnd fixed by heat, may be employed, as i9 well known to the art.
~129481 It will be understood by thos~ sXllled in the art that, since the photoconductor now bears an enhanced latent electrostatic image having a strong field, it may be de-veloped in any appropriate manner known to the art.
A selenium photoconductor i8 very ~ensitlve to blue light and, accordingly, photocopylng machine~ using a qelenium photoconductor do not produce copies from blue-colored originals with high contra~t. That ls to ~ay, a selenium photoconductor "sees" blue light as almost white.
My method will reproduce bluo originals as if they were black. A yellow original is very light in color and re-flects considerable light, 80 that it appears faint in the copie~ made by photocopying machines. My method of ~mage-enhancing reproduces yellow effectively. Furthermore, gradations in density -- that is, a gray scale -- are also achieved with my method.
Owing to the wide latitude of effective photo-conductive sensitivity which my process provides, when uslng the automatic bias of Schaefer et al Patent 3,892,4al, I can adjust the illumination in the step whlch forms the low-contrast image prior to thc mask-forming step to produco a satisfactory reproduction with the brlghtest background --that is, a purc white background -- in the original document.
Thia will produce an image of contrast too low to be effec-tively developed to a aatisfactory copy, but such that the 1~29481 mask-forming ~tep can be easily performed. The bla~
potential applied to the applicator roller will be well above that required to eliminate background potential of the latent electrostatic image completely, but will be S below the potential of the image areas. This permits the image areas to be optically masked by toner during the optical shield-forming step. The illumination of the image-forming step may be readily controlled by varying the intensity of the light source or by a shutter in the optical path of the image-projection system.
It will be further under~tood by those skilled in the art that, while I have shown and described the toning of an image and then its tranqfer to n carrier sheet, my image-enhancing process can be used in any method of electrophotography. For example, the enhanced latent electrostatic image can be transferred to a dielectric sheet and then toned or developed into a visible im~ge on the dielectric sheet, a~ is well known in the art.
It will be qeen that I have accomplished the objects of my invention. I havo increased tho effective sensitivity of photoconductors. I have provided an improved method of electrophotography which will qreatly incroaJe the speed of COpyillg operations. My method achieveJ thiD ln-crease in speed with a reduction of the quantum of energy required. I am enabled to copy originals having poor llZ9481 contrast which will produce copies having surprisingly increased contrast a~ compared with the originalJ. I am enabled to copy originals formed in colors to wh$ch the photoconductor has great sensitivity and which, accord-ingly, do not ordinarily produce copies having the desired contrast. I have provided a novel appAratUs for carrying out my improved method.
It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. ~his is contemplated by and is within the scope of my claims. It i9 further obvious that various change~ may be made in de-tails within the scope of my claim~ without departing from the spirit of my invenition. It is, therefore, to be under-stood that my invention is not to be limited to the specific details shown and described.
Having this described my invention, what I claim
After a photoconductor has been charged, the energy required to produce a latent image in light and shade of the oriqinal of sufficient contrast to produce an acceptable copy is a ~unction of the quantum of light falling upon the photo-conductor and the light sensitivity of the photoconductor.
Ideally, the illumination of the photoconductor should be such that the brightest part of the image will be fully discharged while the darke~t part of the imag~a will leave the photoconductor fully charged. In practice, th~s ls never achieved, owing to the limits of the light response of known photoconductors. In the current state of the art lS of photocopying machlnes, when the speed of producing copies exceeds about 30 copies per minute, the energy required to operate the copier approaches 1500 watts. Since the ordinary potential in office and house wiring is 110 volts, the power from a given outlet is limited to 1500 watts. Accordinqly, to produce satisfactory coples at a higher rate, a ~peclal electrical installatlon will be required. This mean~ that the copying machine cannot be decentralized, but mu~t be located in the region of the hlgher voltage outlet. Purther-more, the high energy will produce thermal pro~lems, both ln respect of the photoconductor and in the environment, aslda from the expense of energy consumption. Becau~e of the~e .
ilZ~
problems, many effort~ are being made to increa~o the light sensit~vity of photoconductor~.
Field of the Invention My invention relstes to a novel method of increasing the effective sensitivity of photoconductors, thus enabling me to increase the speed of electrophoto-graphic reproduction of documents.
Description of the Prior Art The following art is of interest in respect of or i8 referred to in this specification:
Steinhilper ......... Patent 2,756,676 Schaefer et al ...... Patent 3,892,481 Hayashi et al ....... Patent 3,907,423 BrooXe .............. Patent 3,912,3~7 Brooke .......... ~... Patent 3,994,723 Steinhilper, which will be discus~ed more fully hereinafter, propose~ to maXe multiple copie~ of an image produced from a 3ingle light exposuro of an original. He recharges the photoconductor after each transor of n de-veloped image and cnhances the recharged image by sub~ectlng it to illumination. There is no teaching o lncreasing the speed o the xerographic reproduction proces~. The appa-ratus shown by Steinhilper has only one development ~tatlon.
rl~he.re is no o~tica:l maskinc~ station. There is no sho~7illc~
of a biasc~cl toncr app:licator at ~ toni.nc3 station where optical shie:ld.in(l is achievc(~.
Schaefer et al show an aut:omatic control system for bias:incJ a deve,~opment elec-trode. 'l'his system can be used both for the mask-formirlg step, which is a salient feature of my inventi.on, and for the development step as taught by Schaefer et al.
}~ayashi _ al show a revcrse roller desicJIled to remove excess liqui,d from thc pho-tocollcluctor after the latent image has been developed. I employ a roller oE
this type, insulatcd from ground arld biased to a voltage of the same po:l.arity as -the charcJe on the ph(-tc)conductc)r, bu-t: at a potentill hiCJtlC`l- than the backc3roul~cl potential, in order to ellsure -that no -toner is deposited on the bc~ck-q~ound areas oE the i.mac~e when the mat;k~fol-millcl st.ep is performecl.
]3rooke Patent 3,9:L2,387 ancl i.ts cl:ivisional pclterlt 3,99~,72~ show det:c~c-t,:intJ l~ackcJround al-etls wh:i.cil arc~, un(lc~-cxposccl an('l discli~ ;.tl~l t:llcrll b~ :I.iclht: bcro~-c dc~vel.opmellt o~ the :L~LcnL elc~cl:rost,at:i.c imat~e~.
Sulnmary o:l~ t:lle Illvc3nl i.OII
. _ .. ... . , _ _ .. .. _ .... .... .
13.r:o~ld:l~ ~;E)(~(I}~ CJ tllc~ r~ t;(~ lvc~n~ l rc~tlt,~!C; to met:llocl oE elocctro~)llol:ocll-al~lly W]IC.l-C.ill a ~.at,ellt, c~loc~trost:ati.c ,im~ e oE a clocurncrlt: havinc~ l;.qht ancl slladc ~.ll'CclS :iS .fOrlllC'd Orl a photoconcluctor alld thc laterlt :ima~Jc i.s developed to produce a vlsible :ilna~Je. In accordclnce with the metllod of -the inven-tion the ~atellt elcctr.ostatic isll.lc~e it.; toned so as to for.m an .. - . i .
~ pCJ/"; - 5 -llZ9481 Optical sl~i~ld over -the shacle ar~as o:E t,he e]ectl-osta-tic image. ~l~he pl~otoconc~uctor i.s then ~ubject,ccl to lic~ht -Lo dischar-Je tlle l:ic3ht al^~as o:E the imac~e on the phoL()(.onductor b~Eore practicincJ ~he d~velopi.nc~ step~
The appara-tus ~or practicing the above Inethod includes a photoconductor, a station for el.ectrostatically charc3illy tlle photoconduc-tor, an ~xposin~ s-ta-tion for sub-jectinc3 -the charge~ photoconductor to a lic3ht and shade image o:E the document to b~ copied -to ~orm a lat~nt electrostclt.ic imacJ~ on the pllot.ocon~uc-tor and a develop-ing station to ma]{~ the laterl-t ima~ optically visible.
Betwe~n the charging stati.on and th~ dev~lopinc~ station i.s a s-tcltion Eor optica],]y maskirlcJ -the Sllc-l~tO are(ls oE thc latent elc~ctrostcat:i.c i.macJe anc~ a Stclt:iOIl :Eor l.icJht-dischclrc3-ing the unlllasked ar~as o:E tllc pl-lotocoIlcluctor. In addition, means are providcd ~or e:EI-ecti~ re:lat:ivc movemellt oF thC!
photoconcluctox Wi.tll L~Sp(~Ct: to all. o.E l-he c~bc)~o-lllentic)ned ~tation~.
Otll~r c-lIId :~ur~.ll~l~ c-l<.l~(lcl:~; o~ i.nv~ t.ion w~ b~
~l~p~ nt C~ tll~.~ f~ t~ c~ c~ i.ol~.
Br.ie:E l)esc~ .?ti.on o:E the ]~ lWi n(;JS
_ _ ___ . .. .......... . . . __. . .. .. . .. . .. ~ . .. ..... .Ill t~ ccol~l}?~ yil)(~ ill(J~; ~ wh-i.(.~ .toJ-m l~cl.rl: c)I' the i.n~tallt sE)cc~l'.:i.cat:i.on and Wll;.C]I CnIe t.o b-` reC~ j.n COn~
junctioll thcrew:i.l~
FIGUR13 ]. is a :E.~ow cliacJJ-clm showi.ncJ tllc~ steps oE
my improved method o-E el.ectrophotc)(~raplly, in which the P~/ - 6 -full-line arrows indicate necessary steps and the broken-line arrows indicate optional steps of my process.
FIGURE 2 is a diagrammatic view showing apparatus capable of carrying out my invention.
FIGURE 3 is an idealized curve in which tho ordinates are logarithmic and the abscis~ae are linear, showing the potential on the surface of the photoconductor plotted against quantum of light in foot-candle second~
to which the photoconductor has been exposed.
FIGURE 4 i9 a chart plotted with logarithmic ordinates, showing the voltages on the surface of the photoconductor, charged as shown in FIGURE 3, after an exposure of 0.25 foot-candle seconds.
FIGURE S i9 a view similar to FIGURE 4, showing the voltages on the photoconductor after the image areas oS
FIGURE 4 have been masked and the non-image or background areas have been discharged by light in accordance with my invention.
~ Descri~tion of_the Preferred Embodiment In general, my invention contemplntes charging a photoconductor in the dark. The charged photoconductor i-then exposed to a light and shade image of the originnl.
This will form a latent electrostatic image on the photo-conductor. The oriqinal exposure is only a fraction, such as 5% or 10~, of the quantum of light normally required to produce a sati~factory image -- that i~, one having 3uffi-cient contrast so the image areas are dark and the back-ground areas are white. The latent image thus form~d has sufficient contrast, however, 80 that it can be toned --that i~, developed -- with any appropriate toner such a~
resinous powder or, more preferably, by a toner di~persed in an insulating liquid, as i3 well known to the art. This masking step produces a shield substantially OpaquQ to light over the image areas of the original being copied.
A critical feature of the masking step is that a shield will be produced only over the image areas, and not over any of the background areas. This is accomplished by en-suring that the development electrode, or means for applying the toner, during the masking ~tep is biased to a potential above that existing on the background area~ and below that present on the image areas of the latent electrostatic image.
I then expose the photoconductor to a blankot of light. The electrostatic charge of the masked image on the photoconductor will decay marglnally or not at all, while the electrostatic charge of thc background areas will bo discharged to a very low voltage, such as 50 volts or thQ
like. This has the effect of enhancing the charge of tho latent electrostatic image by a very largo percentage with the requirement of about one-tenth of the energy which would normally be neces~ary to produce a latent image having tho 1129~B~
strong contrast now achieved by my method. The cnhanced electro~tatic image can then be developed in any appro-priate manner known to the art and, if de~ired, readily transferred to a carrier sheet.
More particularly, referring now to FIGURE 2 in which apparatus for carrying out my invention i9 ~hown, a metal drum 10 carries a photoconductive layer 12 which may be selenium. The metal drum 10 is supp~rted by apertured di~s 14 which are mounted or. a shaft 16 and keyed theroto for rotation therewith. The shaft 16, which may be grounded, i9 driven by any appropriate means Xnown to the art to ro-tate the drum 10 in the direction of the arrow. A charging corona 18 is adapted to charge the 3urface of the selenium photoconductor 12 to a voltage of between 800 and 1000 volts.
To accomplish this, the charging corona is energized to a positive potential of 5000 or 6000 volts. The element~ of the corona discharge unit cause ionization of the circum-ambient atmosphere and place a uniform positive chnrge over the surface of the selenium. If my proccss were belng practiced with a zinc oxide-coated paper, the corona would be powered to produce a negative charge, a9 will be readily understood by those skilled in the art. ~he photoconductor 12 is then carried past the exposure station indicntod generally by the reference numeral 20. Projection optic~, indicated diagrammntically by the lenu 22, pro~ect an imnge of the original to be copied upon the photoconductor 12.
llZ9481 In my method, the exposure time i8 extremely ~hort. I have made copies with an expo~ure of a~ little as 5X of normal and have been able to achieve completely satisfactory copie~ of the originals.
A selenium photoconductor will generally discharge to about one-fifth of its original charge in about three foot-candle second3. This can be readily seen by reference to FIGURE 3, which shows a surface potential on a selenium photoconductor of 800 volts b~ing discharged to 160 volts in between two to thrce foot-candle seconds. Normally, sufficient energy is employed in the light source of photo-copying machine~ so that the bacXground areas of the photo-conductor will be discharged to about 50 or 60 volts. Thi~
will require about five foot-candle seconds. Five percent of this quantum of light is about 0.25 foot-candle second~.
By referring to FIGU~ES 3 and 4, it will be noted that, after this short exposure, the background area~ (B) will have dropped in voltage about 100 volts from the 1mage areas ~I). If this image were toned, a very low-contrast image would be achieved. If thi~ imago, after boing toned, were to be transferred onto papor from the drum, thc density of the toned image would be so small that a poor transfer or a failure to transfer would re~ult and only a faint image would appear. The low-contrast image, however, when de-veloped, has sufficient optical density so that it provides a ma~k or shield for the latent electrostatic image which is sufficiently dense for the practice of my proce~.
~lZ948~
Referring again to FIGURE 2, I show apparatu~ for providing a mask or shield for the latent electrostatlc image. It comprises a tank 24, from which a developlng liquid 26 containing dispersed toner particles, whlch may be S charged, is drawn through pipe 28 and pumped by pump 30 through pipe 32 to nozzle 34, adapted to discharge the de-veloper between the photoconductor and a reverse roller 36.
If the toner particles are conductive, they may acquire a charge by induction, owing to their passage ad~acent the latent image under the action of its electric field. In this case, the clectrostatic charge pattern serves first to charge the particles and then to trap them. In the case of a liquid-carried toner particle, the continuous phase is an insulating liq~id -- such as a hydrocarbon liquid, a fluorinated hydro-carbon liquid, or the li~e -- having low vapor pressures at room temperature, and the disperse phase is composed of the minute particles of toner adapted to make the latent electro-static image visible. As iB known in the art, the polarity of the charged particles may be controlled by materlals added to the developing liquid. Thcse act by adsorption onto the surface of the particlcs and alter the magnitude and polarity of the charge acquircd by the particles, de-pending on the environment of the particles at the tlmc of their formation and tho method of their preparatlon. Tho toner particles must bc applicd by a dcvelopment electrode biased to a potential of the same polarity a~ that of the latent electrostatic image and to a potentlal greater thnn l~Z9481 that o~ the background areas and below that of the image are~. Preferably, I employ a reverse ro~ler a~ the de-velopment electrode when a liquid developer i~ used. Thlo reverse roller i~ made of metal and is mounted on ~haft 38 for rotation in a direction oppoRite to the rotation of the photoconductor. The reverse roller i8 insulated from ground and is positioned closely adjacent the ~urface of the photoconductor to provide a gap ranging from 0.05 to 0.1 millimeter. The reverse roller is driven by a prime mover and is controlled in speed so as to remove excess developing fluid from the photoconductor. The biasing of the development electrode i~ critical to my process, ~ince there can be no masking of the background areas in my process, as will be pointed out more fully hereinafter.
It will be appreciated that the close proximity of the insulated metal reverse roller to the ~urface of the charged photoconductor is such that it will float to a~sumo tha average potential of the photoconductor and thus be auto-biased. Since the average potential on the rever~e applicator roller 36 will be above the background potentlal on the photoconductor, toner particles will migrate to the applicator roller in~tead of to the background area~ on the photoconductor. If desired, in~tead of pcrmitting the reverse applicator or metering roller 36 to float electrl-cally, it may be biased to a potential from any appropriate D.C. voltage ~ource to above the potential of tho bac~ground areas but below the potcntial of the image areas. The ~ia~
112948~
on the toner applicator roller will substantially elimlnate the deposition of ma~king toner on the background areas of the photoconductor. In the usual developing liquid, how-ever, the carrier liquid has a low boiling point, ~o that it is easily vaporized to ensure that the developed image, when transferred to a carrier sheet such as paper, will pro-duce a copy dry to the touch. In the maskinq step, however, a hydrocarbon carrier liquid having a higher boiling point which will not vaporize may be employed. Thi~, of cour~e, will reduce the danger of atmospheric pollution during the masking step. If desired, instead of a liquid-carried toner, dry toner may be used for the masking step. Such dry toners are well known to the art. One example of a reverse roller which can be u3ed in my invention is shown in Hayashi et al Patent 3,907,423. The excess toner from the masking step will be caught in the tank 24 for recycling. A wiper blade 40 keeps the metal reverse roller clean.
After the optical shield is produced in any appropriate manner such as described, the photoconductor i~
subjected to a blanket of light. This may be accompli~hod by an elongated incandescent lamp 42 placcd adjacent the photoconductor bearing the masked image and extending thore-across. In an office copier, a ~uartz-halogen lnmp h~ving an output of about 500 watts i3 u3ually employed. In my process, a much lower-energy exposurQ lamp to project the image may be employed. Furthermore, the illuminAtion of l~Z9481 'I
the background areas to discharge them to n re~ldual v~ltago of about 50 volts requires comparatively ~mall energy. A8 will be readily appreciated by those skilled ~n the nrt, the exposure step subject~ the photoconductor to an image of the original by reflected light. The illuminated original i~
focused by the optical system upon the charged photoconductor.
Since the light gathered by the optical system i9 a small fraction of the light which illuminates the original, a bright illumination of the original is required. In the background discharging step, after the optical 3hield is in place, the illumination of the photoconductor i8 by direct light, which accounts for the small energy required to discharge the background areas. The image areas ~I) will not be discharged owing to the mask or shield which I have lS provided by my proces~ as jUQt described. The effect of discharging the potential of the background areas (~) whilQ
leaving the image areas (I) substantially undischarged t~
shown in FIGURE 5.
It will be seen that my process has achieved a contrast of substantially 750 volts between the bacXground areas and the image areas and accomplishod tha creation of this strong field nt approximately a tenfold reduction of the energy required to expose thc original. This mean~, as will be readily apparent, that a photocopying machine which prosently uses a 500-watt lamp of the quartz-halogen typo with a tungsten filament could use a 50-watt lamp or, ~2948~
alternatively, employ a fluore~cent light. It will aloo be appreciated that, where a photocopying machine presently i8 able to make only about 25 copies per minute, theoretically, I can easily make a photocopying machine capable of pro-ducing 150 copie~ per minute. A~ a practical matter, however, owing to the inertia of the parts of the photo-copying machine and in order to avoid marginal operation to provide a factor of safety, a photocopying machine which will produce 75 to 100 copies per minute can be made embody-ing my invention. Furthermore, this can be done without having to increase the energy expended significantly, since the only additional energy required will be that employed in the first developing or masking step and that in the light discharging of the photoconductor after the image areas have been masked. It will be further understood that, instead of an incandescent lamp, I may u~e any appropriata light source adapted to flood the photoconductor.
The use of a floodlight to enhance a faint latent image on a photoconductor i8 not new in and of it~elf.
Steinhilper Patent 2,756,676 dcscribes a method of making a plurality of xcrographic reproductions from a single expo-sure of an origin~l. In Stcinhilper, however, the effective speed of the photoconductor is not increased, owing to the fact that Steinhilper must go through a first development step which produce3 a fully-toned image. There i9 no m~king step a~ described in my process. After the first image i~
l~Z9481 developed, it is transferred to a carrier sheet ~uch as paper. The faint image which is left on the photoconductor is of a potential too low to be enhanced by light or to be redeveloped. Steinhilper does not erase this image on the - 5 photoconductor, but recharges the photoconductor. He then discharges the background areas by light. Since the faint image does produce a shield, an enhanced latent image will be produccd. The salient feature of my proces~, however, is absent from Steinhilper. He does not form a lcw-contrast latent e1ectrostatic image in such a manner a~ to leave the bacXground areas free of developer, owing to the fact that his development electrode is never biased but always at ground. Steinhilper must carry out his proce~s to form the residual image from the first transfer of the developed image at the normal ~low rate. Thus the unobv~ou~ re~ult which I achieve -- namely, the increasing of the effective sensitivity of the photoconductor -- is not taught, nor can it be achieved, by Steinhilper.
Owing to the tremendous range through which I am enabled to obtain sufficient optical density to produce a mask, I can employ a single exposure and a biased ~etting in the developer system and obtain a sharp, clear copy from any orlginal, whether the bacXground is ultra white or dingy gray.
It will be readily apparent to tho~e ~killed in the art that, with the contra~t potential shown in FIGURB 5, , ~lZ9~81 there is no problem in obtaining a ~harp, clear im~ge.
After the background potential has been di~chargod by tho lamp 42, the optical shield may be wiped from the enhanced latent image thus formed by a cleaning roller 44 made of sponge rubber or the like, if desired. This wiping action can take place with either a liquid-toned mask or a dry developer-toned ma~k. The enhanced latent electro~tatic image may then be toned by any usual method known to the art.
In FIGURE 2, I have shown the toning system described in Schaefer et al Patent 3,892,481, employing a tank 46 from which a liquid toner 48 circulates from pipe 50 to a toner supply tank ~not shown) and back through pipe 52 to the tank 46. A development electrode 54 i9 controlled to bias any residual voltage left on the background of the photoconductor. It will be readily appreciated, however, that since I have discharged the background potential by my method, I can use a fixed bia~ slightly nbove the average residual bias of the background. This will produce a cluar white background and enable me to eliminate, if deaired, the sensing and biasing method shown in the Shaefer et al patent.
After development with a liquid-carried toner, a rever~e roller 56, sucll ag shown in ~}ayashi et al Patent 3,907,~23, i9 positioned to remove excess developer from the developed image, The reverse roller 56 i8 provided with a ... .
wiper 5~. The reverse roller 56 i9 positioned and rotate~
at ~peeds as described in the llayashi et al patent.
The image is now ready to be transferred to a carrier sheet such as plain paper. A plain paper sheet 60 i9 fed by rollers 62 to a roller 64, past a transfer-charging corona 66. It will be recalled that the toned image still comprises a visible image over a high positive charge on the surface of the selenium drum in the image areas which haye not been discharged by light or by the bia~ applied to remove the residual background potential.
To transfer the developed image from the drum to the paper carrier sheet, a high positive charge is applied to the back of the copy paper. As a result of the application of the high positive charge to the sheet, the toner particles are pulled from the drum surface onto the paper. A pic~-off 68 ensures that the paper leaves the drum, and the end of the paper 70, now carrying the image, may be dried and passed to a receiving tray tnot shown). A cleaning roller 72 wipes the drum clean of any particle~ of toner which have not been removed from the drum, and a wiper blade 74 complates the drum-cleaning operation.
It will be understood, of course, that lf my process i9 applied to a ~inc oxide-coated paper, the im~ge will remain on the coated paper. It will al80 be under-stood that a dry toner, made of fusible resinous powder nnd fixed by heat, may be employed, as i9 well known to the art.
~129481 It will be understood by thos~ sXllled in the art that, since the photoconductor now bears an enhanced latent electrostatic image having a strong field, it may be de-veloped in any appropriate manner known to the art.
A selenium photoconductor i8 very ~ensitlve to blue light and, accordingly, photocopylng machine~ using a qelenium photoconductor do not produce copies from blue-colored originals with high contra~t. That ls to ~ay, a selenium photoconductor "sees" blue light as almost white.
My method will reproduce bluo originals as if they were black. A yellow original is very light in color and re-flects considerable light, 80 that it appears faint in the copie~ made by photocopying machines. My method of ~mage-enhancing reproduces yellow effectively. Furthermore, gradations in density -- that is, a gray scale -- are also achieved with my method.
Owing to the wide latitude of effective photo-conductive sensitivity which my process provides, when uslng the automatic bias of Schaefer et al Patent 3,892,4al, I can adjust the illumination in the step whlch forms the low-contrast image prior to thc mask-forming step to produco a satisfactory reproduction with the brlghtest background --that is, a purc white background -- in the original document.
Thia will produce an image of contrast too low to be effec-tively developed to a aatisfactory copy, but such that the 1~29481 mask-forming ~tep can be easily performed. The bla~
potential applied to the applicator roller will be well above that required to eliminate background potential of the latent electrostatic image completely, but will be S below the potential of the image areas. This permits the image areas to be optically masked by toner during the optical shield-forming step. The illumination of the image-forming step may be readily controlled by varying the intensity of the light source or by a shutter in the optical path of the image-projection system.
It will be further under~tood by those skilled in the art that, while I have shown and described the toning of an image and then its tranqfer to n carrier sheet, my image-enhancing process can be used in any method of electrophotography. For example, the enhanced latent electrostatic image can be transferred to a dielectric sheet and then toned or developed into a visible im~ge on the dielectric sheet, a~ is well known in the art.
It will be qeen that I have accomplished the objects of my invention. I havo increased tho effective sensitivity of photoconductors. I have provided an improved method of electrophotography which will qreatly incroaJe the speed of COpyillg operations. My method achieveJ thiD ln-crease in speed with a reduction of the quantum of energy required. I am enabled to copy originals having poor llZ9481 contrast which will produce copies having surprisingly increased contrast a~ compared with the originalJ. I am enabled to copy originals formed in colors to wh$ch the photoconductor has great sensitivity and which, accord-ingly, do not ordinarily produce copies having the desired contrast. I have provided a novel appAratUs for carrying out my improved method.
It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. ~his is contemplated by and is within the scope of my claims. It i9 further obvious that various change~ may be made in de-tails within the scope of my claim~ without departing from the spirit of my invenition. It is, therefore, to be under-stood that my invention is not to be limited to the specific details shown and described.
Having this described my invention, what I claim
Claims (18)
1. In a method of electrophotography in which a photoconductor is charged in the dark in a charging step, subjected to a light and shade image of a document to be copied to produce a latent electrostatic image of the document on the surface of the photoconductor in an exposing step, the latent electrostatic image is developed to provide a visible image on the photoconductor in a development step, the improvement comprising reducing the duration of the exposing step by a major portion of the time required to form a latent electrostatic image of satisfactory contrast to present a latent electrostatic image having a first contrast, toning said first-contrast electrostatic image to provide an optical shield for the first-contrast latent electrostatic image while preventing deposition of toner on the background areas of the photoconductor, flooding the shielded first-contrast electrostatic image with light to discharge the unshielded areas of the photoconductor whereby to enhance the first-contrast electrostatic image to produce an electrostatic image having a contrast higher than said first contrast, removing the optical shield from the enhanced electrostatic image before practicing said developing step, and then transferring said developed image to a carrier sheet.
2. In a method of electrophotography in which a photoconductor is charged in the dark in a charging step, subjected to a light and shade image of a document to be copied to produce a latent electrostatic image of the document on the surface of the photoconductor in an exposing step, the latent electrostatic image is developed to provide a visible image on the photoconductor in a development step, the improvement comprising reducing the light energy of said exposing step by a major amount to form a latent electrostatic image having a low contrast, applying toner to said low-contrast electrostatic image to provide an optical mask for the low-contrast latent electrostatic image while preventing deposition of toner on the background areas of the photoconductor, subjecting the masked low-contrast electrostatic image and the photo-conductor to the action of a low-intensity direct light to discharge the unmasked areas of the image whereby to enhance the low-contrast electrostatic image to produce an electrostatic image having a high contrast.
3. An improved method of electrophotography including the steps of charging a photoconductor, exposing the charged photoconductor by reflected light to a light and shade image of a document to be copied to form a latent electrostatic image of the document on the surface of the photoconductor, applying toner to the latent electrostatic image by a biased toner applicator to form an optical mask over the image while leaving the background areas of the photoconductor free of toner, subjecting the photoconductor to direct light to discharge the non-image areas of the photoconductor to enhance the latent electrostatic image, removing the mask from the enhanced electrostatic image, developing the enhanced electrostatic image, and transferring the developed image to sheet material.
4. An improved method of electrophotography including the steps of charging the surface of a photo-conductor, forming a relatively low-contrast latent electrostatic image of a document to be copied by projecting onto the surface of the photoconductor a light and shade image of the document having less .illumination than required to produce an image of satisfactory contrast, masking the low-contrast latent electrostatic image with an optical mask by a first toning step, immediately discharging the light areas of the image on the photo-conductor by light to increase the electrostatic field differential between light and shade areas, subjecting the resultant electrostatic image to a second toning step, and then transferring the toned visible image to sheet material.
5. In a method of electrophotography in which a photoconductor is electrostatically charged in a charging step, the charged photoconductor is exposed to a light and shade image of a document to be copied in an exposing step to form a latent electrostatic image of the document being copied on the photoconductor, and the latent electrostatic image is developed to produce a visible copy of the document on the photoconductor, the improvement comprising interposing a toning step to form an optical shield for the latent electrostatic image and a light-discharging step to discharge the non-image areas of the latent electrostatic image to enhance the same between said exposing step and said developing step.
6. An improved method of electrophotography including the steps of forming a latent electrostatic image of a document having light and shade areas on the surface of an electrostatically charged photoconductor, toning said latent electrostatic image in a first toning step to provide an optical shield for the shade areas of the latent electrostatic image, discharging residual potential from the light areas of the photoconductor by light to enhance the contrast of the electrostatic image, and then developing the enhanced electrostatic image in a second toning step to produce a visible image of the document.
7. In a method of electrophotography wherein a latent electrostatic image of a document having light and shade areas is formed on a photoconductor and the latent image developed to produce a visible image, the improvement which comprises the steps of toning the latent electrostatic image to form an optical shield over the shade areas of the electrostatic image, and subjecting the photoconductor to light to discharge the light areas of the image on the photoconductor before practicing the developing step.
8. In an apparatus for making copies of a document by electrophotography having a photoconductor;
means for charging the photoconductor exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image; means for developing the latent electrostatic image; means for transferring the developed image to a carrier sheet; and means for moving the photo-conductor past the charging moans, the exposing means, the developing means, and the transferring means in succession;
the improvement comprising an electrically conductive roller for applying liquid-carried toner to said latent electrostatic image to provide an optical shield for the shade areas of the same, said roller being insulated from ground so as to float electrically to assume the average potential on the surface of the photoconductor, a light source for discharging the unshielded areas of the photo-conductor, and means for positioning said roller and said light source between the charging means and the developing means.
means for charging the photoconductor exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image; means for developing the latent electrostatic image; means for transferring the developed image to a carrier sheet; and means for moving the photo-conductor past the charging moans, the exposing means, the developing means, and the transferring means in succession;
the improvement comprising an electrically conductive roller for applying liquid-carried toner to said latent electrostatic image to provide an optical shield for the shade areas of the same, said roller being insulated from ground so as to float electrically to assume the average potential on the surface of the photoconductor, a light source for discharging the unshielded areas of the photo-conductor, and means for positioning said roller and said light source between the charging means and the developing means.
9. In an apparatus for making copies of a document by electrophotography having a photoconductor;
means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image; means for developing the latent electrostatic image; means for transferring the developed image to a carrier sheet; and means for moving the photo-conductor past the charging means, the exposing means, the developing means, and the transferring means in succession;
the improvement comprising means for toning the latent electrostatic image to provide an optical shield for the shade areas of the same, a light source for discharging the unshielded areas of the photoconductor to enhance the contrast of the latent electrostatic image, means for developing said enhanced electrostatic image, and means for positioning said light source between the toning means and the developing means.
means for charging the photoconductor; exposing means for subjecting the charged photoconductor to a light and shade image of the document being copied to form a latent electrostatic image; means for developing the latent electrostatic image; means for transferring the developed image to a carrier sheet; and means for moving the photo-conductor past the charging means, the exposing means, the developing means, and the transferring means in succession;
the improvement comprising means for toning the latent electrostatic image to provide an optical shield for the shade areas of the same, a light source for discharging the unshielded areas of the photoconductor to enhance the contrast of the latent electrostatic image, means for developing said enhanced electrostatic image, and means for positioning said light source between the toning means and the developing means.
10. In an apparatus for making copies of a document by electrophotography having a photoconductor, a station for electrostatically charging the photoconductor, an exposing station for subjecting the charged photo-conductor to a light and shade image of the document to be copied to form a latent electrostatic image on the photoconductor, a developing station to make the latent image optically visible, and means for providing relative movement of the photoconductor with respect to the charging station, the exposing station, and the developing station, the improvement which comprises a toning station for optically masking the shade areas of the latent electrostatic image and a station for light-discharging the unmasked areas of the photoconductor both interposed between the charging station and the developing station.
11. A method of electrophotography including the steps of charging the surface of a photoconductor to a first potential, subjecting said surface to a light and shade image of a document to be copied to produce a latent electrostatic image having image areas and background areas of a potential appreciably greater than half the magnitude of said first potential, toning said image areas to provide an optical mask therefor, subjecting said surface to light to reduce said background area potential to a magnitude appreciably less than half the magnitude of said first potential, and developing said image areas.
12. In a method of electrophotography in which a photoconductor is charged in the dark in a charging step, subjected to a light and shade image of a document to be copied to produce a latent electrostatic image of the document on the surface of the photoconductor in an ex-posing step, the latent electrostatic image is developed to provide a visible image on the photoconductor in a development step and said visible image is then trans-ferred to a carrier sheet, the improvement comprising reducing the duration of the exposing step by a major portion of the time required to form a latent electro-static image of satisfactory contrast to present a latent electrostatic image having a first contrast, toning said first-contrast electrostatic image with liquid-carried toner to provide an optical shield for the first-con-trast latent electrostatic image while preventing deposi-tion of toner on the background areas of the photoconductor, flooding the shielded first-contrast electrostatic image with light to discharge the unshielded areas of the photoconductor whereby to enhance the first-contrast electrostatic image to produce an electrostatic image having a contrast higher than said first contrast, re-moving the optical shield from the enhanced electrostatic image before practicing said developing step.
13. In a method of electrophotography in which a photoconductor is charged in the dark in a charging step, subjected to a light and shade image of a document to be copied to produce a latent electrostatic image of the document on the surface of the photoconductor in an ex-posing step, the latent electrostatic image is developed to provide a visible image on the photoconductor in a development step, the improvement comprising reducing the light energy of said exposing step by a major amount to form a latent electrostatic image having a low contrast, applying liquid-carried toner to said low-contrast electro-static image to provide an optical mask for the low-contrast latent electrostatic image while preventing deposition of toner on the background areas of the photoconductor, sub-jecting the masked low-contrast electrostatic image on the photoconductor to the action of a low-intensity direct light to discharge the unmasked areas of the image whereby to enhance the low-contrast electrostatic image whereby an electrostatic image having a high contrast.
14. An improved method of electrophotography includ-ing the steps of charging a photoconductor, exposing the charged photoconductor by reflected light to a light and shade image of a document to be copied to form a less than normally exposed latent electrostatic image of the document on the surface of the photoconductor, applying liquid-carried toner to the latent electrostatic image by a biased toner applicator to form an optical mask over the image while leaving the background areas of the photoconductor free of toner, subjecting the photoconductor to direct light to dis-charge the non-image areas of the photoconductor to enhance the latent electrostatic image, removing the mask from the enhanced electrostatic image, developing the enhanced electro-static image, and transferring the developed image to sheet material.
15. An improved method of electrophotography in-cluding the steps of charging the surface of a photo-conductor, forming a relatively low-contrast latent electrostatic image of a document to be copied by pro-jecting onto the surface of the photoconductor a light and shade image of the document having less illumination than required to produce an image of satisfactory con-trast, masking the low-contrast latent electrostatic image with an optical mask by a first toning step per-formed with liquid-carried toner while preventing de-position of toner on the non-image areas on the photo-conductor, immediately discharging the light areas of the image on the photoconductor by light to increase the electrostatic field differential between image areas and non-image areas, subjecting the resultant electro-static image to a second tening step, and then transfer-ring the toned visible image to sheet material.
16. A method of electrophotography including the steps of charging the surface of a photoconductor to a first potential, subjecting said surface to a light and shade image of a document to be copied to produce a latent electrostatic image having image areas and back-ground areas of a potential appreciably greater than half the magnitude of said first potential, toning said image areas with liquid-carried toner to provide an optical mask therefor while preventing deposition of toner on the background areas, subjecting said surface to light to reduce said background area potential to a magnitude appreciably less than half the magnitude of said first potential, and developing said image areas.
17. Apparatus for making copies of a document by electrophotography including in combination a photoconductor, means for charging said photoconductor, exposing means for projecting an image of the document being copied onto the photoconductor to form a latent electrostatic image having image areas and background areas, means for masking the image areas of the latent electrostatic image, said masking means including means to prevent masking of said background areas, illuminating means for illuminating the surface of the photoconductor, developing means for developing the masked electrostatic image, transfer means for transferring the developed image to a carrier sheet, and means for moving the photoconductor past the charging means, the masking means, the illuminating means, the developing means, and the transfer means in succession.
18. Apparatus for making copies of a document by electro-photography including in combination a photoconductor, means for charging said photoconductor, exposing means for pro-jecting an image of the document being copied onto the photo-conductor to form a latent electrostatic image having image areas and background areas, means for masking the image areas of the latent electrostatic image, said masking means in-cluding means to prevent masking of said background areas, illuminating means for illuminating the surface of the photoconductor, means for removing the mask from the image areas, developing means for developing the electro-static image, transfer means for transferring the developed image to a carrier sheet, and means for moving the photo-conductor past the charging means, the masking means, the illuminating means, the mask-removing means, the developing means, and the transfer means in succession.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US90835578A | 1978-05-22 | 1978-05-22 | |
| US908,355 | 1978-05-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1129481A true CA1129481A (en) | 1982-08-10 |
Family
ID=25425651
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA324,587A Expired CA1129481A (en) | 1978-05-22 | 1979-04-02 | Method of electrophotography and apparatus for practicing the same |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS554084A (en) |
| CA (1) | CA1129481A (en) |
| DE (1) | DE2920464A1 (en) |
| FR (1) | FR2426930A1 (en) |
| GB (1) | GB2021284B (en) |
| IT (1) | IT1166754B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4278884A (en) * | 1978-11-09 | 1981-07-14 | Savin Corporation | Method and apparatus for xeroradiography |
| DE10301735B4 (en) * | 2003-01-18 | 2007-01-18 | Deutsches Zentrum für Luft- und Raumfahrt e.V. | Method and device for metering microscale particles |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2219005A1 (en) * | 1971-04-20 | 1972-12-21 | Iwatsuki Koki Co Ltd | Development process for use in electrophotography |
| US3914609A (en) * | 1974-03-04 | 1975-10-21 | Xerox Corp | Signal amplification by illumination of a partially developed latent electrostatic image |
-
1979
- 1979-04-02 CA CA324,587A patent/CA1129481A/en not_active Expired
- 1979-04-04 GB GB7911785A patent/GB2021284B/en not_active Expired
- 1979-04-19 IT IT22017/79A patent/IT1166754B/en active
- 1979-05-09 JP JP5580179A patent/JPS554084A/en active Granted
- 1979-05-15 FR FR7912355A patent/FR2426930A1/en active Granted
- 1979-05-21 DE DE19792920464 patent/DE2920464A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| IT7922017A0 (en) | 1979-04-19 |
| IT1166754B (en) | 1987-05-06 |
| DE2920464A1 (en) | 1979-11-29 |
| GB2021284A (en) | 1979-11-28 |
| JPS6251466B2 (en) | 1987-10-30 |
| GB2021284B (en) | 1982-08-04 |
| DE2920464C2 (en) | 1990-03-08 |
| FR2426930B1 (en) | 1985-04-05 |
| FR2426930A1 (en) | 1979-12-21 |
| JPS554084A (en) | 1980-01-12 |
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