Journal of the

O PTICALof Ak

SOCIETY[ERICA

VOLUME 38, NUMBER 12 DECEMBER, 1948

Xerography: A New Principle of Photography and Graphic Reproduction

R. M. SCHAFFERT AND C. D. OUGHTON

Battelle Memorial Institute, Columbus, Ohio '

(Received August 20, 1948)

A new process of graphic reproduction and photography has been developed. The basicprocess, known as xerography, is dry, direct positive-to-positive, rapid, and has a number ofpotential applications in graphic arts and related fields. As a photographic process, xerographyemploys a re-usable plate consisting of a thin layer of a photo-conductive material on anelectrically conductive base. The plate is sensitized by electrostatic charging immediately be-fore use. After exposure, the image is developed by dusting the plate with a micronized powder.Prints are made by transferring and fixing the powder image to paper or other materials. Theplate can be used many times to produce additional images. Xerography also provides a uniqueand simplified process of graphic reproduction, particularly in printing and duplicating.

INTRODUCTION

THE object of this paper is to introduce thesubject of xerography and to present a

general explanation of the basic principles andtechniques involved in the process.

Xerography was originated by Chester F.Carlson' in 1938, and first publicized by NicholasLanger2 in 1944. Since 1944, the process has beena subject of research and development in theGraphic Arts Research Laboratories of BattelleMemorial Institute, Columbus, Ohio, underprojects sonsored initially by Battelle, andsince January 1, 1947, by Battelle and theHaloid Company of Rochester, New York.

Xerography has a number of characteristicswhich distinguish it as being an entirely newprocess. The process is completely dry. Nochemical reactions are involved. It is a direct

I U. S. Patents No. 2,297,691 and No. 2,357,809.2 Nicholas Langer, "Electrophotography," Radio News,

Eng. Dept. 32, 22 (1944),

positive-to-positive operation. The plates orfilms are not destroyed by exposure and may bere-used many times. The process is economicaland versatile; prints may be prepared on almostany type of paper or on any other materialhaving a reasonably smooth surface. Finished,permanent prints may be completed in a matterof seconds.

PHOTOGRAPHIC ASPECTS OF XEROGRAPHY

Description of Process

The application of xerography to photographyinvolves the formation of an electrostatic latentimage on a layer of photo-conductive insulatingmaterial and the development of the image witha finely divided powder, which adheres to theelectrically charged areas of the plate. The plateis sensitized by applying an electrostatic chargeto the surface of the photo-conductive coating.The sensitized plate is then exposed to the lightimage which it is desired to reproduce. Impinge-

991

R. M. SCHAFFERT AND C. D. OUGHTON

I-

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a.

250

200 7/111

DC0 - /--

500

0 2 4 8 12 16

CHARGING TIME (SECONDS)

FIG. 1. Plate potential as a function of charging time.

ment of light on the photo-conductive coatingcauses electrical charges to leak away or to bereduced in quantity in proportion to the amountof light falling on the particular area, thus leav-ing an electrostatic latent image. The image isdeveloped by dusting the plate with a finelydivided powder which adheres only to the elec-trically charged areas in proportion to the amountof charge remaining on the plate after exposureto light. Prints are made by transferring andfixing the powder image to paper.

The photosensitive plate consists of a photo-conductive coating on a conductive supportingmedium, such as metal. The developer consistsof a finely divided powder, mixed with a coarserpowder. The coarser powder serves to generatean electrostatic charge on the fine powder. Thecharged powder is then used to develop a latentelectrostatic image on the photo-conductivecoating.

The process involves photo-conductivity andelectrostatics. Photoconductivity has receivedconsiderable attention from both physicists andchemists. Images have been formed inside ofcrystals as discolorations directly associatedwith photoconductivity,3 and an image has beenobserved by Selenyi4 on selenium-coated platesas a result of exposure to light. Also, pictureshave been produced by electrolytic reaction onthe surface of a photoelectrically sensitive layer. 5

3A. L. -lughes, "Photo-conductivity in crystals," Rev.Mod. Phys. 8, 312 (1936).

4 P. Selenyi,, "Photography on selenium," Nature 161,522 (1948).

5 Goldman, German Patent No. 674999.

Various electrical and electrolytic photo-proc-esses have been reviewed by Yates.6 Selenyi7

reported a process of forming an electrostaticimage on insulating plates by various methodsof scanning and of developing that image bydusting it with a finely divided powder. Variousobservations have been published concerning thegeneration of an electrical charge on powderparticles from frictional or tribo effects.9-" Thereaction of the powder particles in electricalfields is well known and is the basis of processesfor the removal of dust particles from the airfor electrostatic separation of powders12 and forthe deposition of powders in a pattern form.3

Carlson'4 introduced the method of formingan electrostatic image on a photo-conductingmaterial and developing the latent electrostaticimage with a finely divided powder.

Five steps are involved in making a photo-graphic print by xerography: (1) Sensitizing theplate with an electrostatic charge, (2) exposingthe plate to form an electrostatic image, (3) de-veloping the latent image with fine powder,(4) transferring the powder image to paper orother materials, and (5) fixing the image byfusing the powder. The plate can then be cleaned,after which it can be used again for taking addi-tional photographs.

Preparation of the Plates

Xerographic plates for use in photography areprepared by depositing a smooth layer of aphoto-conductive material on a supporting me-dium. Evaporation is a suitable method for pre-paring such layers. The supporting medium issuspended in a bell jar, the air evacuated, andthe photo-conductive material evaporated byconventional methods. Photo-conductive coat-

6 R. F. Yates, "Photo-electrography-tomorrow's mir-acle," Radio-Craft, p. 334 (March, 1943).

7 P. Selenyi, "On the electrographic recording of fastelectrical phenomena," J. App. Phys. 9, 637 (1938).

8 P. Seleyni, "Methoden, Ergebnisse und Aussichtendes elektrostatischen Aufzeichungsverfahrens (Elektro-graphie)," Zeits. f. tech. Physik 16, 607 (1935).

9 Leonard B. Loeb, "The basic mechanisms of staticelectrification," Science 102, 573 (1945).

10 H. F. Richards, "The contact electricity of solid di-electrics," Phys. Rev. 22, 122 (1923).

11 F. E. Shaw, "Experiments in tribo-electricity," Proc.Roy. Soc., A94, 16 (1917).

12 F. Fraas and 0. C. Ralston, "Electrostatic separationof solids," Ind. Eng. Chem. 32, 600 (1940).

13 U. S. Patents Nos. 2,097,233 and 2,152,077."4 U. S. Patents No. 2,297,691 and No. 2,357,809.

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I

XEROGRAPHY

ings of materials such as anthracene, sulphur,and selenium have been prepared by thistechnique.

Studies of the electrical properties of thephoto-conductive layers indicate that they musthave specific dark resistances of approximately1015 ohm-centimeters to retain electrical chargeson their surfaces. When the photo-conductivematerials are illuminated, this specific resistancemust be reduced to something like 1010 or 1012

ohm-centimeters to account for observed ex-posure times. Photo-conductivity is not an effectwhich lends itself to expression in terms ofohmic resistances so that the above values areto be used only in expressing qualitatively thecharacteristics involved.

Numerous materials have been used success-fully as the supporting media for the photo-conductive coatings. To serve as a conductivebacking for the plates, it is only necessary for amaterial to have a specific electrical resistancebelow the equivalent resistance of the illuminatedphoto-conductive layer. Thus, materials havingspecific resistances of approximately 1010 ohm-centimeters can serve as backing materials. Suchmaterials might ordinarily be considered to beelectrical insulators. Plates functioned satis-factorily when the photo-conductive materialwas deposited on various types of paper, glass,and plastics having electrical resistances belowthat of the photo-conductive material.

Sensitization of the Plates

Before the plate can be used, it must be sensi-tized by depositing an electrostatic charge onthe photo-conductive surface. Plates were origi-nally sensitized by rubbing with materials suchas fur and cloth. When a swab of fur is brushedacross the surface of the plate, an electrostaticcharge is generated on the surface. A more con-venient method of sensitizing the plate, and onewhich gives higher plate potentials, is the applica-tion of a corona discharge from a row of needlesor from fine wires. The plate is passed under thewires in the field of the corona discharge. Re-versing the polarity of the voltage applied to thewires reverses the sign of the electrical chargeon the plate. Electrical potentials from 4000 to7000 volts are ordinarily used to produce thecorona discharge with one to three 0,004-inch

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FIG. 2. Graph showing decay of plate potential undervarying conditions of illumination.

wires suspended approximately one centimeterabove the photo-conductive surface of the plate.Half-wave, rectified, unfiltered power sourcesare convenient, but electrostatic generators canbe used.

The magnitude of the electrical charge on theplate resulting from the sensitization is measuredby the potential to which the surface of theplate is raised. In the laboratory this potentialis measured by an electrometer of the Lindemanntype. The plate is brought to a definite positionin relation to a probe connected to the elec-trometer, and the deflection of the electrometer isread and interpreted in terms of a calibrationmade with an electrically conducting plate inthe same position. The maximum quantity of

DEVELOPER UNIT

FIG. 3. Schematic diagram of xerophotographicdeveloper unit,

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R. M. SCHAFFERT AND C. D. OUGHTON

charge, which a plate will acquire during thesensitizing operation, is dependent on severalvariables. One of these variables is the time ofcharging of the plate under the corona discharge,as shown in Fig. 1.

Because of the light-sensitivity characteristicsof the plate, the sensitizing operation must beperformed in the dark or under a suitable safelight. After one use, the plate can be resensitizedand used over again. The primary limitation onthe use-life of a plate appears to be the mechani-cal abrasion to which the plate might be sub-jected. If handled carefully, the plate may bere-used many times. Some plates in the labora-tory have been used over 500 times and stillyield satisfactory reproductions.

Exposure of the Plates

The sensitized plate may be exposed by anyof the conventional techniques. The plate may be

mounted in a standard plate holder and exposedin a camera; images may be projected onto theplate by an enlarger; or the plate may be ex-posed by passing light through copy held inclose contact with the plate. In the area wherelight strikes the plate, the photo-conductivecoating acts as a conductor for the electrostaticcharge and disperses the charge into the sup-porting medium, leaving a latent electrostaticimage on the plate surface. The light transmis-sion of most papers is sufficient to permit thereproduction by contact exposure of such itemsas typed letter, drawings, ink or pencil memo-randa, and printed matter.

A summary of what has happened to the plateduring the sensitizing and exposing operations isshown in Fig. 2. The plate was sensitized to aninitial potential (A). Laboratory measurementsshow that a slow decay of the charge takes placein the dark. This is indicated by section (B) on

OriginalFIG. 4.

Xerographic copy

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XEROGRAPHY

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THE FORMATION OF AN ELECTROSTATIC IMAGEON A XEROGRAPHIC PLATE

Original

THE FORMATION OF AN ELECTROSTATIC IMAGEON A XEROGRAPHIC PLATE

Xerographic copyFIG. 5.

the curve. At (C) the plate is exposed to thelight image. The potential of the areas of theplate exposed to light drops to a low value asthe charge is conducted from the plate in thoseareas. The potential appears to decrease in pro-portion to the intensity of the light striking thephoto-conductive surface though the relationshipis not exactly a linear one. At point (D) the de-cay of potential ceases, leaving a minimum chargeon the plate. Thus, the latent electrostatic imagemay have a potential which varies from nearzero to a maximum over different parts of theplate. Indications are that tone gradations canbe reproduced and that the process has fairphotoregressive properties. The latent electro-static image can be retained on the plate forseveral days without losing the image quality.

Image Development

The developer, used to develop the latentelectrostatic image on the plate, is a mixture ofa micronized resinous powder and a relativelylarge particle size granular carrier (Fig. 3). Themicronized resin particles, which are used todevelop the image, are referred to as the powder,the larger granular particles as the carrier. Thesizes of powder particles now used are from 0.1to 20 microns for the powder and approximately300 microns for the carrier. These sizes may bevaried over wide ranges. In the developing opera-tion, the granular particles of carrier serve to

generate an electrostatic charge on the powderparticles, thus causing the finer powder particlesto cling to the coarser carrier particles. As aresult of this mutual electrical attraction be-tween the two powders, the carrier is able tosweep the finer developer powder across theplate surface. The electrification of the two ma-terials is caused by triboelectricity. Formulationof the proper developer for use on a plate is de-termined by the relationship of various materialsin the triboelectric series. An illustrative andabbreviated series is as follows:

Corn meal-positive end of seriesCorkCalcium carbonatePigmented lycopodiumRosinSandCupric sulphideTartaric acid-negative end of series

A triboelectric series consists of a list of ma-terials arranged in such a way that each materialwill acquire a positive electrical charge whencontacted or rubbed against any material belowit in the list. In general, the magnitude of theelectrical charge generated on each of two ma-terials on contact will depend on the relativepositions of the two materials in the series. Ifthe materials are close together in the series, themagnitude of the charge will be small; if the ma-terials are far apart in the series, the magnitude

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R. M. SCHAFVFERT AND C. D. OUGHTON

of the charge will be large. Thus, by combiningthe proper materials, one in the form of a mi-cronized resin powder and the other in the formof a granular carrier, it is possible to control themagnitude and the sign of the electrostaticcharge generated on the powder. Two developerssuitable for reproducing high contrast line copyare; (1) micronized tartaric acid powder andcorn meal carrier, and (2) pigmented lycopodiumand sand carrier. The former is for positivelycharged images, the latter for negatively chargedimages.

The image on the plate is developed by flow-ing the powder over the surface of the plate. Inthe laboratory, the plate bearing the latentimage is placed in a tray and developer cascadedback and forth over the surface of the plate bytilting the tray.

Transfer of Image

The resulting powder image on the plate mustbe transferred to some other material to form apermanent record. To transfer the powder tosome medium such as paper, the sheet of paperis placed in contact with the powder image onthe plate and the paper is charged with an elec-trostatic charge of opposite sign to the chargeon the powder. The powder is attracted from theplate to the paper and adheres to the paper whenit is removed from the plate. The transfer opera-tion can be accomplished with the same equip-ment used in the plate sensitizing operation.With this technique the powder image may betransferred to almost any type of material (wood,plastic, metal, cloth, paper, glass, etc.).

Fixing the Image

The powder image may be made permanenton the paper (or other media) by exposing thepowder to heat. The micronized powder is usu-ally a low fusing-temperature resin. When themedium bearing the powder image is placed be-tween heated platens, the resin is quickly fusedto form a permanent image. Powder images havebeen transferred and fixed to over 70 differenttypes of paper in the laboratory. With a fewexceptions, all produced equally satisfactoryresults. Illustrations of the original and the fin-ished xerographic copy are shown in Figures 4,

5, and 6. Figures 1, 2, and 3 are reproductions ofxerographic prints.

Cleaning the Plate for Re-Use

The residual powder remaining after the trans-fer operation must be removed before the platecan be re-used. Rubbing techniques cannot beemployed because of the resulting abrasion. Theplate is cleaned by placing the plate in a tiltingtray and cascading a coarse granular powder,such as sodium chloride, across the plate surface.This operation removes the remaining micronizedpowder particles. The same results may be ob-tained by dipping the plate vertically into a boxof the material.

Resolving Power

Present plates and powders have a resolvingpower of between 7.5 and 10 lines per millimeter.Because the latent image on the plate is of anelectrostatic nature, it is believed that the re-solving power of the process will be limited onlyby the size of the powder particles used to de-velop the image.

Photo-Regression

Experiments to determine the photo-regres-sion on various photo-conductive surfaces haveindicated that some photo-conductors will re-tain the latent electrostatic image before de-veloping for a period of 100 hours without ap-preciable loss of contrast in the image and for240 hours without loss of resolution.

Spectral Sensitivity and Photographic Speed

The spectral sensitivity of the plates, usingselenium as the photoconductor, is approximatelyequivalent to orthochromatic film. The speed ofsuch a coating is about 0.3 ASA (Tungsten).

Multiple-Copy Reproduction by Xerography

The cycle of operations for obtaining a photo-graphic print by xerography can be repeatedover and over again to obtain as many copies ofa particular design or subject as might be desired.Since the process is positive-to-positive, anyprinted sheet drawing in which the printing orwriting is on one side of the paper, could beused as a positive for contact exposure with a

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XEROGRAPHY

Paramount Pictures, Inc.Original Xerographic copy

FIG. 6.

xerographic plate. If the subject matter to bereproduced is printed on both sides, such asliterature or pages from books, the subject canbe photographed by the xerographic procedure,using a camera. However, after the first copyhas been prepared additional copies can be pro-duced by contact methods.

Where a large number of copies of the samesubject material is desired, the developed powderimage can be fixed onto the plate, either byspraying with a solvent or by fusing the resinouspowder with heat. Multiple copies can then bemade by simply subjecting the plate under lightto the electrostatic discharge which places acharge on the fused image, dusting with powder,and then transferring to paper. This cycle canbe repeated indefinitely for as many copies asmight be required, since the plate with the fixedimage can be used as the master plate. Whenthe plate is to be used again for another subjector design, the resin image can be washed off

with a solvent. Thus, in this process of makingmultiple copies the exposure step is eliminatedexcept in the initial preparation of the masterplate.

Another application of xerography to multiple-copy production involves the use of a plate whichconsists of an insulating image layer on a con-ductive surface. Such a plate can be preparedby a number of different methods. For instance,the design or image can be drawn or painted onthe surface of a metal sheet with an insulatinglacquer. However, a more practical way of pro-ducing an insulating image on such a plate is touse the established photomechanical methodsused in preparing lithographic printing platesand photoengravings, in which a bichromatedcolloid is used as a sensitive coating on the plate.After exposure in contact with a negative orpositive of the subject, the plate is treated bywashing or developing to provide a sharp resistimage on a metal surface. Such a plate might be

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R. M. SCHAFFERT AND C. D. OUGHTON

termed a dry planographic printing plate. Thesteps of obtaining printed copies of such a plateare simply to charge the image, dust with powder,and transfer to paper. This is the basis of a newmethod of printing. The method is unique in asmuch as it does not require pressure or liquidinks. The ink is replaced by dry powder and in-stead of mechanical pressure the impression isobtained by electrical transfer. This techniquehas been incorporated into a laboratory printingmachine in which the printing plate is mountedon a rotating cylinder. The dusting and transferoperations are accomplished at convenient in-tervals around the cylinder. With this equip-ment, it has been possible to print at speeds upto 1200 linear feet per minute, the paper beingfed to the cylinder from a roll.

CONCLUSIONS

Xerography constitutes a new and differentapproach to the recording of light images andthe printing of multiple copies of original de-signs. The process has proven to be applicableto photo-copying-having the distinct advan-tages of rapid and dry processing. Continuous-tone subjects, and even live subjects, have beenxerophotographed with fair results. Color printshave been made from transparencies by usingfilters and complementary colors in developingpowders. The process shows promise of being

applicable to the general field of color reproduc-tion. Since images can be transferred to numerousmaterials, the process should have application inthe field of surface decoration.

Potential applications to the fields of duplicat-ing and printing are indicated, and applicationsto the photographic and photomechanical'5

branches of the graphic arts industry arepossibilities.

ACKNOWLEDGMENTS

The authors wish to acknowledge the co-operation and support of the Haloid Company,Rochester, New York. Its permission to includeresults of research on its project is greatly ap-preciated and the guidance and counsel of Dr.John Dessauer, Haloid's Vice President in chargeof Research and Product Development, havebeen particularly helpful.

Numerous members of the research staff ofBattelle Memorial Institute have contributed tostudies on xerography. Particular acknowledg-ment is due P. G. Andrus, W. E. Bixby, J. M.Chapman, H. E. Copley, R. L. Deubner, J. P.Ebert, D. L. Fauser, R. B. Landrigan, J. J.Rheinfrank, and L. E. Walkup.

The Army Signal Corps is sponsoring in partcontinued research on the process.

1" R. M. Schaffert, "Plate making without film," ShareYour Knowledge Review (Technical Digest) p. 33, Vol.XXIX, No. 5 (February, 1948).

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