US3816115A

US3816115A - Method for forming a plurality of electrostatic latent images on an electrophotographic plate

Info

Publication number: US3816115A
Application number: US00174830A
Authority: US
Inventors: R Gundlach; L Bean
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-06-26
Filing date: 1971-08-25
Publication date: 1974-06-11
Anticipated expiration: 1991-06-11

Abstract

A plurality of electrostatic latent images are formed on the surface of an electrophotographic plate having an electrically insulating overcoating by a method comprising the steps of charging the plate to a uniform surface potential, exposing to a first image pattern, recharging the plate to restore the uniform surface potential and subsequently exposing to a second image pattern. The latent images can be developed with an electroscopic marking material to form visible powder images which can be fixed to the plate or transferred to a recording medium and fixed thereon.

Description

United States Patent [191 Gundlach et al.

[ METHOD FOR FORMING A PLURALITY OF ELECTROSTATIC LATENT IMAGES ON AN ELECTROPHOTOGRAPHIC PLATE [75] Inventors: Robert W. Gundlach, Victor; Lloyd F. Bean, Rochester, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Aug. 25, 1971 [21] App1.No.: 174,830

Related US. Application Data [63] Continuation-impart of Ser. No. 50,216, June 26,

1970, abandoned.

[52] US. Cl 96/l.4, 96/1 R, 117/175 [51] Int. Cl G03g 13/22 [58] Field of Search 96/1, 1.5, 1.4; 117/37,

[56] References Cited UNITED STATES PATENTS 2,624,652 1/1953 Carlson 96/1 R June 11, 1974 3,121,010 2/1964 Johnson et al. 96/1 R 3,438,706 4/1969 Tanaka et al. 96/! X 3.457.070 7/1969 Watanabe et al 96/l.5

Primary Examiner-Charles E. Van Horn Attorney, Agent, or Firm-James J. Ralabate; David C. Petre; Gaetano D. Maccarone [57] ABSTRACT to a recording medium and fixed thereon.

17 Claims, 8 Drawing Figures PATENTEflJuu 1 1 m4 SHEET 10F 2 FIG.

INVENTORS. ROBERT W. GUNDLACH LLOYD F. BEAN 7AQCMM ATTORNEY mm .mm m

SHEET 2 OF 2 ZERO T FIG. 3

ZERO v Pl m J 6,24

FIG. 4

ZERO

P2 I H/ i V- cl METHOD FOR FORMING A PLURALITY OF ELECTROSTATIC LATENT IMAGES ON AN ELECTROPHOTOGRAPI-IIC PLATE BACKGROUND OF THE INVENTION This application is a continuation-in-part of earlier copending application Ser. No. 50,216, filed June 26, 1970 and now abandoned.

This invention relates in general to electrostatic images such as are useful in xerographic reproduction and, in particular, to a method for forming a plurality of electrostatic latent images on the surface of a xerographic plate.

In the practice of xerography it is the general procedure to form an electrostatic latent image on a xerographic plate surface generally by charging a photoconveloped with an electroscopic marking material to form a visible powder image thereon which is subsequently transferred to a recording medium and fixed thereto, the cycle being repeated any number of times to produce any desired number of reproductions of the original image. New developments in information reproduction have now made it advantageous to copy two original images on one recording medium such as where computer-generated information in intended to be copied in conjunction with special forms. This has been accomplished heretofore by using overlay transparencies corresponding to the form together with the original information sought to be inserted thereon and exposing a charged xerographic plate to the combination. Highly satisfactory results are obtained according to this procedure; however, the transparency must initially be made prior to reproduction of the information on the forms thus involving delay and aided expense. There are. also problems with respectto maintaining intimate contact of theoverlay with the opaque reproduction subject during exposurewhich can lead to shadow effects, i.e., double lines in the finished copy. It would be desirable to have a method for copying original images in combination whereby electrostatic latent images could be formed on a xerographic plate directly from the originalimages desired to be reproduced.

SUMMARY OF THE INVENTION Now according to the present invention it is possible to form, on a single xerographic plate, electrostatic latent images corresponding to a plurality of original images. The original images may be positives or it is likely in many instances such as the situation discussed above that at least one image can be as readily made available as a negative rather than as a positive. It is therefore. an object of the invention to provide a novel method for forming electrostatic images whereby the abovementioned needs are fulfilled and the disadvantages of the prior known methods are overcome.

It is another object of the invention to provide a method for forming, on the surface of an overcoated xerographic plate, a plurality of electrostatic latent images corresponding to a plurality of original images.

A further object of the invention is to provide such an imaging method wherein development of the visible images may be carried out by any development system.

Still another objectof the invention is to provide such an imaging method wherein development of the visible images is accomplished by a development system which responds to absolute differences from a reference potential.

Yet a further object is to provide a xerographic reproduction method wherein a plurality of images can be reproduced concurrently on a recording medium.

The foregoing objects and advantages are realized according to the invention by employing an overcoated xerographic plate comprised of a photoconductive insulating layer on a conductive substratewith a film of electrically insulating material applied over the photoconductive layer and forming on the plate a plurality of electrostatic latent images corresponding to a plurality of original images. Briefly stated, the xerographic plate is uniformly electrostatically charged in the absence of illumination, exposed to activating radiation in a pattern corresponding to a first original image, charged a second time in the absence of illumination to restore the plate surface to a uniform potential and subsequently exposed to activating radiation in a pattern corresponding to a second original image whereby the electrostatic latentimages are recorded on the plate as will become clearly apparent.

According'to a preferred embodiment of the inven- I tion the first and second original image patterns may be both positive in image sense whereas in another preferred embodiment the first image pattern may be negative in image sense and the second pattern a positive.

der images which can be fixedto the plate or which can be transferred to a recording medium and fixed thereon, thus allowing the plate tobe reused.

Other objects and advantages of the invention will become readily apparent from the following detailed description of the preferred embodiments thereof when read with reference to the drawings in which:

FIG. 1 is a side view of a xerographic plate employed .in the practice of the invention;

dition of various areas of the plate upon completion of FIG. 4 is a graphical illustration representing the condition of various areas of the plate upon completion of the practice of still another embodiment of the invention; and

FIG. 5 is a graphical illustration representing the condition of various areas of the plate upon completion of the practice of yet another embodiment of the invention.

The xerographic plates which are suitable for use according to the invention are constructed generally as illustrated in FIG. 1 and comprise a photoconductive insulating layer on a conductive backing 12 with a highly electrically insulating film 14 coated over the photoconductive layer. The film 14 may be a coating of the nature disclosed in U.S.Pat. No. 2,860,048 to Deubner. The coating maybeformed by any bonding method thatwill eliminate space between the film and the layer without significantly altering the electrical or photoelectrical characteristics of either. In one embodiment of the plate the overcoating is transparent to the activating radiation thus permitting it to reach the photoconductive layer when the plate is illuminated from the film side; or in another embodiment the plate can have a substrate 12 which is transparent to the activating radiation allowing the plate to be illuminated from the substrate side as is known in the art. Insulating film 14 can be any of many coating materials such a polyvinyl chloride, polystyrene, polytetrafluoroethylene, polyethylene terephthalate or like resins or plastic materials and may be opaque or transparent. When the film is opaque exposure of the photoconductive layer may be effected through a transparent substrate, etc. The film material is preferably resistant to 4 ate shape in which case it can be secured to a rotating cylinder as is well-known in the art.

In a particularly preferred embodiment of the invention it is utilized to combine computer-generated inforcleaning and abrasion and nonreactive with the electroscopic marking material employed to develop the latent electrostatic images whereby the plate can. be utilized in a recyclible method for xerographic reproduction to form a multiplicity of reproductions. Photoconductive insulating layer 10 maybe constructed from any standard photoconductive materials such as, for

example, vitreous selenium, sulfur, anthracene and tel-- lurium; or it can be a finely ground photoconductive insulating material dispersed in a high resistance electrical binder such as are disclosed in U.S. Pat. No. 3,121,006 to Middleton et al., or an inorganic photoconductive pigment dispersed in a photoconductive insulating materialsuch as are disclosed in U.S. Pat. No. 3,121,007 to Middleton et al., or an organic photoconductor such as phthalocyanine in a binder; or generally any photoconductive insulating material which is suitable for use in xerographic reproduction techniques.

The material selected for use as photoconductive insulating layer 10 in any instance will be, to some extent, determined by the particular embodiment of the invention being practiced as .will become apparent further below. The overcoated xerographic plates which can be employed in the method of the invention may have relative electrical thicknesses of the insulating film and the photoconductive layer of from about 4:1 to about 1:4 with the preferred range being from about 2:1 to about 1:2 and the optimum embodiment consisting of a plate which has about a 1:1 relationship. The xerographic plate, which may be rigid or flexible, may be flat or it can have other configurations such as an arcumation on standard forms. In such a situation the computer-generated information typically would be negative in image sense. Additionally, it is particularly preferred that the initial and subsequent charging steps be of the same polarity because materials considerations, e.g., the particular photoconductive insulating materials used are simplified and more latitude is provided with respect to the type of development systems which can be employed. Therefore, for purposes of illustration, the invention will be described in detail with respect to the embodiment wherein the first original image pattern is negative in image sense, the second original image pattern is positive in image sense and both charging steps apply a potential of the same polarity.

The invention will be more easily understood when described in relation to FIG. 2A through FIG. 2D wherein the charge density 0' and the potential y at any position x on the free surface of the plate are shown by dotted and solid lines respectively. Referring now to FIG. 2A it is seen that the charge density and potential across the surface of an overcoated xerographic plate of the type previously described are uniform after the plate has been electrically charged in the absence of il- Iurnination by a conventional method such as that disclosed in U.S. Pat. No. 2,777,957 to Walkup but which may be any other plate charging method. The plate may be charged positively or negatively with the charge density placed on the plate preferably being the charge giving an internalfield of about 25 volts/micron, but which may vary from a surface charge giving internal fields of about 5 to 10 volts/micron'to an upper limit of about volts/micron or the maximum electric field that the given dielectrics will effectively support without breakdown. The uniformly charged plate is then exposed to a pattern of. activating radiation corresponding to at least one original negative image, that is, bright characters on a dark background, the source of illumination being any to which the photoconductive insulating layer 10 is sensitive. As shown by FIG. 2B the charge density remains uniform across the plate as will be understood by those skilled in the art whereas the areas of the photoconductor which are exposed to the activating radiation, represented by E are discharged causing the potential of the free surface in these areas to drop. The non-exposed portions of the plate surface experience a negligible drop in potential. Thus, voltage gradients representative of the pattern of activating radiation incident upon the plate are created thereby giving rise to the formation of an electrostatic latent image. The condition of the area represented by E is intended to be illustrative of the situation which exists at any area of the plate where the activating illumination has reached the photoconductive layer.

The negative image or images may typically be projected from a cathode ray tube (CRT) display or from a microfilm transparency. Where a number of original negative images are'desired to be combined in the final copy they may be projected simultaneously or sequentially on the plate. It should be appreciated by those skilled in the art that any number of original negative images may be projected onappropriate areas of the plate during this step of the method. For example, it is possible to project certain information on various parts of the plate sequentially or simultaneously with the projection of a standard form thereon.

The xerographic plate is then charged again to a uniform potential in the absence of illumination in the manner previously described. According to this preferred embodiment of the invention the polarity of the charge applied in this charging step is the same as that of the charge applied in the initial charging step. FIG. 2C illustrates the condition of the plate after the second charging operation has been effected. The potential across the plate surface is again made uniform but the charge density hasnow become non-uniform in a pattern representative of the activating illumination which reached the photoconductive layer during the previous exposure step. The areas of the plate represented by E that is, those which had been exposed to the activating radiation, have a greater charge density than those portions which did not receive any illumination. Subsequently the plate is exposed to a second pattern of activating radiation, this corresponding to an original positive image, that is, dark characters on a white background. The positive image is positioned in a manner such as to be in registration with the original negative image or images to which the plate had previously been exposed. FIG. 2D shows the condition of the plate at this point. There now exist two sets of electrostatic latent images on the plate as represented by E and D the latter intended to be illustrative of the portions of the plate which did not receive any illumination during the second exposure, namely the image areas of the positive image, each of the electrostatic latent images having a positive potential with respect to the background areas. Electroscopic marking material having a charge of polarity opposite to that of the charge placed on the plate is brought into contact with the plate surface as by cascading the material across'the plate, although it should be recognized that any other xerographic development method including, for example, magnetic brush and liquid development methods may be employed, thus forming visible powder images which can be fixed to the plate or which can subsequently be transferred to a recording medium by any conventional transfer method and fixed thereto. While it is often desirable to develop the electrostatic latent images with toner material the images may be used in a host of other ways as, for example, electrostatic scanning systems may be employed to read the latent images or the images may be transferred by TESI techniques to other materials and stored.

Where the xerographic plate is to be reused to make additional reproductions as in a recyclible xerographic' method any residual charge remaining on the plate after the visible powder images have been transferred to a recording medium must be removed therefrom prior to each repetition of the cycle. Generally the residual charge can be removed from the plate by ionizing the air above the insulating film while the photoconductive layer is uniformly illuminated and grounded. For example, the charge removal could be accomplished by AC. corona discharge in the presence of illumination from a light bulb or preferably a conductive brush can be brought into contact with the plate surface in the presence of such illumination. This latter mode is preferred because it also cleans any toner particles remaining. The insulating layer can also be made sensitive to other light, e.g., ultraviolet light and the residual charge removed by general exposure to the appropriate illumination.

According to another embodiment of the invention the firstand second original image patterns to which the overcoated xerographic plate is exposed are positive in image sense "and the initial and subsequent charging steps apply a potential of the same polarity. The condition of the plate after the practice of the method according to this embodiment is illustrated in FIG. 3. Referring now to FIG. 3 it can be seen that there exist two sets of electrostatic latent images on the plate as represented by A and A, with A, being representative of areas of the plate which do not receive any illumination during the first exposure and A being illustrative of those portions of the plate which do not receive any illumination during the second exposure. Of course, in both exposures the areas of the plate which do not receive any illumination are those corresponding to image areas of the positive images. It is clearly evident that the potential contrast, with respect to a reference potential P, between the image and background areas for each respective latent image is opposite in sign with one being positive with respect to background and the other negative with respect to background. Thus, the electrostatic latent images can be developed simultaneously by any development system which develops absolute differences from a reference potential. Typical suitable development systems which exhibit this characteristic behavior include polar liquid ink development and conductive powder development such as is described in US. Pat. No. 3,166,432 to Gundlach.

FIG. 4 illustrates the condition of the plate after the practice of another embodiment of the invention wherein the first exposure is to negative image input and the second exposure is to positive image input with the initial and subsequent charging steps applying po tentials of opposite polarities from each other. In this illustrative instance the first charging step is carried out with a positive potential while the second is made with a negative potential. Of courseit will be understood that the respective directions of field may be applied in any order. Referring now to FIG. 4 it is seen that two sets of electrostatic images are formed on the plate as represented by B, and B B is intended to be illustrative of the condition'of the plate at any area where activating illumination reaches the. photoconductive layer during the first exposure step, i.e., any area of the plate corresponding to image areas of the negative image. B is representative of the plate areas which do not receive any illumination during the second exposure step, i.e., the plate areas corresponding to image areas of the positive image. Again it is seen that the potential contrast, with respect to a reference potential P between the image and background areas for each respective latent image is opposite in sign. As was described above these images can be developed simultaneously by a development system which develops absolute differences from a reference potential.

FIG. 5 is descriptive of the condition of the plate after the practice of another embodiment of the invention wherein both exposure steps are carried out with positive images and the initial and subsequent charging steps apply potentials of opposite polarities from each other. Although, for purposes of illustration, the first charging step is carried out with a positive potential and the second with a negative potential, the reverse order may be used. Referring now to FIG. it is seen that two sets of electrostatic images are formed on the plate as represented by C, and C with C being representative of areas of the plate which do not receive any illumination during the first exposure and C being illustrative of portions of the plate which do not receive illumination during the second exposure. Both sets of electrostatic images are negative with respect to reference potential P It should be recognized that these latent images may be developed by any development system.

with respect to the embodiments of the invention wherein potentials of opposite polarities are applied in the respective charging steps it will be appreciated by those skilled in the art that the photoconductive material employed in the xerographic plate typically should be one which acts as an insulator for both directions of field in the dark. Typical suitable photoconductive materials which are capable of holding both polarities of charge in the dark include, for example, selenium and organic binder plates containing cadmium sulfoselenide such as are described in copending application Ser. No. 94,072 filed Dec. 1, 1970.

Although the present method is intended to be practiced in a manner such that registration of the respective images projected on the xerographic plate will preclude any overlap in the reproduced images it should be recognized that overlap may occur without substantially affecting the results obtained. with respect to the reproduction of line copy which typically comprises the bulk of the copy reproduced, substantial areas of the original material are comprised of background. For example, a US. patent, on pages completely filled with single space information has a background area in the order of about 90 percent with about percent of the total imaging area of the plage taken up by character information. Thus, it will be clearly evident to those skilled in the art that any overlap, between, for example, character information and a standard form would not present any significant problems in the practice of the invention.

It should be noted that the above described method has proved to be very effective through experimentation and any inaccuracy in the theoretical operation thereof as described and illustrated isnot to be construed as being limiting of the invention.

The invention will now be further described in detail with respect to specific preferred embodiments thereof it being understood that these are illustrative only and not intended to limit the scope of the invention to the specific materials disclosed. All parts and percentages are by weight unless otherwise indicated.

. EXAMPLE I A xerographic plate is constructed by arranging a 50 micron thick layer of amorphous selenium on a 50 mil thick aluminum sheet by vacuum evaporation and coating the exposed selenium layer surface with a 12.5 micron thick film of poly-N-vinyl carbazole by solvent coating techniques using toluene. The poly-N-vinyl carbazole film and the selenium layer have a 1:2 relationship in electrical thickness. The plate is charged positively to a potential of 1,000 volts by A.C. corona charging in the absence of illumination and then exposed to actinic light from a CRT, in the range of 450-500 nm, in a pattern corresponding to an original negative image of a form on which information is to be 8 reproduced thus discharging the illuminated or exposed areas of the plate surface to a potential of about 350 volts with negligible discharge in the background or non-image areas. The plate is again charged positively to a uniform surface potential of 1,000 volts by A.C. corona discharge in the absence of illumination. Subsequently the plate is exposed to a camera projection of an original positive opaque image of informa tion which is to be reproduced on the form, illuminated by a fluorescent lamp. The positive image is positioned 'in a manner such as to be in registration with the original negative image to which the plate has been previously exposed. Two latent electrostatic images are formed on the plate, that corresponding to the original negative image and the other corresponding to the original positive image having potentials of approximately 222 volts and 667 volts above the background areas respectively. The latent electrostatic images are then developed by cascading an electroscopic marking material having a negative charge of polarity across the plate surface thereby resulting in the formation of visible powder images which are transferred to a paper sheet and fixed thereto thus providing a good quality reproduction of the original images with a clean background.

EXAMPLE I! The procedure described in Example I is employed with respect to a xerographic plate constructed with a 12.5 micron thick layer of 5 percent mixture of metalfree phthalocyanine (Commercially available under the tradename Monolite Fast Blue from Holland Suco Co., Holland, Mich.) in vinyl acetate, vinyl chloride (commercially available under the tradename VYNS from Union Carbide Co.) arranged on a 50 ml thick backing sheet of aluminum and overcoated with a 38 micron thick film of Mylar. The Mylar is bonded to the photoconductive binder plate by heating to a temperature at which it is soft (about 250C) and rolling the softened material under pressure over the plate. The Mylar film has a 4:1 electrical thickness relationship with the phthalocyanine layer.

A negative microfilm transparency is first projected on the plate employing incandescent tungsten illumination. The second exposure is to an impact-printed black on white subject using incandescent illumination. Two latent electrostatic images are formed on the plate, that corresponding to the original negative image and the other corresponding to the original positive image having potentials of about volts and 800 volts above' the background areas respectively. A good quality reproduction with clean background is formed.

EXAMPLE llI The procedure followed in Example I is again followed with the exception that two original negative images are projected on the plate during the first exposure step. Initially the plate is exposed to actinic light from a CRT display followed by exposure to a negative microfilm transparency. A copy of comparable quality as that made in Example I is obtained.

EXAMPLE IV The procedure of Example I is followed with respect to a xerographic plate constructed by arranging a 30 micron thick layer of 30 percent mixture of cadmium sulfoselenide (CdSSe) (commercially available under the tradename 1020 Red Pigment from General Color Company, Ft. Wayne, Ind.) in a glass binder (identified as Harshaw Commercial Frit, Sample N862 available from Harshaw Chemical Company, Cleveland, Ohio) on a 50 mil thick stainless steel backing plate and coating the exposed surface of a CdSSe layer with a 30 micron thick film of polystyrene. The polystyrene film and CdSSe layer have about a 1:1 relationship in electrical thickness. Negative corona charging is used in this example.

The illumination of the plate is carried out in the same manner described in Example 11. Two latent electrostatic images are formed on the plate, that corre sponding to the original negative image and the other corresponding to the original positive image having potentials of approximately 250 volts negative and 500 volts negative above the background areas respectively. A goodquality reproduction with clean background is obtained.

EXAMPLE V A xerographic plate is constructed by arranging a 50 micron thick layer of a photoconductive composition made up of about 5 parts cadmium sulfoselenide, about 5 parts phthalocyanine and about 5 parts selenium dispersed in about 100 parts of poly-N-vinyl carbazole sensitized with 2,4,7-trinitro-9-fluoroenone on a 50 mil thick aluminum sheet. The exposed surface of the photoconductive layer is coated with a 12.5 micron thick film of poly-N-vinyl carbazole. The poly-N-vinyl carbazole film and the photoconductive layer have a 2:1 relationship in electrical thickness. The plate is charged positively to a potential of 600 volts by corona charging in the absence of illumination and then exposed to a camera projection of a first original positive opaque image thus discharging the illuminated or background areas of the plate surface to a potential of about 200 volts with negligible discharge in the image areas. The plate is then charged negatively to a uniform surface potential of 600 volts by corona discharge in the absence of illumination. Subsequently the plate is exposed to a camera projection of a second original positive opaque image illuminated by a fluorescent lamp. Two electrostatic latent images of negative polarity are formed on the plate, that corresponding to the first original positive image and the other corresponding to the second original positive image having contrast potentials of approximately 133.3 and 466.7 volts below the background areas respectively. Both electrostatic images are negative with respect to the background potential.

EXAMPLE Vl The procedure described in Example V is again used with the exception that the first exposure of the plate is made to activating light from a CRT, in the range of 450-500-nm, in a pattern corresponding to an original negative image ofa form on which information is to be reproduced. Thus after the first exposure step the ex- V10 ground potential. In otherwords' the former is-positive with respect to the background potential and the latter is negative withrespect to the background potential. The electrostatic images arethen developed by a polar liquid ink development system.

EXAMPLE VII The procedure described in Example V is again followed with the exception thatin the second charging step the plate is charged positively to a potential of 600 volts. Two electrostatic images of positive polarity are formed, that corresponding to the first original positive image having a potential of about 133.3 volts below the background areas and that corresponding to the second original positive image having a potential of about 277.7 volts above the background areas. In other words the former is negative with respectto the background potential whereas the latteris positivewith respect to the background potential. The electrostatic images are then developed by a polar liquid ink development systern.

Although the invention has been described with relation to various specific and preferred embodiments thereof, it is not intended to' be limited thereto but rather those skilled in the art will recognize that variations and modifications may be made therein which are within the spirit of the invention and-the scope of the appended claims. For example, while the invention has been described with relation to the combination of information on certain forms it can be utilized to reproduce many other combinations of original images. In actual commercial practice it is preferred to secure the overcoated xerographic plate to a rotating drum in accordance with well known techniques whereby it is possible to make multiplicity of reproductions by a recyclible xerographic reproduction method.

What is claimed is;

l. A method for forming a plurality of electrostatic latent images on a xerographic member comprising the steps of a. applying a first electrostatic charge to a xerographic member in the absence of illumination, said member comprising a conductive substrate and a photoconductive insulating layer with an electrically insulating film applied over said layer;

images by depositing electroscopic marking particles thereon.

3. The method asjdefined in claim Zand further including the step of fixing said developed images to said xerographic member.

4. The method as defined in claim Zand further includingthe steps of transferring said developed images to a receiver material and fixing said developed images thereto. I

5. The method as defined in claim 1 wherein the insulating film and photoconductive insulating layerof the xerographic member have an electrical thickness relationship of from about 2:1 to about 1:2.

6. The method as defined in claim 1 wherein the insulating film and the photoconductive insulating layer of the xerographic member have an electrical thickness relationship of about 1:1.

7. The method as defined in claim 1 wherein the electrically insulating film is transparent to said activating radiation.

8. The method as defined in claim 1 wherein the conductive substrate is transparent to said activating radiation.

9. The method as defined in claim 1 wherein said first and said second electrostatic charges applied to said xerographic member are of opposite polarities from each other.

10. The method as defined in claim 9 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to an original positive image.

ll. The method as defined in claim 9 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to at least one original neg- 12 ative image. I

12. The method as defined in claim 11 and further including the step of developing said electrostatic latent images with a development system which develops absolute differences from a reference potential.

13. The method as defined in claim 1 wherein said first and saidsecond electrostatic charges applied to said xerographic member are of the same polarity.

14. The method as defined in claim 13 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to an original positive image.

IS. The method as defined in claim 14 and further including the step of developing said electrostatic latent images with a development system which develops absolute'differences from a reference potential.

16. The method as defined in claim 13 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of radiation corresponding to at least one original negative image.

17. The method as defined in claim 16 and further including the step of developing said electrostatic latent images by depositing thereover electroscopic marking material having a charge of polarity opposite to that of said first and second electrostatic charges.

Claims

2. The method as defined in claim 1 and further including the step of developing said electrostatic latent images by depositing electroscopic marking particles thereon.
3. The method as defined in claim 2 and further including the step of fixing said developed images to said xerographic member.
4. The method as defined in claim 2 and further including the steps of transferring said developed images to a receiver material and fixing said developed images thereto.
5. The method as defined in claim 1 wherein the insulating film and photoconductive insulating layer of the xerographic member have an electrical thickness relationship of from about 2:1 to about 1:2.
6. The method as defined in claim 1 wherein the insulating film and the photoconductive insulating layer of the xerographic member have an electrical thickness relationship of about 1:1.
7. The method as defined in claim 1 whereIn the electrically insulating film is transparent to said activating radiation.
8. The method as defined in claim 1 wherein the conductive substrate is transparent to said activating radiation.
9. The method as defined in claim 1 wherein said first and said second electrostatic charges applied to said xerographic member are of opposite polarities from each other.
10. The method as defined in claim 9 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to an original positive image.
11. The method as defined in claim 9 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to at least one original negative image.
12. The method as defined in claim 11 and further including the step of developing said electrostatic latent images with a development system which develops absolute differences from a reference potential.
13. The method as defined in claim 1 wherein said first and said second electrostatic charges applied to said xerographic member are of the same polarity.
14. The method as defined in claim 13 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of activating radiation corresponding to an original positive image.
15. The method as defined in claim 14 and further including the step of developing said electrostatic latent images with a development system which develops absolute differences from a reference potential.
16. The method as defined in claim 13 wherein step (b) is carried out by selectively illuminating said charged xerographic member with a pattern of radiation corresponding to at least one original negative image.
17. The method as defined in claim 16 and further including the step of developing said electrostatic latent images by depositing thereover electroscopic marking material having a charge of polarity opposite to that of said first and second electrostatic charges.