US3756811A - Fferent dynamic ranges electrophotographic process employing photoconductive materials of di - Google Patents

Fferent dynamic ranges electrophotographic process employing photoconductive materials of di Download PDF

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US3756811A
US3756811A US00628017A US3756811DA US3756811A US 3756811 A US3756811 A US 3756811A US 00628017 A US00628017 A US 00628017A US 3756811D A US3756811D A US 3756811DA US 3756811 A US3756811 A US 3756811A
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plate
photoconductor
xerographic
selenium
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R Gundlach
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Xerox Corp
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
    • G03G5/02—Charge-receiving layers
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
    • G03G5/02—Charge-receiving layers
    • G03G5/04—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor

Definitions

  • a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer.
  • Development of the latent image is effected with an electrostatically charged finely divided material, such as an electroscopic powder referred to in the art as toner, that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image.
  • the developed, toned xerographic powder or toner image is usually transferred to a support surface, for example paper, to which it may be afiixed by any suitable means.
  • Amorphous selenium has been found to be a preferred photoconductive insulating material because of its extremely high quality image making capability, relatively high light response, capability to receive and retain charge areas at different potentials and of different polarity and because it may be reused over and over again in quick succession.
  • a characteristic of an amorphous selenium xerographic plate is that it reproduces line copy and other contrasty originals in an excellent fashion because amorphous selenium in combination with conventional xerographic development processes, for example cascade, development, has a relatively short dynamic range of about 0.6.
  • R equals the ratio of reflected light to incident light.
  • a density of 1.3 is where about of the incident light is reflected back to the viewer.
  • a density of anywhere from about 1.2-1.5 or above appears to the unaided human eye as a very dense black.
  • an amorphous selenium plate in combination with a particular development system has a dynamic range of about 0.6 is to say that assuming sufiicient exposure of the original to just substantially completely discharge areas of the plate corresponding to white background portions of the original where density is about 0, which substantially completely discharged areas of the plate after xerographic development and transfer would appear as a density of about 0, the plate is only capable of producing changes in D (densities of the reproduction) for D (densities of the original) up to about 0.6. For all D greater than about 0.6, no change in density will be shown on the reproduction since for these D there is substantially no discharge of charge from corresponding portions of the xerographic plate, thus the marking material used in development is maximumly attracted to these areas.
  • any D greater than about 0.6 will appear as a D of a single density between about 1.2-1.4.
  • a xerographic print from a selenium plate under the exposure conditions described of a black and White photograph original of a black ribbed sweater on a girl with dark hair would probably not show definition of the ribs or any hair information, but would appear as a smooth black sweater and smooth black hair because D in the sweater and hair region is above about 0.6 and any changes in D above about 0.6 reproduce as a single density in the region of from about 1.2-1.4.
  • the dynamic range is by successive selected exposures moved across the range ofD desired to be reproduced each print produced from, for example, an amor phous selenium platepicking up tonalcontrast fora different 0.6 increment of D
  • this patent offers an answer to the problem, it does have certain-limitations such as the necessity for successive xerographic imaging cycles to produce a single print, careful adjustment of exposure for each successive print taking operation, registering of successive image transfers and other factors which make it less than a completely satisfactory solution to the problem.
  • Halftone optical screening techniques have been utilized to modulate the intensity of radiation incident to a plate thereby providing alternating discrete areas of the plate one area to produce tonal differences in a particular D increment and the immediately adjacent area to produce tonal differences in a different dynamic range increment of D
  • a primary disadvantage of such a system is that it is light inefficient in that light from a radiation source is first optically screened and much light is absorbed before reaching the photoconductor and thus greater exposure is required compared to an unscreened process.
  • the optical screen absorbs so much light that the overall sensitivity of the plate is usually reduced by at least about four times.
  • optical screening adds to the complexity, bulk and expense of the optical system.
  • a xerographic plate and process comprising an electrically conductive backing and overlying said backing a photoconductive insulating layer having a multiplicity of a plurality of alternating discrete small areas of photoconductive insulating material, each of said pluralities of small areas having a different photosensitivity to a given exposure.
  • FIG. 1 shows a side view of three preferred xerographic plate embodiments according to the invention.
  • FIG. 2 is a graph of D density of various image areas in the reproduction, v. D density of various image areas in the original, for the two photoconductors used in a plate according to one embodiment of the invention and a graph of these two values for the resultant viewable, xerographic image produced.
  • FIG. 1A there is shown a preferred xerographic plate according to one embodiment 10 of this invention comprising electrically conductive substrate 12, screen 14 of a first photoconductive insulating material and an overlying layer of a second photoconductive insulating material 16.
  • the screen may also be laid on top of a layer of photoconductor as shown in FIG. 1B.
  • Electrically conductive substrate 12 is generally employed in the make up of xerographic plates in order to facilitate the charging or sensitization of the photoconductive insulating surface of the plate and to provide for the dissipation of electrical charge from those portions of the photoconductive surface which are rendered electrically conductive upon exposure to actinic radiation.
  • this support member is also sufiiciently strong to provide mechanical support for the remainder of the plate so as to make the plate suitable for use in xerographic copying machines.
  • any suitable electrically conductive material may be used, which includes most metals. For example, aluminum is often used in commercially available xerographic plates.
  • the xerographic imaging member is spoken of as a plate throughout this specification, the plate need not be a rigid planar member, but may consist of a web, foil or the like in the form of a cylindrical surface, endless belt, moebius strip or other shape.
  • the electrically conductive support may comprise two or more layers depending upon the desired characteristics of the total layer.
  • an electrically conductive transparent support 12 may be formulated by using a layer of Nesa glass available from the Pittsburgh Plate Glass Co.
  • a transparent electrically insulating film such as polyethylene terephthalate polyester film available under the trademark Mylar from the E. I. du Pont de Nemours & Co. overcoated with a transparent electrically conductive layer usually substantially thinner than the underlying Nesa or Mylar layer.
  • a transparent electrically conductive layer usually substantially thinner than the underlying Nesa or Mylar layer.
  • a thin layer of copper iodide as more fully described in Lyon Pat. 2,756,165 may be used.
  • thin, transparent and electrically conductive layers of tin oxide may be deposited, on transparent substrates as more fully described in McMaster Pat. 2,429,420, Preston Pat. 2,769,778 and Haayman aPt. 2,772,190.
  • FIG. 1C there is shown anotherpreferred embodiment of a xerographic plate 22according to this invention comprising electrically conductive substrate 12 and alternating areas of a first photoconductor 18 and a second photoconductor 20.
  • a xerographic plate according to this invention comprising a photoconductive insulating portion having a multiplicity of a plurality of alternating discrete small areas of photoconductive insulating material having different sensitivities to a given common exposure may be fabricated in a number of preferred ways.
  • One way is by depositing a screen pattern of a first photoconductive insulating material on an electrically conductive substrate and then overcoating the screen pattern and the interstices with a layer of a second photoconductiveinsulating material to give a free photoconductive surface of the second photoconductor.
  • This type of plate is shown in FIG. 1A.
  • the screen pattern of a first photoconductor may be deposited on the freesurface of a second photoconductor layer as shown in FIG. 1B.
  • the second photoconductor mayajust fill the interstices between the .screen pattern of the first photoconductor to produce a free photoconductive surface of flush, separate and distinct coplanar portions of first and second photoconductors.
  • a smooth photoconductor surface is preferred in xerography in order to aid in the xerographic transfer and cleaning steps. This type of plate 10 is shown in FIG. 1C.
  • the screen pattern of the first photoconductive insulating material may be in the form of a dot pattern with round, elliptical, square,'triangular, or any other regular or irregular dot shape or it may be in the form of a'line pattern, or any other broken pattern whether regular, irregular, or random in shape and/or spacing. It should be noted that instead of making the pattern in the form of dots an outline of a dot pattern may be used.
  • the screen may comprise an integral layer of photoconductive insulating material with a pattern of holes or voids through it.
  • the percentage coverage of the screen pattern of the first photoconductor over the electrically conductive backing layer will vary depending upon the first and second photoconductors used, their photosensitivity difference, the amount of exposure, Whether the screen pattern is sharp edged (hard in the language of the art) or soft. and other factors.
  • the dimensions of the alternating areas of at least a first and second photoconductive insulating material can be varied over a rather wide range, although it is desirable to keep the maximum dimension of any particular discrete area not more than about 0.02 inch to insure more natural and higher quality halftone reproductions.
  • a coarse screen pattern of a photoconductor having 50 or 60 dots or lines or spacings of discrete areas to the lineal inch will be useful for some purposes, finer screens and spacings such as those having 100, 200, 300, or 400 or even more dots, lines or spacings to the lineal inch will give a more nearly continuous tone appearance to the finishedprint.
  • amorphous selenium is the less photosensitive or slower photoconductor used in the plate construction; This same procedure maybe generally followed to choose first and second photoconductors to construct a-plate according to this invention.
  • Sensitive, sensitivity-and photosensitivity as used herein, are intended to mean the rate of charge dissipation or lowering of surface potential of a xerographic plate when exposed to a given actinic radiation.
  • amorphous selenium plate xerographic systems generally have a dynamic range of about 0.6. This phenomenon may be shown graphically by reference to FIG. 2.
  • Curve A is a graph of D v.
  • D for a typical amorphous selenium xerographic plate in a conventional xerographic imaging system.
  • This amount of exposure is shown graphically by the start of curve A from point b, which is also the origin of axis D and D,..
  • the second photoconductor should be about four times more sensitive than said first photoconductor.
  • photoconductors faster than amorphous selenium are available in the.
  • amorphous selenium is relatively insensitive to light of wavelength beyond about 5,500 angstrom units.
  • preferredmore sensitive photoconductive materials when used with lesssensitive amorphous selenium, include selenium alloyed with tellurium, selenium alloyed with arsenic, and phthalocyanine, all of which are substantially more sensitive than amorphous selenium to the red and near infra red region of the spectrum.
  • selenium alloys may be found in Ullrich Pat. 2,803,542; Mayer et al. Pat. 2,822,300; Mengali Pat. 2,745,327 and Paris Pat. 2,803,- 541.
  • Phthalocyanine binder systems are described in more detail in copending application Ser. No. 375,191, filed June 15, 1964, now abandoned.
  • amorphous selenium may be used as the more sensitive photoconductor in a plate according to the invention, but then the slower or less sensitive photoconductor should preferably be about A as fast which overall, produces a plate which is about A as sensitive as a plate where amorphous selenium is used as the slower photoconductor. Since a variety of photoconductors faster than selenium are available, and since generally it is preferred to fabricate the more sensitive plate in order to use lower magnitude exposures, it will generally be preferred, other factors being equal, to employ amorphous selenium, if it is being used as one of the photoconductors according to this invention, as the slower or less sensitive photoconductor.
  • the rate of emission of charged particles from a portion of the screen pattern is generally dependent, at least in a certain thickness range, on the thickness of the screen or dot, and since in a soft screen pattern, the screen portions vary in thickness gradually from points of maximum thickness to points of no deposition or no thickness there is a gradual rather than an abrupt, as in a hard dot pattern, variation in the charge dissipating effect of the screen pattern, thereby providing a mechanism to produce at least some variation in D in any D gap between points a and d on curves A and B respectively.
  • Another important aspect of the invention is the percentage of the whole plate to be taken up by the first and by the second photoconductor.
  • amorphous selenium is the slower photoconductor and its D v. D curve corresponds approximately to curve A in FIG. 2 and where the faster photoconductor is about four times faster than the selenium and thus corresponds approximately to curve B of FIG. 2, and taking a period area of this plate encompassing one typical area of amorphous selenium and the typical adjacent area of the second photoconductor, it is seen that this period area must be capable, due to the contribution of the slower amorphous selenium portion, to reproduce highlights up to a D of about 0.6.
  • the screen pattern of the faster photoconductor covers about 25% of the area of the conductive backing with the amorphous selenium covering about the remaining 75
  • the dense fully toned areas are not totally absorbing but are of a D of about 1.3, or about 5% reflecting, slightly more than 75% of the period area, about must be taken by the slower amorphous selenium photoconductor to give a total period D of about 0.6.
  • the remaining 25 is actually less bright, because some of its brightness is due to light scattering from adjacent areas.
  • the first xerographic processing step is to form a latent electrostatic image on said plate.
  • One method of forming the latent image is to uniformly charge the plate and selectively dissipate charge in light struck areas during an image exposure. Plates may be charged in a wide variety of ways including vigorously rubbing the plate surface with a softened material such as a cotton or silk handkerchief or a soft brush or fur chosen to impart charge of the desired polarity, induction charging, for example, as described in Walkup Pat. 2,934,649, roll charging as described in Straugham, Mayer, Proc. Nat. Electronics Conf.
  • corona discharge devices which generally can apply either positive or negative charge of varying potentials and which may be adapted for many'appliactions, is found to be .a preferred charging mechanism for use herein.
  • corona discharge devices of the general description and generally operated as disclosed in Vyverberg Pat. 2,836,725 and Walkup Pat. 2,777,957 have been found to be excellent sources of corona useful in the charging of xerographic plates.
  • a xerographic plate should be charged when it is at its highest insulating value or when there is an absence of electromagnetic radiation that would make the photoconductive insulating layer photoelectrically conductive. To allow the charge to remain on the surface of the layer once deposited there, charging must, of course, take place in the absence of that wavelength radiation or light to which the particular photoconductive material is sensitive.
  • the image is rendered visible by devleopment techniques. Although a wide range of such techniques may be used, the dynamic range of the component photoconductors and of the plates of this invention may be found to vary somewhat, depending on the particular development process used.
  • the latent electrostatic image is rendered visible or developed by contacting the latent image areas with a finely divided marking material that is brought into surface contact with the free surface of the photoconductor or photoconductors and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image.
  • a finely divided marking material that is brought into surface contact with the free surface of the photoconductor or photoconductors and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image.
  • the system of cascade development has found extensive commercial acceptance and is suitable herein and generally consists of gravitationally flowing developer material consisting of a two component material of the type disclosed in Walkup et a1. Pat. 2,638,416 over the xerographic plate bearing the latent image.
  • the two components consist of an electroscopic powder termed toner and a granular material called carrier and which by mixing, acquire triboelectric charges of opposite polarity.
  • the toner component In development, the toner component, usually oppositely charged to the latent image, is deposited on the latent electrostatic image to render that image visible.
  • Other typical developing systems include magnetic brush development, for example see Giamo Pat. 2,930,351; skid development, for example see Mayo Pat. 2,895,847; fluid development systems, for example see Carlson Pats. 2,221,776, 2,551,582, 2,690,394, 2,761,- 416, 2,928,575; Thompson Pat. 3,064,622; Gundlach Pats. 3,068,115 and 3,084,043 and Metcalfe Pats. 2,907,674, 3,001,888, 3,032,432 and 3,078,231 and other development processes known to those skilled in the art.
  • Example I A Lektromesh grid avialable from C. O. Jelliff Mfg.
  • a mixture of about 82 /z% amorphous selenium, about 17 /2% arsenic and about 0.1% iodine is then vacuum evaporated onto and through the screen to form about a 0.2 micron thick pattern of photoconductor deposit on the Nesa substrate corresponding to the interstices of the grid.
  • the xerographic plate thus formed is then xerographically processed by charging, exposing and developing.
  • the plate is uniformly charged positively by a corona discharge device available commercially as a part of the Model D Processor available from Xerox Corporation, to a substantially uniform surface potential of about +800 volts and removed from the Processor in the absence of actinic radiation.
  • the sensitized plate is then put in a camera and exposed from the substrate side to a photographic original which contains highlights where D approaches zero, dense image areas where D is about 1.2 or more and varying inbetween tones.
  • the radiation source is an incandescent photoflood lamp, and exposure at the back surface of the plate is about 1.5 f.c.s.
  • the latent electrostatic image is then developed by cascade development and transferred to a sheet of paper to yield a high quality halftone reproduction with excellent tonal response, i.e. a dynamic range of from about 0 to about 1.4.
  • This extended dynamic range results from the amorphous selenium portions providing for tonal response in the D region from about 0 to 0.6 to reproduce highlights in the original, and the arsenic-selenium portions being about four times more photosensitive than the amorphous selenium portions alone providing for tonal response in the D region of from about 0.6 to 1.4 to reproduce shadow tones and the denser portions of the original.
  • Areas of the reproduction where D about 0, result because the exposure being sufficient to substantially completely discharge selenium areas of the plate, is also sufficient to substantially completely discharge selenium overcoated screen portions of the more sensitive photoconductor arsenic-selenium.
  • the maximum thickness of the screen portions is less than about 0.2 microns, since it is found that the electron and hole range of this material is relatively short, so that charge injection into the amorphous selenium from the arsenicselenium screen pattern diminishes at greater thicknesses requiring increasingly greater exposures to maintain the screens hole or electron injection capability. From this preferred maximum thickness, the hole or electron injecting capability of the screen may be thought of as roughly declining linearly with the thickness.
  • a layer thickness not less than about 20 microns is preferred. For thicknesses less than about 20 micron, the seleniums capacity to accept and hold a high surface charge potential preferred for quality xerographic prints, begins to drop.
  • holes migrate from the arsenic-selenium screen to discharge charge and lower surface charge potential at the surface of the amorphous selenium layer.
  • Example II Example I is followed except that (a) the electrically conductive substrate is about 50 mil thick aluminum,
  • the overcoating layer of selenium is deposited to a thickness of about 75 microns;
  • the resultant plate is exposed after charging from the top, i.e. from the amorphous selenium side, and exposure at the free surface of amorphous selenium is about 1.5 foot candle-seconds.
  • amorphous selenium layers are substantially transparent to radiation in the red and near infrared region of the spectrum, thus transmitting this portion of the incident radiation to the red and near infrared sensitive screen of arsenic-selenium.
  • An amorphous selenium layer thickness of not greater than about 75 microns is preferred in the embodiment of this example, since it is found that for greater thicknesses the layer begins to substantially block out some of the incident red and near infrared radiation.
  • the amorphous selenium layer is substantially more opaque to the blue, green and ultraviolet region of the spectrum, which renders the selenium layer more electrically conductive, thus discharging the selenium in proportion to the intensity of this radiation incident thereto.
  • Example III Polyvinyl carbazole available under the trademark Luvican M170 from Winter, Wolff & Co. is mixed with dry weight, of the Lewis acid 2,4,7-trinitrofluorenone in enough solvent to give good coating viscosity.
  • the sensitized polyvinyl carbazole is coated with a Bird applicator onto a sheet of about 50 mil thick aluminum and dried to a thickness of about microns.
  • a solution of a 1 to 1 mixture is prepared of 2,5-bis (p-aminophenyl)-l,3,4-oxadiazole available under the trademark TO 1920 from Kalle & Co., Weisbaden- Biebrich, Germany and a resinous binder material Vinylite VYNS, a copolymer of vinyl chloride and vinyl acetate available from Union Carbide Corp. in a 1 to 2 mixture of cyclohexanone and 3-pentanone. Enough X-form metalfree phthalocyanine made as described in copending application Ser. No. 505,723, filed Oct. 29, 1965, now
  • a grid of the type used in Example I is placed on the free surface of the polyvinyl carbazole layer.
  • the mixture is sprayed onto the screen to coat the polyvinyl carbazole layer in screen interstitial areas to a dried thickness of about 2 microns.
  • the grid is removed.
  • the xerographic plate thus formed is then xerographically processed by charging, exposing and developing.
  • the plate is uniformly charged positively by a corona discharge device available commercially as a part of the Model D Processor available from Xerox Corporation, to a substantially uniform surface potential of about +600 volts and removed from the Processor in the absence of actinic radiation.
  • the sensitized plate is then put in a camera and exposed from the photoconductor side to a photographic original which contains highlights where D, approaches zero, dense image areas where D is about 1.2 or more and varying inbetween tones.
  • the radiation source is an incandescent photofiood lamp and exposure is about 1.5 f.c.s.
  • the latent electrostatic image is then developed by cascade development in the presence of a development electrode biased positively to about volts.
  • the toner image is transferred to a sheet of paper to yield a high quality halftone reproduction with excellent tonal response, i.e. a dynamic range of from about 0 to about 1.4.
  • Example IV A Lektromesh grid of the type used in Example I is placed on about 50 mil thick brass.
  • a phthalocyanine pigment organic binder photoconductor about 4 times more photosensitive than selenium, for the particular light source used in this example, is prepared as in Example III.
  • This mixture is then deposited on the brass through the interstices of the grid by spraying, the deposits drying to a thickness of about 2 /2 microns.
  • the grid is removed and about a 20 micron layer of amorphous selenium is then vacuum evaporated over the screen pattern of the phthalocyanine binder photoconductor.
  • the xerographic plate thus formed is then xerographically processed as in Example II to yield a high quality halftone reproduction with a dynamic range of from about 0 to about 1.4.
  • the exposure source was the same and that if the radiation source is changed to provide a radiation source with a different spectral curve and specifically more or less output in the infrared range, then the makeup of the two photoconductors used in plate fabrication in the examples can be changed to make the more sensitive photoconductor if that photoconductor is amorphous selenium, or to otherwise assure, for example by using a soft dot pattern, that there is no increment of D between the dynamic range curves of the two photoconductors, which is not tonally reproduced.
  • the plate may remain the same, but spectral input may be varied to obtain the preferred dynamic range, narrow, intermediate or broad, for a particular copying situation.
  • the system hereof may be advantageously used for superimposing two separate images on one copy sheet. For example, one might wish to copy a business form and variable information from another source, both onto a single document. If both original images exist as negatives (white characters on black background) sequential or even simultaneous exposure of the two images onto a conventional prior art xerographic photoreceptor yields a single high contrast image. However, double exposure of two positive images seriously reduces image contrast.
  • An imaging process comprising providing a xerographic plate having an electrically conductive backing overcoated with a multiplicity of alternating discrete areas of at least first and second photoconductive insulating material having different photosensitivities to a given exposure such that the photosensitively slower material has a dynamic range that ends at about where the dynamic range of the photosensitively faster material begins,
  • said first image pattern of light being formed by steps including directing light of wavelengths from a first spectral region onto a first positive original and said second image pattern of light being formed by steps including directing light of wavelengths from a second spectral region substantially different from said first spectral region onto a second positive original,

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4175957A (en) * 1977-01-14 1979-11-27 Olympus Optical Company Limited Electrophotographic process using insulating dot overlayer

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4175957A (en) * 1977-01-14 1979-11-27 Olympus Optical Company Limited Electrophotographic process using insulating dot overlayer

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