US3558307A - Electrostatic image reproduction with pre-exposure - Google Patents

Abstract

A METHOD FOR FORMING A LATENT ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVE PLATE WHEREIN THE PLATE IS UNIFORMLY PREEXPOSED TO ELECTROMAGNETIC RADIATION HAVING A LOW INTENSITY SUCH THAT THE CHARGE ACCEPTANCE OF THE PLATE TO AN APPLIED ELECTROSTATIC CHARGE IS AT LEAST ABOUT 80% OF ITS CHARGE ACCEPTANCE IF THE PLATE WERE NOT SO PRE-EXPOSED. PREFERABLY SUCH PRE-EXPOSURE SHOULD BE USED IN CONJUCTION WITH AN IMAGE EXPOSURE IN WHICH SUBSTANTIALLY ALL THE LIGHT STRUCK AREAS OF THE PLATE RETAIN A MEASURABLE AMOUNT OF CHARGE.

Description

Jan. 26, '1971 I 1.. A. CARLSON 3,553,307 ELECTROSTATIC IMAGE REPRODUCTION WITH FEE-EXPOSURE Filed 001:,7. 1966 3 Sheets-Sheet 1 6, L. A. CARLSON T 3,553,307 I ELECTROSTATIC IMAGE REPRODUCTION WITH PIKE-EXPOSURE Filed Oct-7, 1966 v s Sheets-Sheet z 2.5 3 [XPOSl/Rf 77 [56(0/VD5) Jan. 26,1971 L. A. CARLSON' 3,558,307 A ELECTROSTATIC IMAGE REPRODUCTION WITH PIKE-EXPOSURE" Filed 0on7} 19 6' I 1 s Sheets-Sheet 5 Q Q Q o' Q Q Q a s (5110A) yrs'v United States Patent Olfice 3,558,307 Patented Jan. 26, 1971 3,558,307 ELECTROSTATIC IMAGE REPRODUCTION WITH PRE-EXPOSURE Lee A. Carlson, Ashland, Mass., assignor to Dennison Manufacturing Company, Framingham, Mass., a corporation of Nevada Filed Oct. 7, 1966, Ser. No. 585,108 Int. Cl. G03g 13/22 US. Cl. 961 Claims ABSTRACT OF THE DISCLOSURE A method for forming a latent electrostatic image on a photoconductive plate wherein the plate is uniformly preexposed to electromagnetic radiation having a low intensity such that the charge acceptance of the plate to an applied electrostatic charge is at least about 80% of its charge acceptance if the plate were not so pro-exposed. Preferably such pre-exposure should be used in conjunction with an image exposure in which substantially all the light struck areas of the plate retain a measurable amount of charge.

This invention relates to xerography or electrophotography and more particularly to improvements in image reproduction, especially images of low contrast.

Electrophotography as normally practiced includes the steps of providing a uniform electrostatic charge to the surface of an electrophotographic plate, exposing the thus charged plate to a pattern of light and shadow to discharge light struck areas and produce a latent electrostatic image, and development of the latent image to a visible image. The plate usually comprises a photoconductive insulating layer, such as selenium or a zinc oxide binder layer on a relatively conductive backing such as metal or paper. Common forms of electrophotographic apparatus provide a relatively movable path between the plate and stations for performing the operating steps, a plate moveable past fixed stations being most common although apparatus providing a fixed plate and sequentially moveable stations is known.

In the above image reproduction, difiiculties have arisen in reproduction of continuous tone, faint originals such as pencil documents, large image areas, and in reproducing originals having a variety of indicia, sometimes in various compositions, colors and intensities. One exposure for such originals often fails adequately to reproduce all such indicia.

It is accordingly the principal object of the present invention to provide improved xerographic image reproduction, especially in solid image areas, low contrast originals, and from originals having varying indicia in color, composition and intensity, especially spirit, diazo and NCR copy, all with less critical exposure limits.

Pre-exposure of xerographic plates comprising photoconductors such as Zinc oxide to light to which the photoconductor is sensitive has been studied. Generally, preexposure is considered undesirable because it decreases charge acceptance and contrast and decreases the .rate of light decay, although US. Pat. No. 3,249,430 appears to suggest pre-exposure to high intensity radiation as a means of controlling contrast.

In accordance with the present invention, it has been found that notwithstanding its disadvantages, pro-exposure of xerographic plates to uniform illumination of low intensity improves image rendition of originals, preferably when combined with under-exposure of the plate, after electrostatic charging, to an image pattern of light and shadow. Thus, in one aspect, the invention comprises the xerographic method providing a photosensitive xerographic plate having a photosensitive surface comprising a photoconductor, uniformly pre-exposing the photosensitive surface to electro-magnetic radiation which can be absorbed by said photoconductor and to which said surface is sensitive, providing a substantially uniform electrostatic charge on said surface, and, preferably, underexposing said charged surface to a pattern of light and shadow to produce a latent electrostatic image in which substantially all light struck areas retain measurable apparent surface voltage charge. Preferably the photoconductor is dye-sensitized zinc oxide, the exposures are to visible, infra-red and/or ultraviolet light, the under-exposure comprises no more than the foot candle seconds necessary fully to discharge non-image background areas, and is more preferably about thereof, and the pre-exposure to about 5 times, in foot candle seconds, the under-exposure.

In a further aspect, the present invention comprises a apparatus for pre-exposing a xerographic plate uniformly to radiation comprising a lamp in a housing attached to a corona charging device relatively moveable with respect to said plate into position to provide an electrostatic charge thereon, said housing having a slit directing radiation from said lamp onto said plate prior to relative movement into said position. Preferably the lamp is an incandescent tungsten lamp and is disposed to provide about 60 foot candle seconds of pre-exposure radiation on the xerographic plate.

The present invention can be better understood by reference to the accompanying drawings in which:

FIG. 1 is a plan view with parts broken away, of a corona charging device with attached pre-exposure lamp and housing;

FIG. 2 is a section on the line 2-2 of FIG. 1;

FIG. 3 is a plot of apparent surface voltage (A.S.V.) in volts on a xerographic plate versus time of exposure to light passing through filters of varying density, but without use of pre-exposure; and

FIG. 4 is a plot of A.S.V. versus time of exposure through the same filters but after uniform pre-exposure of the xerographic plates.

Referring to the drawings, FIGS. 1 and 2 illustrate a corona charging device for a flexible xerographic plate to which a pre-exposure lamp and housing are attached. The corona device itself is more fully described in copending application of Mie'kka and Smith, entitled Corona Charging Device, and filed on about even date herewith. Generally it comprises substantially identical front and back halves 11 and 12 releasably joined by means of electrically conductive tab and lock means 13. Each half comprises corona wires, half 12 having corona wires 15 and 17 and half 11 having wires 16a and 17a, the wires being spring mounted to electrical end leads (not shown) for attachment to a high voltage source. The wires are shielded by conductive shields 25 and 25a to which conductive baffles 24 and 24a are conductively attached as by welding or the like. Across the open face of shields 25 and 25a lacings of spaced dielectric guide strands 30 and 31 are employed as more fully disclosed and claimed in the above mentioned copending applicatlon.

Halves -11 and 12 define a slot 14 through the corona device for a xerographic plate such as zinc oxide-binder coated paper which moves through the device along a path 15 in the direction indicated by the arrow in FIG. 2.

Assembled to half 11 is a housing for a pre-exposure lamp 51 terminating in socket means 52 for connection to a source of electrical power. Housing 50 is attached to half 11 by means of tabs 53-53 which receive locking detents 54-54 on shield 25a. Halves 11 and 12 and housing 50 are also assembled by means of end plates 55-55.

In operation, the xerographic plate such as paper coated with particles of dye-sensitized photoconductive zinc oxide dispersed in an insulating binder is passed along path 15 with the coating side facing half 11 and pre-exposure lamp 51. Lamp 51 is turned on for a time sufiicient to preexpose the plate coating to the desired intensity just prior to entry into slot 14 as shown by ray arrows in FIG. 2, exposure being through slot 56 in housing 50, slot 56 being, for example, of a width to exposure a transverse section of the plate surface about one inch in width.

As indicated above, it is known that high intensity preexposure of dark-adapted photoconductors such as zinc oxide to radiation which the photoconductor can absorb and to which the plate is sensitive fatigues the photoconductor so as to decrease its charge acceptance and its rate of light decay, the decreases generally becoming greater with increasing intensities and duration of pre-exposure. Pre-illumination useful herein is directed onto the photosensitive surface and is of low intensity, preferably an intensity by which the initial charge acceptance of the plate, at constant corona, is at least 80% and preferably at least 90% of the initial charge acceptance without preexposure. Most preferably, the pre-exposure is about 5 times the subsequent image exposure to light and shadow in the xerographic process, for example 60 foot candle seconds of pre-exposure for about 12 foot candle seconds of image exposure. Thus an illustrative process comprises exposing a zinc-oxide-binder photoconductive layer to 60 foot candles of pre-illumination from lamps 51, thereafter charging the layer by means of corona wires 16-1611 and 17-17a to a negative potential of about 500 volts, and subsequently illuminating the thus charged layer to a pattern of light and shadow of maximum intensity of about 12 foot candle seconds, all as more fully explained hereinafter. The thus formed latent electrostatic image can be developed or otherwise used by conventional xerographic techniques such as treatment with liquid toner or the like. As is also well known, the image exposure can be formed in any suitable way, for example by passing the illumination through a transparency, reflecting it from an opaque original or the like.

The image exposure according to this invention is preferably an under-exposure. Normally, it is desired that the charged xerographic plate receive sufiicient illumination in background, non-image areas to substantially reduce the electrostatic charge at such areas to zero, thus to provide least background development and greatest image contrast. However, to obtain the advantage of better image resolution, especially from weak or low contrast originals, it has been found that, with pre-illumination, exposures to lesser illuminations are normally necessary.

FIG. 3 of the attached drawings illustrates a series of curves showing apparent surface voltages (A.S.V.) in volts on a zinc oxide-binder coated paper zerographic plate versus time of exposure to about 12 foot candle seconds of illumination, each curve representing the light decay when exposed from a white original through no filter curve) or through neutral filters of various density indicated by the numbers 0, .1, .2, 1.0 beside each curve, 0 representing 100% reflectance, 0.1 about 80%, 0.2 about 63%, 0.3 about 50%, etc. From the curves it can be seen that, with an initial A.S.V. of greater than 500 volts, the 0 curve declined to zero in about -6 seconds, the higher density curves retaining greater voltages for longer times. However, it will be noted that the low density cunves 0.1, 0.2, etc., corresponding to weak originals, decay almost as fast so that little developable charge remains when background in the 0 curve is carried to zero. Since the curves are so closely spaced, underexposure would not materially increase contrast even if background development were tolerated. Thus low contrast gray originals are poorly reproduced.

In the measurements shown in FIGS. 3 and 4, A.S.V. was measured by known apparatus and techniques dis- 4 closed in an article by E. C. Giaimo in R.C.A. Review, vol. 22, No. 4, pages 780-790.

FIG. 4 illustrates data similar to FIG. 3 but wherein the plates of each curve were each pre-exposed to 60 foot candle seconds from a showcase, 8T, 40 watt, 120 volt tungsten filament bulb giving illumination in both the visible and infrared wavelengths disposed about 2% inches from the photosensitive plate moving along path 15 (FIG. 2) at a rate of about 4 /2 feet per second. As the curves show, light decay is decreased so that about 9 seconds are now required to discharge the 0 curve to approximately zero. However, low density filter curves 0.1, 0.2, etc. are spaced apart further than in FIG. 3. Thus the charge difference, for example, between curves 0 and 0.1 at 7 seconds is much greater than at 9 seconds so that under-exposure produces a greater contrast although also producing some greater background. It has also been discovered, however, that pre-exposure produces more uniform images in large areas such as background (in a black and white original, for example) so that the slight greying of the background obtained in prints with under-exposure is actually often less visually noticeable than with less grey and less uniform prints without either pre-exposure or underexposure. Further, large image areas appear to be more uniformly developed, improving legibility even though contrast may actually be decreased. Moreover, in some document reproduction, some increase in background may be tolerable to obtain better legibility of weak images. The more uniform image resolutions in larger areas may be due to a more equalized surface potential obtained in the corona, after pre-exposure.

As indicated above and by reference to FIG. 4, about or more preferably of the exposure necessary to reduce the zero curve to zero potential is preferred as under-exposure for optimum resolution of light gray tones with minimum tolerable background print development.

It has been thus discovered that the combination of pre-exposure, at relatively low intensities which do not decrease charge acceptance in volts more than about 20 percent and more preferably not more than about 10%, at a constant corona exposure, and preferably with underexposure, produces advantages which often outweigh the known disadvantages of pre-exposure alone. This is especially true in increasing the range of reproduction of varying original indicia at a given exposure.

The effect on charging rate and acceptance of increasing pre-exposure time to a foot candle tungsten source on a zinc-oxide-binder paper xerographic plate is shown in Table'I on next page.

TABLE 1 Charging Charge Light rate, volts/ acceptance, decay Pre-exposure time, sec. sec. volts time see.

None (control) 610 550 7 575 520 7 575 520 8 555 500 9. 5 500 500 10 480 480 10 Preexposure was found to follow the reciprocity law, at least within the range of 10 to 100 foot candles, the effect of 6.0 seconds at 10 foot candles being substantially the same as 0.6 second at 100 foot candles.

It should be understood that the foregoing description is for the purpose of illustration and that the invention includes all equivalents falling within the scope of the appended claims.

What is claimed is:

1. The xerographic method of forming a latent electrostatic image on a photoconductive xerographic plate with improved rendition of low contrast images comprising (a) providing a photosensitive xerographic plate having a photosensitive surface comprising a photoconductor,

(b) uniformly pre-exposing the photosensitive surface to electromagnetic radiation which can be absorbed by said photoconductor the intensity of said radiation being such that the charge acceptance of said plate to a uniformly applied electrostatic charge is less than, but at least about 80% of, the charge acceptance of said plate without said uniform pre-exposure,

(c) providing a substantially uniform electrostatic charge on said surface substantially immediately following said pre-exposure, and

(d) exposing said charged surface of a pattern of light and shadow to produce a latent electrostatic image in which substantially all light struck areas retain measurable apparent surface voltage charge.

2. The method according to claim 1 wherein said preexposure is to an intensity such that said charge acceptance is less than, but at least about 90% of, the charge acceptance of said plate without said uniform pre-exposure.

3. The method according to claim 1 wherein said photo-- conductor is particulate zinc oxide dispersed in an insulating binder.

4. The method according to claim 3 wherein said preexposure and said image-exposure are to visible or ultraviolet light.

5. The method according to claim 3 wherein the time duration of said image-exposure is no more than about of the time required, at constant intensity, to reduce the apparent surface voltage of fully light struck areas substantially to zero.

6. The method according to claim 3 wherein the time duration of said image-exposure is no more than about of the time required, at constant intensity, to reduce the apparent surface voltage of fully light struck areas substantially to zero.

7. The method according to claim 5 wherein said preexposure is about five times said image-exposure when both are measured in foot candle seconds.

8. The method according to claim 7 wherein said preexposure is about foot candle seconds and said imageexposure is to about 12 foot candle seconds.

9. The xerographic method of forming a latent electrostatic image on a photoconductive xerographic plate with improved rendition of low contrast images comprising (a) providing a photosensitive xerographic plate having a photosensitive surface comprising a photoconductor,

(b) uniformly pre-exposing the photosensitive surface to electromagnetic radiation which can be absorbed by said photoconductor the intensity of said radiation being such that the charge acceptance of said plate to a uniformly applied electrostatic charge is less than, but at least about of, the charge acceptance of said plate without said uniform preexposure,

(c) providing a substantially uniform electrostatic charge on said surface substantially immediately following said pre-exposure, and

(d) exposing said charged surface to a pattern of light and shadow to produce a latent electrostatic image.

10. The method according to claim 9 wherein said photoconductor is dye-sensitized zinc oxide.

References Cited UNITED STATES PATENTS 2,638,416 5/1953 Walkup et a1 96-117.5X 3,052,540 9/1962 Greig 96-1.7 3,103,445 9/1963 Bogdonofi et a1 117-17.5 2,863,767 12/1958 Vyverberg 96-1 2,979,403 4/ 1961 Giaimo 961 3,041,167 6/1962 Blakney et a1. 96-1.4 3,249,430 5/1966 Metcalfe et a1. 96-1.3 3,268,331 8/1966 Harper 961 3,355,289 11/1967 Hall et a1. 961.4

CHARLES E. VAN HORN, Primary Examiner U.S. Cl. X.R.

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