Patent Publication Number: US-3880514-A

Title: Ion producing source for electrostatic recording apparatus

Description:
United States Patent 11 1 Kuehnle 1 1 ION PRODUCING SOURCE FOR ELECTROSTATIC RECORDING APPARATUS [75] Inventor: Manfred R. Kuehnle. Lexington.  
 Mass.  
 [73] Assignee: Coulter Information Systems. Inc..  
 Bedford. Mass.  
 [22] Filed: Sept. 14. 1973 [21] Appl. No.: 397.307  
 [52] U.S. Cl 355/3 R: 96/1 C: 250/324:  
 [51] Int. Cl. G033 15/00 [58] Field of Search 355/3 R: 317/262 A; 250/324. 326: 96/1 C [56] References Cited UNITED STATES PATENTS 2.543.051 2/1951 Oughton et a1. 317/262 A 2.803.177 8/1957 Lowrie 355/3 2.993.787 7/1961 Sugarman.... 355/3 X 3.1 17.5(11 1/1964 Oliphant 355/3 3.411.846 11/1968 Naroff... 355/3 3.557.367 1/1971 Roth 317/262 A 3.566.110 2/1971 Gillespie 317/262 A 3.660.656 5/1972 Frank et a1. 317/262 A 3.675.096 7/1972 Kicss 317/262 A 1451 Apr. 29, 1975 Primary E.\&#39;amincrRichard L. Moses Allurm&#39;y. Agent. or Firm-Silverman &amp; C ass. Ltd.  
 [57 I ABSTRACT An electrophotographic film member is disposed in darkness for charging the photoconductive coating thereof so that it may subsequently be exposed to a light pattern for forming an electrostatic latent image of said light pattern on the coating. Means are provided for charging the coating in such a manner that there is a minimum of radiation produced by the charging means. specifically so that ultra violet radiation is minimized. This is accomplished by the use of some form of ultra violet radiation shielding means in association with a corona wire or array of corona wires. Alternatively ionization means such as a highspeed high-voltage spark gap may be utilized.  
 The method of the invention comprises charging the photoconducti e coating of the electrophotographic film member in darkness while excluding ultra violet radiation therefrom. A modified method comprises producing ions in a step which. while initially producing ultra violet radiation. discontinues the emission of such radiation a substantial time before the ions recombine to dissipate. and charging the photoconductive coating thereby.  
 7 Claims. 11 Drawing Figures ION PRODUCING SOURCE FOR ELECTROSTATIC RECORDING APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONS Reference will be made herein to the electrophotographic film which is disclosed and claimed in a copending application entitled: ELECTROPHOTO- GRAPHIC FILM. METHOD OF MAKING AND USING THE SAME AND PHOTOCONDUCTIVE COATING USED THEREWITH. Ser. No. 378.l8(l. filed July ll. I973.  
  Reference will be made herein to a camera construction which is disclosed andclaimed in a copending application entitled. SHUTTERLESS CAMERA SYS- TEM. Ser. No. 397.309 filed Sept. 14. 1973.  
  The basic method of and apparatus for utilizing peak potential surface charge for adjusting the sensitivity of the electrophotographic film of the said copending application Ser. No. 378.180 in a camera construction are disclosed in two copending applications entitled IMAGE RECORDING METHOD FOR ELECTRO- PHOTOGRAPHIC FILM, Ser. No. 389.l49 filed Aug. 17. I973 and &#34;IMAGE RECORDING APPARA- TUS FOR ELECTROPHOTOGRAPHIC FILM&#34;. Ser. No. 389.124 filed Aug. 17. 1973. I  
  All of the above applications as well as the present one are owned by the same assignee.  
 BACKGROUND OF THE INVENTION The invention herein is concerned with the art of electrostatic recording and more particularly is concerned with a novel apparatus and method for effecting such recording.  
  Electrostatic apparatus and techniques which are known at the present time are characterized by the relatively slow speed with which the method is carried out and the apparatus is used. The basic member. which is called an electrophotographic member herein for certain reasons which will appear. is a layer of photoconductive material based upon a support that includes at least a conductive member. In the xerographic process as known, the photoconductive layer is amorphous selenium and the conductive layer is actually a selfsupporting metal drum. In the most familiar of the electrofax processes. the photoconductive layer is a coating of zinc oxide and resin and the support&#39;is a sheet of conductive paper. often comprising a simple paper substrate with a thin metal foil disposed between the photoconductive coating and the substrate.  
  The electrophotographic film of the copending application Ser. No. 378.180 is an improvement over the two electrostatic members mentioned as well as others known thus far for many reasons. Among these reasons are greatly increased charge acceptance and high gain. rendering the electrostatic film of said copending application Ser. No. 378.180 extremely fast. In fact. the film is so fast that it is comparable with conventional high speed photographic films. but it obviates many of the disadvantages of conventional film. For details of the said film, reference may be had to the said copending application Ser. No. 378.180, but in summary the following attributes can be mentioned briefly:  
  It is inorganic, transparent, grainless, and when made on a thin plastic substrate such as Mylar is extremely flexible. It is durable, with a hard abrasion-resistant surface. It has a dark resistivity of the order of 10 ohms per centimeter and in undoped condition will normally have a ratio of dark to light resistivity of the order of IO. It exhibits no photoconductive persistence or fatigue no matter how repeatedly used. It can be charged at very high rates above saturation. will discharge in light to practically zero residual charge and thus. in view of its high dark resistivity. will provide extreme whites and blacks when toned. with an almost infinite gradation of greys between.  
  The basic construction of the film of said copending application Ser. No. 378.180 comprises a layer or coating of photoconductive material of about 3000 Angstroms thick deposited on a plastic film which has a thickness of a fraction of a millimeter. there being an intervening ohmic layer about 300 to 500 Angstroms thick. The preferred photoconductive material is cadmium sulfide. sputtered by means of r.f. to form a crystalline coating as explained in said copending application; and the preferred ohmic material is indium oxide which is sufficiently transparent to pass the desired amount of light. Absorbance of light by this film is from about 707: to 85%.  
 The cadmium sulfide coating which has the characteristics above mentioned is in a pure state. Doping&#39; with suitable dopants such as hydrogen iodide. copper and the like can increase the gain with accompanying variations in other characteristics.  
  Other materials are described in the said copending application.  
  To provide an appreciation of the charge acceptance of the said electrophotographic film. and its high gain. it is feasible to use the film in a shutterless camera in which the photoconductive surface of the film is charged while the film is simultaneously exposed to a light pattern. When the surface&#39;potential reaches a predetermined value at a darkened incremental area of the film. the value having been built into circuitry that relates it to the average light flux of the pattern being viewed by the film. the charging is discontinued and the film is toned. These latter two functions can be effected simultaneously. This last-mentioned circuitry in effect adjusts the sensitivity of-the film in accordance with the light.  
  The film is capable of assuming a net charge. with a full range of grey tones. even though the charging and exposing occur at the same time. Prior electrostatic members. so far as known. are too slow and cannot accept charge fast enough to accomplish this.  
  The construction and operation of the shutterless camera are disclosed in said copending application Ser. No. 397.309.  
  The method and apparatus which provides for adjustable sensitivity are disclosed in said copending applications Ser. No. 389.149 and Ser. No. 389.124.  
  The known methods and apparatus for charging the electrostatic members of the prior art as well as the electrophotographic film member of said copending application involve the use of corona generators. A fine wire. a few thousandths of an inch in diameter. is stretched over the photoconductive surface of the electrostatic member. being usually quite close. and a high voltage power supply is connected between the wire and the conductive layer of the electrostatic member. Sometimes there isa series of wires side by side and sometimes there is a grid between the wire or wires and the photoconductive surface. The high voltage at the wire produces a corona which ionizes the air in the vicinity of the wire and the ions move towards the coating to charge it. The charge may be positive or negative depending upon the type of material. Selenium is usually charged positively and the cadmium sulfide of the said electrophotographic film is charged negatively. Many other kinds of electrostatic members are also charged negatively.  
  The charge, say for example negative comprises electrons which are located at or just below the surface of the coating. But for the shutterless type of camera mentioned above. the charging takes place in complete darkness. In the case of prior electrostatic members. their charge acceptance is so slow and the voltage to which they must be charged is so high (while still producing a &#39;much lower field strength than the electrophotographic film of the copending application charged to less than one tenth the normal surface voltage of selenium, for example) that a shutterless camera is out of the question. The charging must occur in complete darkness in order to achieve a reasonably charged photoconductive coating.  
  Once charged the film is exposed to a light pattern and the impinging photons will cause the photoconductive surface to discharge selectively and proportionally to&#39; the amount of light. if any, which reaches the respective increments of area. The discharge. in the case of a negatively charged coating. represents the electrons moving toward the ohmic member and combining with holes during such movement. When the exposure is completed. the light pattern is cut offQthe photoconductive surface once more placed in darkness and the discharging is stopped at a certain point. The&#39;film now carries a latent image in the form ofa pattern of charge. Thereafter, the increments discharge in &#34;accordance with the dark decay characteristics of the electrostatic member or electrophotographic film member. Electrical anisotropy of the photoconductive coating maintains the differences in charge exhibited by the respective increments thereof. Toning is accomplished as soon after the completion of exposure as possible and carried on for a time which is normally chosen to give a good visible image. Such time may be adjusted in accordance with the value of surface potential reached.  
  After toning. the toned image may be transferred to another member or fixed on the electrostatic or electrophotographic film member itself.  
  As mentioned above, prior commercial electrostatic members have photoconductive coatings which are very slow; they accept charge at a very slow rate and they discharge slowly in light and hence require bright light patterns to produce any reasonable gradation. They do not have large dark resistivities and hence they do not retain charge even in darkness for a considerable time without loss. Improvements in these characteristics have been proposed in the literature but so far as known, very little if any of the improved coatings have been commercialized because of many practical problems.  
  I have discovered that the corona which is produced by the ordinary corona producing&#39;mean s, comprising a fine wire or wires maintained at a high potential (of&#39;the order of kilovolts) besides ionizingthe air in its vicinity, produces substantial, emission inthe ultra violet region. This radiationjhaving high energ&#39;y,wilI effect any film that readily responds to ultraviolet &#39;r adiation&#39; &#39;lhave found that ordinary silver halide film canbe&#39;readily &#34;ex- 4 posed by such radiation. and the said electrophotographic film of the copending invention also responds readily to such ultra violet radiation, being panchromatic. r  
  In the caseof the prior electrostatic members which are in popular use. the extreme slowness of charge acceptance and the small gain and speed of the photoconductive layers or coatings thereof signified that the characteristics of the members were gross. In short, the members could not be used with the speed and flexibility of conventional photographic film, could not have the photographic quality of such photographic film and hence such gross characteristics masked or were not critical enough to be affected by ultra violet radiation during charging in darkness. If there was any appreciable effect. so far as I am aware, this has not heretofore been detected. Either there has been a sufficient net charge remaining after the charging period to give an acceptable image of the known quality of xerography or electrofax; or the charge acceptance and light decay characteristic are so slow that the ultra violet emissions dont&#39; substantially discharge the charge being applied to make any appreciable difference in later exposure;  
 or both.  
  In the case of the electrophotographic film of the said copending application Ser. No. 378,180 the characteristics which are detailed are a substantial improvement over the prior art. Such improvement has been .for a very short time to give the excellent results described.  
 It will be appreciated from the above that if the ultra violet radiation can be lessened to a substantial degree or even eliminated entirely, the electrophotographic film will&#39;exhibit characteristics vastly improved over those which it was believed to have had. A lower corona voltage can be used to charge the film; the charge will be accepted faster and reach a higher surface potential for any given corona producing voltage; the photoconductive surface will exhibit greater sensitivity; it will tone easier and faster. The entire process of charge, expose and tone can be materially speeded up. Some improvement would be achieved in almost any electrostatic member or plate that has some sensitivity to ultra violet&#39;radiation. According to the invention, a method and means are provided for decreasing if not wholly preventing ultra violet emission from affecting the photoconductive coating of the electrophotographic film in the course of charging the same. In this manner I achieve vast improvements in the characteristics of said electrophotographic film and can increase its utility.  
  The environment in which the invention is used preferably is apparatus in which the charging of the photoconductive coating takes place in complete darkness although use in a shutterless camera where charging and exposure occur simultaneously is not excluded from the scope of said invention.  
 SUMMARY OF THE INVENTION Apparatus is provided in which there are means for disposing an&#39;electrophotographic film member in darking. Charging means are provided of such construction as to eliminate or substantially decrease the ultra violet radiation which reaches the photoconductive coating such that there is a substantial net charge remaining on the coating when the charging means are disabled. The charging means may take the form of shielded corona producing means which have provision for blocking the paths of ultra violet emission while permitting the ions produced to reach the said photoconductive surface. Such means may also take the form of a high speed. high potential spark gap which produces ionization persisting for a substantial length of time after the arc has decayed so that the thus-persisting ions can settle onto the photoconductive coating and charge the same.  
  The method of the invention comprises charging the photoconductive coating in such a manner as to substantially decrease or eliminate ultra violet radiation from affecting the charge remaining on such surface.  
  The method and apparatus may also be used in cases where the charging is done while exposing the electrophotographic member.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the steps of the method of the invention;  
  FIG. 2 is a similar block diagram of one variation of the invention;  
  FIG. 3 is a similar block diagram of another variation of the invention;  
  FIG. 4 is a diagrammatic view showing a form of the apparatus of the invention using a single corona wire;  
  FIG. 5 is a diagrammatic sectional view through a corona wire showing one manner of blocking the ultra violet radiation from the bottom thereof;  
  FIG. 6 is a view similar to that of FIG. 4 but showing the use of multiple corona wires;  
  FIG. 7 is a view similar to that of FIG. 4 but showing another form of the invention;  
  FIG. 8 is a diagrammatic view showing a form of the apparatus of the invention using a high speed spark gap and a bias device to move the charged ions toward the electrophotographic film member;  
  FIG. 9 is a chart used to explain the apparatus of FIG.  
  FIG. 10 is a diagrammatic view of still another form of the apparatus of the invention; and  
  FIG. 11 is a diagrammatic view ofa form of the apparatus in which the shielded corona wire is physically moved across the area of the electrophotographic film member.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention, the improvements represented thereby are concerned with charging the photoconductive surface or coating of an electrophotographic plate or film member preferably in darkness without the discharging effects of ultra violet radiation normally generated by known ionizing devices. In this manner the maximum of charge is applied to the coating in a minimum of time.  
  Considering first FIG. 1, the blocks there shown represent the basic components of apparatus for recording electrostatic images on a suitable member. The basic apparatus is designated by the reference numeral and is shown to comprise four blocks 22, 24, 26 and 28 which represent the steps or apparatus to perform the steps in their order of use from left to right. The usual method and apparatus as known would comprise these same blocks but for the construction and operation of the first. According to the prior art, this block 22 which is marked CHARGE WITHOUT ULTRA VIOLET&#34; in FIG. 1 would merely comprise a step and apparatus to charge the electrophotographic member irrespective of whether it was exposed to ultra violet radiation during charging. So far as known, all prior apparatus and methods of necessity resulted in the exposure of the photoconductive coating of the electrostatic or electrophotographic film members to ultra violet radiation during the charging period. Thus. although not known to the user. when the film was ready to be exposed it was already at least partially discharged.  
  As seen in FIG. 1, the block 22 comprises a step in which some technique and/or apparatus is used by reason of which the ultra violet radiation is substantially decreased or eliminated from the ionized gas (normally air) that is applied to the electrophotographic film member. Thus the charged film member is exposed as indicated by the block 24 in any conventional manner or as taught in copending applications. toned in some manner as indicated by the block 26 and thereafter further processed.  
  In any given apparatus, the processing after toning may take place within the apparatus or outside of the apparatus. Such processing may comprise transferring the toned image and re-using the electrophotographic film; or it may comprise fixing the toner directly onto the film or other electrostatic member; or it may comprise photographing the toned image or electronically reading, storing. etc. said image. The block representing any and all of these processes is designated 28.  
  The apparatus or series of steps designated 10 and representing the most basic form of the invention could utilize any ofa number of forms of apparatus and modifications of the process. In FIGS. 2 and 3 two variations are diagrammatically illustrated. FIG. 2 represents apparatus or a process 30 in which the blocks designated 24, 26 and 28 may be considered the same as or substantially the same as those similarly designated in FIGS. 1 and 3. The two blocks 32 and 34 are marked PRODUCE CORONA&#34; and SHIELD FROM UV respectively. Both blocks are shown connected to the block 36 which is marked CHARGE IN THE DARK&#34;. All three blocks 32, 34 and 36 are the equivalent of the single block 22 of FIG. I.  
  What is intended to be inferred from the manner of setting up the diagram with the blocks 32 and 34 both leading to the same block 36 is that these two steps are performed simultaneously, either by some kind of fixed apparatus or by some function produced by dynamic apparatus. Such function may be effected automatically or even manually in some instances.  
  Further to provide explanation in connection with FIG. 2, reference may be had to FIG. 4 which illustrates a relatively simple form of the apparatus of the invention. Omitted from the view are components which enable the performance of the exposing, toning or processing since the very heart of the invention lies in the method and apparatus for achieving the most complete and efficient charge on the electrophotographic film.  
  The view illustrates an electrophotographic film member 38 which is mounted in an enclosed light-free chamber, symbolically designated by the reference character 40 along with the means 22 for producing the desired kind of charge. This latter means include a fine corona wire 42, a. high voltage power supply 44 and leads 46 and 48 for connecting the high voltage supply 44 between the corona wire 42 and the ohmic layer 50 of the electrophotographic film 38. The photoconductive coating 52 which is to receive the charge faces toward the wire 42 and the substrate 54 faces away from the wire. The entire apparatus 42. 44, 46 and 48 comprise the means 32 for producing corona..  
  When the power is applied. the wire 42 will generate corona around its surface which ionizes the air in the immediate vicinity. In known apparatus. the charge is permitted to settle onto the photoconductive surface 52 of the electrostatic member 38, whether of the prior art or of the copending application without any effort being made to exclude ultra violet radiation. Since it has been determined that a substantial amount of such radiation is produced by corona. the charging ofth&#39;e&#39; photoconductive coating 52 is simultaneously accom panied by a discharging of the same so that when the power is cut off and the corona ceases, what isleft on the coating is a net charge substantially less than would and extent of the arc of the shield 34 are chosen in view of the spacing from the wire 42 and the spacing of the shield and wire .from the surface 52 so that an umbra 56 is created to shade or shield the coating 52 from the ultra violet radiation which emanates from the wire 42. The radiation is direct while the ionized particles produced by the wire 42 form clouds along the wire and can move or settle down upon the surface 52 in nonrectilinear paths. Suitable bias means as explained in connection with FIG. 8 could also be used to drive the selected particles to the coating 52. For example, if it is desired to charge the surface 52 with electrons and hence negatively, a field with the negative polarity above the wire and the positive pole at the ohmic layer 50 would tend to drive the electrons to the surface 52. In FIG. a structure for shielding the wire 42 is shown which is less cumbersome and more economical than that of FIG. 4. Here, instead of a physically independent shield 34, the wire 58 has a coating of paint, enamel or resin on its bottom surface as shown at 60. In FIG. 4, the shield 34 can be any material that will block ultra violet emission and could comprise ordinary glass, synthetic resins or metal.  
  Problems arise in the use of a shielded wire. The shield acts as a plate of a capacitor and accumulates charge thus upsetting the homogeneity of the cloud of ions so that the charge on the coating 52 could be laid down unevenly. Also the umbra 56 excludes ions as well as the ultra violet radiation thus further aggravating the condition of charge non-uniformity.  
  One way of attackingthis problem is to have an array of wires each with its own shield as shown in FIG. 6. Here the wires 62 have respective shields 64 which produce a plurality of overlapping umbrae 66 that more effectively exclude the ultra violet radiation but in addition provide multiple overlapping spilLover areas for the ions thus tending to lay down a more uniform charge on the photoconductive coating.  
  Variations of the corona producing means 32 and the shield means 34 are illustrated in FIGS. 7, l0 and II. In FIG. 7, the, wire 68 is located laterally and the shield 70 is angular or arcuate and located to one side of the wire. The umbra 74 is arranged to shade the entire coating 52 and the ions will spill over onto the entire surface. An air jet can be used to distribute the ions .uniformly. In FIG. 10 a knife edge 76 with a shielding tential there is no problem of capacitive accumulation of charge.  
  In FIG. 11, the wire 82 and its shade or shield 84 are carried on a small carriage 86 riding on a track 88 that enables the wire to be moved over the area of the film 38 while producing corona. Thus, the ions for charging are laid dowri in a progressive sweep with a maximum of uniformity. The carriage for the electrophotographic film of the copendingapplication would be required to move at great speed inorderlto take advantage of the high charge acceptance, presenting mechanical and in ertia problems. In slower acceptance films the carriage .could be moved mechanically or manually. It provides an additional advantage where it is desired to get the charging wire out of the path of the toning apparatus.  
  Reverting nowto FIG. 3, the apparatus and method 90 represented by the block diagram illustrated uses a slightly different technique for eliminating the ultra violet radiation. As seen, the first step represented by the block 92 is the production of ahigh speed arcin darkness. FIG. 8 shows a high voltage supply 94 connected across a pair of points 96 and 98 spaced above the film 38. These constitute a spark gap. The are 100 is produced at&#39;a very high voltage, say of the order of 12,000 volts and lasts for a short time, say of the order of 0.5 millisecond. While the are no doubt produces ultra violet emission, this stops with extinguishment. The ions produced by breakdown of the air have a substantially longer persistence; hence before recombining to any substantial degree and dissipating their charges they will settle onto the coating&#39;52 of the film 38 and charge such coating. The discharge effect of the short burst of ultra violet radiation is exceeded by the subsequent charging effect of free ions with no ultra violet radiation present. An &#39;added feature is represented by the independentfield established between the grid 102 by the direct current voltage supply 104 and the ohmic layer 50&#39; to repel the negative ions, for example&#39; driving them toward the surface 52. v i  
  The chart of FIG. 8 shows at the left the voltage and duration of the are 100 represented by the ultra violet output at 106. Note that the fluxhas subsided in about half a millisecond. The curve 108 represents the effective surface potential and, &#39;as will be seen, the maximum charge is achieved in justa few milliseconds. This represents a substantial improvementover the charge acceptance characteristic of the electrophotographicfilm of the copending application&#34;, a large degree of which represents improvement in manufacturing techniquestions, the film may be charged above saturation; it may be charged to potentials controlled by the incident light to adjust sensitivity; and it may even be used in the shutterless camera with modifications. Toningis effected in darkness irrespective of the existence of a shutter.  
  Variations can be made without departing from the spirit or scope of the invention as defined in the appended claims.  
  What it is desired to secure by Letters Patent of the United States is:  
  1. Apparatus for recording electrostatic images which comprises means for disposing an electrophotographic member at a predetermined location in an enclosure with its photoconductive coating in position to be charged. charging means comprising an ionproducing source structured to produce and make available to said coating ions for charging the same without subjecting said coating to adversely effective ultra violet radiation. means for exposing the charged coating to a light pattern and means for toning the coating after exposure, said ion-producing source including a corona producing knife edge member spaced from said location with the knife edge facing away from said location said knife edge member including a back blade of enlarged form facing said location and serving as-ultra violet radiation shielding means for an electrophotographic film disposed at said location.  
 2. The apparatus as claimed in claim 1 in which the means for exposing and the charging means are arranged to operate simultaneously upon the same surface.  
  3. The apparatus as claimed in claim 1 which includes means for maintaining said enclosure in darkness while said charging means is operating and until said coating is charged. said exposing means serving to admit said light pattern into said enclosure after said coating is charged.  
  4. Apparatus for recording electrostatic images which comprises means for disposing an electrophotographic member at a predetermined location in an enclosure with its photoconductive coating in position to be charged. charging means comprising an ionproducing source structured to produce and make available to said coating ions for charging the same without subjecting said coating to adversely effective ultra violet radiation, means for exposing the charged coating to a light pattern and means for toning the coating after exposure and said ion-producing means comprise a spark gap spaced above said location and circuitry for producing an arc across said gap whose duration is less than the duration of persistence of ionization produced by said gap.  
  5. The apparatus as claimed in claim 4 in which includes means for maintaining said enclosure in darkness while said charging means is operating and until said coating is charged. said exposing means serving to admit said light pattern into said enclosure after said coating is charged.  
  6. ln electrostatic recording apparatus which includes an enclosure. means for mounting an electrographic film member at a predetermined location in said enclosure. means for charging the surface of said member in darkness. means for exposing said surface to a light pattern from the exterior of said enclosure and means for toning said surface. the invention comprising: means to prevent ultra violet radiation produced by said charging means from adversely affecting the surface charge, said charging means comprising a spark gap spaced from said surface and said means to prevent ultra violet radiation produced by said charging means from adversely affecting the surface charge comprising circuitry for arcing said gap at a speed which will extinguish the arc in substantial time before the dissipation of charge produced during arcing.  
  7. The invention as claimed in claim 6 and including bias means for driving selected polarity ions to said surface.  
  UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. i 3,880,514-  
 DATED April 29, 1975 INVENTORiS) I MANFRED R. KUEHNLE it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 2, line 22 change &#34;absorbance&#34; to -transmittance-.  
 Signed and Scaled this second Day Of March 1976 [SEAL] A ttest:  
 RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofParents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,880,514  
 DATED April 29, 1975 |NVENT0R(5) 1 MANFRED R. KUEI-INLE It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 2, line 22 change &#34;absorbance&#34; to -transmittance.  
 Signed and Sealed this second Day Of March 1976 [SEAL] A lies I:  
 RUTH C. MASON C. MARSHALLDANN Arresting Officer 4 Commissioner ojPatenls and Trademarks