GB1585233A - Charge control system for electrophotographic reproducing machines - Google Patents

Description

(54) CHARGE CONTROL SYSTEM FÒR ELECTROPHOTOGRAPHIC REPRODUCING MACHINES (71) We, XEROX CORPORATION, a corporation organised under the laws of the State of New York, United States of America, of Rochester, New York 14664, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electrophotographic reproduction machines, and more particularly to an improved charge control system for use with such machines.

In an electrophotographic or xerographic reproduction machine or copier, the charge level on the machine photoconductive surface is critical to satisfactory operation of the machine. As known to those versed in the art, the photoconductive surface is first uniformly charged, desirably to a preset charge level preparatory to imaging. The level of this initial charge however, is often critical since too low a charge may result in weak, washed out looking copies whereas too high a charge may result in dark copies and overloading of the machine cleaning mechanism.

Normally the charge as aforesaid is placed on the photoconductive surface by a corona generator. Close control over the corona output of devices of this type is often difficult, particularly as the machine components including the corona generator age.

According to the present invention we pro vide an electrophotographic reproduction machine for producing copies of an original, the machine having a photoconductor, means for charging the photoconductor in preparation for imaging, exposure means for exposing the charged photoconductor to the original whereby to create a latent electro static image of the original on the photo conductor, developing means for developing the latent electrostatic image on the photo conductor, and transfer means for trans ferring the developed image to a sheet of copy material, the machine also including means for detecting a charge on the photoconductor and generating a charge level signal representative of the charge level of said photoconductor following charging thereof by said charging means and-before development, light means between said charging means and said developing means for illuminating said photoconductor to reduce the charge level of said photoconductor, said light means reducing the charge level on said photoconductor in proportion to the intensity of the light produced by said light means, and control means responsive to said charge level signal for regulating the intensity of said light means to adjust the charge level on the photoconductor prior to development.

Preferably supplementary charging means are provided for increasing the charge on said photoconductor and wherein said control means selectively actuates either said light means or said supplementary charging means in response to said charge level signal to decrease or to increase the charge level of said photoconductor respectively.

Preferred embodiments of -the invention will now be described with reference to the accompanying drawings in which : - Figure 1 is a schematic sectional view of an electrophotographic reproduction machine incorporating the charge control system of the present invention; Figure 2 is an insometric view showing details of the charge control apparatus of the present invention; and Figure 3 is a circuit diagram of one embodiment of the photoreceptor charge control system of the present invention for reducing or increasing photoreceptor charge.

Figure '4 is a circuit diagram of another embodiment of the charge control system of the present invention for reducing photoreceptor charge.

Figure 5 is a circuit diagram of an alter -native embodiment wherein a fixed light source with liquid crystal regulator is provided for controlling the intensity of the light, shone upon the photoconductive surface for reducing photoreceptor charge.

- Four a general understanding of the invent tion, an exemplary copier/reproduction machine in which the invention may be incorporated, is shown in Figure 1. The reproduction or copying machine, is there designated generally by the numeral 5.

A document 11 to be copied is placed upon a transparent support platen 16 fixedly arranged in an illumination assembly, generally indicated by the reference numeral 10, positioned at the left end of the machine 5.

Light rays from an illumination system are flashed upon the document to produce image rays corresponding to the information areas.

The image rays are projected by means of an optical system onto the photosensitive surface of a xerographic plate in the form of a flexible photoconductive belt 12 arranged on a belt assembly, generally indicated by the reference numeral 9.

The belt 12 comprises a photoconductive layer 15 of selenium on a conductive backing. The surface of the photoconductive belt is charged by a previous step of uniformly charging the same by means of a corona generating device 13, which is connected by a power source 68.

The belt is journaled for continuous movement upon three rollers 20, 21 and 22 positioned with their axis in parallel. The photoconductive belt assembly 9 is slidably mounted upon two support shafts 23 and 24, with the roller 22 rotatably supported on the shaft 23 which is secured to the frame of the apparatus and is rotatably driven by a suitable motor and drive assembly (not shown) in the direction of the arrow at a constant rate. During exposure of the belt 12, the reflected light image of such original document positioned on the platen is flashed on the surface 15 of belt 12 to produce an electrostatic latent image thereon at exposure station 27.

The electrostatic latent image on the moving belt 12 passes through a developing station 28 in which there is positioned a magnetic brush developing apparatus, generally indicated by the reference numeral 30, and which provides development of the electrostatic image by means of multiple brushes as the same moves through the development zone.

The developed electrostatic image is carried on belt 12 to transfer station 29 whereat a sheet 6 of copy paper is fed between transfer roller 7 and belt 12 at a speed in synchronism with the moving belt to transfer the developed image to sheet 6 without blurring. A sheet transport mechanism, generally indicated at 17, brings sheets 6 from paper supply tray 18 or 18l to the transfer station 29 at the proper time to match the arrival of the sheet with the arrival of the developed image on belt 12.

Following transfer, the image bearing sheet is separated from belt 12 and conveyed to a fuser assembly, generally indicated by the reference numeral 19, wherein the developed powder image on the sheet is permanently affixed thereto. After fusing, the finished copy is discharged - from the apparatus into a suitable collector, i.e. tray 8. Residual toner particles and any other residue left on belt 12 are removed by brush 26 at cleaning station 25.- Further details regarding the structure of the belt assembly 9 and its relationship with the machine and support therefor may be found in U.S.

Patent No. 3,730,623.

As will be understood by those skilled in the art, development of the latent electrostatic image formed on belt 12 is dependent upon the voltage differential between the charged photoconductive belt bearing the light image and the developing means. This voltage differential, which may be described as a xerographic development field, serves to attract toner to the latent electrostatic image in accordance with the image outline and density requirements to faithfully reproduce the original being copied.

Referring now to Figures 2 and 3 of the drawings, the charge control 50 of the present invention preferably includes both a supplementary charging section 52 and a charge reducing section 54. As will appear, supplementary charging section 52 is utilized to automatically increase the charge level on the photoconductive surface 15 of belt 12 where the original charge level provided by the corona generating device 13 is found to be too low while charge reducing section 54 is utilized to automatically reduce the charge on surface 15 where the original charge is found to be too high. In this way an optimized charge is provided on the photoconductive surface 15.

Supplementary charging section 52 includes a corona discharge wire 61 and adjoining shield 63. Shield 63 is formed from metal and in the arrangement shown, has, when viewed in cross-section, a generally inverted U-shape with top wall 65, depending side walls 66, 67, and end walls 62, 64. Corona wire 61 is strung between end walls 62, 64 of shield 63. To prevent shorting of corotron wire 61 to metal shield 63, suitable electrical insulators 69 are provided between wire 61 and the ends 62, 64 of shield 63. It will be understood that where corotron shield is composed of an electrical insulating material such as plastic, insulators 69 and, as will appear conductive layer 80, may be dispensed with.

Charge reducing section 54 of charge control device 50 includes a generally rectangular electro-luminescent panel 70, the length and width dimensions of which are equal to or slightly less than:the corre-spond- ing length and width dimensions of shield 63, mounted within the shield interior on the inside surface of the shield upper wall 65. One suitable electro-luminescent panel is type 94-0150-1 manufactured by Grimes Manufacturing Co., Urbana, Ohio.

To prevent build-up of static electrical charges on the electro-luminescent panel 70, the exposed or lower surface 71 of panel 70 is covered with a clear conductive material, preferably 'a thin layer 80 of NESA glass. Conductive layer 80 is electrically coupled to shield 63 through contact with side walls 66, 67 of shield 63. It will be understood that where shield 63 is formed from a non--conductive material, i.e. plastic conductive layer 80 may be dispensed with A preset reference signal, which appears in lead 91, is developed from a suitable d.c.

voltage source shown in exemplary fashion as battery 110. Battery 110 is coupled across voltage level controller 90, shown as a potentiometer serving to regulate the voltage level of the reference signal in lead 91 in accordance with the setting thereof to provide a preset reference signal. The reference signal in lead 91 is applied via resistor 92 to one input of voltage comparator 93.

Power input to corona discharge wire 61 of supplementary charging section 52 and electro-luminescent panel 70 of charge reducing section 54 is derived from variable d.c. power supply circuit 74, the output of which to either section 52 or section 54 is regulated in accordance with charge conditions of the photosensitive surface 15 of belt 12 as sensed by a d.c. type electrometer 100. Probe 102 of electrometer 100 is mounted in machine 5 in predetermined spaced relationship to the photoconductive surface 15 as will be understood by those skilled in the art. In a preferred embodiment, probe 102 is disposed downstream of the charge control device 50 and exposure station 27 but before the developing station 30. Other probe locations suitable for sensing the charged belt, however, may be contemplated.

The d.c. signal output from probe 102, representative of the charge on the photoconductive surface 15, is fed via lead 103 to the main body 106 of electrometer 100 wherein the signal is suitably amplified. The signal output of electrometer 100 appears in lead 94 to d.c. power supply circuit 74.

One type of d.c. electrometer that can be employed in both embodiments of Figures 3 and 4 are described in U.S. Patent 3,852,668. Other electrometers including those of the a.c. type may instead be contemplated.

Power supply circuit 74 includes the voltage comparator 93, which may comprise any suitable circuit effective to compare voltage levels inpplied thereto and generate an analog signal prbportional to the difference between the input signal voltages. In the exemplary circuit illustrated, comparator 93 comprises an operational amplifier, operative to compare the preset control signal from controller 90 with the signal output of electrometer 100, the latter being representa- tiv'e of 'the charge level on the photoconductive surface 15 of belt 12. Variable resistance 114 controls the gain of comparator 93.

The signal output of comparator 93 is applied via lead 96 to the base electrodes of PNP transistor 97 and NPN transistor 98 tespectively. The ,collector of transistor 97 is connected by lead 99 to electroluminescent panel 70. Lead 101 couples the emitter of transistor 97 to a suitable source of positive potential, represented herein by battery 112.

Lead 104 couples the collector of tran sistor 98 with the input side of a conventional d.c. to d.c. converter 105. The output of converter 105, which is used to drive charging section 52 of charge control device 50, is connected by lead 107 with corona generating wire 61. Lead 108 couples the emitter of transistor 98 with a suitable source of negative potential represented herein by battery 109.

D.C. to d.c. converter 105 serves to amplify the relatively low power variable signal output of transistor 98 to the relatively high power level required to drive charging section 52. Any suitable commercially available d.c. to d.c. converter having the necessary operating specifications may be used for this purpose.

In operation, the optimum charge level of the photoconductive surface 15 of belt 12 is determined, and the signal output of electrometer 100 corresponding to the optimum charge level is matched with the reference signal in lead 91. This may be effected by adjusting the setting of controller 90 until the matching signal potential is reached. So long as the charge level on the photoconductive surface 15 remains at the level desired, the signal inputs in leads 91, 94 to comparator 93 match, and the signal output from comparator 93 to lead 96 holds transistors 97, 98 in a blocking state. As a result, both the supplementary charging section 52 and charge reducing section 54 of charge control 50 are inoperative.

Should the charge level on the photoconductive surface 15 rise above the level desired, as represented by the setting of controller 90, the voltage potential of the output signal from electrometer 100 in lead 94 rises. Comparator 93 responds by generating a positive signal output, the potential of which is proportional to the difference in potential between the input signals two com parator 93 in leads 91, 94. .Transistor. 97 feeds a proportional amount of power S to electro-luminescent panel 70 to turn panel 70 on and illuminate. the photoconductive surface with an intensity proportional to the strength of the signal output from comparator 93.Light from panel 70 reduces the charge level on the photoconductive surface 15 to bring. the charge level back to the optimum level desired.

Should the charge level on the photo conductive surface 15 fall below the optimum ,level desired, the voltage' potential of the output signal from electrometer 100 in lead 94 falls. Comparator 93 responds by generating a negative signal output, the potential of which is proportional to the difference in potential between the signal inputs to comparator 93 in leads 91, 94.

Transistor 98 feeds a proportional amount of power, which is raised to the requisite power level necessary by d.c. to d.c. converter, to corona discharge wire 61 of supplementary charging section 52. The resulting corona emissions from wire 61 add to or supplement the charge previously applied to the photoconductive surface 15 by corona generating device 13 to bring the charge level back to the optimum level desired.

While supplementary charging section 52 and charge reducing. section 54 are combined herein to provide a unitary charge control 50, it will be understood that supplementary charging section 52 and charge reducing section 54 may comprise separate and discrete entities. In Figure 4, charge reducing section 54 has been combined with the primary corona generation device 13 thereby eliminating the need for a separate unit for charge control, as depicted at 50 in Figure 1.

The corona generating device 13 includes thec orona discharge wire 61 and shield 63.

Shield 63 is formed from metal and in the arrangement shown, has when viewed in cross-section, a generally inverted U-shape with top wall 65, depending side walls 66, 67, and end walls (not shown). Corona wire 61, which is electrically coupled to a suitable d.c. power source, represented in exemplary fashion by battery 68, is strung between end walls of shield 63, as shown in Figure 2 shorting of corotron wire 61 to metal shield 63, suitable electrical insulators 69 would be provided between wire 61 and the ends of shield 63.

As in the case of the Figure 3 embodiment, charge control section 54 includes a generally rectangular electro-luminescent panel 70, the length and width dimensions of which are equal to or slightly less than the corresponding length and width dimensions of shield 63, mounted within the shield interior beenath the shield upper wall 65.

Also; panel 70 is electrically connected to a variable power supply circuit 74 by lead 99.

To prevent build-up of static electrical charges on the electro-luminescent panel 70, the exposed or lower surface of panel 70 is covered with a clear conductive material; preferably a thin layer 80 of NESA glass.

Conductive layer 80 is electrically coupled to shield 63 through contact with side walls 66, 67 of shield -.63. Conductive layer 80 is also referenced in Fig. 3.

Power- supply circuit 74 in this embodiment . is similar ,to that of Figure 3 and therefore, like parts have the same reference numerals. A -suitable course of d.c. control voltage is provided in. the form of battery 110. Battery 110 is coupled across voltage level controller 90, which is a potentiometer serving to regulate the control voltage provided to voltage comparator 93 in accordance with the setting thereof.

The output signal of controller 90 in lead 91 thereof, which serves as a reference potential, if coupled through resistor 92 with one input of voltage comparator 93.

Lead 94 couples the other input of comparator 93 with a device which generates an output signal indicative of the charge level on the photoconductive surface 15 of belt 12 following charging by corona generating device 13. In the exemplary arrangement shown, the charge measuring device comprises a d.c. type electrometer 100. Probe 102 of electrometer 100 is mounted in machine 5 in predetermined spaced relationship to the photoconductive surface 15 as will be understood by those skilled in the art. In a preferred embodiment, probe 102 is disposed downstream of the corona generating device 13 and exposure station 27, but before developing station 30.

The d.c. signal output from probe 102, representative of the charge on the photoconductive surface 15, is connected by lead 103 to the main body 106 of electrometer 100, wherein the signal is suitably amplified.

The signal from electrometer 100 is provided to the input gate of comparator 93 through lead 94.

Comparator 93 may comprise any suit able circuit effective to compare voltage levels supplied thereto and generate an analog signal proportional to the difference between the input signal voltages. In the exemplary circuit illustrated, comparator 93 comprises an operational amplifier, operative to compare the signal outputs of controller 90 and electrometer 100, the latter signal being representative of the charge level on the photoconductive surface 15 of belt 12.

The signal output of comparator 93 is supplied through lead 96 to the base elect trode of control transistor 79. Lead 99 couples the emitter of transistor to panel 70 while lead 101 couples the collector of transistor 79 to a suitable power source shown here as battery 112. Control transistor 79 regulates the power input to panel 70 in response to the output signal from comparator 93 to control the level of illumination of panel 70. Variable resistor 114 controls the gain of comparator 93.

During the operational cycle of reproduction machine 5, the photoconductive surface 15 of belt 12 is charged by the corona generating device 13 and exposed at exposure station 27 to the original 11 being copied to produce a latent electrostatic image of original 11 on the surface 15 of belt 12. The latent electrostatic image so formed is carried past developer station 28 whereat the image is developed. The developed image then passes to transfer station 29 where the developed image is transferred to a sheet 6 of copy paper brought forward from supply tray 18 or 181 by transport 17 at the proper time so as to assure registration of the developed image on belt 12 with the sheet 6. The copy sheet 6 bearing the developed image is thereafter transported to fuser 19 where the image is fixed, following which the final copy is discharged into tray 8.

Electrometer 100 monitors the charge level on the portion of the photoconductive surface 15 of belt 12 viewed by probe 102.

The signal output of electrometer 100 is fed to comparator 93 where the signal from electrometer 100 is compared with the preset reference signal from voltage level controller 90. So long as the voltages of the signal inputs to comparator 93 are substantially identical, transistor 79 does not conduct and there is no flow of current to panel 70. As a result, electro-luminescent panel 70 is not illuminated.

Where the signal inputs to comparator 93 are unbalanced, reflecting charging of the photoconductive surface 15 to a level greater than that represented by the reference voltage in lead 91, transistor 79 conducts and provides power to panel 70 in proportion to the voltage level of the signal on lead 96 being proportional to the difference in potential between the signal on lead 91, and the signal on lead 94. The resulting current flow in lead 99 to the electro-luminescent panel 70 energizes panel 70 to produce an illumination whose intensity will be proportional to the power supplied thereto.

Illumination from panel 70 reduces the charge on the photoconductive surface 15 of belt 12 in proportion to the amount of illumination.

While the charge generating device 13 and charge reducing section 54 may be combined into one unitary device, they may comprise -separate discrete entities, -as in the arrangement shown in Figure 5. There, the charge generating section comprises a corona generator device 131 while the charge reducing section comprises a modified version 541 of the variable illumination device shown in Figure 4.

Referring now to Figure 5, wherein like numerals refer to like parts, electroluminescent panel 70 is mounted within a generally inverted U-shaped shield 65 beneath the shield upper wall 166. Shield 165 is supported by suitable means (not shown) in transverse relationship to belt 12 at some convenient point along the belt run. In the arrangement illustrated, shield 165 is disposed adjacent to and downstream of corona generator 131.

As will appear, panel 70 serves as the source of illumination with control over the amount of light directed onto the photoconductive surface 15 of belt 12 being effected by means of liquid crystal 170 in response to the charge conditions of the photoconductive surface. In this embodiment, the side of electro-luminescent panel 70 facing the photoconductive surface of belt 12 is overlayed by a liquid crystal 170, crystal 170 preferably being sized so as to cover the entire side of panel 70. Suitable light polarizers 168, 169 are disposed on opposite sides of the liquid crystal 170.

Liquid crystal 170 comprises any suitable liquid crystal of the so-called field effect type wherein the light transmissitivity thereof varies in response to the electric current applied thereof. A liquid crystal suitable for this purpose is manufactured by Hamlin, Inc., Lake Mills, Wisc.

In the arrangement shown, the light transmissitivity of liquid crystal 170, and hence the amount of light directed onto the photoconductive surface, is regulated in response to the charge conditions of the photoconductive surface 15. The output of variable power source 74, which is representative of the charge level of the photoconductive surface 15 of belt 12 has described heretofore, is applied via lead 98 to liquid crystal 170 to control the light transmissitivity thereof. Panel 70 in this embodiment serves as the light source with a constant intensity of illumination and is driven from a suitable power source such as battery 172 through lead 173. On I-off switch 174 in lead 173 permits panel 70 to be turned off, as .during periods when machine 5 is not in use.

Corona generator device 13, like device 13 of Figure 1, comprises any suitable d.c., a.c., or a.c./d.c. type corona charging device as known to those skilled in the copier arts.

In the arrangement illustrated, the corona generating device 131 includes a corona emitting wire 83 -supported within shield 84 and coupled to a suitable source of power, exemplified by battery 85.

In operation, switch 174 is closed to ener gize electro-luminescent. panel 70 continuously. The amount of light, if any, transmitted by liquid crystal 170 onto the photoconductive surface 15 is varied in response to the strength of the signal in output lead 98 of variable power supply 74, which in turn is representative of the charge level on the photoconductive surface 15.

Where the charge on surface 15 is at the level desired, the signal in lead 98 to crystal 170 causes molecular turbulence which renders crystal 170 opaque with the result that light from panel 70 to the photoconductive surface 15 is partially or completely blocked. Where the signal in lead 98 reflects overcharging of the photoconductive surface 15, molecular reorientation of the molecules within crystal 170 proportional in degree to the signal strength occurs with the result that crystal 170 transmits a proportional amount of light from panel 70 therethrough onto the photoconductive surface 15. The light, thus, reduces the charge level on the photoconductive surface 15 in proportion to the light intensity.

The corona generating device 131 and charge control 541 may be combined as a single unit. In that event, shield 165 would be dispensed with and the liquid crystal 170 would instead be disposed inside shield 84 of corona generator 131. In the event shield 84 is formed from a conductive material, a conductive transparent layer, such as the conductive layer 80 shown and described in the Figure 4 embodiment, would preferably be disposed over the polarizer 169 facing the photoconductive surface 15.

While the light source has been illustrated and described as comprising an electro-luminescent panel 70, other suitable light sources whose intensity may be varied can be employed alternatively.

WHAT WE CLAIM IS:- 1. An electrophotographic reproduction machine for producing copies of an original, the machine having a photoconductor, means for charging the photoconductor in preparation for imaging, exposure means for exposing the charged photoconductor to the original whereby to create a latent electrostatic image of the original on the photoconductor, developing means for developing the latent electrostatic image on the photoconductor, and transfer means for transferring the developed image to a sheet of copy material, the machine also including means for detecting a charge on the photoconductor and generating a charge level signal representative of the charge level of said photoconductor following charging thereof by said charging means and before development, light means between said charging means and said developing means for illuminating said photoconductor to reduce the charge level of said photoconductor, said light means reducing the charge level on said photoconductor in proportion to the intensity of the light produced by said light means, and control means responsive to said charge level signal for regulating the intensity of said light means to adjust the charge level on the photoconductor prior to development..

2. A reproduction machine according to claim 1 in which said control means includes means providing a predetermined reference signal reflecting an optimum charge level of said photoconductor, and comparator means for comparing said reference signal with said charge level signal, said comparator means producing a control signal for regulating the intensity of said light means.

3. A reproduction machine according to claims 1 or 2 in which said light means comprises a variable intensity lamp.

4. A reproduction machine according to claim 3 in which transparent conductive means are provided between said lamp and said photoconductor for reducing static charge build-up.

5. A reproducing machine according to claims 1 or 2 in which said light means comprises a constant light source and wherein variable light conducting means are disposed between said light source and said photoconductor, said control means including means for controlling the light transmissitivity of said light conducting means in response to said charge level signal.

6. A reproduction machine according to claim 5 in which said variable light conducting means comprises a liquid crystal.

7. A reproduction machine according to claims 1 to 5 in which said charging means includes at least one corona generating device and a housing for said device, said light means being incorporated in said housing.

8. A reproduction machine as claimed in any preceding claim wherein supplementary charging means are provided for increasing the charge on said photoconductor and wherein said control means selectively actuates either said light means or said supplementary charging means in response to said charge level signal to decrease or to increase the charge level of said photoconductor respectively.

9. A reproduction machine according to claim 8 in which said supplementary charging means and said light means are combined in a unitary structure.

10. A reproduction machine according to claim 8 or claim 9 when appendant to claim 2 in which said comparator means generates a control signal selectively operable to control said light means and said supplementary charging means.

11. A method of adjusting the charge

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. gize electro-luminescent. panel 70 continuously. The amount of light, if any, transmitted by liquid crystal 170 onto the photoconductive surface 15 is varied in response to the strength of the signal in output lead 98 of variable power supply 74, which in turn is representative of the charge level on the photoconductive surface 15. Where the charge on surface 15 is at the level desired, the signal in lead 98 to crystal 170 causes molecular turbulence which renders crystal 170 opaque with the result that light from panel 70 to the photoconductive surface 15 is partially or completely blocked. Where the signal in lead 98 reflects overcharging of the photoconductive surface 15, molecular reorientation of the molecules within crystal 170 proportional in degree to the signal strength occurs with the result that crystal 170 transmits a proportional amount of light from panel 70 therethrough onto the photoconductive surface 15. The light, thus, reduces the charge level on the photoconductive surface 15 in proportion to the light intensity. The corona generating device 131 and charge control 541 may be combined as a single unit. In that event, shield 165 would be dispensed with and the liquid crystal 170 would instead be disposed inside shield 84 of corona generator 131. In the event shield 84 is formed from a conductive material, a conductive transparent layer, such as the conductive layer 80 shown and described in the Figure 4 embodiment, would preferably be disposed over the polarizer 169 facing the photoconductive surface 15. While the light source has been illustrated and described as comprising an electro-luminescent panel 70, other suitable light sources whose intensity may be varied can be employed alternatively. WHAT WE CLAIM IS:-

1. An electrophotographic reproduction machine for producing copies of an original, the machine having a photoconductor, means for charging the photoconductor in preparation for imaging, exposure means for exposing the charged photoconductor to the original whereby to create a latent electrostatic image of the original on the photoconductor, developing means for developing the latent electrostatic image on the photoconductor, and transfer means for transferring the developed image to a sheet of copy material, the machine also including means for detecting a charge on the photoconductor and generating a charge level signal representative of the charge level of said photoconductor following charging thereof by said charging means and before development, light means between said charging means and said developing means for illuminating said photoconductor to reduce the charge level of said photoconductor, said light means reducing the charge level on said photoconductor in proportion to the intensity of the light produced by said light means, and control means responsive to said charge level signal for regulating the intensity of said light means to adjust the charge level on the photoconductor prior to development..

2. A reproduction machine according to claim 1 in which said control means includes means providing a predetermined reference signal reflecting an optimum charge level of said photoconductor, and comparator means for comparing said reference signal with said charge level signal, said comparator means producing a control signal for regulating the intensity of said light means.

3. A reproduction machine according to claims 1 or 2 in which said light means comprises a variable intensity lamp.

4. A reproduction machine according to claim 3 in which transparent conductive means are provided between said lamp and said photoconductor for reducing static charge build-up.

5. A reproducing machine according to claims 1 or 2 in which said light means comprises a constant light source and wherein variable light conducting means are disposed between said light source and said photoconductor, said control means including means for controlling the light transmissitivity of said light conducting means in response to said charge level signal.

6. A reproduction machine according to claim 5 in which said variable light conducting means comprises a liquid crystal.

7. A reproduction machine according to claims 1 to 5 in which said charging means includes at least one corona generating device and a housing for said device, said light means being incorporated in said housing.

8. A reproduction machine as claimed in any preceding claim wherein supplementary charging means are provided for increasing the charge on said photoconductor and wherein said control means selectively actuates either said light means or said supplementary charging means in response to said charge level signal to decrease or to increase the charge level of said photoconductor respectively.

9. A reproduction machine according to claim 8 in which said supplementary charging means and said light means are combined in a unitary structure.

10. A reproduction machine according to claim 8 or claim 9 when appendant to claim 2 in which said comparator means generates a control signal selectively operable to control said light means and said supplementary charging means.

11. A method of adjusting the charge

level on a charged photoconductor in an electrophotographic reproduction machine for producing copies of an original, comprising the steps of detecting the charge level on the photoconductor, generating a charge level signal representative of said charge level of said photoconductor after charging thereof and before development of a latent image created thereon by exposure of the charged photoconductor to the original, and illuminating said charged photoconductor at an intensity dependent on the magnitude of the said charge level signal to reduce the charge level of said photoconductor prior to development.

12. A method according to claim 11 including the steps of monitoring the charge level on said photoconductor to produce a charge level signal indicating if the charge level is above or below an optimum charge level provided by a predetermined reference signal, using said signal to control illumination of the charged photoconductor if the monitored charge level is above the optimum level and using said signal to apply additional charge to said photoconductor to bring the charge level on said photoconductor up to said optimum charge level when said monitored charge level is below said optimum charge level.

13. An electrophotographic reproduction machine substantially as described herein with reference to Figures 1 to 3, 4 or 5 of the accompanying drawings.

14. A method of adjusting the charge level on the photoconductor of an electrophotographic reproduction machine substantially as described herein with reference to Figures 1 to 3, 4 or 5 of the accompanying drawings.