Abstract: An image forming apparatus using an electrophotographic process including a photoconductor, a development device, and a transfer device, a measuring device measures an electrostatic capacitance between said photoconductor and said transfer device in predetermined conditions while a standard toner image is formed. Humidity of ambient environment is also measured. Then, a controller controls image forming conditions such as a development condition or a transfer condition according to the measured electrostatic capacitance and the humidity. Alternately, a resistance of the paper is measured, and a controller controls image forming conditions according to the measured resistance. In a still different way, an amount of toner charges of a standard toner image is measured, and a controller controls image forming conditions according to the measured amount of toner charges.

Abstract: An image forming apparatus has an image bearing member, an image forming unit adapted to form a toner image on the image bearing member and having a first charge member for charging the image bearing member with the same polarity as normal polarity on toner, an intermediate transfer member, a voltage apply means for applying voltage to the intermediate transfer member to electrostatically transfer the toner image formed on the image bearing member by the image forming unit onto the intermediate transfer member at a first transfer station, a second charge member for charging residual toner remaining on the intermediate transfer member with polarity opposite to the normal polarity of the toner on the image bearing member, after the toner image on the intermediate transfer member is electrostatically transferred onto a transfer material at a second transfer station, and a controller for variably controlling voltage applied to the first charge member, and the voltage application generates an electric field for tr

Abstract: A method is described to control the maximum density and the pixel profile of microdots produced by a binary or multilevel electrophotographic device. In various embodiments, from the maximum development potential. The working point of the device is established by imposing a relation between charge level, discharge level and saturation voltage level of the photosensitive element. This allows to achieve consistent output densities, irrespective of the environmental parameters, such as relative humidity and temperature.

Abstract: An online printing parameter establishment apparatus is used with a xerographic printing device for printing high quality prints given a target value. The xerographic printing device is capable of producing at least one print control patch on a photoreceptor and is capable of sensing a value of the at least one print control patch. The xerographic printing device has a plurality of sets of inherent performance characteristic values. The online printing parameter establishment apparatus includes a controller device and a switch device. The controller device is operative to produce a first database of different ones of the sensed values. Each sensed value is associated with a respective one of the plurality of sets of inherent performance characteristic values based upon the target value. The controller device is also operative to produce a second database of a plurality of control values.

Abstract: An electrostatographic recording apparatus and method wherein a primary charger establishes a uniform electrostatic charge on an electrophotoconductive image recording member in accordance with a first operating parameter. A recording device imagewise modulates the charge on the image recording member to form electrostatic latent images in accordance with a second operating parameter and also forms a developable process control patch. A development station develops the electrostatic latent images and the control patch on the image recording member with toner in accordance with a third operating parameter. A replenishment device provides toner to the development station at a controlled rate in accordance with a replenishment signal.

Abstract: An image forming device includes an optical system for forming an electrostatic latent image on a photosensitive body by exposure of an original in light, a development system for developing the electrostatic latent image, and a control system for controlling the optical system and the development system. The control system detects the concentration of a toner in a developing agent tank, calculating a difference between the detected toner concentration and a predetermined toner concentration, replenishing the toner to the developing agent tank until the detected toner concentration makes a recovery to the predetermined toner concentration, clocking a time period for the recovery and controlling at least one of the amount of the exposure light and the amount of the toner adhering to a photosensitive body based on the clocked time period for the recovery.

Abstract: The present invention, for example, forms a test pattern image on the surface of a photosensitive member at predetermined periodicity and detects the density of said test pattern image. If the detected density value of the test pattern image is other than said target value, the charging voltage is preferentially adjusted to maintain a stable toner density. When the adjusted charging voltage attains either an upper limit or a lower limit, the toner density is adjusted.

Abstract: An image forming apparatus wherein a developer remaining on a photosensitive member after an image transfer process is removed by developing means, the apparatus includes an electrophotographic photosensitive member; a contact charging means including an electrode contacted to the photosensitive member to electrically charge the photosensitive member, the contact charging means being supplied with a bias voltage form a charging bias voltage source; exposure means for exposing the photosensitive member to form an electrostatic latent image thereon after the photosensitive member is electrically charged by the charging means; developing means for supplying the developer from a developer carrying member to a latent image portion of the photosensitive member to develop the photosensitive member and simultaneously removing the developer remaining on the photosensitive member after the transfer process and conveyed to a downstream of the contact charging means on the photosensitive member, to the developer carrying

Abstract: An image forming apparatus of the type capable of forming a plurality of images in separate areas of an intermediate transfer body is disclosed. When a paper or similar recording medium to which a toner image formed on the intermediate transfer body should be transferred is absent, the transfer of a toner image from the image carrier to the intermediate transfer body is interrupted. At the same time, toner left on the intermediate transfer body is removed by a particular method different from a usual method and capable of enhancing a cleaning ability. The apparatus is therefore capable of fully removing the toner and thereby obviating defective images.

Abstract: The surface of a photosensitive body is evenly charged by a charging member, and then the charged surface is exposed to light by an exposure device in order to produce an electrostatic latent image. A toner is caused to adhere to the latent image by a developing roller and then is transferred to paper. The temperature of the charging member is detected by a temperature detecting device and, in accordance with the detected temperature, a voltage to be applied to the charging member is corrected by a voltage correcting device. An aging change of the photosensitive body is detected by an aging change detecting device and, in accordance with the amount of the aging change, a correction rule for an applied voltage corrected by the voltage correcting device is changed by a correction rule changing device. A potential of the surface of the charged photosensitive body or a developing bias of the developing roller in an image forming process is corrected by a potential correcting device.

Abstract: In a method for forming a digital image with use of electrophotographic process wherein the gradation is expressed by changing the laser intensity according to image information, at least one of the surface electric potential V.sub.O of photoconductor and the development bias electric voltage V.sub.B is controlled, and the laser intensity is controlled for gradation correction to compensate a change in gradation due to the control of V.sub.O and V.sub.B.

Abstract: A feedback control system controls developability of a xerographic imaging device that uses optical sensors for measuring development values based upon a target value. The feedback control system includes a controller device and a feedforward device. The controller device includes an input summing node, a gain device, an integrator and a nominal summing node which are serially connected in communication with each other. The input summing node, the gain device and the integrator are operative in combination with each other to receive and process the measured value and the target value to provide an new corrected actuator value to the integrator summing node. The feedforward device is connected to the nominal summing node and receives and responds to the target value to output a nominal actuator value to the nominal summing node.

Abstract: An electrostatographic recording apparatus and method includes a primary charger that establishes a uniform electrostatic charge on the image recording member in accordance with a first operating parameter. A recording device that imagewise modulates the charge on the image recording member to form electrostatic images in accordance with a second operating parameter and additionally forms a developable process control patch. A development station develops the electrostatic images and the control patch on the image recording member with toner in accordance with a third operating parameter. A transfer station transfers developed images from the image recording member. The transfer station includes a transfer member and power supply providing to the transfer member a transfer current used for transferring developed images. In one embodiment a sensor senses density of the developed control patch.

Abstract: An image forming apparatus has an arrangement wherein a main charger includes a charger line corresponding to a region within a reference range on a surface of a photoreceptor and second charger lines corresponding to regions outside the reference range, and the first charger line within the reference range and the second charger lines outside the reference range are driven independently. The reference range is set on the surface of the photoreceptor as a width of a transported sheet which is most frequently used. In a processing control, in the case where toner patch density within the reference range becomes higher than toner patch density outside the reference range, an applied voltage with respect to the charger line within the reference range is increased relatively higher than an applied voltage with respect to the charger lines outside the reference range, thus uniformalizing the toner density with respect to the entire surface of the photoreceptor drum.

Abstract: The development of the edges of discharged areas on a photoconductor drum in an electrophotographic printing system is controlled by the ratio of the DC component of the developer bias to the magnitude of the difference between the DC component of the developer bias and the DC component of the bias supplied to the photoconductor charge roller. A development system using a developer, a photoconductor charge roller, a high voltage power supply, an optical density sensor, and a controller for controlling the high voltage power supply optimizes the electrophotographic edge development. The optical density sensor is used in a calibration process to determine the value of the DC component of the developer bias necessary to ensure the optical density of developed areas will meet the minimum specified optical density.

Abstract: When an electrophotographic printer is manufactured, the initial electrical resistance of its transfer roller is measured and a corresponding value is stored in a memory device in the printer. During operation, the printer's control program estimates the resistance of the transfer roller from the stored value, taking aging into account, then measures the actual resistance of the transfer roller, and infers ambient conditions from the difference between the estimated and actual resistance values. The electrophotographic printer is controlled according to the inferred ambient conditions.

Abstract: An image forming apparatus includes an image bearing member, a charging station, an exposure station forming an electrostatic latent image on the charged image bearing member, and a developing station for developing the latent image formed on the surface of the image bearing member. A stop control device is provided for immediately stopping a charging operation by the charging device when the image forming apparatus must stop due to the detection of an irregular state during an image forming process. The image bearing member continues moving by at least the distance along that member from the charging to the developing station before the stop control device stops the developing operation and the moving of the image bearing member.

Abstract: An image forming apparatus including a photoconductive drum, a charger which charges the photoconductive drum, an exposing device which irradiates light to the charged photoconductive drum for forming an electrostatic latent image on the photoconductive drum, a developing device which has two component developer which includes magnetic toner and magnetic carriers and a transfer charger to transfer the developed image to a sheet of paper. The bias voltages of the charger, the developing device and the transfer charger are applied from a common high voltage power source. A developing starting voltage of the developing device is higher than a residual voltage of the photoconductive drum.

Abstract: An image forming method and apparatus includes a plurality of processing devices for forming an image on a sheet of paper. When a power source switch is turned on, electric power is supplied to the apparatus and a timer begins to count. If the power source switch is turned on, a controller preconditions an adjustment of the processing devices on a temperature of a fixing roller being not more than a predetermined temperature. Further, if the timer counts for a predetermined amount of time, say 24 hours, the condition of the processing devices is also adjusted.

Abstract: This invention provides for duplex use of densitometers. An image is produced on a substrate, that substrate is brought back through the transfer station in a duplex fashion (that is, the substrate is inverted), and a sensor senses the image on the substrate. Beneficially, the sensor senses one or more parameters such as color registration, color occlusion/interference, black separation in color REaD printing, colorimetry, and the overall transfer and fusing process; and a printing process is controlled based on the sensed parameters.

Abstract: A reproduction apparatus and method particularly suited for an electrophotographic DAD-CAD system having an electrostatic recording member for supporting an electrostatic image, a primary charger establishes a primary charge on the member, the primary charge being defined by a parameter V.sub.o. An exposure device imagewise modulates the primary charge to form an electrostatic image on the recording member. The device has an exposure parameter E.sub.o. A developer station develops the electrostatic image with charged toner. The developer station has an electrical bias to establish a field for urging movement of the toner to the member. The electrical bias has an alternating current component parameter AC and a direct current parameter DC. A controller controls adjustments to the parameters E.sub.o, V.sub.o and AC by measuring a density parameter D.sub.OUT of an exposed and developed area on the member that is formed by operation of the primary charger, the exposure device and the developer station.

Abstract: The image forming apparatus of the invention controls the toner density by any one of the following configurations: by correcting the toner density of the developer in association with the agitation total; by controlling a process parameter so that the density of a toner patch formed on the photoreceptor corresponds to a prescribed density value and determining that developing performance of the developer is improved and canceling the toner density correction when the process parameter reaches the prescribed value; by changing the toner density reference value when the variation as to the charger output is equal to or greater than a first predetermined value and maintaining the changed toner density reference value until the variation of the charger output again becomes equal to or greater than the first predetermined value; or by prohibiting toner supply for a constant duration to prevent excessive toner supply if time from the end of the last operation of the developing unit to the start of a next operation

Abstract: A simulation system is provided which is capable of simulating the behavior of a sheet transported by a transportation system. In response to application of a control sequence formulated for the sheet transportation system, a simulation is performed to check how the sheet is to be transported in the sheet transportation system on the basis of the applied control sequence. Necessary data are extracted from hardware systems constituting the real sheet transportation system. On the basis of the extracted data, a virtual sheet transportation system is constructed in a computer, and the sheet is represented by information indicative of the locations of opposite ends of the sheet and the distance therebetween. The sheet thus represented is moved by a predetermined unit distance in the virtual sheet transportation system on the basis of the control sequence, and data is computed for every movement of the sheet.

Abstract: An image forming apparatus includes an image forming unit for forming an image on a sheet, mode setting unit for setting a test mode for a detection of a cause for a defect in the formed image, a control unit for causing the image forming means to form a plurality of sample images by changing operating conditions of the image forming unit when the test mode is set by the mode setting unit, an input unit for manually inputting a result of comparison of the plurality of sample images and display unit, wherein the control unit determines the particulars of the detection for the cause of the defect in the formed image or contents of the test mode to be subsequently carried out in response to an input of the input unit and, at the same time, causes the display unit to perform a display of the cause for the defect in the formed image or a display of the contents of the test mode to be subsequently carried out.

Abstract: After the jam is cleared or after instruction of the start of printing, in order to remove the residual toner on an endless transfer belt or a secondary transfer roller, the endless transfer belt rotates in a state that the cleaner is contacted with the endless transfer belt and the secondary transfer roller is retracted from the endless transfer belt without applying the voltage by the voltage application means, and after one or more rotations of the endless transfer belt, the endless transfer belt continues to rotate in a state that the cleaner and the secondary transfer roller are contacted with the endless transfer belt with applying the voltage by the voltage application means.

Abstract: A line width controlling method for controlling a line width of a toner image includes adjusting pulse width signals for controlling light beam emission of a laser beam generator; forming a latent patch image, comprised of plural latent line images, on a photoreceptor by irradiating a light beam from the laser beam generator; and developing the latent patch image with toner so as to form a toner patch image comprised of toner line images. The steps of forming the latent patch image and of developing the latent patch image to form the toner patch image are both repeated for a plurality of times. The toner patch image at each time of forming has a different gradation from that at other times of forming, so that a series of toner patch images is formed on the photoreceptor.

Abstract: An electrophotographic image forming apparatus is disclosed that enables both photosensitive member and developer to be used to the full limit of their service lives. The image forming apparatus selectively adjusts one or another of either a surface potential setting of the photosensitive member or a developing bias voltage in accordance with a condition of the photosensitive member and the developer which is determined based on a fog appearance in output image and value of charged potential of the photosensitive member surface.

Abstract: An electrophotographic recording apparatus including a photosensitive drum, a charging section, an exposure section, a developing section, and a transfer section. The charging section is used for recharging the photosensitive drum. In the electrophotographic recording apparatus, the photosensitive drum is recharged after a developed image is transferred to a transfer sheet. Thus, after the transfer section is stopped, the charging and developing sections remain active for at least a rotation of the photosensitive drum. Accordingly, the electric voltage level of the photosensitive drum is uniform.

Abstract: A method of regulating photo induced discharge of an imaging surface without the use of an ESV sensor by providing and developing iterative, incremental exposure patches on the imaging surface. The developed incremental exposure patches are then sensed and the image projecting system is adjusted in order to provide consistent exposure and development levels.

Abstract: A method of setting up an electrostatographic printing machine having image quality attributes and parameters that control the attributes using multivariate modeling and multiobjective optimization. The method includes providing a discrete number of parameter settings and printing test patterns based upon the parameter settings. The test patterns are scanned to produce a set of image quality values. Using a multivariate adaptive regression splines technique, a model of the printing machine image quality is provided in response to the parameter settings and the image quality values. Optimum parameter settings for the printing machine are then determined from the discrete number of parameter settings to produce consistent image quality.

Abstract: An image forming apparatus includes an image carrier on which a static latent image is formed, a main charger to charge the image carrier, an operating portion to direct the output adjustment of the main charger, and a developing device to develop a static latent image by applying developer to it. Further, an image forming apparatus has a charge eliminator arranged between the main charger and the developing device along the rotating direction of the image carrier to fully remove the charge applied to the image carrier through the exposure when the output adjustment of the main charger is directed by the operating portion, and also has a detector to detect the current flowing into the image carrier when the image carrier is charged by the main charger after the output adjustment of the main charger is directed.

Abstract: An image forming apparatus includes a generating section for generating an image signal representing an image of a predetermined density, an image forming section for forming an image on a recording medium on the basis of the image signal generated by the generating section, a measurement section for measuring the density of the image formed on the recording medium, and a control section for determining a condition for image formation on the basis of a measurement result from the measurement section. The control section selects a density of the image signal to be generated from the generating section according to the environment of the image forming section, or to an operation time of the image forming section.

Abstract: An electrostatographic printing machine having an imaging member, operating components, and a control system including a sensor, compensator, and look up table for adjusting the operating components. The sensor signal provides a suitable indication of an operating component condition such as a developer unit or a photoreceptor charging device. A compensator responds to the sensor signal to provide a non-linear adjustment signal and the look up table converts the non-linear adjustment signal to a linear adjustment signal. A device such as a charging corotron or developer power supply responds to the linear adjustment signal to appropriately adjust the operating component.

Abstract: In process control loops by keeping the DMA (Developed Mass per unit Area) under control, the print quality can be satisfactorily maintained to within tolerance in spite of temporal variabilities in subsystem parameters. The DMA is measured by creating patches in the interdocument zones. Three patches are created, one at high area coverage (90% to 100%), one at low area coverage (0 to 20%) and one at mid tone (around 50%). These DMA readings are compared to the setpoints. The errors are processed in the controller to generate the internal process parameters known as the cleaning voltage, discharge ratio and development voltage. These internal parameters have well known meaning to the physical xerographic process. The cleaning voltage is used to indicate the background in printing. The discharge ratio gives an indication of how much dot growth is present in the halftones. Finally, the development voltage is proportional to how much toner is laid on the photoreceptor.

Abstract: A method and apparatus for controlling the image density of a copy of a special document original with a large image density variation. When a special document original is to be copied, the surface potential of a photoreceptor is set to a level lower by a predetermined value than that employed for an ordinary document original copying operation. Thus, the image density of the resulting copy is comprehensively reduced, and the image density variation in the copy is moderated. Further, the amount of light emitted from the light source for illumination of the document original is set to a predetermined level adapted for a low density document original. Thus, the image density of the copy is prevented from being reduced too much. The level of development bias to be applied to a developing unit corresponding to at least a predetermined standard density level is set to a level higher by a predetermined value than that employed for an ordinary document original copying operation.

Abstract: In order to effectively perform image stabilization by control of retaining initial characteristics of a photosensitive body, if correlation between initial and aged values corresponding to bright and dark part signals of the photosensitive body is such that a variation between the initial and aged values of the dark part signal (dark .DELTA.) is greater than a variation between the initial and aged values of the bright part signal (bright .DELTA.), an image stabilizer, first, controls a charging characteristic of the aged values by changing a charging output to the photosensitive body so that the bright .DELTA. and the dark .DELTA. are equal to each other. Next, the image stabilizer makes the aged characteristics of the photosensitive body virtually identical to the initial characteristics by changing an exposure output so that the aged and initial values of the dark part signal become equal to each other.

Abstract: When a control part is restarted, an average of a stored standard operation quantity and a stored operation quantity immediately before a stop of the control part is calculated. The average operation quantity (for instance, laser power) is applied to an image output part, and a resulting control quantity (for instance, a development density) is measured. A control rule retrieval unit calculates adaptabilities of respective control rules with respect to the measured control quantity, thereby synthesizing an application control rule. A new operation quantity is determined based on the application control rule and an error of the measured control quantity from a target value.

Abstract: An exposure step input circuit inputs a desired brightness of a printing image as one of a plurality of exposure steps and a lamp application voltage adjusting unit allows the application voltage of an exposure lamp to, on the bright side of the exposure steps, be increased with a smaller predetermined slope than a dark side each time the exposure step is switched in the bright direction. At this time, a shortage of an exposure is compensated for by lowering a surface potential on a photosensitive drum through the decreasing of a charge current by a charger or raising a developing bias voltage of a developing unit. An image forming apparatus thus provided can obtain a better image at all times by, even on the bright side of the exposure steps in the adjustment of an exposure amount, preventing a lowering in density of an image, deposition of carriers, and other disadvantages.

Abstract: In a apparatus for forming a digital image with use of electrophotographic process wherein the gradation is expressed by changing the laser intensity according to image information, at least one of the surface electric potential V.sub.O of photoconductor and the development bias electric voltage V.sub.B is controlled, and the laser intensity is controlled for gradation correction to compensate a change in gradation due to the control of V.sub.O and V.sub.B. A ratio of variable differences of potential in the apparatus, (V.sub.B -V.sub.I)/(V.sub.O -V.sub.I), is kept constant to control the gamma characteristic in the apparatus.

Abstract: An electrophotographic printer produces a multi-color image on a moving photoconductor (OPC) and includes a laser/scanner arrangement for producing first and second beams to expose the OPC at spaced-apart exposure stations. The OPC is charged to a reference state prior to exposure. First and second developer stations are positioned between the first and second beam exposure stations and third and fourth developer stations are positioned after the second beam exposure station in the direction of movement of the OPC. A controller modulates the laser, during each scan, in accordance with subpixel color data from a pair of color planes to achieve a tri-level charge state (including the reference state) on the OPC. The first and second developer stations apply charged toners, selectively, in accord with the tri-level charge state produced by the scan of the first beam.

Abstract: An electrostatographic printing machine having an imaging member with a surface voltage potential and a control system having changeable set point parameters to provide a dual level of control of the voltage potential. A compensator responsive to a reference signal and the surface voltage potential provides one input signal and one level of control to a summing nod and a look up table responsive to the changing of the set point pa provides a second input signal and a second level of control to the summing node to adjust the surface voltage potential. Two levels of table look up feed forward adjustment are also provided for developer control. Within one use of some form of look up tables to move one operating point by varying nominal values a state space control model of one electrostatic printing machine is developed. Control model is used to design good process loop compensators.

Abstract: An electrostatographic printing machine having an imaging system for projecting and developing images on an imaging member. A process control loop includes a sensor to measure developed mass per unit area on at least three test patches on the imaging member including high area coverage, low area coverage, and mid tone coverage. A comparator responds to the sensor measurements and to developed mass per unit area setpoints to provide error signals. A control unit responds to the error signals to adjust projecting, developing, and imaging member subsystems.

Abstract: Imaging forming apparatus includes an image bearing member, an image formation unit for forming an image on the image bearing member, and a detection unit for detecting a voltage-current characteristic between the image bearing member and the charging member. The image formation unit has a charging member contactable to the image bearing member to charge the image bearing member. An image formation condition for the image bearing member is determined on the basis of a plurality of detection operations of the detection unit.

Abstract: When a status transition occurs, case data are collected and a new control rule is generated based on those case data. A control rule memory can store a plurality of control rules. A plurality of control rules are selectively read out from the control rule memory, and combined in accordance with their adaptabilities into an application control rule. An error between a target value and a measured control quantity is applied to the application control rule, to thereby calculate a correction value of an operation quantity. An operation part of an image forming apparatus main body is controlled based on the correction value so that the control quantity becomes the target value.

Abstract: When a CPU switches the power supply to a fusing lamp to OFF from ON state while an exposing lamp stays on, the CPU suspends a feedback phase control of the exposing lamp and carries out a phase control of the exposing lamp based on the phase control data pre-stored in a RAM for a predetermined period before and after the switching. The exposing lamp is hardly affected by the variance in voltage caused by the switching of the power supply state to the fusing lamp, and therefore can emit exposing light in a constant amount to form an image. Thus, it becomes possible to produce a satisfactory copy image with substantially no inconsistency in copy density.

Abstract: A contact type image transferring system and method incorporated in an image forming apparatus for cleaning residual toner on a transfer roller. The transfer roller is in contact with a photoconductive drum and forms a nip between the roller and the drum. A sheet of paper passes through the nip and a toner image on the drum is transferred to the sheet of paper at the nip. When the sheet of paper is not at the nip, a first transfer voltage is applied to the transfer roller for 3 to 20 seconds after a paper feed jam is corrected. The polarity of the voltage causes the toner on the transfer roller to be removed. Then a second transfer voltage is applied to the transfer roller which has a polarity which is opposite to the polarity of the first transfer voltage. The voltage difference between the transfer roller and photoconductive drum may be formed by applying bias voltages to both the photoconductive drum and the transfer roller.

Abstract: A color image forming apparatus includes a controller to control an image carrier, plural chargers, plural image exposing devices, plural developing devices so that plural color toner images are formed one after another and superimposed on the image carrier, and a regulator to regulate a deviation in position among the plural color toner images.

Abstract: An image forming apparatus provides a photosensitive body, a main charger, an exposing unit, developing unit and a contact-transfer charger. The main charger charges the photosensitive body with a predetermined electric potential. The exposing unit applies a laser beam on the surface of the photosensitive body to form an electric latent image. The developing unit develops the electrostatic latent image to form a toner image. The contact-transfer charger transfers the toner image on an image transfer paper. The image forming apparatus further provides controller for power supplies. The controller controls the power supplies after an image forming operation in a manner to enhance the electric potential of the photosensitive body to a high level by the main charger, drop the surface potential of the photosensitive body to a low level by the contact-transfer charger and then to cut off a bias voltage of the developing unit.