An electrophotographic apparatus

Abstract: An electrophotographic apparatus includes an electrophotographic photosensitive member (1); a charging member (2) contactable to the photosensitive member (1) for charging the photosensitive member (1) at a charging position; exposure means (4) for exposing the photosensitive member (1) to electrically discharge the photosensitive member (1), wherein when a region of the photosensitive member (1) discharged by the exposure means (4) is at the charging position, a voltage-current characteristic between the charging member and the photosensitive member is detected; changing means for changing an incident light quantity to the photosensitive member from the exposure means.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an electrophotographic apparatus having a charging member contactable to a photosensitive member for charging the photosensitive member. It is known to use a contact type charging device such as electroconductive roller, or an electroconductive brush, contacted to a photosensitive member, to charge it in an electrophotographic apparatus such as a copying machine or laser beam printer. A contact type charging device has an advantage over a corona charging device,provided with a shield and a wire, in the voltage reduction of voltage source and small amount of production of ozone. However, with long term use of the electrophotographic apparatus, the photosensitive layer is scraped by a cleaning blade contacted with the contact charging member, then the charge potential of the photosensitive member varies. This is a problem. For the purpose of prevention of the potential variation of the photosensitive member resulting from the decrease of the thickness of the photosensitive layer, EP-A568352 proposes that the decrease of the thickness of the photosensitive layer is predicted or estimated. In EP-A568352, for the purpose of the prediction of the thickness of the photosensitive layer, the current flowing through the charging member when a predetermined voltage is applied to the charging member contacted to the photosensitive member is detected. The current thus detected, increases with the decrease of the thickness of the photosensitive layer. Before the detection of the current flowing through the charging member, the photosensitive member is uniformly discharged by an exposure means in the form of a pre-exposure lamp. The voltage applied to the charging member during image formation is controlled on the basis of the detected current so as to provide a desired potential of the photosensitive member. However, due to a manufacturing lot difference of the respective photosensitive members, the average detected currents for the two photosensitive members having the same thicknesses of the photosensitive layers, can differ in some cases. Namely, even if the thicknesses of the photosensitive layers are substantially uniform, the photosensitivity varies with the result of variation of the average detected current despite the same light quantity applied to the photosensitive member from the pre-exposure lamp. If the average detected current varies, the device predicts erroneously that the thicknesses of the photosensitive layers are different. The erroneous prediction results in excess and deficiency of the voltage applied to the charging member during the image formation, and therefore, improper potential of the photosensitive member and therefore improper image density. It would be considered that photosensitive member having relatively significantly different photosensitivities, are not incorporated in the device, but the reduced yield has then be to accepted. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an electrophotographic apparatus whose photosensitive member can be manufactured with high yield. It is another object of the present invention to provide an electrophotographic apparatus capable of providing a proper image density. It is a further object of the present invention to provide an electrophotographic apparatus capable of stabilizing a potential of a photosensitive member even after long term use. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view, of an electrophotographic apparatus according to a first embodiment of the present invention. Figure 2 is an operation sequence diagram, of the electrophotographic apparatus. Figure 3,(a) and (b) are charging property graphs. Figure 4 is an equivalent circuit diagram formed in a microscopical space at a contact portion between a photosensitive layer and charging roller. Figure 5 is a graph of gap vs. Gap breakdown voltage. Figure 6, (a) is illustrates a contact nip between a photosensitive member and a charging roller. Figure 6(b) shows an electrical circuit representing the electrostatic capacity of the photosensitive drum and charging roller and the resistance of the charging roller. Figure 7, (a) and (b) are graphs of charging property film thickness dependence property. Figure 8 is a graph of detected voltages. Corrected voltage output. Figure 9, (a) and (b) are graphs of potential and CT layer thickness relative to the number of the processed sheets. Figure 10 shows a relation among an average detected current, a corrected lamp voltage output and an increase exposure amount of surface of the drum. Figure 11(a) and (b) are graphs of a potential and a CT layer thickness relative to the number of the sheets processed. Figure 12 shows a relation, between the current and a voltage upon voltage application to the drums in various lots and a selection region of a filter Figure 13 is a sectional view of an image forming apparatus according to embodiment 2 of the present invention. Figure 14 shows a relation between an entering amount L of the film and a before exposure amount of the photosensitive member. Figure 15 shows a relation between a before exposure amount and a current upon application of constant voltage (-1300V) to the photosensitive member in each lot. Figure 16 is a sectional view of an image forming apparatus according to embodiment 3 of the present invention. Figure 17 shows a relation between an entering amount L1 of the reflection member and a current upon -1300V application to photosensitive member in each lot. Figure 18 is a sectional view of an image forming apparatus according to embodiment 4 of the present invention. Figure 19 shows a relation between a thickness of the photosensitive layer of a general photosensitive member and a current upon -1300V application. Figure 20 shows a relation between a thickness of the photosensitive layer in the general photosensitive member and an application voltage necessary for providing the constant drum potential (-680V). Figure 21 is an illustration of a process cartridge detachably mountable relative to an image forming apparatus, according to an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1 Figure 1 is a sectional view of an image formation portion of an image forming apparatus according to the invention. Image formation portion has a cylindrical photosensitive drum 1 as a member to be charged supported rotatably on a main assembly (unshown) of the device. The photosensitive drum 1 is an electrophotographic photosensitive member of drum type having, as basis structural layers, namely an electroconductive base layer 1b of aluminium or the like electrically grounded, and a photoconductive layer (surface to be charged) 1a thereon. The entirety of the photosensitive drum 1 is rotated about a supporting shaft 1d in the arrow R1 direction at a predetermined peripheral speed. A charging member 2 is positioned above the photosensitive drum 1. The charging member 2 is contacted to the surface to be charged of the photosensitive drum 1, and functions to uniformly charge it to a predetermined polarity and potential while it is contacted to the surface to be charged 1a of the photosensitive drum 1. It is a roller type (charging roller). The charging roller 2 is provided with a centre core metal 2c, an electroconductive layer 2b formed on the outer periphery thereof, and a resistance layer 2a 2 , 2a 1 thereon. Charging roller 2-is rotatably supported at the opposite end portions thereof by unshown bearings in parallel with the photosensitive drum 1, and is press-contacted at a predetermined pressure to the surface to be charged 1a of the photosensitive drum 1 by urging means (unshown). It is rotated in arrow R2 direction by the rotation of the photosensitive drum 1 in arrow R1 direction. The core metal 2c of the charging roller 2 is contacted to an electric energy supply sliding contact 3a, and is supplied with a bias voltage from a voltage source 3 through the contact 3a. By this, the surface to be charged of the photosensitive drum 1 is charged to the predetermined polarity, and potential. The charging roller 2 may be driven by the photosensitive drum 1 or may be positively driven codirectionally or counterdirectionally relative to the surface to be charged 1a at a predetermined peripheral speed. Alternatively, it may be fixed without rotation. The photosensitive drum 1 uniformly charged by the charging roller 2, is exposed to image light of object image information by exposure means 4 by laser beam for slit exposure (slit exposure is used in this embodiment). By this, an electrostatic latent image is formed correspondingly to the intended image information on the surface to be charged. The electrostatic latent image is developed with toner by a developing device 6 into a toner image. The toner image is transferred onto a transfer material P by a transfer device 7. The transfer device 7 is provided with a rotatable transfer roller 7a and a voltage source 7b, for charging the rear of the transfer material P with an charge of the opposite polarity from the toner, so that the toner image is transferred from the photosensitive drum 1 to the transfer material P. The transfer material P has been fed from an unshown feeding device to a transfer portion between the photosensitive drum 1 and the transfer device 7 at a proper timing in synchronism with the rotation of the photosensitive drum 1. The transfer material P having received the toner image is separated from the separation, and is fed to an unshown fixing device, where the toner image is fixed thereon. Then, it is discharged to the outside of the main assembly of the device. In the case of a duplex copy, it is fed back to a refeeding means at the transfer portion. The photosensitive drum 1, after the image transfer, is cleaned by a cleaning blade 9a of a cleaning device 9 so that the deposited contamination such as remaining toner is removed, and the charge is removed by a pre-exposure device 10. The description will be made as to the charging roller 2. As shown in Figure 1, the charging roller 2 used in this embodiment, comprises a core metal 2c, an electroconductive drum layer 2b of EPDM or the like having a volume resistivity of 10 4 -10 5 Ohmcm, an intermediate resistance layer 2a 2 of hydrin rubber or the like thereon having a volume resistivity of 10 7 -10 9 Ohmcm approx. and a surface layer (blocking layer 2a 1 ) having a volume resistivity of 10 7 -10 10 Ohmcm, made of a Nylon type substance such as Torejin (trademark of Teikoku Kagasku Kabushiki Kaisha, Japan). It has a hardness of 50-60 approx. as measured in Asker-c measurement. The charging roller 2 is contacted to the photosensitive drum 1 at a total pressure of 1600 g so as to be driven by the photosensitive drum 1. The resistance of the charging roller 2 is preferably such that when the photosensitive drum is replaced with an aluminium drum, and the charging roller is supplied with a voltage of 300V, it is 10 5 -10 7 Ohm per nip of 1cm 2 between the charging roller and the drum. At the upstream and the downstream of the contact portion N formed between the surface to be charged 1a of the photosensitive drum 1 and the charging roller 2, a first gap G 1 and second gap G 2 are formed, respectively. The gap G 1 and G 2 formed between the surface to be charged 1a of the photosensitive drum 1 and the outer peripheral surface of the charging roller 2 are such that while the first gap G 1 which is upstream with respect to the movement direction (arrow R1 direction) of the surface to be charged 1a, gradually decreases toward the contact portion N, the second gap G 2 which is downstream gradually increases away from the contact portion N. The contact portion N is formed substantially over the entire length (axial direction), and therefore, the first gap G 1 and second gap G 2 are formed over the entire length of the charging roller 2. Figure 2 is an example of an operation sequence of the device of Figure 1. This example deals with a continuous print on two transfer materials. In Figure 2, during drum rotation period (device warming-up period) executed upon the actuation of the voltage source for the purpose of image fixing device temperature rise or the like, the charging roller 2 is subjected to a DC constant voltage control, during which DC current detection is effected to the charging roller 2. During the warming-up period, drum 1 is discharged substantially to OV by the pre-exposure device. By the detection of the current through the charging roller 2, the current from the charging roller 2 to the drum 1 can be detected, so that the thickness of the photosensitive layer can be estimated. After the completion of the warming up, rotation of the drum and the discharging exposure by the pre-exposure device 10 are stopped, and the subsequent stand-by state is maintained until a print start signal is produced. Upon print (copy) start signal, the rotation of the photosensitive member 1 is started so that the pre-rotation period starts. Simultaneously with the rotation start of the drum 1, the discharging exposure lamp 15 is turned on, so that one full circumference or more of the drum 1 is discharged. In the period C1, the charging roller 2 is subjected to a constant voltage control with a corrected DC voltage determined on the basis of the current through the charging roller 2 detected during the warming-up period. At this time, the drum 1 is charged by the charging roller 2 for the image formation. Thus, the charging roller 2 charges the image formation region in which the toner image can be formed, when the drum 1 is rotated. It is preferable that the timing of the detection of the current through the charging roller 2 is such that it is for the non-image region in which the toner image is not formed. The current detection operation may be carried out during the pre-rotation period which is before the charging operation for the image formation and after the production of the print start signal. Upon start of the constant voltage control for the charging roller with the corrected voltage, the image formation is carried out for the first sheet with the image exposure. Upon the charging roller 2 of the image formation for the second print, the drum 1 is subjected to a post-rotation period, during which one full circumference or more of the drum 1 is discharged by the discharging exposure lamp 10, and the rotation of the drum 1 and the discharging exposure are stopped. Then, the apparatus is placed under the stand-by state until the input of the next print start signal. With the above-described structure, when the drum surface is scraped by the operations so that the photosensitive layer thickness is reduced, the detected DC current in the DC constant voltage control period B1 when the charging roller 2 is opposed to the surface of the non-image-formation region, increases, and the drum 1 is charged by the charging roller 2 under the charging roller DC constant voltage control with the corrected voltage based on the decreased voltage due to the increase of the detected DC current. Thus, the drum 1 is charged to the potential. substantially equal to that at the initial stage where the drum is not scraped. If the resistance of the charging roller 2 increases under a low humidity ambience (for example, 15°C, 10%), the detected DC current during the charging roller DC constant voltage control in the period B1, decreases. The charging by the charging roller 2 is carried out under the charging roller DC constant voltage control with the increase-corrected voltage in accordance with the detected DC current, and therefore, the k0 charge potential of the drum 1 is constant despite the resistance variation of the charging roller 2 due to the ambience change. The DC current and the corrected voltage thus detected is held until the voltage source of the image forming apparatus is shut off. For the purpose of the image density stabilization, it is effective to carry out the detection once a day, for example, the first start in the morning. When, for example, the voltage source of the device is stopped for a short period to permit jammed sheet clearance, the current detection is carried out again, and the corrected voltage is renewed upon the reactuation of the voltage source. Therefore, it is probable that the corrected voltages before and after the deactuation of the voltage source are different from each other depending on the detection accuracy of the current. If even small amount of the change in the corrected voltage in a short period, the operator feels strange, and therefore, the temperature adjustment value is reset upon the image formation operation. In order to improve the operativity of the image forming apparatus, the charging roller constant voltage application, the current detection and the corrected constant voltage control are carried out at the time of start up first in the morning, and the corrected constant voltage is maintained during the same day. As the method for discriminating the "first in the morning", the results of practical tests exhibit that it is recognized when the fixing roller detected temperature is lower than a predetermined level. The predetermined temperature is 30°C-130°C, further preferably approx. 100°C. Voltage correction method Next, the description will be made as to a charging method using a DC voltage source 3. First, the charging mechanism when the charging roller 2 is supplied with a DC voltage from the DC voltage source. The used photosensitive member 1 is an OPC photosensitive drum of negative type. More particularly, it comprises a photosensitive layer which includes a CGL layer of azo- pigment (carrier generating layer), a CTL layer (carrier transfer layer) of a mixture of hydrazone and resin material of a thickness of 24µm (negative organic semiconductor layer (OPC layer) thereon. Such a OPC photosensitive drum 1 is rotated, and the charging roller 2 is contacted to the surface thereof. The charging roller 2 is supplied with DC voltage V DC , and is contacted to the OPC photosensitive drum 1 in the dark to charge it. The investigation was made as to the relation between the surface potential V D of the OPC photosensitive drum 1 and the applied DC voltage to the charging roller 2. In Figure 3, the line " 24µm " indicates the results of the investigation. The charging includes a threshold for each drum film thickness relative to the application DC voltage V DC , as will be understood from Figure 3, (a). The charge starting voltage is defined as follows. An image bearing member having a 0 potential is charged by a charging member supplied only with a DC voltage, and the DC voltage is gradually increased. Application DC voltages. The surface potential of the photosensitive member as the image bearing member is plotted on a graph. The data are taken for each 100V of the DC potential. The DC potential point corresponds to the first surface potential appearing on the surface, 10 points are obtained therefrom. From the 10 plots, a straight line is drawn using least square approximation in statistics. The value of the application DC voltage at surface potential 0 on the line is defined as the charge starting voltage. The line on the graph of Figure 3 is made on the basis of the least square approximation. Thus, the DC application voltage V DC to the charging roller 2 and the surface potential on the OPC photosensitive drum 1 surface and the charge starting voltage V TH are related as follows: V D =V DC -V TH This equation can be from Paschen (Paschen) law. Figure 4 shows an equivalent circuit constituted by the charging roller 2, OPC photosensitive layer and the microscopical space Z in the contact portion therebetween. When the total resistance R r of the charging roller 2 is small, the voltage drop I D Rr resulting from the current I D into the photosensitive layer 1a is so small as compared with V DC that it can be ignored. If R r is neglected, the voltage Vg across the space Z is: Vg=V DC x Z/ (L S /K S + Z ) V DC : application voltage Z: gap L S : photosensitive layer thickness K S : photosensitive layer dielectric constant From Paschen law, the discharge breakdown voltage Vb can be approximated by the following first order equations (3) and (4) when the z=8 µm larger: Vb=312+6.2Z (Vb>0) Vb=- (312+6.2Z) (Vb<0) Since Vb<o, equations (2) and (4) are expressed in Figure 5. The abscissa represents gap distance Z, and the ordinate represents a gap breakdown voltage. The convex-down curve (1) is the Paschen curve, and convex-up curves (2)-(4) are properties of the gap voltage Vg with the parameter z. When the curve (1) and the curve (2)-(4) intersect, the discharge occurs. At the point of the discharge start, a discriminant obtained by Vg=Vb is 0. This is the discharge start limit, and therefore, V DC =V TH . The Paschen law is related to the discharge development in a gap. In the charging process using the charging roller 2, ozone is produced although the amount is small in the proximity of the charging portion (10 -2 -10 -3 , as compared with the corona discharge), and therefore, the charging by the charging roller is considered as involving the discharge development. Therefore, in order to control V D by V DC , The use is made with V DC =V R +V TH V R : target surface potential The potential target value V R is set, and V TH is obtained by equation (5), and is added thereto, by which the V D approaches V R . As will be understood from equation (5), threshold voltage V TH is determined by: D=L S /K S The dielectric constant K S of the photosensitive layer changes due to the temperature, humidity or the like around the photosensitive member, and the thickness L S of the photosensitive layer decreases with the use. Thus, the surface potential V D changes with the change of threshold voltage V TH depending on the circumference ambience or the degree of use. In other words, if the K S and L S are known, the DC corrected voltage V DC for providing the proper value of the surface potential V D can be determined. Here, the electrostatic capacity C P of the photosensitive drum 1 plus charging roller 2, as shown in Figure 6 (a) and (b), is provided by the nip n at the contact portion between the photosensitive drum 1 and the charging roller 2. When the contact area at the nip is S P , the following results from the equivalent circuit of Figure 6, (b). C P =S P xK S /L S =S/D Namely, C P is proportinal to 1/D. So, if C P is obtained, proper DC voltage V DC can be obtained by equation (5). In this embodiment, in place of determining the C P of the drum(photosensitive member), the simple measurement is effected for the change of the charging property due to the change of the discharge impedance due to the film thickness of the charge transfer layer (CT layer) (L S )of the drum, as shown in Figure 6, by which the change of the photosensitive member C P is estimated, and the application voltage is corrected. Figure 3, (a) is plots of the application voltage to the charging roller 2vs. The drum surface potential for each drum CT layer thickness. Similarly, the DC current at the time is given in Figure 3, (b). As will be understood from this Figure, the charging property, voltage/current property and discharge start voltage change depending on the drum CT layer thickness. The properties are expressed in Figure 7, (a), (b), as the drum surface potential and the DC current for the drum CT layer thickness upon a given constant voltage application. The relation between the drum surface potential and DC current in accordance with the CT layer thickness can be seen. With increase of the CT layer thickness, the drum surface potential (black portion potential V D and white potential V L ) and the DC current increase. By measuring the DC current upon a specific constant voltage application, the surface potential can be estimated or predicted in accordance with the drum C P . Figure 8 show a relation between the average detected current and the corrected voltage output for the control of the drum surface potential even if the C P change occurs due to the drum CT layer thickness change, on the basis of the above analysis. The correction is effected such that the voltage output decreases with the increase of the average detected current. Figure 9, (a) and (b) shows the experiment result with the use of the correction. Abscissa represents the number of processed sheets ("K" means 1000 sheets) namely the number of image forming operations, and the change of the drum surface potential is shown. The surface potential change when a specified constant voltage is applied always to the charging member irrespective of the number of image forming operations, is shown by L: When the DC current upon the constant voltage application to the charging member is detected, and the corrected constant voltage application in accordance with the detected current is effected (the charging power of the charging member is determinated in accordance with the average detected current), a constant drum surface potential can be maintained despite the increase of the number of sheets processed, as indicated by M. In these experiments, the OPC photosensitive drum 1 described above was used. Using the image forming apparatus shown in Figure 1, a large number of sheets are processed. It is preferable to increase the voltage applied to the original illumination lamp 4 to increase the image exposure amount in accordance with the increase of the average detected current. Under the low humidity ambience, the resistance of the charging roller 2 increases, and therefore, the detected DC current during the charging roller DC constant voltage control in the period B1 and the period B2 decreases. Under the charging roller DC constant voltage control with the increase-corrected voltage in accordance with the detected DC current, the image formation region of the drum 1 is charged by the charging roller 2, and the image formation is carried out by the exposure using the corrected lamp voltage, and therefore, the k0 charge potential of the drum 1 is made constant despite the resistance variation under the ambience around the charging roller 2. Figure 10 shows an interrelation among the average detected current, corrected lamp voltage output applied to the original illumination lamp 4 for the image exposure, and the increased exposure amount for the drum surface. Figure 11, (a), (b), show the experiment results with this correction. The abscissa is the number of the processed sheets, namely, the number of image forming operations, and the change of the drum surface potential is indicated. The surface potential change when a specified constant voltage is applied always to the charging member despite the film thickness decrease of the photosensitive member, is indicated by dark portion potential V D and L, light portion potential V L being O. When the DC current upon the constant voltage application to the charging roller is detected, and the application voltage to the charging roller is corrected in accordance with the detected current, and the constant voltage control is effected with this voltage. As shown by M, P, a constant drum surface potential can be provided despite the increase of the number of the processed sheets. Additionally, the voltage applied to the image exposure lamp is increased in accordance with the increase of the average detected current to increase the exposure amount, by which the light portion potential decreases to Q, and as a result, the dark portion potential V D is Q, and light portion potential V L is Q. By the decreasing tendency control for the dark portion potential V D , the rising ratio of the dark portion potential V L is suppressed, and the width of the exposure amount change can be suppressed to a low level. With the increase of the average detected current of the charging roller beyond the predetermined value, the film thickness of the photosensitive member is discriminated as being reduced from the initial thickness. As shown in Figure 11, (a), by M, the corrected voltage for the charging roller is determined such that the dark portion potential decreases with the decrease of the film thickness. The charging roller is constant-voltage controlled by the charging roller with this corrected voltage, so that the portion on which the image is going to be formed is charged to V D . As shown in Figure 10, when the average detected current is not higher than a predetermined value (60µA), the lamp light quantity is made constant. Namely, it is discriminated that the film thickness change of the photosensitive member does not occur, and the resistance change of the charging roller occurs. The charging member 2 may be a roller type, blade-like type, block-like type, rod-like type, beltlike type or the like. The description will be made as to means 30 for changing the light quantity from the pre-exposure device 10 to the photosensitive member 1. As for the changing means 30, color filters having various transmission factors are inserted between the pre-exposure device and the photosensitive member, as an example. The color filters have light transmittance of 50%, 70%, 90% (three filters). In Figure 12, three photosensitive members (lot A-C) having different photosensitivities and the same thickness were prepared. The voltage applied to the charging member 2 is changed, and the current through the charging member 2 is detected. Before the measurement of the current, the photosensitive member is uniformly exposed without the filter. The filter used actually in the device was selected from the three filters on the basis of the current at the initial stage of use of the photosensitive member when the constant voltage (-1300V) is applied to the charging member, as shown in Figure 2. As shown in Figure 12, the filter having the transmission factor of 70% is used for the photosensitive member (lot B) having current I of -38--42µA upon -1300V application, and the filter having the transmission factor of 90% is used for the photosensitive member (lot C) having -38 micro-ampere or large, and the filter having the transmission factor of 50% is used for the photosensitive member (lot A) having -42 micro-ampere or smaller. With the increase of the average detected current, the light quantity incident on the photosensitive member by the pre-exposure device 10 is reduced. To permit insertion of the filter, the pre-. exposure light quantity is increased by approx. 50% as compared with usual case. Thus, the average detected current at the initial stage is surely made equal for each of the photosensitive members (lot A-C) having the same thicknesses and different sensitivities. By changing the light transmittance of the filter for each photosensitive member in this embodiment, the average detected current value I, at the initial stage upon -1300V application and the drum potential V D provided by the charging roller 2 for the image formation are converged to I=35±2µA, V D =-680± 30V, respectively. The other conditions are set so as to provide proper images by -680V of the drum potential. The filter may be selected in accordance with the lot of the photosensitive member used in the device, from three kinds of filters. When the incident light quantity to the photosensitive member by the pre-exposure device is made the same irrespective of the kind of the photosensitive member without use of the filter, as in the prior art, I=-35±4µA, V D =-680±70V result, and several of the drums exhibit slightly darker or thinner images Embodiment 2 The description will be made as to a method of insertion of the filter according to another embodiment of the present invention. The structures and operations other than the filter are the same as in embodiment 1, and therefore, the detailed description thereof is omitted. In this embodiment, as shown in Figure 13, the entering amount L of the filter by the changing means 30 is made variable. Figure 14 shows a relation between the entering amount L of the filter and the pre-exposure light quantity on the surface of a new photosensitive member by the pre-exposure device. Figure 15 shows a relation between the light quantity on the new photosensitive member by the pre-exposure device and the current through the charging member when a voltage of -1300V is applied to the charging member. Prior to the detection of the current through the charging member, the photosensitive member is uniformly exposed by the pre-exposure device. As shown in Figure 15, the entering amount L of the filter is adjusted so that the'current through the charging member is constant (-35µA) despite the (lot A-C) of the new photosensitive members. More particularly, the thickness of the photosensitive layer of the photosensitive member at the initial stage is discriminated as being the same despite the kind of the photosensitive member. A filter having a transmission factor of 70% was used. By changing the entering amount of the filter for each photosensitive member in this manner, the current and the voltage are converged to I=-35± 0.5µA, V D =-680±10V, so that better images can be provided than in embodiment 1. The cost can be reduced because only one kind of the filter is required. In embodiments 1 and 2, as shown in Figure 16, there is not reflection member 31 for inserting the light into the gap G 1 at the upstream with respect to the motion of the photosensitive member surface to improve the charging property. More particularly, in the embodiment 2, I=-35±5µA, V D =-680±80V without the filter, but they can be converged to I=-35±0.5µA, V D =-680±10V. In the prior art, the photosensitive members with which I<-39.5µA, I>-30.5µA, are deemed as rejects. But, this embodiment is effective to improve the yield. Embodiment 3 Another embodiment of the changing means for changing the light quantity incident on the photosensitive member, will be described. In this embodiment, in place of using the filter in embodiments 1 and 2, the use is made with a reflection member 31 for reflecting the light emitted from the pre-exposure device toward the gap G 1 at the upstream side. The other structures and operations are the same as in embodiment 1, and therefore, the description is omitted. In this embodiment, as shown in Figure 16, the length L1 of the reflection portion 31 of the reflection member for improving the charging uniformity is made variable, so that the nip exposure amount to the first gap surface G 1 is made variable. Figure 17 shows a relation between the length of the reflection portion and the current through the charging member upon application of constant voltage (-1300V) to the charging member. Similarly to embodiment, L1 is set such that the current through the charging member is the same (-35µA) for each photosensitive member. By doing so, the initial current and drum potential of a new photosensitive member can be made to be I=-35±0.5µA, V D =-680±10V. In this embodiment, there is no need of using transparent material such as a filter, and therefore, the control is simple. In place of changing the length of the reflection portion, the angle of the reflection portion may be changed, or the reflectance of the reflection portion may be changed. These modification provide the similar advantages. In another modification, the reflection member is made light transmissive, and the light transmitted through the reflection member and the light reflected by the reflection member among the light rays emitted by the pre-exposure device may be incident on the photosensitive member. Embodiment 4 The description will be made as to a further embodiment of the changing means for changing the light quantity incident of the photosensitive member. In this embodiment, in order to adjust the pre-exposure amount by the light incident on the photosensitive member, the turn on voltage 32 of the pre-exposure lamp 10 is made variable in accordance with the photosensitive property of the photosensitive member used, thus permitting control of the exposure amount of the surface of the photosensitive member. The other structures and operations of the electrophotographic apparatus are the same as in embodiment, and therefore, the description will be omitted. In this embodiment, the initial current and potential of the photosensitive member are converged to I=-35±0.5µA, V D =-680±10V. - This embodiment is effective irrespective of the presence or absence of the reflection member for improving the charging property. In this embodiment, only electrical control is used without the use of additional members, and the structure is simple. The foregoing embodiments may be used in combination. In embodiments 1-4, the use is made with a photosensitive member having a diameter of 30mm, and a length of 320mm, which was rotated at a process speed of 100mm/sec. Figure 21 shows an example, wherein a filter having a light transmittance of 70% as the changing means 30 is press-contacted to a top surface of the cleaning device 9, wherein the entrance amount of the filter is changeable. The changing means, the photosensitive member, the charging member, the cleaning device, and the developing device are constituted into a process cartridge 100 as an unit. The process cartridge 100 is detachably mountable relative to the electrophotographic apparatus. By doing so, even if the photosensitive member is damaged, or the service life thereof is reached, the proper images can be produced by simply replacing the cartridge without adjustment of the main assembly. In place of making variable the entering amount, the light transmittance of the filter may be changed in accordance with the kind of the photosensitive member in the cartridge. It is a possible alternative to provide two kinds of cartridges, one with the filter and the other without the filter. The process cartridge may be provided with a reflection member as described in embodiment 3, and the structure of the reflection member may be changed in accordance with the photosensitive property of the photosensitive member. In this embodiment, the changing means, the photosensitive member, the charging member, the cleaning device, and developing device or the like are integral as an unit cartridge. However, the same advantageous effects can be provided if the at least the changing means and the photosensitive member are contained in the cartridge. Preferably, the cartridge comprises also at least one or more of the charging member, the cleaning device and the developing device. In all of the embodiments, when the photosensitive member is changed with a fresh one, the light quantity incident on the photosensitive member may be changed in accordance with the property of the fresh photosensitive member by using filters, entering amount, reflection member of the filter, the reflection property of the reflection member, the application voltage for the pre-exposure lamp. When the photosensitive member is exchanged, the kind of filter, entering amount of the filter, the reflection property of reflection member, the application voltage for the pre-exposure lamp may be selected on an operation panel or the like on the electrophotographic apparatus in accordance with the property of the new photosensitive member determined beforehand, by the user or serviceman. When the photosensitive member is in the process cartridge, the kind of the filter, the entrance amount of the filter, the reflection property of the reflection member is determined preferably beforehand, in accordance with the property of the new photosensitive member. By doing so, the user does not need to input the kind of the filter, entering amount of the filter, reflection property of reflection member for each exchange of the process cartridge. In all of the foregoing embodiments, in order to discriminate the thickness of the photosensitive layer, the constant voltage control is carried out with a predetermined voltage to the contact charging member, and the current flowing through the contact charging member is detected. In another alternative, a constant current control is effected to the contact charging member with a predetermined current, and the voltage inputted to the contact charging member may be detected. In a further alternative, the charging member may be subjected to a constant current control during image formation. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An electrophotographic apparatus comprising: a contact-type charging member (2) for charging an electrophotographic photosensitive member (1) whilst arranged in contact therewith; detecting means (3) for detecting the voltage-current characteristic of the electrophotographic photosensitive member whilst said charging member is arranged in contact therewith; correction means (3) responsive to said detecting means, and which is co-operable with said charging member to correct the amount of charge applied by said charging member in dependance upon the detected voltage-current characteristic; image exposure means (4) for exposing the electrophotographic photosensitive member, charged by said charging member, to image defining light, to form an electrostatic image; developing means (6) for developing the electrostatic image to form a developed image; transfer means (7) for transferring the developed image onto a transfer material (P); cleaning means (9) for cleaning the electrophotographic photosensitive member following transfer; and pre-exposure means (10), including a light source, for exposing the electrophotographic photosensitive member to light of a fixed amount for electrically discharging residual potential of the electrophotographic photosensitive member preceding each next charging operation performed by said charging member; characterised by : variable changing means (30;31;32), co-operable with said pre-exposure means (10), for setting at installation of the electrophotographic photosensitive member (1) and at replacement with a new electrophotographic photosensitive member (1), the fixed amount of light for pre-exposure by said pre-exposure means, according to the respective photosensitivities of respective installation and replacement electrophotographic photosensitive members (1) available for use in said electrophotographic apparatus. Apparatus according to claim 1 including the installation or replacement electrophotographic photosensitive member (1). Apparatus according to claim 2 wherein the fixed amount of light for pre-exposure is of set amount set according to the photosensitivity of the included installation or replacement electrophotographic photosensitive member (1). Apparatus according to any preceding claim wherein said variable changing means comprises a set of interchangeable light transmissive filters (30) having each a respective different transmissivity, which are selectable for setting the fixed amount of light for pre-exposure to said respective different amounts. Apparatus according to any preceding claim 1 to 3 wherein said variable changing means comprises a light transmissive filter (30), the light transmissivity of which is variable and selectable for setting the fixed amount of light for pre-exposure to said respective different amounts. Apparatus according to any preceding claim 1 to 3 wherein said variable changing means comprises a light transmissive filter (30) the position of which is adjustable in the path of light from said pre-exposure means (10) to vary the entrance amount (L) thereof, for setting the fixed amount of light for pre-exposure to said respective different amounts. Apparatus according to any preceding claim 1 to 3 wherein said variable changing means comprises a light reflective member (31) which is to reflect light from the light source of said pre-exposure means (10) towards the contact portion (N) of said charging member (2) and the electrophotographic photosensitive member (1), wherein any of the reflective width, light reflection angle, or reflectance thereof is variable and selectable for setting the fixed amount of light for pre-exposure to said respective different amounts. Apparatus according to any preceding claim 1 to 3, wherein said variable changing means comprises a voltage controller (32) to change and select the voltage applied to the light source of said pre-exposure means (10) for setting the fixed amount of light for pre-exposure to said respective different amounts. Apparatus according to any preceding claim wherein said variable changing means (30;31;32) is cooperative with said detecting means (3) to set the fixed amount of light for pre-exposure to said respective different amounts on the basis of the voltage-current characteristic detected by said detecting means at installation or replacement, which voltage-current characteristic is dependant upon the photosensitivity of the respective installation or replacement electrophotographic photosensitive member (1). Apparatus according to any preceding claims 1 to 8 including an operation panel, co-operable with said variable changing means (30;31;32), for setting the fixed amount of light in accordance with a measured photosensitivity or a related property of each replacement electrophotographic photosensitive member (1) determined beforehand. Apparatus according to any preceding claim wherein said detecting means (3) is operable to measure the charging current at constant voltage as means of detecting the voltage-current characteristic of the electrophotographic photosensitive member (1). A method of manufacturing or of maintaining the electophotographic apparatus of claim 1, comprising steps of: providing an installation or replacement electrophotographic photosensitive member (1); incorporating said installation or replacement electrophotographic member (1) in said apparatus; and using said variable changing means (30;31;32) to set, at installation of the installation electrophotographic member (1) or at replacement with the replacement electrophotographic member (1), the fixed amount of light for pre-exposure by said exposure means (10) to an amount in accordance with the photosensitivity of the installation or replacement electrophotographic photosensitive member (1). A method of maintaining the electrophotographic apparatus of claim 1 wherein the method of claim 12 is performed repeatedly, once at a time for each of a plurality of replacement electrophotographic members (1) having respective different photosensitivities, which method includes setting at each replacement the fixed amount of pre-exposure light to respective different amounts therefor, each in accordance with the respective different photosensitivities thereof. An electrophotographic apparatus comprising: an electrophotographic photosensitive member (1); a contact-type charging member (2) for charging said electrophotographic photosensitive member (1) whilst arranged in contact therewith; detecting means (3) for detecting the voltage-current characteristic of said electrophotographic photosensitive member whilst said charging member is arranged in contact therewith; correction means (3) responsive to said detecting means, and which is co-operable with said charging member to correct the amount of charge applied by said charging member in dependance upon the detected voltage-current characteristic; image exposure means (4) for exposing said electrophotographic photosensitive member, charged by said charging member, to image defining light, to form an electrostatic image; developing means (6) for developing the electrostatic image to form a developed image; transfer means (7) for transferring the developed image onto a transfer material (P); cleaning means (9) for cleaning the electrophotographic photosensitive member following transfer; and pre-exposure means (10), including a light source, for exposing the electrophotographic photosensitive member to light of a fixed amount for electrically discharging residual potential of said electrophotographic photosensitive member preceding each next charging operation performed by said charging member; characterised by : changing means (30;31) comprising a light transmissive filter (30) or a light reflective member (31) arranged in said apparatus for intercepting light in a path between said light source of said pre-exposure means (10) and said electrophotographic photosensitive member (1), defining a set amount of light as said fixed amount of pre-exposure light for the pre-exposure of said electrophotographic photosensitive member (1); and wherein said changing means (30;31) and said electrophotographic photosensitive member (1) are both incorporated in a process cartridge (100) detachably mounted within said apparatus. An apparatus according to claim 14 wherein said process cartridge also incorporates at least one of, or a combination of any of, said charging member (2), said developing means (6) or said cleaning means (9). An apparatus according to claim 15 wherein said process cartridge includes said cleaning means (9) and said light transmissive filter (30) or said light reflective member (31) is mounted thereon. A process cartridge (100) for replacing the detachably mounted process cartridge (100) of said electrophotographic apparatus of claim 14, incorporating a replacement electrophotographic photosensitive member (1) and changing means (30;31), comprising a light transmissive filter (30) or a light reflective member (31), arranged in said process cartridge (100) to be arranged in said apparatus when said replacement process cartridge is mounted therein for intercepting light in a path between the light source of the pre-exposure means (10) of said apparatus and the replacement electrophotographic photosensitive member (1), to redefine the set amount of light as the fixed amount of pre-exposure light for pre-exposure. A process cartridge (100) according to claim 17 incorporating also at least one of, or a combination of any of: a replacement charging member (2); a replacement developing means (6); or a replacement cleaning means (9). A process cartridge (100) according to claim 18, incorporating said replacement cleaning means (9), wherein said light transmissive filter (30) or said light reflective member (31) is mounted thereon. A stock of process cartridges (100) each as claimed in any of the preceding claims 17-19 each for use as a replacement in the apparatus of claim 14, at least some of which differ in respect of the photosensitivity of respective electrophotographic photosensitive members (1) and any one or more of: the respective transmissivity of the respective co-incorporated transmissive filters (30); the respective arrangements determining the entrance amounts (L) of the co-incorporated transmissive filters (30); the reflective width, orientation reflectivity of respective co-incorporated reflective members (31) matched thereto. A method of maintaining the apparatus of claim 14 performed by replacing the process cartridge (100) mounted therein with a process cartridge (100) according to any preceding claim 17-19, or selected from the stock of process cartridges (100) according to claim 20, wherein the replaced process cartridge (100) and the replacement process cartridge differ in respect of the photosensitivity of respective electrophotographic photosensitive members (1) and any one or more of: the respective transmissivity of the respective co-incorporated transmissive filters (30); the respective arrangements determining the entrance amounts (L) of the co-incorporated transmissive filters (30); the reflective width, orientation reflectivity of respective co-incorporated reflective members (31) matched thereto.

Inventors: NOGUCHI TAKAHIRO, YAMAMOTO TAKEO, NOGUCHI, TAKAHIRO, YAMAMOTO, TAKEO, Noguchi, Takahiro, c/o Canon Kabushiki Kaisha, Yamamoto, Takeo, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G15, G03G21