Electrostatic recorder and electrostatic latent image measuring instrument

Abstract: An electrostatic recorder is provided with an electrostatic latent image measuring instrument (4c) for measuring a state of an electrostatic latent image formed on a photosensitive substance (6), and executes a printing process until an electrostatic latent image formed on a surface of a photosensitive substance (6) based on measured data is transferred onto a blank form as a visual image by means of a transfer device by adjusting control factors such as exposure, exposure time, electrostatic charge voltage, development bias, temperature and humidity. Further, the electrostatic latent image measuring instrument (4c) is provided with distance sensors (1), which when a measuring electrode (3a) is made close to the photosensitive substance (6), keeps the distance therebetween constant. It is thus possible to measure an electrostatic latent image even if the photosensitive substance (6) is eccentric. Furthermore, spark between the measuring electrode (3a) and the photosensitive substance (6) is prevented by filling with a gas having a high spark starting voltage between the measuring electrode (3a) and the photosensitive substance (6) of the electrostatic latent image measuring instrument (4c).

Description: BACKGROUND OF THE INVENTION The present invention relates to an electrostatic recorder provided with an electrostatic latent image measuring instrument, and more particularly to an electrostatic recorder controlled in accordance with results of measurement of an electrostatic latent image, and an electrostatic latent image measuring instrument capable of measuring directly an electrostatic latent image formed on a photosensitive substance. In general, the exposure, the development bias and the like have been heretofore determined at predetermined design values having sufficient allowance, and the fine adjustment has been made by an operator for every device. An electrostatic recorder is known that controls an electrostatic charge system, an exposure system and a development system by means of a fatigue correction program based on the number of printed sheets as fatigue correction of a photosensitive substance (JP-A-62-27390). Further, an electrostatic recorder is known that determines the life of a photosensitive substance by the number of printed sheets and the like (JP-A-62-505). In addition, electrostatic images have been measured by methods such as ion motion, DC amplification, a revolving sector method, an electro-optical effect, electric power (Kazutoshi Asano: Electric Field and Potential Measuring Method in Electrostatic Engineering, Journal of Institute of Electrostatic Engineers, 10, pp. 205 - 212 (1986)) and the like, as a method of measuring an electrostatic latent image. JP-A-60-254061 relates to an electrostatic recorder according to the preamble of claim 1, in which a surface potential sensor is employed to detect the potential of an electrostatic latent image formed on a photosensitive drum, so as to maintain a stable record density. Since an electrostatic latent image on a photosensitive substance has several hundreds to several tens µm in size, an electrostatic latent image measuring technique having sufficiently high resolution is required to measure the distribution of the electrostatic latent images. In order to improve resolution, the area of a measuring electrode has to be made small, and the measuring electrode and the photosensitive substance have to be made come close to each other. Several reports in which an electrostatic latent image has been measured by using an electrode having a very small measurement area have been proposed already. (Mitsuru Matsui: Very Small Area Surface Electrometer, Journal of Institute of Electrostatic Engineers, 10, pp. 217 - 224 (1986)). In a report it has been proposed to measure the electrostatic latent image by an electron microscope using a measuring electrode (G. F. Fritz, D.C. Hoesterey and L.E. Brady: Observation of Xerographic Electrostatic Latent Images with a Scanning Electron Microscope, Appl. Phys. Lett. 19, pp. 277 - 278 (1971)). Since there has been no means for observing an electrostatic latent image in an electrostatic recorder, it is difficult to carry out the fine adjustment for adapting the device to an individual photosensitive substance, and it is also difficult to fully demonstrate the performance of the photosensitive substance in the case of the conventional electrostatic recorder. Further, it is impossible to accurately grasp the life of a photosensitive substance since the electrostatic latent image cannot be observed whether the latent image is present or not. Furthermore, no consideration has been given to measurement of an electrostatic latent image in an electrostatic recorder in the conventional electrostatic latent image measuring instrument in which the area of a measuring electrode is made small. There is eccentricity and the like in a photosensitive drum on an electrostatic recorder, which has made it impossible to have a measuring electrode come close to a photosensitive substance at a very small distance. Moreover, the electric potential of an electrostatic latent image on a photosensitive substance sometimes reaches close to 1 KV, thus causing such a problem that discharge occurs when the measuring electrode is brought close to the photosensitive substance. Further, the above-mentioned conventional devices for measuring an electrostatic latent image using the electron microscope are incapable of measuring a latent image on a photosensitive substance of the electrostatic recorder due to special measuring environment. SUMMARY OF THE INVENTION It is an object of the present invention to provide an electrostatic recorder capable of carrying out optimum control which is suited to a state of a photosensitive substance. This object is achieved by an electrostatic recorder according to claim 1. The electrostatic recorder of the present invention has a measuring electrode for measuring an electrostatic latent image that can come close to a photosensitive substance even in case of eccentricity of the photosensitive substance. For this purpose, means are provided for controlling a distance between the measuring electrode having a very small area and the photosensitive substance to a constant value. Moreover, for the purpose of preventing spark discharge between the measuring electrode and the photosensitive substance, means can be provided for setting a common electric potential of a measurement circuit for measuring an electrostatic latent image to a reference power source potential between the highest potential and the lowest potential of the electric potential of a measurement object. Furthermore, means can be provided for introducing a gas having a high spark starting potential between the measuring electrode and the photosensitive substance. It is possible to detect a state of an electrostatic latent image on the photosensitive substance by providing electrostatic latent image measuring means. The printing process and the latent image are optimized by carrying out appropriate control which is suited to the state of the photosensitive substance. Further, it is also possible to accurately determine the life of the photosensitive substance because the state of the latent image is observed. It is possible to identify the width of charge pattern formed by an exposure portion and a non-exposure portion and the state of the electrostatic latent image formed with repetitive dot pattern which repeats on and off for every several dots to several tens of dots, and also, the repetitive dot pattern which repeats on and off for every single dot with each other as an exposure pattern for measuring latent image. Since exposure, exposure time, development bias and photosensitive substance heating time are controlled based on the state of the electrostatic latent image, it becomes possible to carry out optimum control of the printing process. Further, the life of the photosensitive substance can be detected accurately from determining the state of the electrostatic latent image. Plural distance measuring means are arranged on concentric circles with the measuring electrode as the center so that the spacing between the plural distance measuring means is constant. Therefore, assuming that the photosensitive substance surface is a plane, the distance between the measuring electrode and the photosensitive substance surface is given by the mean value measured by the distance measuring means. Further, in case that the measurement object of a latent image is a photosensitive drum, two distance measuring means are adopted and a line connecting these two distance measuring means is made parallel to the axis of the photosensitive drum whereby the distance between the photosensitive substance and the measuring electrode is given by the average of the two distance sensors. It is possible to maintain the distance between the measuring electrode and the photosensitive substance constant by maintaining these mean values constant. It is possible to prevent discharge when the distance between the photosensitive substance and the measuring electrode becomes narrower by setting the common potential of the latent image measurement circuit to a potential between the highest potential and the lowest potential of the measurement object, thereby reducing the potential difference between the measuring electrode and the photosensitive substance. Further, the output of the measurement circuit becomes very small when the measuring electrode area is reduced, which causes the output of the measurement circuit to become unreadable due to ripples and the like of the reference power source. If a battery is used as the reference voltage, however, a stabilized reference voltage is obtainable. Further, a measuring electrode point is surrounded by a first case having an opening portion in a direction facing the photosensitive substance and a second case having an opening portion in a direction facing the photosensitive substance and surrounding the first case, and it is possible to fill the space between the measuring electrode and the photosensitive substance with a gas having a high spark starting voltage without releasing the gas having a high spark starting voltage outside the second case by filling up the first case with the gas having a high spark starting voltage and intaking the gas having a high spark starting voltage in the first case together with outside air by means of the second case. Further, it is possible to prevent spark in case the distance between the measuring electrode and the photosensitive substance becomes short by filling with a gas having a high spark starting voltage between the measuring electrode and the photosensitive substance by such a technique as described above. In a charge measurement circuit of a direct-current amplification type in which electric charge induced in a measuring electrode is converted into voltage using a measuring capacitor, and measurement of electric charge is performed by impedance conversion, it is assumed that a discharge switch has a two-stage structure comprising a main discharge switch and a sub-discharge switch. The main discharge switch performs connection and disconnection between the measuring electrode and the common potential. The sub-discharge switch supplies a drive voltage for bringing the main discharge switch into a connected state and the common potential to the main discharge switch at the time of non-measurement (at the time of spark), and supplies an output obtained by impedance conversion of a measuring electrode potential output through a buffer amplifier to all terminals which are not connected to the measuring electrode of the main discharge switch at the time of measurement. With this, it is possible to eliminate potential difference between potentials applied to the measuring electrode side of the main discharge switch and other terminals at the time of measurement, thus making it possible to make the resistance in open circuit of the whole discharge switch apparently very large. As a result, it is possible to make the drift generated because of the fact that leakage current at the time of measurement is accumulated in the measuring capacitor very small. Further, the sub-discharge switch is composed of a first sub-discharge switch which is closed at the time of measurement only and a second sub-discharge switch which is closed at the time of non-measurement only, and switching is made from a non-measurement state (spark state) through a state in which both the first sub-discharge switch and the second sub-discharge switch are closed. Thus a plunging current which is generated when the first sub-discharge switch is connected can be dumped via the second sub-discharge switch, and unnecessary current may be prevented from accumulating in the measuring capacitor when the non-measurement state is changed over to the measurement state. Further, it is possible to prevent noises caused by capacity variation between measurement circuit wirings due to external vibration at the time of measurement by placing the measurement circuit in a measurement circuit case having a vibration absorbing member. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A, Fig. 1B and Fig. 1 C are trihedral views for explaining an embodiment of arrangement of respective parts of an electrostatic latent image measuring instrument; Fig. 2 is a view for explaining an embodiment of structures of an inner case and an outer case for filling up a gas having a high spark starting voltage in the vicinity of a measuring electrode; Fig. 3 is a diagram for explaining an embodiment of a measurement circuit; Fig. 4 is a diagram showing results of computing a measuring gap vs. an operational amplifier input voltage for a plurality of electrostatic capacity values of a measuring capacitor for measurement when photosensitive substance surface potential at the time of measurement is 1 KV; Fig. 5A is a graph showing the result of measuring output variation on an experimental substrate when the non-measurement state (discharge state) is changed over to the measurement state; Fig. 5B is a graph showing the result of measurement in case the sub-discharge switch 11 b is closed after the sub-discharge switch 11c has been opened; Fig. 5C is a graph showing the result when the sub-discharge switch 11c is opened after the sub-discharge switch 11 b has been closed; Fig. 6A and Fig. 6B are views for explaining a method of manufacturing a point portion of a measurement circuit; Fig. 7 is a diagram for explaining the composition of the extemal portion of a circuit case; Fig. 8 is a flow chart of a control program of a precision pulse stage by a distance sensor output, in which a contact position between a measuring electrode and a photosensitive substance is detected; Fig. 9 is a flow chart of a program controlling a precision pulse stage by detecting a contact position between a measuring electrode and a photosensitive substance; Fig. 10A, Fig. 10B and Fig. 10C are explanatory views showing states of detecting contact positions between a measuring electrode and a photosensitive substance; Fig. 11 is a flow chart of a distance control program for explaining control of the distance between a measuring electrode and a photosensitive substance; Fig. 12 is a flow chart of a precision pulse stage control program including contact position measurement and distance control between a measuring electrode and a photosensitive substance; Fig. 13 is a graph showing a result of measuring an electrostatic latent image of an electrostatic recorder; Fig. 14 is a diagram for explaining control circuit construction of an electrostatic recorder in which an electrostatic latent image measuring instrument is incorporated; Fig. 15 is a flow chart of latent image measurement and device control of an electrostatic recorder in which an electrostatic latent image measuring instrument is incorporated; Fig. 16A, Fig. 16B and Fig. 16C are graphs showing results of measuring electrostatic latent images formed on a photosensitive substance by means of repetition of exposure and non-exposure for every several dots and repetition of exposure and non-exposure for every dot; and Fig. 17 is a graph showing results of computing contrast potentials in a repetition pattern portion of exposure and non-exposure for every dot or every two dots. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention are described with reference to the drawings, relating to an electrostatic latent image measuring instrument and an electrostatic recorder including the electrostatic latent image measuring instrument. First, an embodiment of an electrostatic latent image measuring instrument capable of being mounted on an electrostatic recorder will be described with reference to Fig. 1A thru Fig. 13, and then an electrostatic recorder having the electrostatic latent image measuring instrument mounted on it will be described with reference to Fig. 14 and Fig. 15, and lastly, a control method of the printing process will be described with reference to Fig. 16A thru Fig. 17. Fig. 1 A, Fig. 1 B and Fig. 1 C are views for explaining an embodiment of respective parts of an electrostatic latent image measuring instrument. An electrostatic latent image measuring instrument 4C has laser type non-contact distance sensors 1 on both sides of a probe 2, in which a measuring electrode (an electrostatic latent image measuring sensor portion) is provided at a point portion 3 of the measurement circuit, and a gas chamber 4 composed of a double case formed of an inner case 4a and an outer case 4b, which surrounds the measuring electrode is fitted in order to be filled with a gas having a high discharge starting voltage in the vicinity of the measuring electrode. An output line 9 from the measurement circuit case 2 is connected to a measurement circuit controller or A-D converter (both are not shown on the drawings). The gas chamber 4 which surrounds the probe 2, laser type non-contact distance sensors 1 and the measuring electrode are fitted onto a precision pulse stage 5. Output lines 8 of the laser type sensors are connected to a computer, and the computer controls the precision pulse stage 5 based on this output so that the distance from a photosensitive substance 6 may be maintained constant. A control line 10 from the precision pulse stage 5 is connected to a pulse stage controller (not shown on the drawings). Fig. 2 is a view for explaining an embodiment of a construction of the gas chamber 4 for filling with a gas having a high spark starting voltage in the vicinity of the measuring electrode. A passage 7a through which SF 6 gas having a high spark starting voltage is supplied via a pressure reducing valve is formed in the inner case 4a, and a passage 7b connected to an exhaust pump is formed in the outer case 4b. These passages 7a and 7b are connected to the exhaust pump in a form of pipes as shown in Fig. 1A. As is understood from the above, it is possible to fill with the SF 6 gas having a high spark starting voltage in the inner case 4a, and also, to recover the SF 6 gas so as to prevent the SF 6 gas from leaking to outside of the outer case 4b. In case the electrostatic latent image measuring instrument is used in an electrostatic recorder, bad influence is exerted on operations in which discharge of an electric charge device and the like are utilized if the SF 6 gas having a high spark starting voltage leaks from the instrument. Therefore, such recovery means of the gas having a high spark starting voltage as described above is required. Fig. 3 is a diagram for explaining an embodiment of a measurement circuit in the probe 2. A basic principle of the measurement circuit is to convert electric charges induced in a measuring electrode 3a by having the measuring electrode 3a come close to the photosensitive substance 6, into voltage by means of a measuring capacitor 12, which is amplified by an amplifier having high input impedance. The output voltage of this amplifier is influenced very strongly even by inflow of a very small current from the outside, for it is adapted to measure very small electric charges. Therefore, the amplifier has a two-stage construction of a buffer amplifier 13a and an amplifier 13b of an amplification factor of 20 times. The voltage induced in the measuring electrode 3a is subjected to impedance conversion of the output voltage in the buffer amplifier 13a (amplifier having an amplification factor of 1), and this voltage is applied to a shield 14 which surrounds a guard electrode 3b of the measuring electrode 3a, thereby to keep current inflow from the outside as small as possible. The output of the buffer amplifier 13a is amplified by the amplifier 13b having an amplification factor of 20, and is output outside the probe 2 at the buffer amplifier output. The amplifier 13b serves the purpose of reducing the influence by extemal noises when an output signal is sent out of the probe 2, and it is recommended to use an appropriate amplification factor in accordance with the output voltage of the buffer amplifier 13a. When it is assumed that the potential at the surface of the photosensitive substance 6 is V S and the electrostatic capacity of the photosensitive substance 6 is C P , an electric charge quantity Q P existing on the photosensitive substance 6 is: Q P = V S · C P When the measuring electrode 3a is brought very close to the photosensitive substance 6, the surface potential of the photosensitive substance 6 is lowered. This is due to the fact that the electrostatic capacity on the surface of the photosensitive substance 6 is varied because the measuring electrode 3a is brought very close thereto. When it is assumed that an electrostatic capacity of a very small gap between the measuring electrode 3a and the photosensitive substance 6 is C A and an electrostatic capacity of the measuring capacitor 12 is C M , an electrostatic capacity C PM at the time of measurement is expressed by the following expression: C PM = C P + C A · C M C A + C M The surface potential V SM of the photosensitive substance 6 at the time of measurement when the measuring electrode 3a is very close to the photosensitive substance 6 is given by the following expression: V SM = C P C PM V S = ( C P (C A + C M ) C P (C A + C M ) + C A · C M ) V S The detected electrode voltage at this time, viz. the input voltage V IN of the buffer amplifier 13a is given by the following expression based on th very small gap between the measuring electrode 3a and the photosensitive substance 6 and the electrostatic capacity of the capacitor 12 for measurement. V IN = C A C A + C M V SM = C P ·C A C P (C A + C M ) + C A · C M V S When the diameter of the measuring electrode 3a is at 50 µm, the measured area is approximately 1.96 x 10 -3 mm 2 . When the photosensitive substance 6 to be measured is an organic photosensitive substance having a thickness of approximately 20 µm, the dielectric constant of the photosensitive substance 6 is approximately 4, and the electrostatic capacity C P of the organic photosensitive substance having the thickness of 20 µm on the measured area is approximately at 3.5 x 10 -15 F. Further, when the measurement gap is assumed to have a width G (µm), the electrostatic capacity of the small gap between the measuring electrode 3a and the photosensitive substance 6 will be C A = 1.74 x 10 -14 /G(F). Fig. 4 shows results of computing input voltages of the buffer amplifier 13a as a function of the width of the measurement gap and electrostatic capacity of the measuring capacitor 12 when the surface potential of the photosensitive substance 6 is at 1 KV at the time of measurement. In view of the stability of the output against noises and the like, of an input voltage of the buffer amplifier 13a of approximately 100mV is required if the surface potential of the photosensitive substance 6 is 1 KV. When it is assumed that the measurement gap is 50 µm wide, the measuring capacitor 12 must have a capacity of several pF and below. If the measuring capacitor 12 has a capacity of 5 pF, the input, voltage of the buffer amplifier 13a becomes approximately 63 mV if the surface potential of the photosensitive substance 6 is 1 KV. In a circuit system of the present embodiment, a discharge switch 11 is required for discharging the capacitor 12 prior to measurement. Attention should be paid to the discharge switch 11 so that electrostatic capacity and leakage current are not generated at the time of measurement. When an ordinary analog switch or relay and the like is used as the discharge switch 11, rapid drift caused by a leakage current from a power source line is observed. Furthermore, the measuring electrostatic capacity is increased by the electrostatic capacity of the discharge switch 11, so that a sufficient electrode voltage cannot be obtained. Accordingly, the discharge switch 11 is constructed to have two-step structure including a main discharge switch 11 constituted by photocouplers and subdischarge switches 11b and 11c constituted by analog switches so that the output of the buffer amplifier 13a is fed back to all the terminals of photocouplers at the time of measurement. Fig. 5A shows results of measuring output variation on an experimental substrate when a non-measurement state (spark state) is changed over to a measurement state in this construction. At the time of mode change-over, a sudden output change of approximately 0.5 mV is observed which is foreseen to be caused by plunging current from the sub-discharge switch 11b and the sub-discharge switch 11 c. If such voltage rise occurs at the time of mode changing over, the output is saturated promptly at the time of amplification in the amplifier 13b on the second stage and sufficient amplification can not be executed. Fig. 5B shows results of measurement in case the closing of the sub-discharge switch 11b is delayed with respect to the opening of the sub-discharge switch 11c by means of a delay circuit. It will be understood from the figure that there is a voltage rise of 0.05 to 0.5 mV due to the plunging current of the sub-discharge switch 11c, and one of 0.35 to 0.4 mV due to the plunging current from the sub-discharge switch 11b. Fig. 5C shows the result of delaying the opening of the sub-discharge switch 11c with respect to the closing of the sub-discharge switch 11 b. As described, it is realized that the influence by the plunging current from the sub-discharge switch 11b may be eliminated by opening the sub-discharge switch 11c after closing the sub-discharge switch 11b. This is because of the fact that the current generated from the sub-discharge switch 11b flows to a common power source in a state that both switches are closed. Accordingly, in the circuit construction of the present embodiment, the sub-discharge switch is divided into the sub-discharge switch 11b which is closed at the time of measurement only and the sub-discharge switch 11c which is closed at the time of non-measurement only, and a delay circuit is provided so that both switches are changed over through the closed state when the non-measurement state is changed over to the measurement state. Fig. 6A and Fig. 6B are views for explaining a method of manufacturing a point portion 3 of the probe 2. It is preferable that the point portion 3 of the measuring electrode includes a guard electrode 3b having a sufficient area so that an electrostatic latent image is not disturbed at the time of measurement, or the surface of the photosensitive substance 6 is not damaged when the point portion 3 of the measuring electrode comes into contact with the photosensitive substance 6. The point portion 3 of the measuring electrode having such a large guard electrode 3b may be manufactured as described hereunder. As to the point portion 3 of the measuring electrode, first, a fine hole having a diameter which is a little bigger than the diameter of the measuring electrode 3a is processed in a base material 15 of the point portion of the measuring electrode. An electric discharge machine and the like may be used for the processing. Next, a very fine wire 16 with the surface coated for insulation with resin and the like is inserted, and bonding is performed thereafter with epoxy resin 17 and the like. After cutting the inserted very fine wire 16 at the surface of the measuring electrode 3a, flattening is applied to the surface of the measuring electrode 3a with a lathe or a milling machine and grinding is applied thereto, thereby to smooth the surface of the measuring electrode 3a. With this, it is possible to manufacture the point portion 3 of the measuring electrode having a sufficiently large area of the guard electrode 3b. Incidentally, Fig. 6B is an enlarged view of a part shown at "A" in Fig. 6A. Fig. 7 is a diagram for explaining a circuit composition outside the probe 2. A common potential of the probe 2 is applied to a reference potential power source 19 at an intermediate potential between the highest voltage and the lowest voltage of the photosensitive substance 6, and a measurement signal is applied to a data storage means 21 through an insulation amplifier 20. The data storage means 21 stores measurement data for a certain time interval setting to a timing signal from a timing control unit 23. Further, a photocoupler (p.c) receives a discharge switch control signal 22a from a timing control unit 23, and transmits this timing signal 22a to the probe 2. The probe 2 executes measurement operation on the electrostatic latent image based on the timing signal 22a. The reason why the common potential of the probe 2 is set at an intermediate potential between the maximal voltage and the minimal voltage of the photosensitive substance 6 is to make electric field intensity between the photosensitive substance 6 and the measuring electrode 3 small to the utmost, so that sparks can be prevented between the measuring electrode 3a and the photosensitive substance 6. A reference potential power source 19 which gives the common potential must have a very small power source ripple. Therefore, a higher voltage battery and the like may be used. In order to prevent sparks, the above-mentioned SF 6 gas having a high spark starting voltage may be employed, or a bias may be applied using a reference power source, either one of them or none of them may be used in some cases in accordance with the electric field intensity generated between the measuring electrode 3a and the photosensitive substance 6. Fig. 8 is a chart for explaining a technique for detecting a contact position between the measuring electrode 3a and the photosensitive substance 6, and is a flow chart of a control program for controlling the precision pulse stage 5 by the outputs of the distance sensors 1. First, a set distance H 5 between the measuring electrode 3a and the photosensitive substance 6 is inputted. Next, distance data L 1 are taken in as a mean value of distances which the distance sensor 1 can measure. Next, the precision pulse stage 5 equipped with the electrostatic latent image measuring instrument 4c is advanced by a very short distance (for example, a portion corresponding to one pulse for advancing the precision pulse stage 5) toward the photosensitive substance 6. Distance data L 2 are taken in from the distance sensor 1. Next, the difference of these distance data (L 2 - L 1 ) is computed, and if this value is found to be smaller than a predetermined value ΔL, the distance data at that time are stored in a data storage means 21 as a position H 0 of the photosensitive substance 6 assuming that the output value of the distance sensors 1 at that time is the distance between the measuring electrode 3a and the photosensitive substance 6. In case the difference between distance data (L 2 - L 1 ) is larger than the predetermined value ΔL, the precision pulse stage 5 is advanced toward the photosensitive substance 6 by replacing L 2 with L 1 , and above-mentioned operation is repeated. After the contact position between the measuring electrode 3a and the photosensitive substance 6 is measured, the precision pulse stage 5 is made to retreat. A distance H between the measuring electrode 3a and the photosensitive substance 6 may be computed as H = H L - H 0 from the distance data H L by the distance sensor 1. This operation utilizes a fact that the precision pulse stage 5 stops to advance even if a pulse is applied to the precision pulse stage 5 when the measuring electrode 3a and the photosensitive substance 6 come into contact with each other. When respective parts of the electrostatic latent image measuring instrument 4c, the photosensitive substance 6 are deformed by advancing operation of the precision pulse stage 5, however, accurate measurement can no longer be made. Accordingly, the driving voltage of the precision pulse stage 5 in case of distance setting is made smaller than at the time of distance control, thereby to reduce the driving force and make deformation of respective parts at the time of contact between the measuring electrode 3a and the photosensitive substance 6 as small as possible. The above-mentioned value ΔL for the contact determination may be determined experimentally considering such very small deformation. Fig. 9 is a chart for explaining another embodiment for detecting the contact position between the measuring electrode 3a and the photosensitive substance 6, and is also a chart for explaining the flow of a control program of the precision pulse stage 5. A stress sensor or a distortion sensor is provided at such a position that a distortion may be generated when the measuring electrode 3a of the electrostatic latent image measuring instrument 4c comes into contact with the photosensitive substance 6. The precision pulse stage 5 is made to advance while monitoring the output variation of this sensor, and distance data of the distance sensors 1 at a position where the distortion caused by the contact between the measuring electrode 3a and the photosensitive substance 6 is detected are stored in the data storage means 21 as the position H 0 of the photosensitive substance 6. After the contact position between the measuring electrode 3a and the photosensitive substance 6 is measured, the precision pulse stage 5 is made to retreat. The distance H between the measuring electrode 3a and the photosensitive substance 6 may be computed as H = H L - H 0 from the distance data H L of the distance sensors 1. Fig. 10A, Fig. 10B and Fig. 10C are views for explaining the operation for detecting the contact position between the measuring electrode 3a and the photosensitive substance 6. The probe 2 is fitted to the precision pulse stage 5 in a state that the probe 2 is slidable in the stage movement direction. in Fig. 10A, the probe 2 and the photosensitive substance 6 are made to touch each other at a position where the precision pulse stage 5 has been retreated. In Fig. 10B, the precision pulse stage 5 is made to advance slowly toward the photosensitive substance 6 side up to the predetermined distance data position H 0 of the distance sensors 1 so as to push the probe 2 against the photosensitive substance 6. In Fig. 10C, the contact position between the measuring electrode 3a and the photosensitive substance 6 reaches the predetermined distance data position H 0 of the distance sensors 1 by retreating the precision pulse stage 5 thereafter. Since the fixing strength of the probe 2 is of importance at this time, the fixing strength of the probe 2 is arranged to be changed easily. Fig. 11 is a chart for explaining an embodiment of distance control between the measuring electrode 3a and the photosensitive substance 6, and shows the flow of a distance control program. Distance data H D of the sum or the mean value of distance which can be measured by the distance sensors 1 are loaded and the difference between the distance data H D and the distance data H 0 of the photosensitive substance 6 is obtained, thereby to compute the distance H (= H D - H 0 ) between the measuring electrode 3a and the photosensitive substance 6. Then, the precision pulse stage 5 is made to advance or retreat so that the distance H becomes equal to a distance set value H S . Fig. 12 shows an embodiment of the whole composition of a control program of the precision pulse stage 5 including measurement of the contact position and control of the distance between the measuring electrode 3a and the photosensitive substance 6, and is a chart showing the flow of the program. Fig. 13 shows the result of measurement of an electrostatic latent image formed on the photosensitive substance 6. Also, Fig. 13 shows the result of the measurement of the actual electrostatic latent image by forming an exposure pattern having the resolution of 300 dpi (dot per inch) in the case d using the electrostatic latent image measurement instrument. The horizontal axis represents a time (5 microsecond/section) indicating a movement of the photosensitive substance 6. The vertical axis represents an electric potential (0.5 v/section) measured by the instrument. It is noted that the accurate measurement is repeated by two of each exposure and non-exposure in the order of one-dot, two-dot and four-dot from the loft and of the graph, and also, by one of each exposure and non-exposure for sight-dot, again one-dot then two-dot as shown in Fig. 13. This is the result of exposing an organic photosensitive substance with an LED, and the exposed pattern is a repetition of "8 dots exposure - 8 dots non-exposure - 4 dots exposure - 4 dots non-exposure - 4 dots exposure - 4 dots non-exposure - 2 dots exposure - 2 dots non-exposure - 2 dots exposure - 2 dots non-exposure - 1 dot exposure - 1 dot non-exposure - 1 dot exposure - 1 dot non-exposure". Fig. 14 is a block diagram of a control circuit of an electrostatic recorder in which the electrostatic latent image measuring instrument 4c is incorporated. To the electrostatic latent image measuring instrument 4c, a precision pulse stage control unit 28, a gas supply/exhaust mechanism portion 24, a gas supply/exhaust control unit 27 and a timing control unit 26 are connected, and control of latent image measuring operation is carried out. An output signal of the probe 2 is transmitted to the data storage means 21 through an insulation amplifier 20 to send a waveform data to data analyzing means 25. The operation of the precision pulse stage control unit 28 for controlling the pulse stage means, the gas supply/exhaust control unit 27 for controlling start and stop of the gas supply and the timing control unit 23 indicating the timing control of the respective circuits is made by means of an electrostatic latent image measurement control unit 30, and the result of analysis by the data analyzing means 25 is sent to the electrostatic latent image measurement control unit 30 and then sent to a process control unit 31 of the electrostatic recorder as control information. Incidentally, an exposure control unit 32 carries out ordinary exposure operation based on an exposure control signal from the timing control unit 23. A line from the timing control unit 23 to the probe 2 corresponds to the line via photocoupler (P.C) as shown in Fig. 7. Also, a surface potential measuring unit 4d is arranged on the surface of the photosensitive substance 6 so that it measures an absolute value of an electric potential on the whole surface thereof. Fig. 15 is a flow chart showing latent image measurement and device control of an electrostatic recorder in which the electrostatic latent image measuring instrument 4c is incorporated. First, a photosensitive drum of the electrostatic recorder is stopped, and the distance between the measuring electrode 3a and the photosensitive substance 6 is set, thus conducting measurement gap control. Next, a spark preventive portion with a gas having a high spark starting voltage or a common potential as a reference power source voltage is operated. The electrostatic recorder is started, and exposure of diagnosis exposure pattern from an exposure system, switching of the mode of the probe 2 of an electrostatic latent image from a spark preventive state to a measurement state, and loading of measurement data to the data storage means 21 are controlled by means of three timing signals of the timing control unit 26. The measurement data stored in the data storage means 21 are analyzed by means of the data analyzing means 25, thus controlling exposure, exposure time, development bias, photosensitive substance heater and the like. Measurement and control are repeated until the result of analysis of the latent image state becomes normal. Further, in case normal latent image measurement results are unobtainable even if control range of operation of the printing process is exceeded, the life of the photosensitive substance is determined to be over. When the latent image state becomes normal, the photosensitive drum is stopped, the operation of the spark preventive portion is stopped, distance control is stopped, and the measuring electrode 3a is removed from the vicinity of the photosensitive substance 6, thus completing a series of latent image measurement and instrument state setting operation. Further, when the distance is preset, it is also possible to perform control of a printing process without stopping the printing operation of the electrostatic recorder by providing an exposure location for measurement and control at a part of the photosensitive substance during printing. The above-mentioned embodiment relates to an electrostatic recorder using the electrostatic latent image measuring instrument 4c shown in above-mentioned embodiment, and the operation and the construction described above may be simplified depending on the construction and the performance of the electrostatic latent image measuring instrument 4c. Fig. 16A, Fig. 16B, Fig. 16C and Fig. 17 are diagrams for explaining a control technique of the exposure system and the development system by an electrostatic latent image. Here, Fig. 16A shows a characteristic when the exposure or the exposure time are little, Fig. 16B shows a characteristic when the exposure or the exposure time are appropriate, and Fig. 16C shows a characteristic when the exposure or the exposure time are much. Fig. 16A, Fig. 16B and Fig. 16C show the results of measuring the latent images formed on the photosensitive substance 6 having a long latent image portion 35a in which repetition interval of exposure and non-exposure is several dots wide and a short latent image portion 35b in which repetition interval of exposure and non-exposure is one dot wide. The electrostatic latent image varies according to the variation of the exposure or the exposure time as shown in Fig. 16A, Fig. 16B and Fig. 16C. In order to obtain a beautiful picture image, it is only required to perform control so that the width 34b of the latent image at the exposure position and the width 34a of the latent image at the non-exposure position are identical with each other as shown in Fig. 16B. Further, it is also possible to obtain absolute potentials at respective parts of the latent image by using both the electrostatic latent image measuring instrument 4c and a surface potential measuring means at the same time. In the photosensitive substance 6, potentials V RA and V RD in the exposure region and the non-exposure region measured by the surface potential measuring means correspond to the output voltages V A and V D of the latent image portion 35a having sufficiently wide exposure width of the electrostatic latent image measuring instrument 4c shown in Fig. 16A, Fig. 16B and Fig. 16C. The practical maximal potential V RB and minimal potential V RC in the latent image portion 35b which is formed by means of an exposure pattern having a narrow latent image width are obtainable with following expressions from output voltages V A , V B , V C and V D of the electrostatic latent image measuring instrument 4c and measured voltages V RA and V RD by the surface potential measuring means. V RB = (V B - V D ) (V A - V D ) × (V RA - V RD ) + V RD V RC = (V C - V D ) (V A - V D ) × (V RA - V RD ) + V RD Fig. 17 shows results of computing contrast potential (difference between the maximal potential and the minimal potential) in the latent image portions 35a and 35b by repetition of exposure and non-exposure for every dot or every two dots by means of above-mentioned operation with the expressions (1) and (2). In order to form a beautiful picture image, it is only required that the exposure or the exposure time takes a position shown at "B" where the potential difference in the exposure pattern for every dot becomes maximal. The above-described embodiment shows a case in which the electrostatic latent image measuring instrument 4c detects an electrostatic latent image on the photosensitive substance 6 as a relative variation of a photosensitive substance surface potential and an exposure. In case the electrostatic latent image measuring instrument 4c is able to detect an electrostatic latent image on the photosensitive substance 6 as absolute value variation of the photosensitive substance surface voltage, absolute voltage value conversion for respective parts of the latent image by computation based on the expressions (1) and (2) is not required, and also only the electrostatic latent image measuring instrument 4c (high precision surface potential measuring means) is required as the measuring means of the electrostatic latent image. In the next place, a control technique of development bias by an electrostatic latent image will be described. In case of an inversion phenomenon such as a printer, a fine line is defected to turn black if the development bias is over the maximal potential V RB indicated by the output voltage V B in Fig. 16A, Fig. 16B and Fig. 16C. Further, a fine line is not developed when the development bias is at the minimal potential V RC and below indicated by the output voltage V C in Fig. 16A, Fig. 16B and Fig. 16C. It is possible to develop the electrostatic latent image of a picture to an extent of a fine line by setting the development bias at a potential between the maximal potential V RB and the minimal potential V RC of the fine line, and the picture image may be formed most beautifully at a potential near the middle of the maximal potential V RB and the minimal potential V RC . By controlling so that the development bias comes near to the middle of the maximal potential V RB and the minimal potential V RC of the electrostatic latent image of a fine line, it is possible to stabilize the development process regardless of the type and elapsed variation of the photosensitive substance 6. It has been known that a-Si photosensitive substance and the like produce image flowing of a latent image depending on humidity or temperature. Since it is possible to find out a state of an electrostatic latent image on the photosensitive substance 6 by using the electrostatic latent image measuring instrument 4c, it is also possible to control operation stop of a heater and the like of the photosensitive substance 6. Further, since it is possible to know a state of an electrostatic latent image on the photosensitive substance 6 by using the electrostatic latent image measuring instrument 4c, it is possible to determine the life of the photosensitive substance 6. In practice, it may be determined that the life of the photosensitive substance 6 has been exhausted when a sufficient potential difference cannot be applied to the latent image of a fine line within the control range of the exposure system, when the control range of the development bias is off state between the maximal potential V RB and the minimal potential V RC of the fine line and so forth. In case of the photosensitive substance 6 such as a-Si photosensitive substance in which a latent image produces image flowing depending on humidity and temperature, above-described life determination may be made at a prescribed temperature and humidity or when the heater of the photosensitive substance 6 has operated for a sufficient period of time. As explained above, it is possible to measure the latent image state on the photosensitive substance 6 by using an electrostatic latent image measuring instrument in an electrostatic recorder. Thus, it is possible to control a printing process rolated to latent image formation such as exposure, exposure time, development bias and heater for photosensitive substance under optimum conditions. Since means for maintaining the distance between the measuring electrode 3a and the photosensitive substance 6 constant is provided, the electrostatic latent image can be measured even if eccentricity is produced in a photosensitive substance 6. Further, it is possible to control the distance of the measuring electrode 3a portion by means of a plurality of distance sensors 1. Furthermore, it is possible to prevent spark between the measuring electrode 3a and the photosensitive substance 6 by setting the common potential of the probe 2 at an intermediate potential between the maximal potential and the minimal potential of the photosensitive substance 6. Further, it is possible to prevent spark between the measuring electrode 3a and the photosensitive substance 6 by providing means of filling with a gas having a high spark starting voltage between the measuring electrode 3a and the photosensitive substance 6. It is possible to reduce a plunging current at the time of switch change-over and a leakage current at the time of measurement so as to measure an electrostatic latent image stably by constructing a discharge switch 11 of the probe 2 as shown in Fig. 3. It is also possible to measure an electrostatic latent image without having noises caused by vibration at the time of measuring an electrostatic latent image by supporting the probe 2 with a vibration absorbing member. In the embodiments, the SF 6 gas is used for reducing the electrostatic spark, but a dry air or a dry N 2 gas can be used as well.

Claims: An electrostatic recorder comprising electrostatic charge means for uniformly charging a surface of a revolving photosensitive substance (6), exposure and latent image forming means for carrying out exposure and for forming an electrostatic latent image on the surface of said photosensitive substance, development means for forming a visible image, transfer means for transferring said visible image onto a blank, an electrostatic latent image measuring unit, and control means (21, 25, 30, 31, 23, 32) for varying control factors related to stages of a printing process from electrostatically charging the surface of said photosensitive substance (6) by said electrostatic charge means to transferring the visible image onto a blank by said transfer means in accordance with the results of measurement of the electrostatic latent image, characterized in that the exposure and latent image forming means are arranged to form a dot pattern of exposed parts having a width of one dot and non-exposed parts on the surface of said photosensitive substance, the electrostatic latent image measuring unit is adapted to measure said pattern and to output signals in accordance with said pattern, distance control means (1) are provided for maintaining a distance between a measuring electrode (3a) of the latent image measuring unit and the revolving photosensitive substance (6) constant, the distance control means being composed of a distance measuring means (1) and a sensor portion driving means, and spark prevention means are provided for preventing discharge between a measuring electrode of the latent image measuring unit and the photosensitive substance (6), said spark prevention means including a reference power source having a potential between the highest voltage and the lowest voltage of the photosensitive substance (6), and a common potential for a measurement circuit (33) is adopted as a reference power source voltage for the electrostatic latent image measuring unit. An electrostatic recorder according to claim 1, wherein control factors related to the printing process include exposure, exposure time, electrostatic charge voltage, development bias, transfer voltage. temperature, humidity and the like. An electrostatic recorder according to claim 1, wherein said electrostatic latent image measuring unit measures variation of electric charge quantity or of electric potential from a repetitive dot pattern of the electrostatic latent image, formed by exposure, along the surface of said photosensitive substance (6) in revolving direction. An electrostatic recorder according to claim 3, wherein said electrostatic latent image measuring unit measures variation of electric charge quantity or electric potential from a several-dots pattern and a single dot pattern of said electrostatic latent image, along the surface of said photosensitive substance (6) in revolving direction. An electrostatic recorder according to claim 3, wherein said control means (21, 25, 30, 31, 23, 32) controls either one of the exposure or the exposure time of an exposure portion of said repetitive dot pattern so as to form an exposure portion and a non-exposure portion of identical width in said electrostatic latent image actually formed on said photosensitive substance. An electrostatic recorder according to claim 2, wherein said control means (21, 25, 30, 31, 23, 32) controls temperature and humidity of said photosensitive substance (6) until the results of measurement of said electrostatic latent image measured by said electrostatic latent image measuring unit indicate a predetermined temperature and humidity. An electrostatic recorder according to claim 4, wherein said electrostatic latent image measuring unit determines a difference W between the maximum and minimum values of output voltages measured from said several dots pattern, determines a difference S between the maximum and minimum values of output voltages measured from said single dot pattern, and generates a replacement signal for replacing said photosensitive substance (6) when the value of S/W is smaller than a threshold value. An electrostatic recorder according to claim 4, wherein said control means (21, 25, 30, 31, 23, 32) measures at least one of operation time, temperature and humidity of said photosensitive substance (6) : and when the measured value is in a predetermined range, said electrostatic latent image measuring unit determines a difference W between the maximum and minimum values of output voltages measured from said several dots pattern, determines a difference S between the maximum and minimum values of output voltages measured from said single dot pattern, and generates a replacement signal for replacing said photosensitive substance (6) when the value of S/W is smaller than a threshold value. An electrostatic recorder according to claim 3, wherein said electrostatic latent image measuring unit includes a surface potential measuring unit for measuring an absolute value of the electric potential in a relatively wide area having a width of several dots along the surface of said photosensitive substance (6) in revolving direction. An electrostatic recorder according to claims 4 and 9, wherein said surface potential measuring unit determines a difference V between the electric potentials of the whole surface of said photosensitive substance (6) at time of exposure and of non-exposure, respectively; said electrostatic latent image measuring unit determines a difference W between the maximum and minimum values of output voltages measured from said several-dots pattern, and determines a difference S between the maximum and minimum values of output voltages measured from said single dot pattern; and said control means (21, 25, 30, 31, 23, 32) adjusts at least one of exposure and exposure time so as to maximize the value of V x S/W. An electrostatic recorder according to claims 4 and 9, wherein said surface potential measuring unit determines a difference V between an electric potential of the whole surface of said photosensitive substance (6) at time of exposure and of non-exposure, respectively; said electrostatic latent image measuring unit (4c) determines a difference W between the maximum and minimum values of output voltages measured from said several dots pattern, and determines a difference S between the maximum and minimum values of output voltage measured from said single dot pattern; and said control means (21, 25, 30, 31, 23, 32) adjusts temperature and humidity of said photosensitive substance (6) until the value of V x S/W reaches a threshold value. An electrostatic recorder according to claims 4 and 9, wherein said surface potential measuring unit determines an electric potential on the whole surface of said photosensitive substance (6) at time of exposure and of non-exposure, respectively: said electrostatic latent image measuring unit determines the maximum and minimum values of output voltages measured from both said several dots pattern and said single dot pattern; said surface potential measuring unit determines the maximum potential V 1H and the minimum potential V 1L of said single dot-pattem of said electrostatic latent image based on the maximum and minimum values determined from said electrostatic latent image measuring unit, respectively, and said control means (21, 25, 30, 31, 23, 32) controls a development bias so as to become smaller than the maximum potential V 1H and larger than the minimum potential V 1L . An electrostatic recorder according to claims 4 and 9, wherein said surface potential measuring unit determines a difference V between an electric potential on the whole surface of said photosensitive substance (6) at time of exposure and of non-exposure, respectively; and said electrostatic latent image measuring unit determines a difference W between the maximum and minimum values of output voltages measured from said several dots pattern, determines a difference S between the maximum and minimum values of output voltages measured from said single dot pattern, and generates a replacement signal for replacing said photosensitive substance (6) when the value of V x S/W is equal or smaller than a threshold value. An electrostatic recorder according to claims 4 and 9, wherein said control means (21, 25, 30, 31, 23, 32) measures at least one of operation time, temperature and humidity of said photosensitive substance (6); said surface potential measuring unit determines a difference V between an electric potential on the whole surface of said photosensitive substance (6) at time of exposure and of non-exposure, respectively, when the value measured by said control means is in a predetermined range; and said electrostatic latent image measuring unit determines a difference W between the maximum and minimum values of output voltages measured from said several dots pattern and a difference S between the maximum and minimum values of output voltages measured from said single dot pattern, and generates a replacement signal for replacing said photosensitive substance (6) when the value of V x S/W is equal or smaller than a threshold value. An electrostatic recorder according to claim 3, wherein said electrostatic latent image measuring unit includes a surface potential measuring unit for measuring an absolute value of the photosensitive surface voltage of said electrostatic latent image formed on the surface of said photosensitive substance (6). An electrostatic recorder according to claim 15, wherein said repetitive dot pattern is a single dot pattern, said electrostatic latent image measuring unit determines a contrast potential indicative of a difference between the maximum and minimum potential portions of output voltages measured from said single dot pattern; and said control means (21, 25, 30, 31, 23, 32) adjusts at least one of exposure and exposure time so as to maximize said contrast potential. An electrostatic recorder according to claim 15, wherein said repetitive dot pattern comprises a single dot pattern, said electrostatic latent image measuring unit determines a contrast potential indicative of a difference between the maximum and minimum potential portions of output voltages measured from said single dot pattern, and said control means (21, 25, 30, 31, 23, 32) adjusts temperature and humidity of said photosensitive substance (6) so that said contrast potential becomes larger than a threshold value. An electrostatic recorder according to claim 16, wherein said electrostatic latent image measuring unit measures the maximum potential V 1H and the minimum potential V 1L from said single dot pattern; and said control means (21, 25, 30, 31, 23, 32) adjusts a development bias so as to be smaller than the maximum potential V 1H or larger than the minimum potential V 1L . An electrostatic recorder according to claim 15, wherein said electrostatic latent image measuring unit determines a contrast potential indicative of a difference between the maximum and minimum potential portions of output voltages measured from said single dot pattern, and generates a replacement signal for replacing said photosensitive substance (6) when said contrast potential is equal or smaller than a threshold value. An electrostatic recorder according to claim 18, wherein said control means (21, 25, 30, 31, 23, 32) adjusts temperature and humidity of said photosensitive substance (6). An electrostatic recorder according to claim 1, wherein said control means (21, 25, 30, 31, 23, 32) include exposure means, data storage means (21) for storing measurement data of electric charge quantity or electric potential of said electrostatic latent image, data analyzing means (25) connected to said data storage means (21) for analyzing measurement data, and timing control means (23) for generating a timing signal to indicate operation of said respective means, whereby said exposure means is operated by the timing signal, said electrostatic latent image measuring unit is operated so as to measure the electric charge quantity or electric potential of the electrostatic latent image, the measured data are temporarily stored in said data storage means (21), the measured data are analyzed by said data analyzing means (25), and control factors related to the printing process are adjusted until the electrostatic latent image is transferred to a blank based on the result of the analysis. An electrostatic recorder according to claim 1, wherein said electrostatic latent image measuring unit (4c) includes an electrostatic latent image measuring sensor portion kept away from the surface of said photosensitive substance (6) by a distance larger than the eccentricity of said photosensitive substance (6) at a time of non-measurement of the electrostatic latent image, while kept closely to the surface of said photosensitive substance (6) by a predetermined distance at a time of measurement of the electrostatic latent image, thereby maintaining the distance therebetween constant. An electrostatic recorder according to claim 1, wherein there are provided two sets and more of distance measuring means, and the distance measuring means are arranged on a circumference with the sensor portion (3a) as the center and arranged so that a spacing between distance measuring means is constant. An electrostatic recorder according to claim 23, wherein two sets of distance measuring means are provided and the photosensitive substance (6) is on a revolving drum, which is arranged so that an axis connecting two sets of distance measuring means and the revolution axis of the drum are in parallel with each other. An electrostatic recorder according to claim 1, wherein a sum or a mean value of output values of the distance measuring means is computed, and the measuring electrode driving means is controlled so that the sum or the mean value becomes constant, thereby to maintain the distance between the measuring electrode and the photosensitive substance (6) constant. An electrostatic recorder according to claim 1, comprising means for changing over the common potential of the measurement circuit (33) between a voltage of one or more reference power sources and a ground potential. An electrostatic recorder according to claim 1. wherein a reference power source voltage is adopted as the common potential of the measurement circuit (33) when the distance between the measuring sensor portion (3a) and the photosensitive substance (6) is at a predetermined constant value and below. and a ground potential is adopted when the distance between the measuring sensor portion (3a) and the photosensitive substance (6) is at a predetermined constant value and above. An electrostatic recorder according to claim 1, wherein a battery is used as a reference power source. An electrostatic recorder according to claim 1, comprising a mechanism for introducing a gas between a measuring electrode (3a) of the electrostatic latent image measuring unit and the photosensitive substance (6). An electrostatic recorder according to claim 29, wherein a measuring electrode (3a) is surrounded by a first case (4a) having an op ening portion in a direction facing the photosensitive substance (6), and the first case (4a) is surrounded by a second case (4b) having an opening portion in a direction facing the photosensitive substance (6) and having an opening area larger than the first case, and means (7a) for supplying a gas is connected to the first case (4a) and exhaust means (7b) is connected to the second case (4b). An electrostatic recorder according to claim 29, wherein sulfur hexafluoride (SF 6 ) is used as a gas to be introduced between the measuring electrode (3a) and the photosensitive substance (6). An electrostatic recorder accordin g to claim 1, comprising, in an electrostatic latent image measurement circuit (33) of a direct current amplification type which consists of a measuring electrode (3a) having a guard electrode (3b) which is made close to a photosensitive substance (6), a measurement capacitor (12) for converting electric charges on the photosensitive substance (6) into voltage, a buffer amplifier (13a) for applying impedance conversion to an output signal, a discharge switch (11) for removing electric charges of the measurement capacitor prior to measurement, a main discharge switch (11 a) which changes over connection to disconnection with respect to a common potential and sub-discharge switches (11b, 11c) which are changed over so that an output potential of the buffer amplifier (13a) is applied to all terminals other than those terminals that are connected to the measuring electrode (3a) of the main discharge switch at the time of measurement in order to reduce leakage current of the main discharge switch (11a) in a cut-off state. An electrostatic recorder according to claim 32, wherein a switch which is brought into an open state when all driving terminals are set at the same potential is used as the main discharge switch (11a). An electrostatic recorder according to claim 32, wherein the sub-discharge switches (11b, 11c) are composed of a first sub-discharge switch (11b) which is closed only at the time of measurement and a second sub-discharge switch (11c) which is closed only at the time of non-measurement. An electrostatic recorder according to claim 34, comprising means for delaying the operation timing of the first sub-discharge switch (11b) behind the operation timing of the second sub-discharge switch (11c). An electrostatic recorder according to claim 32, wherein the measurement circuit (33) is held with a vibration absorbing member in the measurement circuit case.

Inventors: IGAWA TATSUO, MIYASAKA TORU, NAGATA TETSUYA, UMEDA TAKAO, IGAWA, TATSUO, MIYASAKA, TORU, NAGATA, TETSUYA, UMEDA, TAKAO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: G03G15
IPC codes: G03G15, G01R29, G03G21