DE2136696A1 - Method and device for developing an electrostatic charge image - Google Patents

Description

PATENTANWÄLTE DR .-! NG. WOLFF, H. BARTELS, DR. BRANDES, DR.-ING. HERO

2136

H / 93

Munich, 22.18., ..... Y «ni ... l.971 THIERSCHSTRASSE 8 TELEPHONE: (0811) 293297

Registration number. 123 006

Eastman Kodak Company, 343 State Street, Rochester, New York State, United States of America

Method and apparatus for developing a electrostatic charge pattern

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-2- 2136698

The invention relates to a method and a device for Development of an electrostatic charge image on the surface an electrophotographic film composed of an electrically insulating layer and an electrically conductive support Recording material in which the surface of the electrophotographic recording material is slightly Distance at a developer electrode of a magnetic brush developer is passed.

Electrophotographic imaging methods are also described loading W detected and extensively in both the patent and other literature in the "z. In U.S. Patents 2,221,776, 2,277,013, 2,297,691, 2,357,809, 2,551,582, 2,825,814, 2,833,648, 3,220,324, 3,220,831, and 3,220,833. In these methods For example, an electrostatically charged, photoconductive, insulating recording material is generally used which is responsive to imagewise exposure to electromagnetic radiation by selective discharge to form an electrostatic charge image. The electrostatic charge image is then made visible by bringing the charged surface of the photoconductive recording material into contact with suitable developer toner particles.

One of these known development methods involves cascading the developer over the surface bearing the image trickle as described in U.S. Patent 2,618,551. This cascade development process is useful for the development of ordinary line art, however, its applicability is limited when developing large areas is required. For the development of large areas, a magnetic brush development process has proven itself as

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proved to be more suitable «In such a magnetic brush development process becomes a developer disapproval that typically consists of ferromagnetic iron carrier grains and colored resin toner particles, with the help of a device, as described in US Pat. No. 3,003,462, applied to an electrostatic charge image. In In such a device, the iron particles are held in a bristle-like arrangement with the aid of a magnet, which resembles a brush, whereby the toner particles adhere to the iron through electrostatic attraction Iron particles are electrically conductive and favor the transfer of the toner to the surface carrying the charge image, A magnetic brush of this type is preferably electrically grounded, that is, it is electrical with the equipment connected to which it is attached. This process generally gives a very satisfactory development of Charge images in which the voltage of the background or the exposed areas is reduced to zero. In the Development of charge images in which the background voltage has never been reduced to zero appears at the Enibsrieklimg a considerable background density.

This developed density is undesirable and has hitherto been the result eliminates the fact that the magnetic brush is subjected to an electrical bias with a certain fixed value above the Grounding set. Such a system generally works satisfactorily, when the background density of the original to be reproduced is more or less constant. With many However, for templates, the background has a variable density or even different colors. Templates of this type can be used

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not correctly and evenly free of background density when using a fixed bias system reproduced.

There has therefore long been a need for a magnetic brush development system that in which these disadvantages do not occur and which is capable of consistently good reproductions of originals of the most varied types, which are free of a background tint under the most varied of conditions are.

The object of the invention is to provide a method and a device for developing an electrostatic charge image using a magnetic brush, which makes it possible to change the background density of an electrophotographic Reproduction according to the surface tension of the electrostatic charge image scanned during development to control and thereby making it possible to exert an excessive sensitivity to change sensed Surface tension significantly lower, making it possible is to apply an almost uniform development bias according to the overall image voltage.

It has now been found that this object can be achieved by an electrophotographic Development method can be solved by using an electrically conductive developing electrode which is a short distance from the image-bearing surface of an electrophotographic recording material is opposite, with toner material being moved through the gap and with the developer electrode

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Bias of a resistance track with a predetermined one Resistance between the electrode itself and a voltage source, which can optionally be grounded, derives and which prevents the downward fluctuations of the thus applied bias voltage from becoming one beforehand exceed the specified value; In doing so, the most commonly used tension is developed from the surface tension of the Recording material derived. It has been found that this development method using an apparatus can be done showing the changes in surface tension of the electrophotographic recording material samples, compares the sampled voltage with a stored voltage and, if necessary, an additional one Correction voltage supplies.

The invention now relates to a method for developing an electrostatic charge image on the surface of an electrophotographic recording material consisting of an electrically insulating layer and an electrically conductive layer support, in which the surface of the electrophotographic recording material is guided past a developer electrode of a magnetic brush developing device at a small distance is characterized in that a developer electrode with controlled bias is used, the bias of which is controlled in such a way that

a) an electrical charge with a voltage dependent on the voltage of the charge to be developed is stored and

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b) the voltage of the developer electrode on the through the stored voltage held a predetermined value will.

The invention also relates to a device for implementation of the method characterized above, which is achieved by a resistance track with a resistance between 10 and 10 Ohm between the developer electrode and the electrically conductive substrate of the electrophotographic recording material s is arranged and connected to it, is marked.

According to a preferred embodiment of the invention, the carrier for the toner particles and the developer electrode are involved around the same part as it is known in magnetic brush development systems. To carry out the procedure Other electrode development systems are also suitable for the invention, as detailed below.

The invention is explained in more detail below with reference to the accompanying drawings. Therein mean:

Fig. 1 is a "echeaatische representation of a manual method of magnetic brush development, embodying a method of the invention;

2 shows a vertical cross section through a mechanically driven one Magnetic brush suitable for automatically performing the method of the invention; and

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Fig. 3 is a partially sheared cross-section through a Development assembly necessary for carrying out the method of the invention using the apparatus of the invention is suitable and which explains the circuit schematically, with the help of which prevents it is intended that the bias voltage during the development of an electrostatic image in the process shown in FIG illustrated developing arrangement drops below a predetermined value.

1 shows a photographic recording material 102 which consists of a layer support 106 and an electrically insulating layer 108 which is applied to one side and which has an electrostatic charge image 104 on its outer surface which is to be made visible. The electrically insulating layer 108 consists of a material that is not sensitive to the activating radiation, for example an insulating polymeric material, which can also contain a substance ₉ which is normally insulating but which, however, can become conductive in the presence of activating radiation, as in FIG .B. a photoconductive material. The electrically conductive layer carrier is grounded by means of a suitable conductor 110. A magnetic brush, which has been suitably made using a bar magnet 112 with magnetically attracted iron support particles 114 of a suitable size and carrying toner particles electrostatically adhered thereto as described above, is allowed to wipe over the electrostatic charge image 104 in a manner known per se How to apply toner particles to the charge image * During development, a voltage occurs between the developer electrode 114 and

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the earth on. It is believed that this is a result of the Removal of the toner from the magnetic brush on the surface of the layer carrying the charge image, by induction, triboelectrics and other effects. When the electrically conductive magnetic brush is in close proximity to the charge image is placed on the surface of the layer 108, the toner is transferred from the brush to the surface. then an excess of charge remains on the brush 112 opposite to that of the electroscopic toner particles Polarity back. The brush thus takes a tension * opposite to that of the removed toner particles Polarity on. Since. * There is a resistance element between the magnet 112 and the reference voltage or earth 116 is located, a current flows between the brush and the reference voltage source and occurs at the resistance element Tension on in the form of a bias. The greater the resistance, the greater the voltage on the brush. If vice versa the value of the resistance decreases, the charge generated on the brush can be neutralized faster and the Brush therefore assumes a lower voltage.

Contrasted with the bias from an external DC power source the voltage generated between the developer electrode and the earth varies directly with the voltage of the contacted one Image districts · In this way, the image districts result in the maximum, medium and minimum densities in the original represent, from electrostatic image voltages of 600, 300 and 76 volts in the corresponding local surface area the developer electrode voltages of 300, 130 or 30 volts, but at any given time a considerable area of the electrostatic image with the

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winder is in contact, represents the respective average Voltage determined, for example, by reading the voltage drop across the resistor Average of all individual voltages on the surface of the developer contacted at this moment, For example, in a typical electrophotographic process in which an electrostatic image passes through In addition, stroking a magnetic brush is developed, as a result of the above range of 300 to 30 volts during the development of voltage differences along the external resistance can be measured within the range of, for example, 200 to 120 volts. By appropriate choice of the value of the resistor 116 within the above range, it is possible to select the developing conditions. If the resistance is relatively low, for example below about 10 ohms, the formed Bias is relatively low and changes little with changes in the surface tension of the developed local picture districts. This results in a relatively narrow exposure latitude together with good filling of the large areas in those districts with a practically even tension. If the resistance is proportionate is high, e.g. B. is about 10 ohms, the bias voltage formed is relatively high and it depends heavily on the Surface tensions of the developed local image areas. This reduces the breadth (spread) of the effective bias values, resulting in a reduction in the overall contrast and leads to an increase in the exposure latitude. Then the filling is less in large developed areas and the development is largely of the hem-type. (Peripheral area development)

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With resistance values between these extreme conditions, there is a continuous gradation and you get one '' Compromise between a filling with large-area development with a concomitant tendency to form a background density if the exposure is not precisely maintained and extreme fringing development with wide exposure latitude, no background density and poor Filling for large areas of development.

Figure 2 of the accompanying drawings illustrates an apparatus for automatically performing the method of the invention. This device is a magnetic brush assembly 120 which includes a storage container 122, which is closed at its ends with lids (not shown) to accommodate the developer 124. In the end caps are the ends of a concentric to the ends of a cylindrical magnet 128 which is magnetized over a diameter is rotatably attached to fixed shafts. 326. the rotatable mounting of the shaft 126 in the end caps allows the magnet to be adjusted for optimal brush formation. Two circular end caps (not shown) are rotatably attached to the tips on the shaft, between them concentrically a cylindrical, non-ferromagnetic one Sheath 130 rotatable therewith is attached. The surface of the shell 130 is preferably parallel to its axis of rotation grooved to facilitate the carrying of the developer on their surface during rotation. The end cap and shell 130 completely encircles magnet 128 to allow contact between the magnet and the developer 124 to avoid. The storage container 122 is essentially

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filled with a dry magnetic developer of the type described above. The shaft 126 is so in the End caps attached that extend approximately 1/4 of sleeve 130 above the average level of developer 124 in Extending out of Reservoir 122 · Wires 132 and 134 are electrical to shaft 126 and reservoir 122, respectively connected, which are led to a common attachment point at one end of the resistor 136, the other end with an arbitrarily set reference voltage, for example the earth, is electrically connected »

During operation, a recording material to be developed, denoted by the numeral 138, is moved in the direction of the arrow 139. The recording material 138 consists of an electrically conductive layer support 140 and an insulating layer 142, which is described in connection with FIG. 1, applied to a surface thereof. The insulating layer 142 consists of a material that is not sensitive to activating radiation, for example an insulating polymeric material that can also contain a substance that is normally insulating, but which is electrically conductive in the presence of activating radiation can be, such as B. a photoconductive material. The layer 142 bears on its Oberfläefee an electrostatic charge image 144. The sheath 130 is normally allowed in the direction of arrow 131 rotierenj opposite to the direction of franslationsbewegung of Aufgeiefeiangsmaterials 138 _β when the tray rotates, it carries on itsrer surface bristles of the developer 124 ₉ along the the lists of magnetisehen flow are formed, the ©! &"128 connects the N wad S"? of the magnet,

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the bristles over the charge image 144 on the surface of the Strip away layer 142 are borrowed from the wearer Removed toner particles and deposited in a known manner on the charged areas of the charge image. During development, the same occurs between the surface of layer 142 and the surface of envelope 130 Manner and for the same reasons as explained above, a tension

Fig. 3 of the accompanying drawings shows a cross section through a magnetic brush assembly similar to Fig. 2, the is generally designated by the numeral 2 and comprises a storage container 4 which at its ends with covers (not shown) and contains the developer 6. With the shaft 3 and the storage container 4, the wires 12 and 14 electrically connected to a common attachment point at the electrical connection point A to the circuit of Flg. 3.

During operation, a recording material 16 to be developed is moved in the direction of arrow 18. The recording material 16 can consist of an electrically conductive layer carrier 24 and one applied to one of its surfaces insulating layer 20 exist. The surface of the layer 20 carries the electrostatic charge image 22 to be developed. The electrically insulating layer 20 consists of a material that is not sensitive to the activating radiation, for example fine insulating polymeric material that may additionally contain a substance which is normally insulating, but which in the presence of the activating radiation

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- 13 - 21

development can be electrically conductive, such. B. a photoconductive one Material. The sheath 10 is usually rotated in the direction of arrow 26, opposite to the direction of Translational movement of the recording material 16. As the sleeve rotates, it carries bristles on its surface of the developer 6, which along the lines of the magnetic flux that the N and S foIe of the magnet 8 with each other connects, be formed. When the bristles graze over the charge image 22 on the surface of the layer 20, are typical of the carrier particles of the developer Way toner particles are removed and deposited on it according to the charge image. Develops during development another stress between the surface of the shell 10 and the surface of the layer 10, as above was explained in connection with FIG.

One by using the method of the invention to achieve The advantage achieved with a bias is that the exposure latitude is increased and the image-free areas are increased remain free of undesirable density while maintaining the reproduction of the fine details will. Since the effective bias is not constant over the entire image area, but rather from the local respective one Depending on the value of the surface tension, the advantages mentioned above can be achieved. The one used here The term "effective preload" refers to the voltage difference A distinction must be made between the brush and the surface of the recording material to be developed from the "bias" specified in the usual procedures, which is the voltage between the developer electrode or brush

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and represents the earth. The "effective bias" determines whether there is a background density in a given image area is developed or whether a voltage is generated that is high enough to cause a sparkover between the Brush and cause the surface of the recording material.

A useful portion of the induced voltage appears at point A and is applied to resistors 40 and 42 in series created. By properly choosing the value for resistors 40 and 42 it is possible to control a range of development conditions to be selected beforehand. When the combined resistance is relatively low, for example below about 10 ohms, the self-bias generated is relatively low and fluctuates only slightly when the Surface tension of the local image area to be developed. This leads to a relatively narrow exposure latitude together with a good filling when developing large areas in the image areas, which creates a practically uniform tension exhibit. If the resistance is relatively high, for example around 10 ohms, the self-bias created is relatively high and depends heavily on the surface tension of the developed local picture districts. This reduces the width (spread) of the effective preload values, which leads to a reduction in the overall contrast and an increase in the exposure latitude. The filling in the large-scale development districts is then less and the development is largely one those of the seaming type. Resistance values between these

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extreme conditions provide continuous gradation and compromise between good filling large-scale development with an accompanying tendency to form a background density if the exposure is not exactly right and an extreme fringing development a large exposure latitude without background density and poor filling in the case of large-area development.

The following description refers to that part of the figure of the accompanying drawing in which a circuit diagram is explained, which is used in connection with a magnetic brush, to prevent the value of the bias voltage from falling below the predetermined value. The circuit is in place at point A in communication with the magnetic brush developing device. The circuit of Fig * 3 works on the in the manner described below. A pre-determined part of one on the brush through contact with the the charge bearing surface of the recording material generated initial tension is applied to a voltage storage device. At a predetermined point in time, the Voltage storage device containing circuit is separated from the brush and the stored voltage is through a Choke coil amplifier chain with high power consumption (impedance amplifier) amplified to the value that is necessary that the increased voltage is equal to the initial voltage. The increased voltage is compared with the respective voltage.

It should be noted that this device is not a closed loop, i.e., "to achieve no feedback is used for stabilization, since

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the power supply from the brush is interrupted when the circuit is in operation. This is necessary because it is required when using a closed loop Feedback would be positive, which is easily seen. Assuming that a positive junction voltage is opposite the reference voltage or the earth occurs at the brush, then a positive transition voltage would be fed to the amplifier, reinforced and applied to the brush. However, it is very difficult to find an amplifier that is a positive Feedback used for more than a few seconds or

* (Reinforcement)

to keep stable for a maximum of minutes if the energy gain / of the feedback loop one unit or more than one Unity is. As a result, an open loop amplifier is preferred.

The operation of the circuit shown in FIG. 3 is explained in more detail below. The values of resistors 40 and 42 are selected from values which provide a partial voltage (sample voltage) when arranged to form a voltage divider between the magnetic brush and the voltage reference point 44 by a small percentage of that applied by the magnetic brush at its Contact with the charge image 22 generated voltage. If the polarity of the charge image is negative, the reference value is earth, as explained below. For example, if the percentage chosen is 1 % , the value of resistor 42 will be 1/100 de * the value of resistor 40. This voltage is applied through the normally closed contacts of section B of relay 46, shown in the off position “Capacitor 48 applied. With the help of

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Relay 46 it is possible to electrically connect the capacitor 48 the voltage tap point of the resistors 40 and 42 to be connected. As can be seen from considering the contacts of section C of relay 46, takes place below these Conditions of no power supply to the amplifier 72 so that the amplifier is not effective on the power supply signal appeals to. The image-bearing recording material 16, which is an electrostatic Carrying charge image 22 is moved in the direction of arrow 18 over the surface of the rotating casing 10 of the magnetic brush. During this movement, the recording material comes with the thin layer of on the magnetic brush surface located developer 6 in contact, with the result that toner particles are deposited in accordance with the charge image. A voltage is established between point "A" and the reference line 44 during development. To one before specified time, for example when the lead edge of the recording material 16 actuates the switch 30, is the relay 46 switched on. The movable relay leaves then take the positions indicated in dashed lines a. The section B of the relay 46 interrupts the energy supply of the amplifier 50 from the voltage divider, which is formed by the resistors 40 and 42, so that this only is still in connection with the capacitor 48. The section A of the relay 46 is in communication with the resistor 41 between the magnetic brush and the reference value of the line 44 to avoid an undesired accumulation of voltage on the brush, the otherwise would result from contact with an area of abnormally high stress on surface 20, too impede. The section C of the relay 46 closes the

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stronger chain and allows that in the capacitor 48 stored constant voltage is applied to the brush after it has been appropriately amplified.

It is assumed that initially a charge 22 with negative polarity on the surface of the insulating layer 20 to be scanned. The four sections of switch 52, a polarity selection switch, are then turned into the in positions shown by solid lines, where-

side grounded by the positive power output / of the amplifier 72 and the amplifier 64 is switched into the circuit. If the relay 46 asserts as just described becomes, the capacitor 48 is removed from its power supply source separated by opening the relay contacts 46B. The power supply of the amplifier 50 is then based on that in the capacitor 48 stored voltage, which is determined by the voltage divider resistors 40 and 42. The amplifier 50 must have a choke coil with high power consumption (impedance) in order to prevent it from occurring during operation Intervals the capacitor 48 discharges excessively. In general, its power supply impedance must be more in most cases than about 10 ohms. The value of the energy output is determined by suitable selection of the values of the resistors 58 and 62 and by appropriately setting the variable resistor 60 and by appropriately selecting the values of + LV and -LV /. The voltage supplies for these last-mentioned two values can expediently be between + and -10 volts to + and -50 volts, with the circuit described here a range from J15 to 25 volts is preferred. The tension

remediation
Yeriiuydes amplifier 50 is controlled by adjusting the feedback resistor 54. In the case of the amplifier 50

+) LV «■ Low voltage source, the positive pole of which with the Resistor 58 and whose negative pole is connected to the resistor.

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supplied positive energy does not occur phase reversal, so that the phase of the sign does not change.

The energy output from amplifier 50 is then fed to phase reversing amplifier 64 which is set to that a gain occurs ι of a unit. The amplifier 64 need not have as high a power supply impedance as amplifier 50 because it is within the circuit is not placed in such a position that any voltage storage device is discharged. The one from the amplifier 64 output energy is then passed through switch section 52B and variable resistor 70 into the energy input of amplifier 72 is introduced. The amplifier 72 becomes

The amplification is chosen so that it has a voltage amplification of about 200 volts so that with an energy supply of 1 or 2 volts an energy output of 200 volts can be achieved. The in The particular amplifier 72 shown in the examples has a power delivery capacity of up to 200 volts, so the Resistor 70 switched on to weaken the energy absorption signal accordingly, so that the energy output to about 500 volts is limited. The amplifier 72 has two energy outputs, one of which is positive and the other is negative. Either of the two energy outputs can be grounded and the other can be used as a signal energy output, depending on the desired polarity of the energy output. If the energy output is negative, the positive energy output is grounded. Then the emitted negative energy appears at the cathode of the diode 74. If the level of the voltage at the point A drops, i.e. becomes less negative, allow the diode lines 74 to the amplifier 72 to restore the correct bias. This occurs, for example, when

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the trailing edge of the recording material 16 slides over the brush, so that part of the scanned area carries no charge and the sensed average voltage is less than the required bias voltage. The amplifier then keeps the bias at the correct level Value until the rear edge 34 of the insulating layer 20 slides past the switch 30, this and thus that too Relay 46 enables. At this point the recording material has 16 probably passed the surface of the developer brush 10, so that it is no longer necessary to bias ) automatically.

In the event that a positive initial polarity is to be scanned on the surface of the insulating layer 20, the switch 52 is switched to the other position, that is to say it is switched to the position indicated in dashed lines. Thus, the amplifier 64 from the circuit is removed, and another, turned, the gain \ regulating resistor 56 for the amplifier 50th This is because another effective reinforcement may be required for this operation. The negative energy output of amplifier 72 is grounded and the positive energy released is introduced into the anode of diode 76. A drop in the positive voltage sensed by the surface of the brush lowers the voltage of cathode diode 76 so that it is conductive The amplifier 72 then restores the correct bias by the same mechanism as described above in connection with the use of negative surface tension.

Each of the diodes 74 and 76 must be capable of a high

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Peak reverse voltage (piv.), I.e. with positive cathode opposite the anode to withstand. The value of the peak reverse stress must be at least equal to the expected maximum surface tension since the brush is at a high voltage before the relay 46 is actuated while the amplifier output has practically the earth voltage. You must also have a high back resistance, / preferably more than 10 Ohms. In general, silicon diodes meet both requirements in a satisfactory manner.

The invention is explained in more detail below with reference to specific preferred embodiments.

Example 1 (comparative example, fixed pretension)

A photoconductive recording material was prepared by laying a 0.127 mm (0.005 inches) thick polyethylene terephthalate) film Carrier coated with an electrically conductive layer, the copper (l) iodide in one Poly (vinyl formal) binder. A barrier layer as described in the US patent was applied to the electrically conductive layer 3 245 833 described type, which in turn carried a photoconductive layer, the 4,4'-benzylidene-bis (N, N-diethyl-m-toluidine) The recording material thus obtained was contained in a binder was negatively charged to a surface tension of about 600 volts measured with the aid of an electrometer probe and in a photocopy camera with a composite reflection test image obtained from a photomontage of different "sizes and handwritten "typewritten" in different colors and printed characters and halftone images on different

+) Resistance of diodes 74 and 76 when the current direction is reversed.

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which consisted of white and colored backgrounds, exposed imagewise. The recording material was then developed by contacting it with a magnetic brush of the type shown in FIG. The preload on the brush was O _r, ie it was grounded. The brush contained electrolytic iron powder (Glidden 475 from SCM Corporation) mixed with 3% by weight of a black pigmented, thermoplastic styrene copolymer powder having positive polarity and an average particle size of about 8 microns. After development, the toned image was transferred to flat white paper by placing a sheet of paper on top of the developed image and passing the combination briefly under a negative corona discharge source. The paper was stripped from the recording material and then passed face up under a radiant heater in order to permanently fix the powder image on its surface. The remaining powder was removed by any suitable method from the recording material and the element was in the required dimensions some charged again the ^s and exposed so that a small loss occurred at fine lines and details of the bright image areas after development. This degree of exposure was arbitrarily set as the logarithm of the exposure (log-Ε) of 0.75. Then further exposures with values corresponding to log-of 0.60, 0.65, 0.30, 0.15 and 0.0 were carried out or the quadrature values were taken from decreasing levels of exposure after the previous one developed image was transferred and the recording material was recharged. The same series of exposures was made using negative bias voltages on the brush versus the electrically conductive one

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Layer of the photoconductive recording material at values of 60, 120, 180, 240 and 300 volts. The condition of bias 0 (grounded) became the "best" copy obtained at the exposure value of "0.60". Continuous tone development was observed in the areas of maximum image density observed with excellent even black tones. However, there was excessive background tinting itself in Districts that reflect the highly reflective white areas of the Originals corresponded. The background became only marginal by increasing the exposure by 40% at the value of "0.75" suppressed at the expense of fewer details in the bright areas of the image and there was a serious loss of fine Lines up. In the direction of underexposure, the minimum density rose very quickly. Even a copy with one exposure of "0.45" was totally unacceptable. When a bias was introduced and gradually increased, the best copy became obtained at progressively lower exposures that had an increasingly cleaner background. At all bias settings such as zero bias or the grounded State, an undesirable background density occurred in all exposures below the optimal exposure, while the details in the bright areas of the image were compromised at higher exposures. At the highest preload values, -240 and -300 volts, the maximum density was lowered and the image showed some of the characteristics fringing or outline development, the image density being in the inner parts of the broad areas of uniform surface charge It can be seen from this that there was a very narrow exposure latitude, i.e. with only one certain value of exposure for each value of bias

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was able to produce an image labeled "acceptable" will.

Example 2 (resistor bias)

The recording material of Example 1 was used in a similar manner as in Example 1 to make further series of copies. Instead of using fixed bias voltages, the bias voltage was established by switching on one of a series of high resistors in place of the bias voltage source. The resistances had the values IQ ⁶ , 10 ⁷ , 10 ⁸ , 10 ⁹ , 10 ¹⁰ and 10 ohms. The resistors were obtained using a Keithley Decade Shunt Resistor Box in which ι. one terminal was connected to the electrically conductive layer of the recording material and the other terminal was connected to the roller of the magnetic brush. During the development of each copy, the voltage across the resistor was measured using a high impedance electrostatic voltmeter having a power input impedance of 10 ohms. The voltage values obtained as a function of exposure and resistance are given in Table I below. This table shows that the preload at values

the external resistance of more than about 10 ohms remained practically constant. When using a resistor that is far was above this value, only a small profit was achieved, unless an extreme seam development is desired was. The table below also shows that an increase in exposure contrasts with a decrease in development bias was accompanied. At higher exposure in electrophotographic

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see process the surface tension decreases, the remains effective bias, which is the difference between the surface tension and the voltage generated on the brush, better constant than is the case when the Bias voltage is obtained from a fixed source. It follows that eti achieves much wider latitude in exposure than can be achieved using conventional electrode development techniques.

Tabel I. io ⁶ 26th 23 20th the exposure 15th and modification the self-prestressing ⁺ in the event of a change ! O ⁷ 132 118 100 70 of the external electrical circuit resistance io ⁸ 360 308 255 17th 163 13th 10 ⁹
1 r \ 450 390 315 83 200 60 io ¹⁰ 455 400 325 205 220 140 io ¹¹ 455 400 325 242 220 180 Outer
Contrary 0 0.15 0.30 250 0.60 195 was standing 250 195 (Ohm) 0.45 0.7

Relative logarithmic exposure ⁺ maximum voltage drop along the external resistance.

The copies were oriented in a rectangular arrangement according to the arrangement of the table and for their overall quality and examined their exposure latitude. It has been found that the best exposure latitude is the best maximum density and no background density with an outside

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Resistance of IO ohms were obtained. Acceptable results were obtained using a resistor between

7 9 '

10 and 10 ohms. Over the entire exposure range was

get a good development of the large areas and a high maximum density (D). The background density was only Max

noticeable at the lowest exposure at which it became detrimental, but legibility was not affected, even in the extremely bright picture areas. The highest exposures resulted in a loss of the very bright ones and the very fine ones Lines; this could be done by switching on a resistor with a lower value, e.g. B. 10 ohms, slightly reduced will. Most of the originals, with average gradation and detail, were about the acceptable copies are obtained over the entire range of 0.15 to 0.60 relative logarithmic exposure. To achieve a comparable copy quality and a comparable exposure latitude according to the usual system with fixed Pre-tensioning according to Example 1 was six different settings of the pre-tensioning and a coordination by a person skilled in the art necessary.

Example 3 (negative surface tension)

The values for the suitable Operation of the circuit of Fig. 3 have been selected, along with the respective amplifiers.

Resistances:

40 10 ⁸ Ohm 41 10 ⁷ ohm 42 10 ⁶ ohm

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54 50 000 ohm Potentiometer 56 50 000 ohm Potentiometer 58 47 000 ohm 60 10 000 ohm Potentiometer 62 47 000 ohm 66 10 000 ohm 68 10 000 ohm 70 50 000 ohm variable capacitor

48 0.0068 microfarads

Diodes

74 2-IN629 diodes in series

JC It Il Il Il It Il amplifier

50 operational amplifier model 14IA (Analog Devices, Inc.,

Cambridge, Massachusetts)
64 Model 111 (same manufacturer) 72 High-voltage operational amplifier (Kepco Model OPS-2000 from Kepco Inc., Flushing, New York)

Sources of energy

The values chosen for + LV and -LV were + and, respectively -15 volts.

The other operating voltages are specified by the manufacturer.

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The gain control resistor 70 was originally adjusted so that the output voltage of the amplifier 72 was 500 volts with a 2 volt power supply. The gain control resistor 54 was then adjusted so that a voltage of 500 volts has been obtained at the output of amplifier 72 when the input voltage of amplifier 50 Was 5 volts, i.e. the overall gain of the amplifier chain precisely compensated for the attenuation of the sampled voltage, which was formed by the voltage divider comprising the resistors 40 and 42. When using an insulating surface a negative charge image corresponding to that produced by exposure of a photographic positive the bristle was developed, it was found that the circuit kept the bias constant over the entire development interval I. This applies to a surface tension any value within the interval between 150 and 600 volts.

Example 4 (positive surface tension)

The components used were identical to those used to detect (sense) a negative surface tension (Example 3), except that - as already mentioned above - the amplifier 64 and the associated resistors are switched off from the circuit became. The device was designed to develop a positive electrostatic charge image corresponding to that formed by exposing a photographic negative, i.e. used to develop such an image, in which the background districts were charged instead of discharged.

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" ²⁹ " 2136698

It has been found that using the same reinforcement, as applied in connection with example 1, the bias provided only about half that required for optimal development. To make up for that the gain control resistor 56 was adjusted to give a total gain of 200 or twice those required in Example 1 was obtained was. This means that the power output from amplifier 72 was 500 volts when the power to amplifier 50 was applied Was 2.5 volts instead of 5 volts. It has been found that the circuit (circuit) provides the bias voltage for the kept voltages sampled constant within a range of about 100 to about 500 volts or higher.

From the foregoing it can be seen that the method of the invention does not rely on the use of a magnetic Brush development is limited. It is also suitable for development by any other process, in which an electrode is located a short distance from a surface carrying a charge. she is therefore also suitable for development processes, such as liquid development, aerosol development, a Powder development, a cascade development, a development with palz brushes including the metalized aluminum brushes. For example, for the production of a developer brush, a carrier with an attached plurality of non-metallic threads, such as. B, natural or synthetic Hair materials (fur materials) »are used for their part optionally can carry a thin adhesive layer of metal. The process of making such materia- lien is described, for example, in Belgian patent 757 259.

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In a preferred brush arrangement, two such brushes, arranged one behind the other, are used, of which the one brush has a low electrical conductivity and the other has a high electrical conductivity. These The arrangement is described in more detail in Belgian patent specification 762 379..

A particularly preferred arrangement consists of a double magnetic brush, such as that in the Belgian one Patent 718 099 is described.

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Claims (2)

-31- 2136896 Patent claims 1.) Process for developing an electrostatic charge image on the surface of an electrically insulating layer and an electrically conductive substrate existing electrophotographic recording material in which the surface of the electrophotographic Recording material at a small distance past a developer electrode of a magnetic brush developing device is, characterized in that a developer electrode is used with controlled bias, the Bias is controlled in such a way that a) an electrical charge is stored with a voltage that depends on the voltage of the charge to be developed and b) the voltage of the developer electrode is kept at the value predetermined by the stored voltage will. 2. Device for performing the method according to claim 1, characterized by a resistance track (116) with a Resistance between 10 and 10 ohms between the developer electrode (114) and the electrically conductive substrate (106) of the electrophotographic recording material (102) is arranged and connected thereto. 109885/1737