US3779203A - Toner concentration control apparatus - Google Patents

Toner concentration control apparatus Download PDF

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US3779203A
US3779203A US3779203DA US3779203A US 3779203 A US3779203 A US 3779203A US 3779203D A US3779203D A US 3779203DA US 3779203 A US3779203 A US 3779203A
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toner
means
signal
mixture
images
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C Altmann
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Eastman Kodak Co
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Eastman Kodak Co
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    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ELECTROGRAPHY; HOLOGRAPHY
    • G03GELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00Apparatus for electrographic processes using a charge pattern
    • G03G15/06Apparatus for electrographic processes using a charge pattern for developing
    • G03G15/08Apparatus for electrographic processes using a charge pattern for developing using a solid developer, e.g. powder developer
    • G03G15/0822Arrangements for preparing, mixing, supplying or dispensing developer
    • G03G15/0848Arrangements for testing or measuring developer properties or quality, e.g. charge, size, flowability
    • G03G15/0849Detection or control means for the developer concentration
    • G03G15/0851Detection or control means for the developer concentration the concentration being measured by electrical means
    • GPHYSICS
    • G03PHOTOGRAPHY; CINEMATOGRAPHY; ELECTROGRAPHY; HOLOGRAPHY
    • G03GELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00Apparatus for electrographic processes using a charge pattern
    • G03G15/50Machine control of apparatus for electrographic processes using a charge pattern, e.g. regulating differents parts of the machine, multimode copiers, microprocessor control
    • G03G15/5033Machine control of apparatus for electrographic processes using a charge pattern, e.g. regulating differents parts of the machine, multimode copiers, microprocessor control by measuring the photoconductor characteristics, e.g. temperature, or the characteristics of an image on the photoconductor
    • G03G15/5041Detecting a toner image, e.g. density, toner coverage, using a test patch

Abstract

A toner concentration control apparatus includes a circuit responsive to the cummulative total charge of electrostatic images and/or toner images which pass a predetermined position of a path for producing a signal which is an accurate measure of the quantity of toner which will and/or has been applied to such images.

Description

United States Patent [191 Altmann 1 Dec. 18, 1973 1 TONER CONCENTRATION CONTROL APPARATUS [75] Inventor: Conrad Altmann, Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, N.Y.

[22] Filed: Aug. 14, 1972 [21] App1. No.: 280,396

[52] US. Cl 118/8, 118/637, 324/72 [51] Int. Cl G03g 13/16 [58] Field of Search 118/7, 8, 637, DIG. 24;

117/175; 346/74 ES; 355/3, 3 DD; 324/32, 72

[56] References Cited UNITED STATES PATENTS 10/1960 Giamo, Jr. 118/637 UX 2/1963 Codichini 6/1963 Sneliing 324/32 X 3,161,882 12/1964 Muilin 117/175 X 3,348,522 10/1967 Donohue [18/637 X 3,367,307 2/1968 Lawes et a1 118/7 X 3,406,334 10/1968 Marquart et a1 324/72 X 3,449,658 6/1969 Robinson et a1 324/72 X 3,674,353 7/1972 Trachtenberg 355/3 DD Primary Examiner-Morris Kaplan AttorneyW. 1-1. .1. Kline et a1.

[57] ABSTRACT A toner concentration control apparatus includes a circuit responsive to the cummuiative total charge of electrostatic images and/or toner images which pass a predetermined position of a path for producing a signal which is an accurate measure of the quantity of toner which wi11 and/or has been applied to such images.

8 Claims, 3 Drawing Figures sir/9.203

PATENTED DEC 18 I975 SHEET 10F 2 Pmmmuu: 1 a mu 3.779.203

SHIT 2 [IF 2 68 FROM 1+ 64 a 7 4 THRESHOLD To DECTECTOR 52 P 76 x 75 DELAY CIRCUIT 34 FIG. 3

TONER CONCENTRATION CONTROL APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrographic development apparatus, and more particularly, to toner concentration control apparatus for maintaining the relative con centration of toner in a mixture of toner and carrier components at a level wherein copies of uniformly good density are consistently producable.

2. Description of the Prior Art In electrographic apparatus, an electrostatic image, formed on the surface of a drum or web, is developed by application of finely divided toner particles thereon to form a toner image. In certain electrographic apparatus, toner images are formed from electrostatic images by brushing a developer mixture of ferromagnetic carrier particles and much smaller toner particles (typically suitably pigmented or dyed resin-based particles) across the electrostatic images. The contact of the ferromagnetic particles with the toner particles charges the toner particles by triboelectrification to a polarity needed so that the toner particles are attracted to the electrostatic images for toning. In this process, toner particles are depleted from the developer mixture, requiring subsequent replenishment to avoid a gradual reduction in density of the toner images. Toner replenishment is accomplished by several different types of apparatus. In one type, a given amount of toner is added to the mixture after a given number of copies is made. This approach is acceptable providing the amount of toner used for each copy is reasonably predictable. In apparatus which do not tone broad, solid areas, however, such as many magnetic brush developing and cascade toning apparatus, the amount of toner used in any copy of group of copies can vary substantially. For this reason, toner concentration monitors have been designed which automatically add toner according to the results .of a monitoring process; see for example U.S. Pat. Nos. 2,956,487 to Giamo; 3,094,049 to Snelling; 3,227,549 to Ullrich, 3,233,781 to Grubbs; 3,399,652 to Gauron; and 3,409,901 to Dost et a1.

Certain toner concentration monitors examine the quality of a toner image before adding toner. These apparatus suffer from the disadvantage that toner is not added until only after copy quality is degraded.

In certain optical monitoring apparatus a reduction in the relative toner concentration in the mixture below a desired level can be detected in the mixture of toner and carrier before image degradation. (See for example U.S. Pat. Nos. 3,233,781 to Grubbs and 3,409,901 to Dost etal). Such optical apparatus have not proven entirely satisfactory in operation, especially over extended periods of time. This may be attributed, at least in part, to the lighting source utilized for illuminating the developer mixture which may be a luminous energy source of relatively low output, unstable intensity and short life, and further to the fact that spectral characteristics of light from the source may be closely akin to the background light, thereby making background discrimination difficult. Also contributing to the instability of such devices is the fact that the photoconductive cell used for detecting developer reflectance may be sensitive to temperature and humidity variations of the type encountered in normal machine operation, or may exhibit an undesirable hysteresis or light history effect. Moreover, when photocells are confronted with the interference of airborne toner particles they often provide inaccurate readings. In general, optical monitoring apparatus require frequent cleaning of components to assure freedom from erroneous measurements.

Another approach is set forth in commonly assigned copending U.S. Patent Application Ser. No. 158,174, filed July l, 1971, now U.S. Pat. No. 3,674,353, entitled, TONER CONCENTRATION CONTROL APPA- RATUS in the name of William Trachtenberg. This invention pertains to an open loop toner concentration monitor wherein monitoring is achieved by sensing the charge of an image before and after development. The monitor produces an analog signal representative of the difference between, before and after image charges. Although such an approach can perform satisfactorily, it is difficult in practice to correlate the analog signal in terms of toner lost to the development station.

SUMMARY OF THE INVENTION An object of the present invention then is to improve electrographic development by providing an apparatus for automatically maintaining the toner concentration in an electrographic developer at a substantially constant level at which high quality toner images can be consistently produced.

Another object of the invention is to provide an inexpensive, yet highly accurate and reliable, apparatus for automatically controlling the level of toner concentration in an electrographic developer.

Another object of the invention is to provide an improved toner concentration control apparatus which does not require the need for monitoring the mixture of toner and carrier components.

In accordance with the invention, it has been determined that the magnitude of the total charge of an electrostatic latent image is directly related to the amount of toner which will be attracted to the electrostatic image and lost to the development station.

In the disclosed apparatus, a toner concentration control apparatus includes means for producing a signal which is a function of the cummulative total charge of electrostatic images passing a predetermined position along a path. The total charge of each electrostatic image is related to the amount of toner which is required to develop such image. The apparatus further includes means responsive to a particular magnitude of said signal for causing toner to be added to the mixture to increase the relative concentration of the toner component and obtain a desired relative concentrations of mixture components.

Other objects of the invention and its various advantages will become apparent from the ensuing detailed description of the preferred embodiment shown below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic vertical section of an apparatus embodying the invention; and

FIGS. 2 and 3 are circuit diagrams partially in schematic and partially in block form which may be utilized with the apparatus shown in FIG. 1.-

DESCRIPTION OF THE PREFERRED EMBODIMENT For a general understanding of an electrographic reproducing or copier apparatus wherein the invention has particular utility, reference is made to FIG. 1

wherein various stations of an electrophotographic ap-' member 16, shown as a web, to selectively dissipate change and form an electrostatic image. The web had previously been uniformly charged at a charging station 20. The photosensitive member 16 includes a photoconductive layer with a conductive backing. The photoconductive layer may comprise, for instance, a heterogeneous mixture of a bisphenol A-polycarbonate binder, a triarylmethane organic photoconductor and a pyrylium sensitizing dye. The apparatus further includes a development station 22 at which the electrostatic image is contacted with finely divided charged toner particles that adhere' to the photoconductive layer in a configuration defined by the electrostatic image; a transfer station 24 in which toner particles are transferred in the image configuration to a receiving surface of a copy sheet 26 on which it can be subsequently permanently fused; and a cleaning station (not shown) inwhich residual toner is removed from the photoconductive layer so that it can be reused. In certain known modifications of this same system, one or more of these stations may be eliminated. For a more complete description of an electrographic apparatus reference may be made to commonly assigned copending patent application Ser. No. 191,304 filed Sept. 21, 1971, now U.S. Pat. No. 3,698,805, entitled CON TROL APPARATUS FOR ELECTROPI-IOTO- GRAPHIC APPARATUS to Hickey et al. An example of an exemplary development station is set forth in U.S. Pat. No. 3,543,720 to Drexler. The development station 22 shown is as a magnetic brush-type apparatus which includes a cylindrical brush member 30 which is rotated in a counterclockwise direction as shown by the arrow, in a magnetic field produced by magnets (not shown). The brush member 30 is effective to transport a portion of a mixture 33 of ferromagnetic carrier particles and toner particles from a developer reservoir 34 which is a conductive member and acts as an electrode across the surface of the web 16 such that toner particles will contact a portion of and then remain on charged image areas on the surface of the web 16. Since a mixture having a high concentration toner would tend to clog, a continuous control mixing paddle-wheel 35 has been provided to uniformly blend the mixture.

As the relative concentration of toner in the mixture decreases, the resulting developed toner images will tend to have a lower density and eventually will appear washed-out. In order to alleviate this condition, a toner concentration monitor has been provided which is shown generally at 40 and is coupled to a charge sensor (disposed at a predetermined position along the web path) such as an electrometer 47 which is coupled to a source of potential shown for convenience of illustration as a battery 48. The probe 47 is also coupled to a development station potential control apparatus 71 (also referred to herein as an auto bias system which is adapted to control the potential of the development reservoir 34 to provide the well known autobias function described in detail in U.S. Pat. No. 2,956,487. An example of an electrometer suitable for use with the invention would be an induction plate or probe fabricated from a highly conductive material such as copper or aluminum, and spaced closely to the web to web to receive an induced charge as an electrostatic image passes by. The electrometer 47 may extend over the whole width of the area and is adapted to scan the electrostatic image and produce an electrostatic signal which is proportional to the charge density of that p0rtion of the electrostatic image passing under the probe 47. In placing the probe 47, it is necessary to take into account the factor that some photoconductive members exhibit a sharp reduction in conductivity for a short time period, usually less than one second, following exposure. Thus, it is desirable to displace the electrometer 47 a sufficient distance from the exposure station 21 so that the effect of such conductivity reduction will have run its course before the undeveloped image arrives at the meter 47. The probe 47 is directly coupled to the auto-bias system 71 which will be described in connection with FIG. 3. I

The signal produced by the probe 47 is also applied to the circuitry 50. The circuitry 50 is effective when the toner concentration of the mixture-will be reduced below a desired level to actuate a solenoid device 52 which rotates a metering wheel 57 releasing a predetermined increment of toner from a toner container 56. Such toner increment, under the influence of gravity, falls into the reservoir 34 wherein the paddle-wheel 35 uniformly blends it into the developer mixture 33, thereby returning the relative concentrations of toner and carrier components to a desired level. One of the advantages obtainable with the toner control apparatus 40 is that any conventional toner 'replenisher mechanism can be actuated by the aforementioned output signal, such as for example that disclosed in U.S. Pat. No. 3,409,901.

Turning now to FIG. 2, wherein the circuit 50 for actuating the solenoid 52 is shown in more detail and includes an integrator 60 shown as an operational amplifier with a feedback capacitor 62 coupled between its output and non-inverting input terminals.

The inverting input of the operational amplifier 60 is coupled to ground. The output signal produced by the amplifier 60 is applied to a charging circuit comprising an adjustable resistor 64 and a charging capacitor 66 coupled to ground. At the electrical junction of the resistor 64 and the capacitor 66 there is provided a threshold detector circuit 68 which may include a conventional unijunction transistor which fires when the capacitor 66 reaches a predetermined charge level. At such time, the threshold circuit 68 will provide a signal to the solenoid 52 causing its actuation.

Briefly, in operation, the circuit 50 functions as follows. The charge level of an electrostatic charge density (viz., field strength) is continuously sampled and the output of the operational amplifier 60 provides a signal representative of the cumulative total charge level of electrostatic images passing by the probe 47. When a predetermined charge level on the capacitor 66 is reached (which may be changed by adjusting the resistance of the resistor 64) the threshold detector circharge on the capacitor 66 and reduce the output potential of the amplifier 60 to ground level. The relay 70 is shortly thereafter de-energized and opens contacts 70a thereby conditioning the circuit 50 to repeat the above operation.

A second embodiment of the invention is shown in H6. 1 wherein a probe 47 (shown in dotted lines) is disposed in a predetermined position along the web path disposed after the development station 22. Assuming the electrostatic images have a negative potential, then toner images corresponding thereto will have positive potential. Accordingly the probe 47 provides a similar function as the probe 47 but will provide a signal to the circuitry 50 which is representative of the charge density of deposited toner. The integrator 60 now provides a signal representative of the total change of toner and the threshold detector 68 will be actuated when a predetermined amount of toner has been lost to the reservoir 34.

The autobias system 71 is shown in FIG. 3 and includes a preamplifier 72 which receives a signal directly from the probe 47 and amplifiers same. The preamplifier 72 also provides an isolation function. A conventional delay circuit 74 receives an input from the amplifier 72 and is adapted to delay the output signal of the amplifier 72 for a time period sufficient to permit an electrostatic image sampled by the probe 47 to advance over the development station for toning. An example of such a circuit is set forth in U. S. Pat. No. 3,61 1,982. The output of the delay circuit 74 is applied through an adjustable resistor 75 to a power amplifier 76 which adjusts the output signal to a potential level suitable for use and applies it to the reservoir or electrode 34.

It will be appreciated by those skilled in the art that the subject invention has utility in a number of electrographic reproduction systems, including xeroprinting, thermoxerography, and xerothermography, and in no way is dependent upon the physical form of the developer (viz. liquid or particulate) or the manner in which it is applied to the electrostatic image to render it visible.

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

1 claim:

1. For use in electrographic apparatus having a member charged with a pattern forming an electrostatic image, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from forming the toner images, an improved toner replenishment control apparatus comprising:

a. means for sensing the charge density on electrostatic images prior to toning and for producing a signal the magnitude of which is representative of the total cummulative charge thereof passing a predetermined portion; and

b. means responsive to a predetermined magnitude of said signal for actuating the toner replenishment means to cause the addition of toner to the mixture.

2. In electrographic apparatus in which a surface carrying an electrostatic charge pattern is contacted by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner concentration apparatus comprising:

a. first means for sensing the electric field strength associated with an electrostatic charge pattern to provide a signal representative thereof; and

b. second means responsive solely to said signal for indicating the amount of toner which will be depleted by producing a second signal the magnitude of which is a function of the amount of toner which will be deposited on the surface.

3. The toner concentration apparatus as set forth in claim 2 including:

c. a container for storing toner particles;

(1. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and

e. said second means including means responsive to a particular magnitude of said second signal for actuating said actuable means.

4. The invention as set forth in claim 3 wherein said second means includes an integrator circuit.

5. For use in electrographic apparatus having a photoconductive member adapted to form electrostatic images, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images respectively, and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from forming the toner images, an improved toner replenishment control apparatus comprising:

a. means for sensing the charge density on toner images and for producing a signal the magnitude of which is representative of the total cummulative charge thereof passing a predetermined portion; and

b. means responsive to a predetermined magnitude of said signal for actuating the toner replenishment means to cause the addition of toner to the mixture.

6. In electrographic apparatus in which a surface car- 5 rying an electrostatic charge pattern is contacted by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner con centration apparatus comprising:

a. first means for sensing the electric field strength associated with a toner image and providing a signal representative thereof; and

b. second means responsive solely to said signal for indicating toner depletion by producing a second signal the magnitude of which is a function of the amount of toner deposited on the toner image.

7. The toner concentration apparatus as set forth in claim 6 including:

c. a container for storing toner particles;

d. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and

e. said second means including means responsive to a particular magnitude of said second signal for actuating said actuable means.

8. The invention as set forth in claim 7 wherein said second means includes an integrator circuit.

k OK I.

Claims (8)

1. For use in electrographic apparatus having a member charged with a pattern forming an electrostatic image, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from forming the toner images, an improved toner replenishment control apparatus comprising: a. means for sensing the charge density on electrostatic images prior to toning and for producing a signal the magnitude of which is representative of the total cummulative charge thereof passing a predetermined portion; and b. means responsive to a predetermined magnitude of said signal for actuating the toner replenishment means to cause the addition of toner to the mixture.
2. In electrographic apparatus in which a surface carrying an electrostatic charge pattern is contacted by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner concentration apparatus comprising: a. first means for sensing the electric field strength associated with an electrostatic charge pattern to provide a signal representative thereof; and b. second means responsive solely to said signal for indicating the amount of toner which will be depleted by producing a second signal the magnitude of which is a function of the amount of toner which will be deposited on the surface.
3. The toner concentration apparatus as set forth in claim 2 including: c. a container for storing toner particles; d. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and e. said second means including means responsive to a particular magnitude of said second signal for actuating said actuable means.
4. The invention as set forth in claim 3 wherein said second means includes an integrator circuit.
5. For use in electrographic apparatus having a photoconductive member adapted to form electrostatic images, developing means for contacting such electrostatic images with a mixture of toner and carrier components to form toner images corresponding to the electrostatic images respectively, and actuable toner replenishment means effective when actuated for adding toner to the mixture to compensate for toner depletion resulting from forming the toner images, an improved toner replenishment control apparatus comprising: a. means for sensing the charge density on toner images and for producing a signal the magnitude of which is representative of the total cummulative charge thereof passing a predetermined portion; and b. means responsive to a predetermined magnitude of said signal for actuating the toner replenishment means to cause the addition of toner to the mixture.
6. In electrographic apparatus in which a surface carrying an electrostatic charge pattern is contacted by a developer mixture of carrier particles and electrostatically attractable toner particles to form a toner image on the surface according to the pattern, a toner concentration apparatus comprising: a. first means for sensing the electric field strength associated with a toner image and providing a signal representative thereof; and b. second means responsive solely to said signal for indicating toner depletion by producing a second signal the magnitude of which is a function of the amount of toner deposited on the toner image.
7. The toner concentration apparatus as set forth in claim 6 including: c. a container for storing toner particles; d. actuable means effective when actuated for causing toner from said container to be delivered to the mixture; and e. said second means including means responsive to a particular magnitude of said second signal for actuating said actuable means.
8. The invention as set forth in claim 7 wherein said second means includes an integrator circuit.
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4021112A (en) * 1975-06-23 1977-05-03 Xerox Corporation Photoreceptor dark current leakage detecting apparatus for xerographic machines
US4524088A (en) * 1983-01-31 1985-06-18 Magnetic Technologies Corp. Method and apparatus for controlling the thickness of developer on an applicator, such as a magnetic brush, in electrostatic reproduction
US4575224A (en) * 1984-12-05 1986-03-11 Eastman Kodak Company Electrographic apparatus having an on-line densitometer
US4607944A (en) * 1985-06-07 1986-08-26 Eastman Kodak Company Apparatus for controlling toner replenishment in electrographic copier

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US2956487A (en) * 1955-03-23 1960-10-18 Rca Corp Electrostatic printing
US3079483A (en) * 1962-01-02 1963-02-26 Xerox Corp Xerographic fixing apparatus
US3094049A (en) * 1961-02-03 1963-06-18 Xerox Corp Xerographic developer measuring apparatus
US3161882A (en) * 1960-08-05 1964-12-15 Minnesota Mining & Mfg Galvanometer using electrostatic orifice recording means
US3348522A (en) * 1962-08-28 1967-10-24 Xerox Corp Automatic toner control system
US3367307A (en) * 1965-06-02 1968-02-06 Arlside Ltd Electrostatographic developing apparatus
US3406334A (en) * 1964-07-27 1968-10-15 Nuclear Corp Of America Apparatus for testing electrostatic copy material
US3449658A (en) * 1966-11-17 1969-06-10 Eastman Kodak Co Method and apparatus for sensitometrically testing photoconductive insulators
US3674353A (en) * 1971-07-01 1972-07-04 Eastman Kodak Co Toner concentration control apparatus

Patent Citations (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2956487A (en) * 1955-03-23 1960-10-18 Rca Corp Electrostatic printing
US3161882A (en) * 1960-08-05 1964-12-15 Minnesota Mining & Mfg Galvanometer using electrostatic orifice recording means
US3094049A (en) * 1961-02-03 1963-06-18 Xerox Corp Xerographic developer measuring apparatus
US3079483A (en) * 1962-01-02 1963-02-26 Xerox Corp Xerographic fixing apparatus
US3348522A (en) * 1962-08-28 1967-10-24 Xerox Corp Automatic toner control system
US3406334A (en) * 1964-07-27 1968-10-15 Nuclear Corp Of America Apparatus for testing electrostatic copy material
US3367307A (en) * 1965-06-02 1968-02-06 Arlside Ltd Electrostatographic developing apparatus
US3449658A (en) * 1966-11-17 1969-06-10 Eastman Kodak Co Method and apparatus for sensitometrically testing photoconductive insulators
US3674353A (en) * 1971-07-01 1972-07-04 Eastman Kodak Co Toner concentration control apparatus

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4021112A (en) * 1975-06-23 1977-05-03 Xerox Corporation Photoreceptor dark current leakage detecting apparatus for xerographic machines
US4524088A (en) * 1983-01-31 1985-06-18 Magnetic Technologies Corp. Method and apparatus for controlling the thickness of developer on an applicator, such as a magnetic brush, in electrostatic reproduction
US4575224A (en) * 1984-12-05 1986-03-11 Eastman Kodak Company Electrographic apparatus having an on-line densitometer
US4607944A (en) * 1985-06-07 1986-08-26 Eastman Kodak Company Apparatus for controlling toner replenishment in electrographic copier

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