US3399652A - Automatic toner concentrate detector - Google Patents

Description

Sept. 3, 1968 s. A. GAWRON 3,399,652

AUTOMATIC TONER CCNCENTRATE DETECTOR Filed June 14, 1967 2 sheets-sheet 1 Sept 3, 1968 s. A. GAWRON 3,399,652

AUTOMATIC TONER CONCENTRATE DETECTOR Filed June 14, 1967 2 Sheets-Sheet 2 I1 ULL United States Patent 3,399,652 AUTOMATIC TONER CONCENTRATE DETECTOR Stanley A. Gawron, Mount Prospect, lll., assigner to Addressograph-Multigraph Corporation, Mount Prospect, Ill., a corporation of Delaware Filed June 14, 1967, Ser. No. 646,056 18 Claims. (Cl. 118-637) ABSTRACT OF THE DISCLOSURE A rotating reective disc located directly in a developer mix containing toner is electrically biased so that it will attract toner from the The amount of toner attracted to the surface of the disc is a function of the concentration of toner in the mix. A light beam is directed to reect off the surface onto a photoelectric unit, with the intensity of the light striking the unit being controlled by the amount of toner on the surface. The photoelectric unit and source of light are mounted in a protective housing in order to render them free of toner dust. The unit is calibrated to signal a dispensing apparatus to operate when the intensity of the reflected light indicates a less than optimum toner concentration.

Background of the invention This invention relates to an apparatus which monitors and controls the feeding of toner to a mass of developer mix in accordance with the concentration of toner in said mix. More particularly, this invention relates to an apparatus for continuously measuring the concentration of toner in the developer mix by optically sensing the amount of toner that is triboelectrically attracted to a sampling medium. In accordance with the well known electrostatic printing process, a surface bearing a latent electrostatic image is developed by selectively applying a developer mix comprising toner and a carrier medium to the image. With repeated use the mix toner is gradually consumed until there is no longer a sufficient concentration of toner in the mix to develop high quality, dense images.

The prior art systems of monitoring toner concentration follow two general schemes of measurement, electrical and optical. Electrical systems depend on measuring the changes in resistance of the mix as the high resistivity toner component varies in relation to the conductive carrier component, and the optical systems depend on measuring the density of a calibratead toner test pattern. In the latter system the test pattern is established in accordance with the concentration of toner in the mix providing a graduated visible scale correlated to the toner concentration as determined by an optical sensing device.

The prior art systems are somewhat deficient in that variations in the toner formulation in the case of the electrical systems require frequent calibration checks to account for any changes in resistance of the thermoplastic resin. Optical systems are confronted with the interference due to the airborne particles falling on the optical sensing components which result in false readings and in general require frequent cleaning maintenance of all the components in order to be sure that no erroneous measurements occur.

Summary of the invention It is the general object of this invention to provide an improved automatic toner feed control device that is accurate, reliable and free of frequent cleaning maintenance.

It is'a further object of'this invention to provide an automatic toner feed control that monitors the concentration directly at the point of addition of the toner so ice that there is little time-lag between the generation of the moniorin'g signal and the activation of the dispensing device in either starting or stopping.

It is a still further object of this invention to provide an automatic toner feed control in which the toner sampling device is self-cleaning.

It is still another object of this invention to provide an automatic toner feed control in which optical components used to measure the amount of toner deposition are protected against the accumulation of the toner from the optical path.

It is a specific object of this invention to provide an automatic toner feed control which is capable of operating under changing temperature conditions. A related object is to provide a temperature responsive control operatively associated with the photoelectric instruments which compensate for fluctuation of the temperature.

-It is still another specific object of this invention to provide a toner feed control device which is particularly well adapted to be used in conjunction with conventional magnetic brush developing assemblies.

The present invention comprises a conductive probe having a reflective or transparent light transmitting surface, a reflective surface being preferred, means for bringing the probe and mix into contact to collect a sample of toner on the surface, means for photoelectrically measuring the amount of toner on the surface, and dispensing means for adding toner to the mix in accordance with the amount of toner attracted to and retained on the probe. The term light transmitting as used herein refers either to the light reflective property of the probe or its transparent property. The probe is connected to a DC potential imparting to the surface a toner-attracting electrostatic charge which is uniformly distributed over the surface to establish an electrical eld of uniform intensity. The range of voltage to be applied can vary widely depending on the nature of the mix formulation and, in particular, the type of resin toner employed. In a typical magnetic brush arrangement where the resistivity of the toner is in the range of from 1010-1 012 ohm centimeters and the carrier is iron, the applied voltage can range from 25 200 volts, preferably in the range of from -40-60 volts. It has been found that, if the biasing potential applied to the probe is within the range of from about 40-60 volts DC, the probe is self-cleaning. It is realized that this device can be biased at higher levels, but as the biasing potential increases, the toner on the disc becomes more ditiicult to remove and reversing the polarity of the biasing potential or other measures must be taken to clean the probe.

The electrical field attracts toner from the mix and deposits the micron-size particles over the probe surface in a uniform manner to provide a homogeneous toner formation thereon. It will be appreciated that the action of the proble is to simulate the developing step of the apparatus so that there is close correlation between the monitoring device and the actual ima-ging process that takes place in a copying machine.

The preferred construction of the probe is that of a rotatable member, such as, for example, a disc. In accordance with the preferred embodiment of this invention, a rotating disc formed into a plurality of radially extending fin portions is partially immersed in a mass of developer mix. It is preferred that the finned disc be located in the mix directly where the toner is dispensed. As the dise rotates, toner is attracted from the mass onto the surface of the fins and withdrawn to a viewing zone where the change in reflectivity on each of the ns, due to the attraction of the toner, is measured. Since the disc is conductive, preferably formed of metal, the charge is unifo-rmly vdistributed over the fin surface. It has been found that, for optimum results, the iin surface should provide a greater area than the viewing area of the photocell. However, it is understood that the continuous rotation of the disc provides continuous sampling of the mix which is monitored by the photocell and provides a sys-tem which, in effect, integrates a large number of samplings so as to provide a fairly accurate control.

` This device has been found to be particularly well adapted for use with conventional magnetic brush de veloping assemblies, although it is contemplated that this device is equally well adapted for use with other types of developing lapparatus, such as cascade developers.

The optical measuring means include a light source, a photodiode which functions as a light-responsive switch in this control, and a temperature control device which is operatively associated with the photodiode to compensate for changing temperature conditions which affect the response characteristics of the photodiode.

To aviod deposition of the airborne toner on the light source, it is enclosed in a protective housing having a light access opening. The housing is formed of materials which are triboelectrically similar to the triboelectric characteristic of the toner. Housings formed of plastics, such as acrylic resins or polyvinyl acetate resins, tend to be triboelectrically positive and, hence, they will repel the positively charged toner. However, it should be understood that other materials of construction may be used which are conductive in which case an external field may be applied to the housing to create the equivalent effect of using triboelectrically oriented plastic materials.

To further reduce the likelihood of toner deposition on the sensing device, the housing construction provides for a pair of light conducting channels which converge at a common opening in the front of the housing juxtaposed the face of the fine portions. The light source and the photodiode are each received in one of the channels at points remote from the common opening. The common opening at which the channels converge serves as an exit way for the light rays which are directed onto the fins from the light source, and at the same time act as an entry way to the other channel in which is mounted the photodiode for sensing the light rays refiected from the fin surface. Accordingly, the light source is mounted at the head of one of the channels and the photocell is located at the end of the second channel. This type of an arrangement serves to protect the emission source and the sensing element from the interfering effects of toner dust which may tend to accumulate thereon.

Brief description of the drawings Other objects and features of this invention will become apparent from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a perspective view of a portion of a conventional magnetic brush developing apparatus showing the toner feed control device of this invention;

FIGURE 2 is a cross-sectional view taken along line 2 2 of FIGURE 1;

FIGURE 3 is an enlarged detail view in section of the optical elements showing the novel housing;

FIGURE 4 is the electrical control circuit diagram.

Description of preferred embodiment According to FIGURES 1 and 2, there is shown a magnetic brush assembly, indicated generally as 1G, and the toner feed control of this invention, indicated generally The assembly is a conventional magnetic brush assembly such as disclosed in detail in U.S. Patent No. 3,003,462, which includes a rotatably mounted cylinder 14 (FIGURE 2) having magnetic pole pieces 16 disposed within and along the longitudinal axis of the cylinder, and a pair of angers 18 and 20 which, working in concert, serve to circulate the mix 22 laterally through the trough 24. As seen from FIGURE 2, the auger 18 4. is rotated in a counter-clockwise direction moving a portion of the mix 22 from the trough 24 to the assembly 10. Simultaneously, the auger is being rotated in a clockwise direction to return the mix from the assembly 10 to the trough 24 for replenishment with fresh toner. The cylinder 14 is rotated in a counter-clockwise direction, and as it rotates, it collects on its outer surface developer mix being fed by the auger 18. Under the influence of the magnetic field produced by the magnetic pole piece 16, the magnetic mix 22 is formed in the configuration of a magnetic brush 26 on the periphery of the cylinder 14. By bringing a sheet bearing a latent electrostatic image into tangential contact with the brush 26, the image is selectively developed.

As the cylinder 14 continues to rotate, the brush 26 which is carried thereon is moved beyond the influence of the magnetic flux field established by the pole pieces 16. The mix is then released from the surface of the cylinder 14 and falls back into the main body of the mix being circulated by the 'auger 20. The toner concentration monitoring assembly, identified generally as 28, is disposed in the trough 24 which serves as the terminal point for the used mix before it is recirculated by the auger 18. It is at this station that fresh toner 30 is dispensed into the trough 24 from a dispensing hopper 32. A detailed description of the operation and construction of the dispensing hopper appears in U.S. Patent No. 3,224,644, issued Dec. 21, 1965 to Robert L. Gunto. The assembly 28 includes a rotating disc member 14 having radially extending fin portions 36. The fin portions 36 have finished face portions 38 (FIGURES 2 and 3) achieved by chrome-plating or similar surface treatment in order to provide a specularly reective surface. The disc member 34 is mounted on a shaft 40 coupled to a drive motor 42. It will be observed that the disc member 34 is situated between the augers 18 and 20 rotating in a plane normal to the plane passing through the axes of rotation of the respective augers. The disc 34 is partially immersed in the developer mix 22 where it is exposed to a representative sampling of the mixture to be analyzed for the concentration of toner therein. Immediately adjacent the specularly reflective face portion 38 of the disc member 34 is a light refiectance sensing unit 44 encased in a dust-protective housing 46 attached by means of a bracket 48 to the trough 24. The housing 46 construction provides for a pair of converging channels 50 and 52 (FIGURE 3) which converge to a common opening 54 at the front portion of the housing juxtaposed the disc 34 immediate the face portions 38. It will be apparent that the rotation of the disc member 34 causes a good deal of toner powder in the mix to become airborne, particularly in the immediate vicinity of the sensing unit 44.

At the head of channel 52 is a light source 56 operat ing in conjunction with a lens element 58 directing the light through the opening 54 which serves as an exit way for the light rays to be impinged on the face portions 38 of the fins 36. The light rays impinge upon the face portion 38 at Ian angle of incidence and are reflected therefrom into the opening 54 at an angle of refiection from the surface which is equal to the angle of incidence from the light source 56. The light proceeds into the channel 52 at the end of which is a light-responsive photodiode 60 and a focusing lens 62. The photodiode 6i) may be a conventional photocell or a siliconduo-diode. The Iangular relationship of the channels `is such as to conform to the angle of incidence and the angle of reliection of the light rays impinging and being re ected from the face surface 38 of the fins 36. It will be appreciated that the location of the light source and the photocell inside their respective channels at a location remote from the opening 54 protects these elements from becoming coated with the toner dust. In this manner the all-important transmission and reception of the light rays vital to the operation of the sensing unit is not impaired.

It has been found desirable for the material of construction of the housing 46 to have triboelectric characteristics the same or similar to the toner. The similarity of triboelectric characteristics prevents the attraction of toner to the housing. A wide variety of plastic materials may be used, such as acrylic ester polymers and polyvinyl acetates, which have been found to be particularly suitable for the application. It is to be understood that, while the preferred materials of construction are plastic, other materials of construction may be used, such as metals, which may be electrically biased by an external source in order to create the necessary charge relationship to the toner particles and thereby obtain the equivalent result as using a triboelectrically oriented plastic material.

In order to achieve consistent and invariant measurement, the light-responsive photodiode 60 includes a temperature-sensing element 66 such as a thermistor located adjacent the photodiode 60. An increase in temperature of the photodiode 60 environment will affect its response because of its inherent characteristic to undergo a decrease in resistance. Accordingly, an increase in temperature would result in an erroneous output lowering the resistance -beyond `what is attributable to reflected energy which is the significant energy to be measured. 'The thermistor control 66 serves to shunt oli a proportional amount of the output c-urrent due to a change in the operating temperature in the environment of the diode 60 so that the control portion of the mechanism receives a signal primarily based on the quantity of radiation being refiected from the specularly .reective face portions 38.

To 'further reduce the possibility of occluding the photodiode 60 or the light source 56 with toner particles, the housing 46 is mounted at an oblique angle with the opening 454 at a lower elevation than the photodiode or the light source. To accommodate this angular position of the housing 46, the face portions 38 of the fins 36 may be formed at an angle so that the light rays from the light sources are directed along a line normal to the face portion. This technique of positioning the housing 46 in relation to the disc 34 keeps out the larger particles which are heavy enough to be prevented by the gravitational forces yfrom moving up into the respect- ive channels 50 and 52.

Referring to FIGURE 4, there is shown the control circuit 68 connected by feed lines to a 11S-volt AC power supply controlled by a double-pull, single-throw switch 72. The control circuit 68 includes the drive motor 42 which drives the disc 34, 'a vibrating coil 74 for vi-brating the hopper dispenser 32 land the toner sampling circuit and monitoring circuit, identified ygenerally as 76 and 78, respectively. The constant voltage transformer 82 is connected across the 11S-volt line which powers the primary winding 83 of the transformer 82. The toner sampling and toner monitoring circuits are connected to the secondary windings 85, 86 annd 88 of the trans- -former 82.

The light source S6 is connected across the winding 85 in series with a 10 ohm variable resistor 87 used to adjust the intensity of the light'source in accordance with the characteristics of the photodiode 60.

A biasing potential of about 50 volts DC is applied to the disc 34 from the winding 86 which feeds the input to a full-wave rectifier 92 which is, in turn, connected through resistors 94 and 96 arranged in parallel connection. The resistors 94 and 96 are rated at 820 kilo ohms. When the toner employed is positively charged, the disc is connected as shown so that a negative charge is applied thereto. When the toner employed is of a negative polarity, the connections between the disc 34 and the rectitier 92 are reversed so that the disc is charged to a positive polarity.

' Winding 88 supplies power to the toner monitoring circuit 78. The winding 88 is connected to the input terminals of a full-wave rectifier 98. Across the' output terminals of the rectier 98 is connected a filter network consisting of a pair of 50 microfarad condensers 100 and 104 across a 33-ohm resistor 102. The output of the filter network is connected to the input line of the photodiode .60. The negative output terminal of the rectifier 98 leads to the thermistor 66. Variable resistor .108 is in series connection with the variable resistor leading to the output side of photodiode 60. As a result of the constant illumination of the photodiode 60 and its proximate location to the light source 56 (FIGURE 3), the temperature of the photodiode gradually increases with use. Since the only variable factor in the monitoring system should be the amount of toner deposited on the fins 36, the temperature control device, such as the thermistor 66, serves to draw ofi a proportional amount of the output of the diode attributable to the increased heat of the system.

As the environment heats up, the photodiode 60 increases in the temperature causing the .drop in resistance, and hence, resulting in a greater current output to the transistors 114 and 116 which function as a high-gain amplifier control. In order to control the current output caused by heat, the thermistor 66, in close proximity to the photodiode 60, also undergoes a decrease in resistance and, hence, serves to shunt that portion of the heat-produced current around the transistor network. The resistors 108 and 110 serve to modify the response characteristics of the thermistor so that it is compatible with the diode characteristics at the given temperaure of operation so that only that portion of the output due to heat is shunted off.

The diode 60 is at its high impedance level when the light being cast on it is at a low intensity level. Since the fin portions 36 are spaced apart as the disc rotates, it produces a discontinuous reflected light pattern. To negate the effect of this intermittent illumination of the photodiode, there is provided a condenser 112 to even out the output signal of the photodiode 60.

When the output signal from the photodiode 60 reaches a predetermined level, it will fire the transistor 114 which will permit current to fiow to the base of transistor .116 which, in turn, at the predetermined level will also fire causing current to flow through resistor to the predriver transistor 122. The firing level of the pre-driver transistor 122 is controlled by the variable resistor 124. Upon reaching the firing level of the transistor 122, it supplies current to the base of the output driver transistor 126 which energizes the coil of the main relay 130 pulling in its contacts 132 energizing the vibrating solenoid 74 to dispense the toner into the trough 24. The 20- microfarad condenser 134 in parallel relation to the coil protects the driver transistor 126 against lock E.M.F. when the coil is de-energized. The resistors 136, 138 and 140 are current limiting resistors; resistor 144 (10 kilo ohms) sets the level of voltage response for transistor 122 with the unsaturated range of the high gain amplifier circuit of transistors 114 and 116. Resistor 142 assures that the base of transistor 122 does not reach ground potential.

To place the apparatus in operation, the switch 72 is closed energizing the drive motor 42 and the primary coil 83 of the constant voltage transformer 82. The disc member 34 begins to rotate at a speed not in excess of 50 r.p.m. It will be appreciated that the particular speed at which the disc 34 is rotated may be varied over a wide range,- the only limitation being that its speed be adjusted so as not to stir up the developer mix to the point where it begins throwing out the particles from the trough 24. Insulating spacers 136 and 138 (FIGURE l)y are provided between the motor and the trough wall, and for the shaft 40, in order to electrically insulate the disc 34 from ground so that the disc is electrically floating. As the disc member 34 moves through the developer rnix 22, toner is attracted to the fins, and in particular to the specularly reflective face portions 38 in accordance' with the concentration of toner on the mix. It will be appreciated that the disc member 34, being of a different material than the developer mix, will generate triboelectric forces such that the powder will be attracted to the specularly reflective surface. However, it is preferred that a biased charge be applied to the disc as provided by the secondary winding 86 through the rectifier 92. The voltage applied is in the range of from 40-60 volts causing the attraction of the electroscopic powder.

Simultaneously, the light source 56 is energized causing radiation in the visible range of the spectrum to be directed on the face portions 38 to which has been attracted a certain amount of toner. The intensity of rellected light from the specularly reflective surface 38 may be correlated to the toner concentration in the mix, since the amount which is attracted to the surface is directly dependent on the concentration present in the mix. By L adjusting the variable resistor 110, the output current level at which the transistor 114 is to be fired may be preset. The variable resistor 110 operating in conjunction with the diode 60 determines the current level at which the transistor 114 will fire causing current to flow to the base of transistor 116 leading to the pre-driver transistor 122. In the circumstance that the current level is insutli cient to fire the transistor 114, it returns to the input side of the photodiode through the resistor (l.5 kilo ohms) 138. The high-gain amplification achieved through the transistors 1.14 and 116 directs the signal to the predriver transistor 122 controlled by the variable resistor (l kilo ohms) 124.

When the pre-driver transistor 122 fires, it permits the passage of current through resistor kilo ohms) 127 to the base of the transistor 126 which then energizes the relay 130 which pulls in its contacts 132 energizing the vibrator mechanism 74 of the hopper 32, which then begins the dispensing of fresh toner into the trough 24.

The calibration of the device will depend on the nature of the material, but in general the operator will determine the concentration in the mix which results in the optimum quality reproduction, and it is this concentration for which the toner sampling circuit 76 and the toner monitoring circuit 78 are adjusted to maintain.

As the fresh toner is dispensed from the hopper 32, it falls directly into the trough 24 and there is intermixed with the existing developer mix by the augers 18 and 20, and the disc member 34 immediately samples the replenished developer mix which in turn is immedi ately monitored by the toner monitoring circuit 78 so that Van instantaneous reading is obtained without any undue delay 0r lag between the addition of toner and the readout.

Although the present invention has been described with reference to an illustrative embodiment thereof, it should be understood that numerous other embodiments and modifications can be devised by those skilled in the art that would fall within the spirit and scope of this invention.

What is claimed is:

1. An apparatus which monitors the concentration of toner in a mass of developer mix and controls the feeding of toner to the mass in accordance with the concentration of toner in said mix, including a probe having a light transmitting surface carrying a toner attracting charge of one polarity which is uniformly distributed over the surface, said charge being capable of attracting from the mix a first amount of toner when the concentration of toner in the mix is at a first level corresponding to an optimum concentration and a second amount of toner when the concentration of toner in the mix is at a second level corresponding to a less than optimum 8 concentration, said attracted toner being uniformly deposited on said surface;

means for circulating the mix and probe relative to one another so that toner is attracted to and retained on said surface;

photoelectric means for detecting the amount of toner retained on said surface to determine the lcvel of toner concentration; and

means for adding toner to the mix when said second amount is detected, said toner being added in a quantity sufiicient to return the concentration of toner in said mix to said first level. 2. In an apparatus which monitors and controls the feeding of toner to a mass of developer mix comprising toner and magnetic carrier particles, said toner being fed in accordance with the concentration of toner in said mix, the combination including a sampling probe having 'a light transmitting surface carrying a toner attracting charge of one polarity which is uniformly distributed over the surface,

means for inserting the probe into said mass and then withdrawing said probe whereby a first quantity of toner as determined by the concentration of toner in said mix is retained on said surface and withdrawn from said mass, said first quantity being retained on said probe until said probe is reinserted into said mass whereupon said first quantity is released and a second quantity of toner as determined by the concentration of toner in said mix is attracted to said surface and separated from said mass;

photoelectric means for measuring the quantity of toner on said surface to ascertain the toner concentration of said mix;

means for adding toner to said mass in accordance with the measured amount of toner on said surface.

3. The apparatus as set forth in claim 2 in which said probe is maintained at a biasing potential within the range of -60 volts.

4. The device as dened in claim 2, wherein said sampling probe is a rotating disc.

5. The device as defined in claim 4 wherein said disc is rotated at a rate not in excess of r.p.m.

6. The device as defined in claim 4 wherein said disc is provided with a plurality of radially extending tins each having a reflective surface capable of intercepting and reflecting light.

7. An improved device for controlling the concentration of toner in a developer mix, said device having a light transmitting sampling surface carrying a toner attracting charge of one polarity which is uniformly distributed over the surface, said charge being capable of attracting toner from the mix in accordance with the concentration of toner in said mix, means for bringing the mix and surface together so that toner is attracted to and retained on said surface, photoelectric means for measuring the amount of toner on said surface including a light source and light responsive control means actuated by the light transmitted by said surface, and means coupled to said control means and controlled thereby for adding toner to said mix, the improvement comprising housing means for the light source which prevent toner in the vicinity of said source from being deposited thereon, thereby avoiding interference with the measurement of toner on said surface as a result of spurious deposition of toner on said source.

8. The device as set forth in claim 7, `wherein said housing means is formed of a material having the same triboelectric characteristic as the toner so that the toner tends to be repelled by said housing.

9. The device as set forth in claim 8, wherein said material is an `acrylic; plastic.

10. The device as set forth in claim 8, wherein said material is a vinyl acetate plastic.

11. The device as set forth in claim 7 wherein said light responsive control means are disposed within said housing, said housing preventing toner in the vicinity of said control means from being deposited thereon, thereby avoiding interference with the measurement of toner on said surface as a result of spurious deposition of toner on said control means.

12. The device as set forth in claim 11, wherein said housing means is provided with a first and second converging light conducting channels which merge into a common opening in said housing forming an exit way for incident light and an entry Way for reflective light.

13. The device as defined in claim 12, wherein said light source and said light responsive control means are respectively mounted in said first and second channels remote from said opening.

=14. The device as set forth in claim 13 wherein said housing means is mounted adjacent said surface and tilted in a manner such that the light source and control means are elevated above said common opening.

15. The device as set forth in claim 14 wherein said light source and light responsive control means are provided with lens means `for focusing the light utilized by said device.

16. An improved device for controlling the concentration of toner in a developer lmix, said device having a light transmitting sampling surface carrying a toner at tracting charge of one polarity which is uniformly distributed over the surface, said charge being capable of attracting toner from the mix in accordance -with the concentration of toner in said mix, means for bringing the mix and surface together so that toner is attracted to and retained on said surface, photoelectric means for measuring the amount of toner retained on said surface including a light source and light responsive control means actuated by the light transmitted by said surface, said control means being sensitive to ambient temperature conditions, and means coupled to said control means and controlled thereby for adding toner to said mix, the irnprovement comprising temperature responsive means adjacent said control means for automatically correcting for changing temperature conditions in the vicinity of said control means thereby avoiding interference with the measurement of toner on said surface as a result of the infiuence of temperature on said control means.

17. In a developing apparatus wherein a mass of developer mix comprising toner and magnetic carrier particles are employed to develop a latent electrostatic image, the combination including a magnetic brush assembly;

a container adjacent said assembly for holding said mass;

first 4means for moving the mix from the container to the assembly;

second means for returning the developer mix from the assembly to said container `for re-enrichment; dispensing means for adding toner to said mass in said container;

a Ilight transmitting charged sampling member having a portion immersed in said mass adjacent said second means so that as developer mix is returned to said container an amount of toner as determined by the toner concentration of the mix is attracted to and retained on said member;

drive means for moving the member through said mass for separating said amount of toner which thereby governs the intensity of light said member is capable of transmitting;

means for controlling the operation of the dispensing means including a light source which directs a beam of light onto said portion retaining said separated amount, and light responsive control means positioned in relation to said member to be actuated by the light transmitted by said member, said control means operating to turn said dispensing means on and off in accordance with the concentration of toner in said mass as determined by the intensity of said transmitted light.

18. The device as defined in claim 17, wherein said member is a rotatably driven disc having a plurality of radially extending reflective fins disposed about the periphery of said disc, with said light beam being directed to reflect off said fins onto said control means.

References Cited UNITED STATES PATENTS 3,003,462 10/ 1961 Streich 118-637 3,094,049 6/1963 Snelling 118-637 XR 3,233,781 2/ 1966 Grubbs 222-57 3,299,787 1/1967 Kolb et al. 118-637 XR 3,348,521 10/1967 Hawk 118-637 3,348,522. 10/ 1967 Donohue 118-637 XR 3,348,523 10/1967 Davidson 118-637 XR CHARLES A. WILLMUTH, Primary Examiner.

PETER FELDMAN, Assistant Examiner.

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