Abstract: A toner concentration sensor senses the concentration of toner in the toner system of an electro-photographic printer or copier. A novel circuit improves the toner concentration sensor. The novel circuit is composed of a resonant circuit, a frequency source, and a logic circuit. The resonant circuit is tuned to a selected frequency and has an inductor which creates a field into which the toner system is introduced. A frequency source excites the resonant circuit. A logic circuit receives a signal from the frequency source and an output from the resonant circuit. The logic circuit output is a signal representative of the phase shift between the signal from the frequency source and the output of the resonant circuit. The phase shift is representative of the inductance of the inductor in the presence of said toner system. The measurement of the inductance is representative of the concentration of toner in said toner system.

Abstract: A copier/duplicator has transparent photoconductor with a first surface on which a latent electrostatic image is formed. The latent image is developed by toner particles applied as the photoconductor is moved through a development station. A toner concentration monitor includes an emitting element and a detector. The emitting element directs a beam of radiation through the photoconductor from a second surface thereof toward an area of the development station where a developer mixture including toner particles is being provided to the first surface of the photoconductor. The detector is located adjacent the second surface of the photoconductor to receive rays emitted by the element and reflected from the developer mixture through the transparent photoconductor.

Abstract: An apparatus in which particles are discharged from the open end of a storage chamber. In the operative mode, a brush mounted movably adjacent the open end of the chamber, is actuated. The brush has at least one row of bristles for dispensing particles from the open end of the storage chamber. In the inoperative mode, the brush is positioned such that the row of bristles seals the open end of the chamber preventing the discharge of particles therefrom.

Abstract: A toner dispensing apparatus consists of a toner receiving portion and a toner container unit to be mounted on the former. The toner receiving portion has an openable cover for opening and closing an opening portion of the toner receiving portion. The toner container unit has container, a seal member for sealing an opening portion of the container, and a slide member connected with a portion of the seal member and slidable with respect to the opening portion. The seal member is torn off from the opening portion when the slide member is moved in conjunction with the opening motion of the openable cover.

Abstract: A cartridge in which marking particles used in an electrophotographic printing machine are stored. The cartridge has an opening in the surface thereof for discharging the marking particles therefrom. A flexible sealing strip is secured removably to the cartridge and seals the opening in the surface to prevent the discharge of the marking particles. The sealing strip is folded back over itself in juxtaposition. A flexible backing strip has the portion of the sealing strip folded back secured thereto. As the sealing strip is removed from the cartridge, successive portions thereof are secured to the backing strip. The foregoing occurs automatically when the cartridge is inserted in a dispensing unit of the printing machine and rotated. In this way, the cartridge is sealed when inserted in the printing machine and, after being rotated, automatically opened to discharge marking particles into the dispensing unit of the printing machine.

Abstract: A toner replenishing device for replenishing toner to a developing device in an electrostatic copying apparatus includes a storing box extending in approximately a horizontal direction, a supply roller provided at a supply port at an end of the storing box, pairs of wheels respectively provided at opposite ends of the storing box, a pair of endless stretched members passed around the wheels, a stirring and supply rod provided between the endless stretched members, and a device for driving the wheels so as to move the lower portion of the endless stretched members toward the supply roller.

Abstract: A developing device mountable into and demountable from an image formation apparatus, includes a housing provided with an aperture, a shutter assuming a position to close the aperture when the device is mounted in the image formation apparatus, and a position to open the aperture when the device is not mounted in the image formation apparatus.

Abstract: A toner cartridge for replenishing toner in a reproducing machine which may be readily inserted in an opening in a toner dispenser, includes a base plate and a toner hopper slidably engaged with the top of the base plate, both the base plate and the toner hopper having a plurality of apertures therein for toner to pass therethrough into the toner dispenser when in the open position. The base plate of the toner cartridge has a hook at one end adapted to lock under the lip of the toner dispenser opening when the cartridge is inserted in it, and the toner hopper has a hole at the opposite end adapted to lock under the lip of the toner dispenser opening when the toner hopper is slidably moved from the closed position to the open position. In the closed position portions of the base plate provide a seal under the apertures in the toner hopper and when the toner hopper is slidably moved to the open position, the apertures in the base plate and toner hopper are open permitting toner to fall into the toner dispenser.

Abstract: A magnetic brush developing apparatus includes a material transport guide member extending from a developing material supply position to a developing region, in addition to a rotatable developing sleeve having an inwardly disposed rotatable manetic roller driven for rotation in the same direction as the sleeve. A clearance is maintained between the guide member and the outer peripheral surface of the developing sleeve to allow transport by the rotation of the magnetic roller of developing material, having magnetic carrier particles and electrically insulative toner particles, from the developing material supply position along the outer surface of the guide member to the developing region in a direction opposite to the direction of rotation of the magnetic roller and developing sleeve.

Abstract: A toner cartridge including a hollow body of cylindrical form made of nonmagnetic material to serve as a container formed at its outer peripheral surface with a toner feeding slot extending lengthwise thereof, a first lid member attached to one end of the hollow body and formed at the outer peripheral surface of its end portion with a flange, a second lid member attached to the other end of the hollow body and formed at its bottom with a toner supply port, a cap member closing the toner supply port, and a sealing member that can be peeled off closing the toner feeding slot formed at the outer peripheral surface of the hollow body. The second lid member is formed of thin sheet metal by bending same at its outer peripheral edge portion in a manner to provide a minuscule clearance between a bent portion and the thin sheet metal and the hollow body is inserted at the other end thereof in the bent portion and secured in place.

Abstract: A developing apparatus is disclosed which has a toner bath with a magnetic toner placed therein, the toner being supplied via a developing role to a developing gap between an image carrying substance and the developing roll, and a toner supply bath for supplying the magnetic toner to the toner bath, a latent image formed on the image carrying substance being developed with the magnetic toner by means of a magnetic brush development. The developing apparatus has a detector member for generating a signal when the magnetic toner is absent in the toner bath, a drive member which responds to the signal generated from the detector member, and a magnetic field generating device driven by the drive member for magnetically shutting off a supply path provided between the toner supply bath and toner bath when the detector member generates no signal, but magnetically opening the supply path when the detector member generates the signal.

Abstract: The disclosure is directed to an improved electrophotographic copying apparatus which includes a detector for detecting amount of toner adhering to a reference latent image, a replenishing device for replenishing toner to a developer in a developing device when the toner amount detected by the detector falls below reference toner amount and a correcting device for changing the reference toner amount, and/or voltage of a developing bias power source at a time when the reference latent image is developed, according to an operation for altering original document projecting magnifications, so that toner concentration of the developer may be maintained in the best condition at a reduced size magnification as well as an equal size magnification.

Abstract: The present invention relates to a developing material supplying control device for an electrostatic copying apparatus which includes an oscillator circuit composed of an oscillation coil provided along a toner supplying passage, a timer, a counter for counting the output signal from the oscillation circuit at every interval predetermined by the timer to supply a discriminating output signal when the counted value is equal to a predetermined value for each of the intervals. A toner supply acts in response to the discriminating output signal from the counter for supplying toner to the toner supplying passage. Since this invention employs the oscillation coil and does not employ a mechanical switch having a movable contact for detecting toner in the toner supplying passage, the toner supplying operation is achieved precisely.

Abstract: Toner dispensing apparatus comprising a toner hopper, the walls of which form an elongated opening from which toner is capable of being dispensed and a dispensing roll to dispense toner from the hopper through the elongated opening into the developer chamber. The dispensing roll is a cylindrical resiliently deformable foam roll having a plurality of depressions of predetermined size, shape and volume uniformly disposed on the roll and being capable of holding finely divided toner. The deformable foam roll including the depressions has a thin toner impermeable surface, the thin toner impermeable surface and the foam roll of the dispensing roll being separate layers bonded together at their interface. In a specific embodiment, the depressions are a plurality of longitudinal surface grooves parallel to the axis of the roll and uniformly spaced around the circumference of the roll.

Abstract: A toner concentration control system for use in electrophotographic copying machines or the like, wherein light from a single light source is simultaneously detected by two photosensors, or by two sensing portions of a single photosensor, the light striking one of the photosensors (or portions) directly and the other after being reflected from developer comprising toner and carrier particles as the developer passes a viewing window in a chute provided for the transport thereof within the copying machine, the intensity of the reflected light being inversely proportional to the concentration of toner in the developer, whereby, by comparison of the output of the photosensors (or portions) by conventional means, the concentration of toner in the developer can be determined and adjusted, the photosensors (or portions) having been carefully selected as a pair of their essentially identical characteristics and response to ambient conditions so that, for example, their parallel deterioration with time or their identi

Abstract: A developing device for electrophotography includes a developer supporter faced to an image bearing member with a constant space maintained therebetween, a magnetic field generator for limiting the thickness of developer on the developer supporter. An element is provided within the magnetic field generated by the generator and in the vicinity of the developer supporter to limit the thickness of developer which is rendered erect by the magnetic field. The element is effective to limit the thickness of developer supported on the supporter to a value not causing contact of the developer with the non-imaged area on the image bearing member.

Abstract: A developer supply device for supplying developing apparatus with a developer consisting of a mixture of toner and carrier, is characterized in that when toner and carrier are supplied separately to the developing apparatus, a magnetic field is provided near a carrier storage device storing carrier therein and the supply of the carrier is controlled by the magnetic field.

Abstract: This specification discloses a developer supply device which supplies separately toner and carrier of a two-component developer and in which supply of toner is controlled in accordance with the variation in volume of the developer within a developing device and supply of carrier is controlled in accordance with the variation in density of the toner.

Abstract: A toner dispensing device has a toner storage chamber provided in its lower portion with an opening for passing toner therethrough, a roller disposed within the storage chamber close to the opening and rotatable to deliver the toner from the storage chamber outward through the opening, an intermediate toner supply chamber in communication with the storage chamber through the opening and for receiving toner to keep it substantially full, and a feeder provided within the intermediate supply chamber for feeding the toner out of the intermediate supply chamber at a constant rate.

Abstract: A development powder dispensing apparatus that is in the form of a carousel. The apparatus has a generally cylindrical configuration that is divided into a number of chambers such that the development powder toner within each chamber may be supplied individually to a magnetic brush unit. Because of the separation of the toner into chambers, no toner compression is experienced even though large amounts of toner may be stored.

Abstract: An automatic development control for use in an electrophotographic apparatus. The control includes a pair of substantially planar members forming a channel therebetween through which developer flows. One of the members is constructed such that an electrostatic charge pattern is periodically formed on the surface thereof, the charge pattern being developed by the attraction of toner thereto which toner has a charge opposite to that forming the charge pattern. The member on which the developed charge pattern is formed includes a uniform, transparent conducting substrate covered with a transparent insulating layer on top of which is deposited a set of spaced-apart, opaque and electrically conducting fingers. Thus, the one member includes alternate opaque and transparent areas.

Abstract: An apparatus which regulates the developability of a conductive developer mixture by controlling the concentration of toner particles therein. The concentration of toner particles in the developer mixture is adjusted in response to the sensed conductivity.

Abstract: A developing device in an electrostatic copying apparatus. The developing device includes a developer container, a developer applicator for holding on its surface a pair of a developer present in the developer container and applying toner particles to a latent electrostatic image to be developed, and a toner particle dispenser for dispensing toner particles therein to the developer container. The toner particle dispenser is controlled such that it performs a toner particle dispensing action in response to the performance of a copying process by the electrostatic copying apparatus. The developing device is provided with a device for inhibiting the dispensing of toner particles which makes the toner particle dispenser unable to start a toner particle dispensing action when a sufficient amount of the developer is present in the developer container.

Abstract: A developer quality monitoring device for monitoring the mixed state of toners and carriers in the developer used for developing a latent image formed on a recording medium, wherein a developer circulating mechanism and a toner density detector are used in common, an electrical signal representative of a decrement of toner density and a magnitude of pulsations is produced, and by using this electrical signal, decay condition of the developer is monitored and decay condition limit is discriminated.

Abstract: An automatic development control system having a sensor providing a signal representative of developed toner mass on a patch sample on the photoreceptor. The signal is conveyed to a comparator through signal processing circuits for comparing with a reference signal. The signal processing circuits include digital to analog circuitry also capable of providing analog to digital conversion. The reference signal represents an undeveloped photoreceptor signal normalized to accommodate changing photoreceptor, sensor and operating environment characteristics. A dispenser solenoid activates a toner dispenser through an interrupt mechanism in given increments of time depending upon the error signals generated by the comparator.

Abstract: A holder for electrographic developing powder comprises an oblong outlet opening for the powder and has the opening covered by a membrane and by a slide valve which is movable along channels bordering the opening and is provided near one of its ends with a cutting device that cuts the membrane as the slide valve is moved from a first position closing off the opening to a second position not covering the opening. The holder comprises a flexible end portion that can be contracted by being pinched in a direction substantially parallel to the plane of the slide valve, and from which projections extend laterally for engagement with notches in guides that support the holder over a powder reservoir. The cutting device is moved toward the flexible end portion as the slide valve is moved from the first to the second position.

Abstract: First and second sensors (37), (39) are provided in a developing tank (36) containing a liquid developer consisting of a liquid carrier or dispersant and toner particles dispersed in the carrier. The first sensor (37) measures the electrical resistivity of the developer whereas the second sensor (39) measures the optical transmissibility thereof. Additional toner is supplied into the developer to maintain the transmissivity at a value which is a predetermined function of the resistivity. The transmissibility value is reduced as the resistivity increases, thereby maintaining the copy image density constant. The first sensor (37) includes two electrodes immersed in the developer and an A.C. or D.C. voltage is applied thereacross. The current flow through the electrodes (37) and thereby the developer is measured to determine the resistivity of the developer. Where the applied voltage is D.C.

Abstract: An apparatus in which particles are dispensed from the open end of a chamber. A member is positioned interiorly of the chamber and periodically agitated in response to particles being dispensed therefrom. Agitation of the member prevents bridging and caking of the particles in the chamber and facilitates the flow of the particles therefrom.

Abstract: The powder material transportation apparatus for transporting powder materials such as developer powder, by a rotating member such as a coil is provided with an agitator for destroying a bridge of powder material which is apt to be formed in an upper portion of the rotating member. The diameter or pitch of a rotating coil member is designed to be greater in a transportation termination portion than in a transportation initiation portion, whereby the powder material transportation force is increased in the transportation termination portion and formation of the bridge of powder material is prevented.

Abstract: A toner feeding device comprising a toner box having a toner outlet port formed at the bottom, a roller housed in the toner box in a manner to close the toner outlet port, and a toner catching plate biting the roller. The roller is formed of a porous elastic material and the toner catching plate is slidable such that the toner feeding rate can be controlled by suitably adjusting the position of the slidable toner catching plate.

Abstract: Disclosed is a device for indicating that the level of a dosable material has fallen below a predetermined level in a dosing container having a shaft mounted horizontally for rotation inside of the container and a dosing brush mounted on the shaft. This device comprises an elastic contact tongue radially fixed to the shaft for rotation therewith, the contact tongue being sufficiently elastic so that it assumes a bent configuration having a first radius when rotated in contact over a predetermined portion of its length with the dosable material and so that it assumes an extended configuration having a second radius when rotated so that it is not in contact over the predetermined portion of its length with the dosable material; a contact pin mounted at a distance from the shaft greater than the first radius but not greater than the second radius, so as to be contacted by the contact tongue in its extended configuration; and means for producing a signal when the contact tongue contacts the contact pin.

Abstract: In a photocopier including a movable member carrying a photoconductive layer, a charging station for charging the layer, an illuminating station for photographically exposing the layer, a developing station provided with a magnetic brush and arranged to supply a magnetic single component toner to the brush for developing the image formed on the layer, a transfer station for transferring the developed image to a record material sheet, and a cleaning station for cleaning the layer, the stations being disposed in succession along the path of movement of the movable member for bringing each portion of the layer into operative association with each station in succession, in combination with a cassette containing a supply of such toner, the developing station is provided with a structure for receiving and holding the cassette in a position for dispensing toner for supply thereof to the magnetic brush, and the cleaning station is provided with a structure for receiving and holding the cassette in a position for coll

Abstract: An improved developing material supplying device for use in a developing apparatus of an electrophotographic copying machine which includes a developing material supplying roller having at least one developing material measuring recess formed by cutting the peripheral surface of the supplying roller into flat shape and rotatably provided at a developing material supplying opening of a developing material storage tank, a driving mechanism for the supplying roller, a developing material measuring regulating member provided where the recess of the supplying roller at the supplying opening is rotated from inside to outside of the storage tank, and a shield member capable of covering the entire surface of the recess and provided where the recess at the supplying opening moves from outside to inside of the storage tank.

Abstract: A developer powder mixing device used in an electrophotographic developing apparatus comprising stationary mixing chutes which convey developer powder across the centerline of the developer station from one side of the station to the other in both directions as excess developer powder is falling under gravity from a developing roll.

Abstract: An electrophotographic apparatus of the two-component, dry developer and reusable photoconductor type, having a developer unit, and replenisher for supplying toner to the developer unit. The replenisher includes a first supply of virgin toner, and a second supply of used toner. A toner feed device operates to simultaneously feed toner to the developer unit from the first and second replenisher supplies. The proportion of toner fed from the first and second supplies is variable; however, such proportion requires that virgin toner must always be fed. Used toner is provided by collecting toner at the apparatus' cleaning station. Toner-low indication is provided for only the first supply, so as to prevent the feeding of only used toner. A toner concentration sensor signals the need to feed the combination of virgin/used toner to the developer unit.

Abstract: Disclosed is a device for measuring the concentration of toner in a developer mixture composed of toner and a ferromagnetic carrier material, e.g., in an electro-photographic developing device. The device comprises a measuring oscillator which comprises an inductance-including component comprising at least one coil wherein the conductor paths of the coil lie substantially in one plane; means for passing at least a portion of the developer mixture into the zone of the inductance; and means for evaluating changes in the frequency of the oscillator resulting from changes in the developer mixture composition passing over the zone of inductance. Also disclosed is an electro-photographic copying apparatus embodying the foregoing device.

Abstract: A toner concentration detecting apparatus comprises a rotatable probe having a toner-receiving face, a case housing the probe, means attached to the case for detecting an amount of toner on the toner-receiving face, and a port formed in the case for discharging stray toner particles from the case. The toner-receiving face is put in contact with a magnetic brush of a developer mechanism, separates toner particles from a developer and receives them. The detecting means includes a light-emitting element for applying light to the toner-receiving face and a light-receiving element for receiving the light reflected by the toner-receiving face and converting the light into an electric signal. The stray toner discharging port extends in the direction in which acts centrifugal force generated by the rotation of the probe and is located near the light-emitting and light-receiving elements.

Abstract: A dry process developing apparatus for use in an electrophotographic copying machine which includes a developing non-magnetizable sleeve and a multipolar magnet member arranged to be rotated in the same direction at different speeds, the speed of rotation of the developing sleeve causing movement of those particles of the developing material which at the most are only slightly affected by the multipolar magnet member in the direction of rotation of the sleeve, and the speed of rotation of the multipolar magnet causing the movement of particles of developing material affected by the magnet around the sleeve in a direction of rotation opposite that of the magnet and at a speed greater than the speed of the developing sleeve. Scraping members are provided for sliding contact with the peripheral surface of the developing sleeve in the directions against and following the direction of rotation of the developing sleeve.

Abstract: In a dry-type developing apparatus for use in electrophotography including a developing sleeve for conveying a two-component developer composed of a toner and a carrier to a developing station, an automatic toner replenishing device comprises a roller to which voltage is applied having recessed toner dispenser portions and a toner recovering peripheral surface portion on its periphery and rotatable in contact with a portion of the developer on the sleeve after the developer portion has passed through the developing station, bias voltage sources connected to the sleeve and to the roller respectively to produce a potential difference between the sleeve and the roller, and a toner tank arranged to cover part of the roller and containing the toner to be replenished.

Abstract: Electrographic apparatus having an improved developer metering construction provides an elongated coil spring that extends across an applicator member's transport path (from the developer supply to the development zone) to meter and mix applied developer. Spring tension can be adjusted to control developer flow. In certain embodiments the spring provides a source of reference potential for and/or triboelectrically charges the developer and can enhance magnetic mixing of the developer.

Abstract: An exposure unit radiates a light image of an original document onto a charged photoconductive member to form an electrostatic image through localized photoconduction. A developing unit applies a powdered developing substance comprising carrier particles and toner particles to the photoconductive member to develop the electrostatic image and produce a toner image which is transferred and fixed to a copy sheet to produce a permanent reproduction of the original document. The tone density, or the ratio of toner particles to carrier particles in the developing substance is electromagnetically measured, and the developing ability of the developing substance is opticaly measured. The developing substance is replaced when the difference between the toner density and developing ability exceeds a predetermined value. Prior to exposure with the light image a reference electrostatic image is formed on the photoconductive member and the electrostatic potential thereof is measured.

Abstract: A system and method for monitoring and maintaining a predetermined concentration of material in a fluid body or carrier includes a control arrangement for detecting the transmissivity of a flow cell in each of two states. One of the states during which the flow cell is observed contains substantially no fluid present therein but provides a basis for determining the degree of existence of external factors affecting measurements made via the flow cell. The measurements for one state are subtracted from the measurements for the other in order to determine the transmissivity of the fluid body itself as distinguished from the combination of fluid body and flow cell.

Abstract: A shaft with a spiral blade is supported in a receptacle for receiving a developer removed from the surface of a photosensitive drum. The developer is conveyed toward a collection box during the rotation and movement of the shaft. When an excess load is applied to a screw shaft due to an overflow or blockage of the developer, the shaft is moved in a direction opposite to that in which the developer is conveyed, causing it to abut against a sensor to permit the sensor to be rendered ON.

Abstract: Apparatus for supplying electrophotographic developer to an electrophotographic developing device is disclosed. The developer supply apparatus includes a cartridge containing therein an amount of developer and supported within the housing of the supply apparatus in a manner that the cartridge is allowed to move rotationally and reciprocally around and along its axis. The cartridge has an opening at its one end and also a plural number of ribs provided within the cartridge. During the movement of the cartridge, the ribs stir the developer contained in the cartridge and also cause the developer to move toward the opening. The cartridge is brought into motion responding to a supply signal so as to effect a supply of developer from the cartridge to the developing device.

Abstract: A powdered toner mixture comprising a non-magnetic toner component and a ferromagnetic carrier component is caused to flow downwardly through a vertical conduit by gravity. An electromagnetic coil is wound around the conduit and energized with an alternating electric signal. An electrically conductive and magnetic mass is disposed inside the conduit. The permeability of the toner mixture decreases as the toner density, or the ratio of the toner component to the carrier component, increases. The lower the permeability of the toner mixture, the greater the proportion of magnetic flux passing through the mass. The flux passing through the mass induces eddy currents therein which, in combination with hysterisis and skin effect losses, dissipate a portion of the electric signal. The amount of power dissipation is a function of the relative proportion of flux through the mass and thereby the toner density and, when measured, provides a measure of the toner density.

Abstract: The invention relates to a toner replenishment system for a dry toner developer unit in a photocopier. Toner is introduced into the developer unit in a canister which is closed at one end and has near its other end an extractor plate mounted for rotation around the axis of the canister and for axial movement along the canister, the toner powder being disposed between the closed end and the extractor plate. This canister can be simply loaded into a developer unit and then, during operation of the developer unit, toner demand is met at a metered rate by rotation of the extractor plate causing an extractor blade to slice toner and transfer it through the extractor plate to the axially outer side thereof for subsequent conveyance to the developer hopper.

Abstract: An electrostatic copier has a rotatable copy drum and a developing unit which can be inserted and removed from the copier in toto. The developing unit is provided with bearing members which at least partly and loosely embrace the shaft of the copy drum.

Abstract: A toner mixture comprising ferromagnetic carrier particles and non-magnetic toner particles are caused to flow through a conduit for measuring the toner density which is the ratio of toner particles to carrier particles in the mixture. The effective inductance of an electromagnetic coil disposed in the conduit varies in accordance with toner density. The coil is designed so that the magnetic lines of force thereof are substantially confined within the conduit and are not influenced by external bodies.

Abstract: The magnetic brush development apparatus for developing a latent electrostatic image on a photoconductive material by a magnetic brush formed on a development sleeve comprises a hopper whose outlet for supplying one-component type magnetic developer to the development sleeve is positioned in proximity to the surface of the development sleeve, and a developer detecting apparatus comprising a light emitting element and a light receiving element is positioned in proximity to the outlet of the hopper and the surface of the development sleeve. The light emitting element and the light receiving element are positioned so as to hold therebetween a transparent container for allowing the developer to pass therethrough.

Abstract: Deterioration of a dry developing substance causes toner particles to be so strongly adhered to carrier particles that they cannot be readily separated therefrom. The present apparatus comprises means for applying a predetermined electrostatic or air suction force to the developer to readily remove toner therefrom except for spent toner particles strongly adhered to the carrier particles. The toner density, or the ratio of toner to carrier particles, is then measured electromagnetically or optically and compared to a reference value. The developer deterioration determines what proportion of toner will be removed from the developer by said force and thereby the subsequently sensed toner density. The higher the sensed toner density, the greater the degree of deterioration.