Abstract:
An apparatus ( 100 ) for measuring toner concentration in a developer contained in a developer housing ( 10 ) includes a developer sample container( 102 ) that receives a portion of developer extracted from the developer housing ( 10 ). A spectrophotometer ( 112 ) measures spectrophotometric data for the portion of the developer in the developer sample container ( 102 ). A processor ( 116 ) estimates the toner concentration based on the measured spectrophotometric data and a pre-determined relationship ( 118 ) between the spectrophotometric data and the toner concentration. A method ( 200 ) for estimating the toner concentration in a developer comprising a toner and a carrier includes measuring ( 220 ) a color characteristic of the developer, comparing ( 222 ) the measured color characteristic with a predetermined relationship between the color characteristic and the toner concentration, and estimating ( 226 ) the toner concentration based on the comparing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the printing and xerographic arts. It finds particular application in conjunction with the monitoring and control of developer materials, and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in other printing and xerographic systems where the concentration of toner or other materials or chemicals used in the printing process is advantageously calibrated, monitored, or controlled.  
           [0002]    In many printing and xerography systems, images are formed on paper or another medium using electrophotographic printing. In this method, a photoreceptive surface is uniformly electrostatically charged, and the image is transferred to the photoreceptive surface through selective exposure to light or other electromagnetic radiation. The light discharges the exposed areas of the photoreceptive surface to form an electrostatic charge pattern known as the latent image. The latent image is developed by exposure to a developer material that selectively coats the charged surface areas. A typical two-component developer includes toner particles comprising a polymer or resin with a color agent, and carrier beads comprising resin-coated spheres of steel or another material. The carrier beads are usually several times larger than the toner particles. The toner particles triboelectrically bond to the larger, spherical carrier beads to form composite developer particles. In the vicinity of the electrostatically charged regions of the latent image, the toner particles are attracted away from the carrier beads and attach onto the photoreceptor due to the greater electrostatic attraction of the photoreceptor versus the triboelectric bonding to the carrier beads. The thusly developed latent image is known as the toner image. The toner image is transferred to the paper or other print medium using a corona discharge to effectuate transfer of the toner particles from the toner image onto the paper. Finally, a fusing process employing heat and pressure permanently affixes the toner onto the paper to form the final printed image.  
           [0003]    In the case of color printing, several toner stations are employed, e.g. in the case of CMYK printing separate toner stations for printing the cyan, magenta, yellow, and black (K) image components. A full color toner image is thus produced which is transferred to the final print medium and fused in a manner similar to that just described. An electrophotographic printing apparatus typically includes additional components to monitor the electrostatic potentials, image characteristics, and other aspects of the complex printing process.  
           [0004]    An important system parameter for obtaining consistently high quality electrophotographic printing is control of the developer composition. The toner concentration is typically defined as the ratio of the weight of the toner to the weight of the carrier in the developer. During printing, toner is gradually depleted whereas the carrier beads do not transfer to the paper. Thus, the toner concentration in the developer decreases over time with usage.  
           [0005]    With reference to FIG. 1, a developer housing  10  that stores, maintains, and applies the developer is described. A mixing wheel  12  rotates in a direction  13  and mixes toner particles  14  and carrier beads  16  in a developer sump  18 . Under the action of the mixing wheel  12 , the toner particles  14  triboelectrically bond to the carrier beads  16  to form composite developer particles  20 , each of which includes a plurality of toner particles  14  surrounding a single carrier bead  16 . Note that the toner particles  14  and the carrier beads  16  are shown schematically in FIG. 1 and are not drawn to scale. In a typical developer, the carrier beads  16  are several times larger than the toner particles  14 , and both are much smaller than they are shown in FIG. 1. A magnetic roll  22  comprising a hollow tube  24  and fixed magnets  26  applies the developer particles  20  to the photoreceptor  28  (shown in part). The hollow tube  24  of the magnetic roll  22  rotates in a direction  29  as shown and the magnets  26  attract the metallic cores of the carrier beads  16  of the developer particles  20  onto the tube  24 . As the hollow tube  24  rotates the attached developer particles  20  are brought into close vicinity with the photoreceptor  28  where the toner particles  14  are pulled off the carrier beads  16  and onto the charged portions comprising the latent image  30 . The photoreceptor  28  is typically embodied in the form of a continuous belt loop that rotates in a direction  31  so as to develop the entire latent image  30 . The toner coating thus formed comprises the toner image  32 . In order to control the thickness of the developer coating on the roll  22 , a baffle or metering blade  34  removes excess developer from the roll  22 .  
           [0006]    As a consequence of the developing process, toner particles  14  are removed from the developer sump  18  to form the toner image  32 . As a result, the toner concentration in the developer sump  18  decreases over time and is advantageously replenished. A toner dispenser  36  includes a toner brush  38  that dispenses toner in a controlled fashion from a toner reservoir  40  into the developer sump  18 .  
           [0007]    The prior art discloses several methods for determining when to replenish the toner, and to determine how much toner to add. In some printing systems, toner dispensing occurs on a fixed schedule, i.e. by a pre-determined use factor, such as one minute of dispensing for every ten minutes of printing. Of course, this type of system rather inflexible. Many printing systems use some sort of automatic dispensing system in which the toner concentration is monitored in some way and toner replenishment occurring responsive to the monitoring. The monitoring process can take place either in the developer housing  10  or on the photoreceptor  28 , e.g. by printing a test patch that is characterized by optical reflectance or other means. Monitoring on the photoreceptor  28  has the disadvantage of introducing additional factors which can affect the toner image  32 , such as variations in electrostatic charge of the photoreceptor  28 . Monitoring in the developer housing  10  involves measurement of the toner concentration in the developer sump  18 . The prior art discloses use of an in situ magnetic permeability sensor, commonly known as a packer sensor (not shown in FIG. 1), for monitoring the toner concentration in the developer sump  18 . The packer sensor detects changes in the magnetic permeability of the developer material due to changes in the average spacing of the metallic cores of the carrier beads l 6  due to changes in the toner concentration. The packer sensor has the disadvantage of being a relative sensor. There remains an unfulfilled need in the art for a convenient method and apparatus for obtaining absolute quantitative information on the toner concentration in a developer which can be used, for example, to calibrate a packer sensor.  
           [0008]    The present invention contemplates a new and improved method and apparatus therefor which overcomes the above-referenced problems and others.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with one aspect of the present invention, an apparatus for measuring toner concentration in a developer contained in a developer housing is disclosed. A developer sample container receives a portion of developer extracted from the developer housing. A spectrophotometer measures spectrophotometric data for the portion of the developer in the developer sample container. A processor estimates the toner concentration based on the measured spectrophotometric data and a pre-determined relationship between the spectrophotometric data and the toner concentration.  
           [0010]    In accordance with another aspect of the present invention, a method for measuring toner concentration in a developer is disclosed. A sample of the developer is extracted. Color characteristics of the developer sample are measured. The toner concentration is estimated based on the measured color characteristics.  
           [0011]    In accordance with yet another aspect of the present invention, a method for estimating the toner concentration in a developer comprising a toner and a carrier is disclosed. A color characteristic of the developer is measured. The measured color characteristic is compared with a pre-determined relationship between the color characteristic and the toner concentration. The toner concentration is estimated based on the comparing.  
           [0012]    Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0014]    [0014]FIG. 1 schematically depicts a developer housing according to the prior art for an electrophotographic printing apparatus;  
         [0015]    [0015]FIG. 2 schematically depicts an apparatus which suitably practices an embodiment of the invention;  
         [0016]    [0016]FIG. 3 shows an experimental graph of the L* color parameter or attribute of a series of developer samples plotted against toner concentration for new developer and for developer which has been used to print 40,000 prints, which data provides a suitable pre-determined empirical relationship or calibration data for inclusion as a component of an apparatus that suitably practices an embodiment of the invention;  
         [0017]    [0017]FIG. 4 shows an experimental graph of the hue color parameter or attribute of the series of developer samples of FIG. 3 plotted against toner concentration;  
         [0018]    [0018]FIG. 5 shows an experimental graph of the chroma color parameter or attribute of the series of developer samples of FIG. 3 plotted against toner concentration;  
         [0019]    [0019]FIG. 6A schematically depicts a method which suitably practices an embodiment of the invention; and  
         [0020]    [0020]FIG. 6B schematically depicts another method which suitably practices an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    With reference to FIG. 2, an apparatus  100  for characterizing the toner concentration in a developer of a printing or xerographic device according to one embodiment of the invention is described. A sample of the developer (not shown) is placed into a sample container, which in the illustrated embodiment comprises a small depression  102  in a flat surface  104 , using an appropriate sample dispenser  106 . For accurate color measurement, a well-defined reflective surface is defined in the sample. In the illustrated embodiment, this surface can be defined two ways. The developer sample surface can be leveled using a leveling device such as a strip of shim stock  108 . In another approach, the developer sample can be mixed with a surfactant or solvent, e.g. dispensed by a syringe  110 , to form a liquid sample. In the latter case, the ratio of developer mass to solvent mass should be determined to enable standardized measurements. The choice of solvent or surfactant material will depend upon the type of developer; however, it will be appreciated that the solvent-diluted sample can advantageously have an improved dynamic range of color attributes which translates into more precise and sensitive toner concentration measurements. Although in the illustrated embodiment the mixing of the developer and the solvent or surfactant occurs in the small depression  102 , depending upon the efficiency of the mixing process it can be preferable to mix the developer with the solvent or surfactant first, e.g. in a beaker or test tube, prior to transferring a portion of the mixture into the small depression  102  for spectrophotometric characterization.  
         [0022]    Once a sample having a well-defined surface is situated in the small depression  102 , one or more color attributes or color parameters are measured using a spectrophotometer  112 . The spectrophotometer can optionally include an integrating sphere (not shown) and can use any optical geometry, e.g. one of the 45/0, 0/45, or diffuse/0 reflection geometries known to the art. Because the developer sample is typically translucent, a reflection geometry is preferred. However, measurements employing a transmission spectrophotometer geometry are also contemplated, particularly in the case of a relatively low concentration of developer mixed into an essentially transparent solvent. In this latter case, the sample container should be essentially transparent in the region of the measurement with well defined top and bottom surfaces. It will be appreciated by those of ordinary skill in the art that spectrophotometric measurements are typically highly dependent upon the optical geometry employed by the spectrophotometer. As a result, a consistent optica geometry, and preferably the same spectrophotometer  112 , should be used for both the calibration measurements and the measurements of developer extracted from the developer housing  10 .  
         [0023]    The spectrophotometer  112  generates conventional color parameters or color attributes, e.g. in the (L*, a*, b*) color space coordinates known to the art. The color attributes or parameter values are communicated to a computer or other electronic data processing device  116  for further processing. Alternatively, the spectrophotometer  112  can measure “raw” spectrophotometric data, e.g. an optical intensity versus wavelength matrix, which is communicated to the computer  116  and is then converted into conventional color space coordinates by processing occurring on the computer  116 . The color space coordinates are compared against a pre-determined relationship (i.e., calibration data)  118  between one or more of the color space coordinates and the toner concentration. Based upon the comparison, toner concentration in the developer sample is estimated from the spectrophotometric measurements. Although the comparison and estimation is done electronically using the computer  116  in the apparatus  100  shown in FIG. 2, it will be appreciated that the comparison could be performed manually, e.g. by comparing the spectrophotometric data with a hard copy of one or more calibration curves.  
         [0024]    It will be recognized by those of ordinary skill in the art that the apparatus depicted in FIG. 2 is readily constructed in typical printing and lithographic shops. The principle components, namely the computer  116  and the spectrophotometer  112 , are often already available in these shops. In particular, spectrophotometers are commonly used in printing and lithography to monitor the color quality and characteristics of color prints, e.g. for generating toner reproduction curves. Thus the apparatus of FIG. 2 enables convenient obtaining of absolute, quantitative toner concentration measurements.  
         [0025]    With reference to FIGS. 3, 4, and  5 , pre-determined empirical relationships between the toner concentration and the L*, hue, and chroma color parameters or attributes that are suitable alone or in combination for use as the calibration data  118  in the apparatus of FIG. 2 is described. The graphs shown in FIGS. 3, 4, and  5  were experimentally obtained in the following manner. Six calibration samples of M4 developer (a well-known developer) were prepared as follows:  
                                       Sample #   Toner Concentration   Usage                   1   3.49%   New       2   4.72%   New       3   5.53%   New       4   3.53%   40,000 prints       5   4.12%   40,000 prints       6   5.48%   40,000 prints                  
 
         [0026]    where the listed toner concentrations of the new developer samples was determined by the mass ratio of the developer and carrier which was mixed to form the new developer sample. The listed toner concentrations of the used samples  4 ,  5 , and  6  which were obtained from developer housings after 40,000 prints were determined by physical separation and weighing of the toner and carrier components of the developer sample. Spectrophotometric measurements of the samples 1-6 were obtained using an X-rite 938 spectrodensitometer (a hand-held instrument having spectrophotometric capabilities).  
         [0027]    As seen in FIG. 3, the L* parameter varied by about 8ΔE cmc (44.5 to 52.5)over approximately 2% change in toner concentration (3.5% to 5.5%). As seen in FIG. 4, the hue parameter varied by about 2ΔE cmc (77.7 to 79.6) over the same approximately 2% change in toner concentration (3.5% to 5.5%). As seen in FIG. 5, the chroma parameter varied by about 4ΔE cmc (74.9 to 79.6) over the aforementioned approximately 2% change in toner concentration (3.5% to 5.5%). It will further be observed that the hue and chroma color attributes show a significant difference for the new and used M4 developer, due to changes in the characteristics of the carrier beads resulting from the usage. From the experimental data shown in FIGS. 3, 4, and  5 , it is apparent that the L* color attribute is the optimal choice for use as the calibration data  118  in the apparatus of FIG. 2. The L* graph shows a linear variation with toner concentration, very little change after 40,000 print usage, and provides a wide 8ΔE cmc  dynamic range for toner concentrations ranging from about 3.5% to about 5.5%. However, the hue and chroma data of FIGS. 4 and 5 also show relatively linear dependencies on toner concentration, particularly for the chroma color attribute, albeit with smaller dynamic ranges and a significant dependence on the developer usage as is apparent by comparing the plots for new and used developer. Thus, the hue and chroma color parameters can also be used as the calibration data  118 , either alone or in combination with the L* data. When using the hue or chroma data, the empirical relationship preferably includes a correction for the number of prints developed by the developer prior to characterization by spectrophotometry.  
         [0028]    Although pre-determined empirical relationships between the toner concentration and the L*, hue, and chroma color attributes are derived in the FIGS. 3, 4, and  5  respectively, the employment of other color attributes in the calibration data is also contemplated. For example, pre-determined empirical relationships can likewise be derived for the a* and/or b* parameters of the conventional (L*, a*, b*) color coordinates. Similarly, pre-determined empirical relationships can be derived for color attributes comprising color differences versus a standard developer color, such relationships advantageously incorporating the CMC color difference formulas.  
         [0029]    With reference to FIG. 6A, a method  200  for characterizing the toner concentration in a developer of a printing or xerographic device according to one embodiment of the invention is described. Broadly speaking, the method includes a step  202  in which one or more empirical relationships or calibration data are obtained which quantitatively relate the color attributes to the toner concentration, and a step  204  in which an actual spectrophotometric measurement of a sample taken from the developer housing is obtained and compared with the calibration data to obtain an absolute, quantitative estimate of the toner concentration.  
         [0030]    To develop the calibration data in the step  202 , a developer sample is prepared with a known toner concentration in a sub-step  206 . This can be done, for example, by weighing out appropriate amounts of toner and carrier material and mixing to form the calibration developer sample. The sample is leveled in a sub-step  208 , e.g. using the shim stock  108  as described with reference to FIG. 2. The spectrophotometric data is measured in a sub-step  210 , e.g. using the spectrophotometer  112  as described with reference to FIG. 2. This process is preferably repeated for a plurality of developer samples having known toner concentrations in a sub-step  212 . The sub-step  212  can also include repetitions with developer samples of various usages, in which case the toner concentration is preferably determined after the usage by physical separation and weighing of the toner and carrier components of the developer sample. Based on the data acquired in the step  202 , one or more empirical relationships relating the toner concentration with one or more color attributes are derived in a step  214 . Exemplary empirical relationships produced by the step  214  for the L*, hue, and chroma color attributes are shown in FIGS. 3, 4, and  5  respectively for the exemplary M4 developer. The steps  202 ,  214  typically are done only once for a given developer material to generate the calibration data for that developer, and preferably occasionally thereafter to verify the calibration and to update it to account for shifts in the spectrophotometer or other sources of measurement drift.  
         [0031]    In the step  204 , an actual sample from a developer housing under test is obtained in a sub-step  216 , and the surface of the sample is leveled in a sub-step  218 , for example using the shim stock  108 . Spectrophotometric measurements are obtained from the sample in a sub-step  220 . In a sub-step  222 , selected spectrophotometric parameters are compared with the empirical relationships that were derived in the step  214 , taking into account the developer usage  224  if the empirical relationships indicate a dependence of the spectrophotometric parameter or parameters on usage. Based on the comparing sub-step  222 , the toner concentration is estimated in a sub-step  226 . It will be appreciated that the step  204  can be repeated for different developer samples from different developer housings insofar as empirical relationships such as those obtained in the step  214  are available for the developer materials involved.  
         [0032]    With reference to FIG. 6B, another method  300  for characterizing the toner concentration in a developer of a printing or xerographic device is described. Broadly speaking, the method includes a step  302  in which one or more empirical relationships or calibration data are obtained which quantitatively relate the color attributes to the toner concentration, and a step  304  in which an actual spectrophotometric measurement of a sample taken from the developer housing is obtained and compared with the calibration data to obtain an absolute, quantitative estimate of the toner concentration. The method  300  of FIG. 6B differs from the method  200  of FIG. 6A in that the developer sample preparation includes mixing with a solvent or a surfactant.  
         [0033]    To develop the calibration data in the step  302 , a developer sample is prepared with a known toner concentration in a sub-step  306 . This can be done, for example, by weighing out appropriate amounts of toner and carrier material and mixing to form the developer. A pre-determined mass of the developer sample is mixed with a pre-determined mass of solvent or surfactant in a sub-step  308 , e.g. using the syringe  110  to add the solvent or surfactant as shown in FIG. 2. The spectrophotometric data is measured in a sub-step  310 , e.g. using the spectrophotometer  112  as described with reference to FIG. 2. This process is preferably repeated for a plurality of developer samples having known toner concentrations in a sub-step  312 . The sub-step  312  can also include repetitions with developer samples of various usages, in which case the toner concentration is preferably determined after the usage by physical separation and weighing of the toner and carrier components of the developer sample. Based on the data acquired in the step  302 , one or more empirical relationships relating the toner concentration with one or more color attributes are derived in a step  314 . The steps  302 ,  314  typically are done only once for a given developer material to generate the calibration data for that developer, and preferably occasionally thereafter to verify the calibration and to update it to account for shifts in the spectrophotometer or other sources of measurement drift.  
         [0034]    In the step  304 , an actual sample from a developer housing under test is obtained in a sub-step  316 , and a pre-determined mass of the sample is mixed with a pre-determined mass of surfactant or solvent in a sub-step  318 , e.g. using the syringe  110  to add the surfactant or solvent as shown in FIG. 2. Of course, for a valid comparison of the sample from the developer housing with the calibration samples processed in the step  302 , the mixing with the solvent in the sub-step  318  should be performed in essentially similar fashion to the mixing with the solvent performed in the sub-step  308  in which the calibration samples were prepared. Spectrophotometric measurements are obtained from the sample in a sub-step  320 . In a sub-step  322 , selected spectrophotometric parameters are compared with the empirical relationships that were derived in the step  314 , taking into account the developer usage  324  if the empirical relationships indicate a dependence of the spectrophotometric parameter or parameters on usage. Based on the comparing sub-step  322 , the toner concentration is estimated in a sub-step  326 . It will be appreciated that the step  304  can be repeated for different developer samples from different developer housings insofar as empirical relationships such as those obtained in the step  314  are available for the developer materials involved.  
         [0035]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.