Abstract:
A development unit used in xerographic printing retains a mixture of at least a first colorant and a second colorant. An illuminator directs toward at least a portion of the mixture a first light of one color, a second light of a second color, and a third light substantially outside of a visible range. A photosensor records a first reflectance signal based on light reflected from the mixture substantially in a visible range, and a second reflectance signal based on light reflected from the mixture substantially outside a visible range. The signals can be used to maintain the mixture at the desired total color and other properties.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The following documents are incorporated by reference in their entireties for the teachings therein: U.S. Pat. Nos. 6,993,272 and 6,931,219, and U.S. Patent Application Publication 2006/0127110. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to electrostatographic or xerographic printing, and in particular to obtaining desired colors in xerographic printing through mixing of primary-color toners. 
       BACKGROUND 
       [0003]    In high-volume xerographic printing, it has recently become of interest to provide “custom color” options, in which a dedicated developer unit dispensing toner of a very specific color is provided. A custom color is typically desired by a customer using a characteristic, and sometimes proprietary, color for letterheads and other purposes. In one business model, a printing or supplies vendor blends two or more commercially-available component color toners to obtain a mixture having the specific desired color. The mixture is then used directly in a single developer unit, along with the standard black developer unit, within a highlight-color printing apparatus or in a “fifth housing” in a full-color printing apparatus. 
         [0004]    In a practical application of a custom color xerographic system, the use of a mixture of color toners in a single developer unit presents certain challenges. Most notably, different types of toner, such as corresponding to different component colors, may be electrostatically drawn from the development unit at different rates: unchecked, the “faster-going” type of toner will be drawn out of the unit toward a photoreceptor at a high rate toward the beginning of use, leaving a high concentration of the “slower-going” toner (of a different color) in the unit. In short, the use of a mixture of toners can cause a drift of the actual color produced by the unit over time. Further, the two types of toner may have significantly different electrostatic properties, and the single developer unit may be controlled assuming a set of electrostatic properties of the predetermined original mixture: as the relative concentration of the two types of toner drifts away from that of the original mixture, electrostatic control of the unit, even with feedback control, becomes uncertain. 
         [0005]    An optical approach for sensing toner concentration has been used in a broad range of noises and under machine operating conditions. However, for sensing of mixtures of toners of different types, using a single LED emission wavelength cannot provide color information. This sensing approach uses the fact that the color of the developer, i.e., its L*a*b* values, monotonically increases or decreases as a function of increasing or decreasing concentration of the toner mixture and of the constituents of the mixture. A spectrophotometer based device can provide the same information, but this approach has some shortcomings, such as a need for a broad band white light source, and a network of fiber optics to transmit the signal to and from the various color-toner sumps within a printing apparatus. 
         [0006]    This disclosure proposes an optical toner concentration sensor that provides an alternative to the spectrophotometer. 
         [0007]    The two patent documents incorporated by reference above demonstrate the overall strategy of using and controlling a custom-color developer unit with a powdered-toner mixture. U.S. Pat. Nos. 5,781,828 and 6,575,096 describe a color control system for xerography using toners suspended in liquids. 
       SUMMARY 
       [0008]    According to one aspect, a printing apparatus comprises a development unit retaining a mixture of at least a first colorant, having a first component color, and a second colorant, having a second component color. An illuminator directs toward at least a portion of the mixture a first light of a first predetermined color, a second light of a second predetermined color, and a third light of a third predetermined color, the third predetermined color being substantially outside of a visible range. A photosensor records a first reflectance signal based on light reflected from the mixture substantially in a visible range, and a second reflectance signal based on light reflected from the mixture substantially outside a visible range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a simplified diagram of a “custom color” xerographic printing apparatus. 
           [0010]      FIG. 2  is a plan view and  FIG. 3  is a side view of an illuminator-photosensor package. 
           [0011]      FIG. 4  is a diagram of a feedback system according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  is a simplified diagram of a “custom color” xerographic printing apparatus, capable of printing images in two colors, such as black and a custom color. As is familiar in xerography, there is provided a belt photoreceptor  10 , entrained around rollers and rotatable in process direction P. At fixed locations are provided the black developer unit  12 , and the custom color developer unit  20 , which will be described in detail below. Images developed in black and in the custom color on photoreceptor  10  are transferred at a transfer station  14  to a print sheet. Other elements familiar in xerography, such as charging, exposure, and cleaning devices, are not shown. 
         [0013]    Turning to custom color development unit  20 , a main housing is used to mix, in this embodiment, two primary color toners (component colorants) to obtain a desired custom color that will be used to develop the suitably-charged surfaces of photoreceptor  10 . Although a system with two component colorants is shown in the illustrated embodiment, other embodiments with three or more mixable component colorants are possible. As mentioned above, in order to maintain the desired custom color within development unit  20 , the two component colorants must be maintained in a predetermined proportion over time while the printer is running. In order to admix extra amounts of each component colorant as needed to the housing of development unit  20 , there are provided distinct component colorant supplies  22 , one for each component colorant, and labeled C 1  and C 2 . Each component colorant supply  22  comprises some sort of container for its component colorant, as well as a mechanism for outputting a predetermined amount of the component colorant into development unit  20  upon an external request; such mechanisms are known in the art. 
         [0014]    Associated with the housing development unit  20  is what can generally be called an “illuminator-photosensor”  200 , which is associated with a control system  300  that ultimately controls the amount of component colorant C 1  or C 2  output by each supply  22 . Illuminator-illuminator-photosensor  200  is placed adjacent to a light-transmissive window  30  in development unit  20  and is thus exposed to a well-mixed mixture of the two component colorants. The window  30  can be adjacent a mixing brush  32  or similar structure inside development unit  20 . 
         [0015]      FIG. 2  is a plan view of an illuminator package  201  that can be used in illuminator-photosensor  200 . A typical size of such a package  201  is about 3 mm per side. The illuminator-photosensor includes a plurality of color-specific light sources  202 R,  202 B,  202 G, and  202 N, which selectably emit light in predetermined ranges of red (600-700 nm), blue (400-500 nm), green (500-600 nm), and neutral wavelengths, respectively. In one embodiment the “neutral” color is infrared, about 940 nm. The light sources  202 R,  202 B,  202 G encompass the entire visible spectrum, or at least a portion of the spectrum as large as a possible gamut of colors obtainable by various combinations of component colorants C 1  and C 2 . In order to expand the sensed color gamut, additional light sources of wavelength of interest can be integrated in the illuminator package. 
         [0016]    The light sources  202 R,  202 B,  202 G, and  202 N are disposed at the bottom of a cavity  208  shaped like a truncated cone. Also associated with light sources  202 R,  202 B,  202 G, and  202 N are any number of terminals  210 . 
         [0017]      FIG. 3  is a side view of the illuminator-photosensor  200 . Photosite  204  is disposed at the end of an angled channel  206 , and thus oriented relative to the light sources such that it will receive only diffuse light reflected from the developer mixture through window  30 . The photosite  204  includes an electronic element having a photosensitive surface, such as a Si-photodiode detector. The arrangement of this embodiment is useful for ensuring light from a selected light source  202 R,  202 B,  202 G, or  202 N is reflected from the window  30  associated with development unit  20  and the reflected light is detected at photosite  204 . 
         [0018]    According to one practical embodiment, when it is desired to determine accurately the color of the mixed component colorants within development unit  20  at a given time, each of the plurality of light sources such as  202 R,  202 B,  202 G, and  202 N is sequentially “lit up” for a brief predetermined period and the reflection response for each color is recorded sequentially at photosite  204 . In one practical embodiment, the light-up period is about one half second for each light source, but the period can be varied, typically depending on the rotational speed of augers or brush  32  within the development unit  20 . 
         [0019]      FIG. 4  is a diagram of a feedback system according to one embodiment. Sensor  200  establishes reflectivity values for a developer mixture of several toners in development unit  20 . The reflectivity signals in the RGB channels, i.e., the reflectivity of the developer mixture when reflecting light from sources  202 R,  202 B, and  202 G, are submitted to a “model” algorithm  220 , which converts the signals to TC values corresponding to each component colorant C 1  and C 2 . (As used herein, the term “TC” refers in this embodiment to a % TC, or percentage of toner to carrier, but can refer to any datum, such as toner-to-carrier ratio, toner-to-total-developer ratio, or any other value, for any purpose, in which relative amounts of toner and carrier are significant components.) Simultaneously, the reflectivity related to the neutral or infrared channel (relating to source  202 N) is used to derive a TC value, which relates to the TC of the total mixture. 
         [0020]    In one embodiment, the model algorithm  220  establishes the relationship between the optical response of the sensor  200  and the TC. The optical responses are given by the following equations: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     i 
                     PD 
                   
                   = 
                   
                     
                       C 
                       i 
                     
                     × 
                     
                       
                         ∫ 
                         
                           λ 
                           0 
                           i 
                         
                         
                           λ 
                           1 
                           i 
                         
                       
                        
                       
                         
                           S 
                           PD 
                         
                         × 
                         
                           E 
                           i 
                         
                         × 
                         
                           R 
                           d 
                         
                          
                         
                             
                         
                          
                         
                            
                           λ 
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    And the total response is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     i 
                     PD 
                   
                   = 
                   
                     
                       C 
                       t 
                     
                     × 
                     
                       
                         ∫ 
                         
                           λ 
                           0 
                           t 
                         
                         
                           λ 
                           1 
                           t 
                         
                       
                        
                       
                         
                           S 
                           PD 
                         
                         × 
                         
                           E 
                           t 
                         
                         × 
                         
                           R 
                           d 
                         
                          
                         
                             
                         
                          
                         
                            
                           λ 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
       Where 
       [0000]    
       
         
           
             i=LED (R, G, B, etc.) 
             t=LED(IR) 
             S PD  is the normalized spectral responsivity of photosite  204   
             E is the normalized spectral density of the LED 
             R d  is the diffuse reflectivity spectra of the developer 
             V PD   i  is the optical response of the developer measured using LED illuminant i 
             V PD   t  is the optical response of the developer measured by using LED illuminant t 
           
         
       
     
       C i  and C t  are constants containing (a) optical path factors, (b) peak responsivity of the photosite  204 , and (c) peak responsivity of each LED  202 R,  202 B,  202 G, and  202 N. 
       [0028]    In referenced U.S. Pat. No. 6,931,219 it is shown how to obtain conversion factors from optical responses to TC i  and TC t , for equations (1) and (2) for developer of single color toners. For developer mixtures it is required to measure the responses for several regions of the optical spectrum. Here we use selected colors, e.g., RGB illuminants. One embodiment involves the transformation to device independent color space, i.e., Lab CIE, to process the optical responses of the mixture. For example, using the Lab CIE color space, a usual transformation from RGB to Lab CIE involves a matrix transformation of the type: 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             L 
                             * 
                           
                         
                       
                       
                         
                           
                             a 
                             * 
                           
                         
                       
                       
                         
                           
                             b 
                             * 
                           
                         
                       
                     
                     ] 
                   
                   ≈ 
                   
                     A 
                     × 
                     
                       [ 
                       
                         
                           
                             R 
                           
                         
                         
                           
                             G 
                           
                         
                         
                           
                             B 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    Where R G B are the optical responses measured by the RGB illuminants, respectively, L* a* b* are the CIE color space values of the sample, and A is a 3×3 matrix with coefficients a ij  determined experimentally from measurements of TC calibrated samples using a spectrophotometer. Another embodiment uses directly the measured RGB values to map the TC of the sample. The values of the responses are uniquely determined by the TC of the developer sample, provided that the chemical and physical compositions of the constituent toners and carriers are not changed. Otherwise, a new set of calibration coefficients has to be determined. 
         [0029]    Further as shown in  FIG. 4 , the values of TC derived for each response V PD   i , as well as the total TC derived from the IR response, V PD   t , is compared to a desired level, and the comparison is used in control system  300  to admix toner of one or another type through component colorant supplies  22 , as shown in  FIG. 1 . 
         [0030]    The following example illustrates how measurements using selected illuminants, e.g., RGB colors, can be used to measure properties of a mixture of a generally green custom color. 
         [0031]    The spectra of a desired green developer (mix of cyan and yellow toners) contain elements of both cyan and yellow responses. By inspection, the blue LED (470 nm) has small response for yellow and maximum response for cyan, whereas for green (565 nm) and red (660 nm) LEDs the situation is reversed. The relationship between the optical response of the mixture and those of the constituents is given by adding the absorbances of the optical responses of the constituent toners. However, in first approximation, and for small TC changes around a given target TC, the optical response of the mixture and those of the constituents can be given by adding the optical responses of the constituent toners. In the following example we use the later approximation. Then, the optical response of a 50:50 mixture of cyan and yellow at 4.5% TC to render 4.5% TC green developer, can be approximately represented by the following relationships: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 V 
                                 R 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_green 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 G 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_green 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 B 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_green 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     ≈ 
                     
                       
                         
                           a 
                           cyan 
                         
                         × 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     V 
                                     R 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_cyan 
                                     ) 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     V 
                                     G 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_cyan 
                                     ) 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     V 
                                     B 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_cyan 
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       + 
                       
                         
                           b 
                           yellow 
                         
                         × 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     V 
                                     R 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_yellow 
                                     ) 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     V 
                                     G 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_yellow 
                                     ) 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     V 
                                     B 
                                     PD 
                                   
                                    
                                   
                                     ( 
                                     TC_yellow 
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    In the equation, the coefficients a cyan  and b yellow  are determined experimentally. Let TC 0     —   green=4.5% be the target TC value for the green developer, and TC_green be the actual optical response obtained from measurements using the IR illuminant (V PD   t ,), e.g., TC_green=4.2%. This indicates a total TC deficit of 0.3%, and unknown deficits in the amounts of cyan and yellow toners. The TC of the cyan and yellow constituents of the green mixture are extracted from the responses [V RGB   PD (TC_green)], using an error minimization fitting procedure and the relationship between TC and optical responses of the individual constituents. The minimization procedure provides actual values of [V RGB   PD (TC_cyan)] and [V RGB   PD (TC_yellow)]: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 V 
                                 R 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_cyan 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 G 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_cyan 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 B 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_cyan 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     ≈ 
                     
                       [ 
                       
                         
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                         
                         
                           
                             0.95 
                           
                         
                       
                       ] 
                     
                   
                   , 
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     
                       5 
                        
                       a 
                     
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 V 
                                 R 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_yellow 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 G 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_yellow 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 B 
                                 PD 
                               
                                
                               
                                 ( 
                                 TC_yellow 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     ≈ 
                     
                       [ 
                       
                         
                           
                             0.96 
                           
                         
                         
                           
                             0.96 
                           
                         
                         
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                   , 
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     
                       5 
                        
                       b 
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    From these results, the following equations describing TC as a function of optical responses can be obtained. For illustration purposes, in these equations, some of the responses were approximated to zero. The actual values are not 0, but show only a small dependence on TC, as one can see from the cyan and yellow spectra, following the calibration reported in reference U.S. Pat. No. 6,931,219: 
         [0000]      % TC_cyan(Red LED)˜0,  equation (6a) 
         [0000]      % TC_cyan(Green LED)˜0,  equation (6b) 
         [0000]      %  TC _cyan(Blue  LED )=7.17 *V   B   PD −2.07,  equation (6c) 
         [0000]      %  TC _yellow(Red  LED )=10.15 *V   R   PD −5.39,  equation (7a) 
         [0000]      %  TC _yellow(Green  LED )=10.15 *V   G   PD −5.39,  equation (7b) 
         [0000]      % TC_yellow(Blue LED)˜0,  equation (7c) 
       For TC=4.5%, equation (1) and (4) gives for the target values of the responses of the primaries cyan and yellow 
       [0032]    
       
         
           
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 V 
                                 R 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _cyan 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 G 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _cyan 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 B 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _cyan 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     ≈ 
                     
                       [ 
                       
                         
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                         
                         
                           
                             0.916 
                           
                         
                       
                       ] 
                     
                   
                   , 
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     
                       9 
                        
                       a 
                     
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 V 
                                 R 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _yellow 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 G 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _yellow 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 V 
                                 B 
                                 PD 
                               
                                
                               
                                 ( 
                                 
                                   
                                     TC 
                                     0 
                                   
                                    
                                   _yellow 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ] 
                     
                     ≈ 
                     
                       [ 
                       
                         
                           
                             0.974 
                           
                         
                         
                           
                             0.974 
                           
                         
                         
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                   , 
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     
                       9 
                        
                       b 
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    The difference between target values, equations (9a-9b), and actual values, equations (6a-7c), provides a measure of the error that is translated into ΔTC for used in the controller. Then, for this simplified case, there is obtained: 
         [0000]      ΔTC_cyan=0.24%, 
         [0000]      Δ TC _yellow=−0.14%, 
         [0000]    These values are processed by a controller of, e.g., integrator type, to obtain the masses of cyan and yellow to be dispensed to adjust the ratio of cyan:yellow and the total TC_green of the development unit  20 . The particular controller design may be of different types, and could have different gains, based on the actual rate of change of the TC. 
         [0033]    Although the above description is directed to a xerographic system using mixtures of powdered colorants, the description can be applied to printing systems of any type in which the colorants are to some extent in liquid or suspension form. As such, the term “development unit” can apply not only to electrostatographic systems, but to any container in any type of printing system (such as offset or ink-jet), in which component colorants are mixed to obtain a predetermined target color. 
         [0034]    The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.