Abstract:
A method of calibrating an electrophotographic machine having an image bearing surface includes depositing a toner patch on the image-bearing surface. The toner patch has a nominal coverage that varies substantially continuously from end to end. Light is emitted onto a selected one of the areas of the toner patch. An amount of light that is reflected off of the selected one area of the toner patch is measured. The emitting and measuring steps are repeated for each of a plurality of different areas of the toner patch. At least one electrophotographic condition is adjusted. The adjusting is dependent upon the measuring steps.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic machine, and, more particularly, to calibration of halftone screens in an electrophotographic machine. 
     2. Description of the Related Art 
     Color laser printers need regular calibration of halftone screens to ensure that printed colors, especially those colors made by mixing multiple color planes, are consistent and accurate. It is known to perform the calibration by measuring a reflection signal from test patches of different shade levels for each color plane. The test patches are sized to either allow multiple measurements on a patch or to sample over a wide area to minimize noise. The number of patches is limited by calibration time and patch size. Thus, the halftone response function is measured at a limited number of points, typically less than twelve. FIG. 1 shows a typical halftone response curve derived by interpolating between ten discrete measurement points. 
     The limited number of points sampled and the noise inherent in the measurement process limits the accuracy of the linearization process. That is, measuring discrete color patches and fitting a response function to the sample colors compromise halftone screen linearization. This misses important details in the shape of the halftone response curve, especially near zero coverage unless a large number of patches are sampled. An obvious alternative is to increase the number and size of the test patches, but this wastes more toner and increases the amount of time that the printer is unavailable for printing. 
     What is needed in the art is a method of performing halftone screen linearization that is accurate, quick, and does not use an excessive amount of toner. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of determining the shape of a halftone response curve by using only a single continuous gradient test patch. 
     The invention comprises, in one form thereof, a method of calibrating an electrophotographic machine having an image-bearing surface. The method includes the steps of depositing a toner patch on the image-bearing surface, the toner patch having a plurality of areas, the plurality of areas having different shade levels; emitting a light beam that extends over a region of multiple areas of the plurality of areas of the toner patch; measuring an amount of light that is reflected off of the multiple areas of the toner patch; repeating the emitting and measuring steps for each of a plurality of different regions of the toner patch; and adjusting at least one electrophotographic condition, the adjusting being dependent upon the measuring steps. 
     An advantage of the present invention is that a large number of test measurements can be performed on a single continuous gradient patch. Thus, the halftone response curve is determined more accurately than can be determined with discrete test patches. 
     Another advantage is that a large number of test points can be measured more quickly than by using discrete test patches. 
     Yet another advantage is a single continuous gradient test patch uses less toner than do multiple discrete test patches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a plot of a halftone response curve determined by measuring ten discrete gradient patches; 
     FIG. 2 is a side sectional view of a multicolor laser printer which can be used in conjunction with the method of the present invention; 
     FIG. 3 is a side, schematic side view of the sensor arrangement of FIG. 2; 
     FIG. 4 is a top view of one embodiment of a continuous gradient toner patch which is illuminated by the light source of FIG. 3; 
     FIG. 5 a  is another embodiment of a light source assembly can be used in conjunction with the method of the present invention; 
     FIG. 5 b  is a top view of the continuous gradient toner patch of FIG. 4 illuminated by the light source of FIG. 5 a;    
     FIG. 6 a  is yet another embodiment of a light source assembly which can be used in conjunction with the method of the present invention; 
     FIG. 6 b  is a top view of the continuous gradient toner patch of FIG. 4 illuminated by the light source of FIG. 6 a;  and 
     FIG. 7 is a top view of another embodiment of a continuous gradient toner patch which is illuminated by the light source of FIG.  3 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2, there is shown one embodiment of a multicolor laser printer  10  including laser printheads  12 ,  14 ,  16 ,  18 , a black toner cartridge  20 , a magenta toner cartridge  22 , a cyan toner cartridge  24 , a yellow toner cartridge  26 , photoconductive drums  28 ,  30 ,  32 ,  34 , and an intermediate transfer member belt  36 . 
     Each of laser printheads  12 ,  14 ,  16  and  18  scans a respective laser beam  38 ,  40 ,  42 ,  44  in a scan direction, perpendicular to the plane of FIG. 2, across a respective one of photoconductive drums  28 ,  30 ,  32  and  34 . Each of photoconductive drums  28 ,  30 ,  32  and  34  is negatively charged to approximately −900 volts and is subsequently discharged to a level of approximately −200 volts in the areas of its peripheral surface that are impinged by a respective one of laser beams  38 ,  40 ,  42  and  44  to form a latent image thereon made up of a plurality of dots, or pels. The photoconductive drum discharge is limited to about −200 volts because the conductive core is biased at −200 volts to repel toner at the beginning of printing when the photoconductive surface touching the developer roll has not yet been charged to −900 volts by the charge roll. During each scan of a laser beam across a photoconductive drum, each of photoconductive drums  28 ,  30 ,  32  and  34  is continuously rotated, clockwise in the embodiment shown, in a process direction indicated by direction arrow  46 . The scanning of laser beams  38 ,  40 ,  42  and  44  across the peripheral surfaces of the photoconductive drums is cyclically repeated, thereby discharging the areas of the peripheral surfaces on which the laser beams impinge. 
     The toner in each of toner cartridges  20 ,  22 ,  24  and  26  is negatively charged to approximately −600 volts. A thin layer of negatively charged toner is formed on the developer rolls of toner cartridges  20 ,  22 ,  24 , and  26  by means known to those skilled in the art. The developer roll is biased to approximately −600 volts. Thus, when the toner from cartridges  20 ,  22 ,  24  and  26  is brought into contact with a respective one of photoconductive drums  28 ,  30 ,  32  and  34 , the toner is attracted to and adheres to the portions of the peripheral surfaces of the drums that have been discharged to −200 volts by the laser beams. As belt  36  rotates in the direction indicated by arrow  48 , the toner from each of drums  28 ,  30 ,  32  and  34  is transferred to the outside surface of belt  36 . As a print medium, such as paper, travels along path  50 , the toner is transferred to the surface of the print medium in nip  54 . Transfer to paper is accomplished by using a positively biased transfer roll  55  below the paper in nip  54 . 
     A sensor arrangement  56  includes a light source  58  and a light detector  60 . Since belts are prone to warp and flutter as they move between rollers, patch sensor arrangement  56  can be located opposite a roller to stabilize the distance between sensor arrangement  56  and belt  36 . An aperture  62  (FIG. 3) defines a light beam  64  produced by light source  58 . Light beam  64  illuminates a continuous gradient toner test patch  66  on intermediate belt  36 , bearing in mind “continuous gradient” may be substantially continuous as having been synthesized from discrete shade levels that can be produced by the printer. Typically, the gradient is synthesized from uncorrected shade levels 0-255. The light reflecting off of toner patch  66  is sensed by light detector  60 . 
     Continuous gradient patch  66  extends over an area on belt  36  that corresponds to the area occupied by conventional toner patches in prior art printers. That is, an eight-inch long continuous gradient patch  66  occupies the space that could otherwise be taken by eight one-inch conventional patches. Continuous gradient patch  66  has a white first end  68 , best seen in FIG. 4, which has no toner coverage, and a solid second end  70  which has 100% toner coverage. The amount of toner coverage gradually increases along the length of gradient patch  66  from white end  68  to solid end  70 . 
     A small solid bar  72  is at a fixed position just beyond and adjacent to white end  68 . Bar  72  can be a distance away from white end  68  that is one-half the diameter of aperture  62  (one-half the diameter of light beam  64 ). Bar  72  marks the start of continuous gradient patch  66 . Bar  72  has a width  74  that is approximately equal to the diameter of aperture  62  (the diameter of light beam  64 ) in order to provide a true solid area reading. 
     For a nominally linear gradient patch, the nominal coverage is given by X/L, where X is a distance between white end  68  and the middle of light beam  64  where it impinges upon continuous gradient patch  66 , and L is the length of continuous gradient patch  66 . For example, a light beam  64  with a diameter of 4 mm covers 2% of a 200 mm long gradient patch  66 . The reading from sensor  56  will give the average color over the area illuminated by light beam  64 , and given that the halftone response is substantially linear within this 2% region, the reading will represent the color of the halftone screen at X/200 nominal coverage. Assuming that sensor  56  can be read every 0.33 mm, almost 600 readings are obtained over the length of gradient patch  66 . 
     In order to set the halftone linearization, each reflection signal is converted into a predicted L* or lightness. The predicted L* value is the lightness value one would expect to measure if the toner patch were transferred to paper and fused. The predicted L* values obtained in this manner are then fit to a piecewise polynomial function using a least squares method. The almost 600 readings are enough so that the least squares fit of the data covers the entire halftone response function, and is enough so that a significant amount of noise will be averaged out. The piecewise polynomial function is then used project L* values beyond and in between the tested conditions and allows more accurate image density control for the toner on intermediate transfer belt  36 . The piecewise polynomial halftone response function is used to determine what percentage of toner coverage is necessary to achieve a desired lightness or L* level. 
     The change in toner coverage along the length of gradient patch  66  is not limited to being linear or monotonic, but the printer&#39;s software and controller  76  must track the position of light beam  64  with the nominal coverage. It is possible for light beam  64  to start at 100% toner coverage on gradient patch  66 , drop to 0, then increase again to 100%. This gives two complete ramps, bounded by solid coverage so that the precise positions of both ends of gradient patch  66  are easily detected. Specific regions of interest may be repeated to reduce noise. For example, toner coverage on a gradient patch might start at 100%, drop to 0, increase to 25%, decrease to 0, then increase again to 100%. 
     There are two important issues with nonlinear or non-monotonic gradients. First, the rate of change of the nominal coverage is limited by the width of light beam  64  and the linearity of the halftone response. If the halftone response within the area illuminated by light beam  64  is nonlinear, then the average toner coverage within light beam  64  will not represent the toner coverage at the center of light beam  64 . Second, if the direction of increasing toner coverage changes within the area illuminated by light beam  64 , then the average toner coverage within light beam  64  will not represent the toner coverage at the center of light beam  64 . 
     An easy solution to the above two issues is to keep the rate of change in toner coverage along the length of gradient patch  66  low enough so that the difference between the average coverage and the coverage at the center of light beam  64  is under the noise level, i.e., is negligible. One way of doing this while minimizing the length of gradient patch  66  is by modifying the shape of aperture  62  and light beam  64 . For instance, a light source assembly including a light source  78  (FIG. 5 a ) having a rectangular aperture  80  can be used to illuminate the sample. The light source assembly is oriented such that a major axis  82  (FIG. 5 b ) of a rectangular area  84  illuminated through aperture  80  is perpendicular to the length of gradient patch  66 . In another embodiment, a light source assembly including a light source  86  (FIG. 6 a ) having an elliptical aperture  88  can be used to illuminate the sample. The light source assembly is oriented such that a major axis  90  (FIG. 6 b ) of an elliptical area  92  illuminated through aperture  88  is perpendicular to the length of gradient patch  66 . In either the embodiment of FIG. 5 or the embodiment of FIG. 6, a smaller portion of gradient patch  66  in the length direction is sampled than with the round light beam  64  of FIG.  4 . 
     Another possible solution to the above two issues is that the software can average the nominal coverage within the light beam at any given position, based on the shape of the aperture and the nominal coverage at that point in gradient patch  66 , and use that as a reference for the measured coverage. 
     Gradient patch  66  is used in the embodiment described herein to set the halftone linearization. However, it is to be understood that gradient patch  66  can also be used to set other imaging conditions, i.e., electrophotographic conditions such as laser power (exposure energy), developer roll bias voltage and/or gamma correction. 
     Gradient patch  66  has been described herein as having a linear increase in shade level between white end  68  and solid end  70 . However, the change in shade level can also be nonlinear between white end  68  and solid end  70 . For example, since L* is relatively insensitive to shade level at higher shade levels, the rate of increase in shade level can be greater on the darker side of the gradient patch, as is the case in gradient patch  94  (FIG.  7 ). Different shade levels are obtained by turning on different numbers of pixels within a halftone cell. In a laser printer this is done by exposing the photoconductive drum at the selected pixel locations. 
     The present invention has been described herein as being used in conjunction with an electrophotographic machine. However, it is to be understood that the method of the present invention can also be used in any printing device that uses a sensor to monitor and correct/linearize the halftone printing response. Such printing devices include inkjet printers, dye sublimation printers and thermal transfer printers. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.