Abstract:
A method of calibrating a printing system is described herein. The method includes measuring media sensitometry at the manufacturing location using controlled test equipment and distributing data via web, mailing, or RFID chip, optimizing exposures of a test pattern, averaging multiple machine calibrations and factory distributed calibration data, adjusting the calibration curve using pivot points and measured media differences

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for calibrating a laser thermal proofer and more specifically optimizing exposures for a calibration target, averaging multiple density and exposure calibrations, measuring media performance at the factory, distributing calibration data, calculation of a calibration curve without the requirement of printing a calibration target, imaging a target for density control charting, and adjusting calibration curves when well known control chart rules have been violated.  
       BACKGROUND OF THE INVENTION  
       [0002]     Color proofing is a process used by the printing industry to simulate proofs generated on a press run. By using color proofing the printing industry saves time and money simulating how the press will look before the costly press run is performed. The advantage of a color proof is that it is a representation of an ideal press run. The color proof should reflect exactly what the printing industry would like to see coming off a press. The press is continually adjusted to match the color of the proof. The color proof therefore needs to be precise color, reproducible from proofer to proofer, and pre-press shop to pre-press shop. Proofs that exhibit color variation are deemed unacceptable.  
         [0003]     One commercially available image processing apparatus, which is depicted in commonly-assigned U.S. Pat. No. 5,428,371, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying thermal energy to the dye donor material, to transfer dye to the thermal print media, thereby forming an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), thermal print media, and dye donor material exit transports.  
         [0004]     The operation of the image processing apparatus comprises feeding a sheet of thermal media from the media roll to the vacuum drum, partially wrapped around the drum, cut to length, then wrapped fully around the drum. A length of dye donor from a roll form is similarly transported to the drum, partially wrapped around the drum, cut to a desired length, then fully wrapped around the vacuum drum. The dye donor material is wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media. The translation drive, part of the scanning subsystem, traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.  
         [0005]     The printhead includes a plurality of laser diodes that are coupled to the printhead by fiber optic cables that can be individually modulated to supply energy to selected areas of the donor in accordance with an information signal. The printhead includes a plurality of optical fibers coupled to the laser diodes at one end and at the other end to a fiber optic array within the printhead. The printhead moves relative to the longitudinal axis of the vacuum imaging drum and dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead to the dye donor material, is converted to thermal energy in the dye donor material.  
         [0006]     Color variation is typically a result of variation of the individual color density used to define the desired color. There are many factors that influence the variation of a color in a proof. Factors include but are not limited to environment variability, density calibration technique, optical noise, thermal media coating quality, densitometer measurement noise, lamination noise, and digital proofer focus errors.  
         [0007]     Due to factors that cause density variation, calibration of density is required to achieve high levels of consistency between a requested or desired density and the average density on an imaged proof. Calibration to the average proof density is desired due to the fluctuation of density within a single proof. To be most color accurate across the entire proof the average density should closely match the requested density.  
         [0008]     Some early digital proofers, such as U.S. Pat. No. 5,268,708 utilized a linear calibration model over a specified density range. Specifications for the output density range were developed from printing standards. The SWOP standard was the model used for the calibration range for U.S. Pat. No. 5,268,708.  
         [0009]     Presses are capable of producing more than cyan, magenta, yellow, and black output colors. Often special colors are added to expand the color of a print. Colors from a press are limited to the various ink colors that can be mixed or created—a virtually endless assortment. Presses therefore had a much larger color gamut than proofers, which are limited by the color gamut produced by thermal dye.  
         [0010]     With the introduction of U.S. Pat. No. 5,428,371, color gamuts were expanded using a concept named “Recipe Color.” The amount of usable density range was increased for each thermal dye donor. Multiple passes of the same bitmap used for a single color plane were imaged using different thermal dye donor material. This allowed for custom color creation, and with the introduction of a few special thermal dye donor material, the color range of a proof is extended to closely match the output of a printing press using many different inks.  
         [0011]     Commonly-assigned copending U.S. patent application Ser. No. 10/649,462 offered improved data processing, an extended calibrated dynamic density range, and improved color calibration through the use of controlled polynomial modeling of density and exposure data. One draw back of U.S. patent application Ser. No. 10/649,462 is the time required to calibrate thermal media was increased over previous calibration models as more density points were required to image and use for modeling the extended color space.  
         [0012]     One drawback of U.S. Pat. No. 5,428,371 is the fact that calibration is required for every thermal media roll change. Customers are requested to spend time and money on calibration for every unique thermal donor and thermal receiver lot. Often customers will calibrate based on a schedule, not based on roll change, therefore the imaging device accuracy is sacrificed.  
         [0013]     A disadvantage of U.S. patent application Ser. No. 10/649,462 is that the exposures for the calibration target are the same and broad in scope for all machines to compensate for machine to machine exposure variation. This disadvantage creates density data points on the calibration target for some machines that are not useable for building an exposure calibration model, therefore the density data points are discarded thus lowering the accuracy of the calibration model.  
         [0014]     A disadvantage of U.S. patent application Ser. No. 10/649,462 is that the thermal media density can drift after an exposure calibration curve has been created causing inaccuracies in the output density.  
         [0015]     U.S. Pat. Nos. 5,229,585; 6,634,814; 6,099,178, and U.S. Patent Application Publication No. 2004/0184801 discuss including readable sensitometric data with customer media supplies to control calibration. Typically the sensitometric data is referred to as a reference exposure. The disadvantage of a reference exposure is that the system that creates a reference exposure may drift over time. Systems that use lookup tables to reference exposure patches will become inaccurate if the reference system drifts and cannot be restored to original operating conditions.  
       SUMMARY OF THE INVENTION  
       [0016]     Briefly summarized, according to one aspect of the present invention, a method for calibration for an imaging apparatus comprises optimizing exposures used for imaging a calibration target such that all density points are used in modeling density and exposure data, averaging multiple calibration curves to create an average calibration curve, measuring thermal media exposure performance difference from average thermal media exposure performance on a control imaging device, distributing exposure calibration data via electronic media or on an RFID chip, adjusting an exposure calibration curve for an imaging device based on control imaging device measured thermal exposure differences, and adjusting an average exposure calibration curve for an imaging device based on averaged control imaging device measured thermal exposure differences.  
         [0017]     According to one embodiment a series of test patches covering the range of desired exposures are imaged in order using randomized exposure values. The randomized nature of the data points, the range of exposures the points cover, and the number of points are used in order to gain high accuracy with the presence of many noise sources. The calibration proof is designed to be scanned using a scanning densitometer allowing for polynomial interpolation between patches for precise location of the requested density. Exposures for desired densities can be calculated for future calibration targets based on an initial exposure calibration curve. Thermal print media can be calibrated and control charted on a control imaging device allowing the calculation of thermal media lot to lot exposure differences. Multiple exposure calibration curves can be collected with correlating data sets of thermal media exposure differences. Using an averaged calibration curve and thermal media exposure differences, the averaged calibration curve can be adjusted to accommodate changes in lot to lot variability of sensitometric performance for thermal media and eliminate the requirement of exposure calibration for every unique thermal media.  
         [0018]     An advantage of the present invention is decreased calibration time.  
         [0019]     An advantage of the present invention is the ability to create thermal media exposure calibration data using a control imaging device that will drift over time.  
         [0020]     An advantage of the present invention is decreased material required for calibration.  
         [0021]     An advantage of the current invention is to bypass thermal media exposure calibration for every roll change.  
         [0022]     An advantage of the present invention is increased accuracy locating the average density request in noisy density regions.  
         [0023]     An advantage of the present invention is an increase in the number of data points used in density and exposure modeling which correlates to an increase in density accuracy.  
         [0024]     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a side view and vertical cross-section of an image processing apparatus usable with embodiments of the present invention.  
         [0026]      FIG. 2  is a perspective view of the lathe bed scanning subsystem usable with embodiments of the present invention viewed from the rear of the image processing apparatus.  
         [0027]      FIG. 3  is a schematic diagram of the method of the embodiments described herein.  
         [0028]      FIG. 4  shows a representation of the test pattern with a set of randomized exposures.  
         [0029]      FIG. 5  shows an example of the calibration curve.  
         [0030]      FIG. 6  shows the data set formed for use in embodiments of the invention.  
         [0031]      FIG. 7  shows an example of optimizing calibration exposures on a calibration target and the resulting densities.  
         [0032]      FIG. 8  shows a factory control chart which monitors control imaging device exposures for a requested density level for control thermal media.  
         [0033]      FIG. 9  shows calibrated exposure for different thermal media lots of one type of thermal media and the calibrated exposure difference for each thermal media lot from an average calibrated exposure for all thermal media lots.  
         [0034]      FIG. 10  shows the process for calculating an exposure calibration curve using an initial exposure calibration curve and exposure adjustments at defined density levels.  
         [0035]      FIG. 11  shows exposure calibration averaging for multiple thermal media lots. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0037]     Referring to  FIG. 1 , there is illustrated an image processing apparatus according to the present invention having an image processor housing  112  which provides a protective cover. A movable, hinged image processor door  114  is attached to the front portion of image processor housing  112  permitting access to a media carousel  116 . A roll of donor roll material  118   a  is connected to a media carousel  116  in a lower portion of image processor housing. Up to seven rolls of roll media can be used  118   a,    118   b,    118   c,    118   d,    118   e,    118   f  and  118   g.  One roll of media  118   g  is thermal print media used to receive the donor material. This thermal print media  118   g  is passed to vacuum imaging drum  130  and is ultimately cut into donor sheet material (not shown). In this regard, a media drive mechanism  115  is attached to the thermal print media  118   g,  and includes three media drive rollers, two rollers  140  and  142  are shown in  FIG. 1 , through which the thermal print media of interest is metered upwardly into a media knife assembly  144 . After thermal print media reaches drum load roller  146 , media drive rollers  140 ,  142  cease driving the donor roll material and at least one media knife blade (not shown) positioned at the bottom portion of media knife assembly to cut the thermal print media roll into thermal print media sheets. Drum load roller  146  presses the cut thermal print media against the vacuum imaging drum  130  while the vacuum imaging drum  130  slowly rotates the cut media (not shown) around vacuum imaging drum  130 .  
         [0038]     Each remaining roll of donor material is a different color, typically black, yellow, magenta and cyan. Special colors include but are not limited to orange, green, blue, metallic, and white. These donor roll materials are passed to the vacuum imaging drum  130  and are ultimately cut into donor sheet materials (not shown) for forming the medium from which colorant imbedded therein are passed to thermal print media resting thereon, which process is described in detail below.  
         [0039]     A media drive mechanism  115  is attached to each roll media of donor roll material, and includes three media drive rollers  112  through which the donor roll material  118   a,    118   b,    118   c,    188   d,  and  188   e  of interest is metered upwardly into a media knife assembly  144 . After donor roll material reaches drum load roller  146 , media drive rollers  140 ,  142  cease driving the donor roll material and the media knife blade is positioned at the bottom portion of media knife assembly to cut the donor roll material into donor sheet materials. Drum load roller  146  presses the cut media against the vacuum imaging drum  130  while the vacuum imaging drum  130  slowly rotates the cut media (not shown) around vacuum imaging drum  130 . The donor sheet material now rests atop the thermal print media (not shown) with a narrow space between the two created by microbeads embedded in the surface of the thermal print media.  
         [0040]     A laser assembly  120  includes a quantity of laser diodes  122  in its interior. Laser diodes  122  are connected via fiber optic cables  124  to a distribution block  126  and ultimately to printhead  128 . Printhead  128  directs thermal energy received from laser diodes  122  causing the donor sheet material to pass the desired colorant across the gap and onto the thermal print media (not shown).  
         [0041]     In operation, vacuum imaging drum  130  rotates at a constant velocity. Printhead  128  begins at one end of the thermal print media and traverses the length of the thermal print media, transferring dye for a particular donor sheet material resting on the thermal print media. After printhead  128  has completed the transfer process, for the donor sheet material resting on the thermal print media the donor sheet material is then removed from the vacuum imaging drum  130  and transferred out of image processor housing  112  via a skive or donor ejection chute  132 . The donor sheet material eventually comes to rest in a donor waste bin  134  for removal by the user. The process is then repeated for the other desired rolls of donor roll media.  
         [0042]     After the color from all desired sheets of the donor materials have been transferred and the donor sheet materials have been removed from vacuum imaging drum  130 , the thermal print media is removed from vacuum imaging drum  130  and transported via a transport mechanism  136  to a exit tray  137 . Thermal print media is then laminated to a paper stock using a 800XL laminator.  FIG. 1  also show translation bearing rods  148 ,  150  which are explained with reference to  FIG. 2 .  
         [0043]      FIG. 2  shows a perspective view of lathe bed scanning subsystem of image processing apparatus, including vacuum imaging drum  130 , printhead  128 , and lead screw  152  assembled in lathe bed scanning frame  154 . Printhead  128  is attached to a lead screw  152  via a lead screw drive nut  156  and a drive coupling (not shown) for moving the printhead along the longitudinal axis of vacuum imaging drum  130  for transferring the dye to create the intended image onto the thermal print media  118   g.    
         [0044]     Vacuum imaging drum  130  is mounted for rotation about an axis X in lathe bed scanning frame  154 . Printhead  128  is movable with respect to vacuum imaging drum  130 , and is arranged to direct a beam of light to the donor sheet material. The beam of light from printhead  128  for each laser diode (not shown in  FIG. 2 ) is modulated individually by modulated electronic signals from image processing apparatus, which are representative of the shape and color of the original image, so that the color on the donor sheet material is heated to cause volatilization only in those areas in which its presence is required on the thermal print media to reconstruct the shape and color of the original image.  
         [0000]     Translation  
         [0045]     Viewing  FIGS. 1 and 2 , printhead  128  is mounted on a movable translation stage member  158  which, in turn, is supported for low friction slidable movement on translation bearing rods  148  and  150 . Translation bearing rods  148  and  150  are sufficiently rigid so as not to sag or distort as is possible between their mounting points and are arranged to be as parallel as possible with axis X of vacuum imaging drum  130  with the axis of printhead  128  perpendicular to the axis X of vacuum imaging drum  130 . Front translation bearing rod  150  locates translation stage member  158  in the vertical and the horizontal directions with respect to axis X of vacuum imaging drum  130 . Rear translation bearing rod  148  locates translation stage member  158  only with respect to rotation of translation stage member  158  about front translation bearing rod  150  so that there is no over-constraint condition of translation stage member  158  which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to printhead  128  during the generation of an intended image.  
         [0000]     Writing  
         [0046]     Printhead  128  travels in a path along vacuum imaging drum  130 , while being moved at a speed synchronous with the vacuum imaging drum  130  rotation and proportional to the width of a writing swath (not shown). The pattern that printhead  128  transfers to the thermal print media  118   g  along vacuum imaging drum  130  is a helix.  
         [0047]      FIG. 3  shows the steps of the embodiments described herein. The embodiments described herein generally include a method of calibrating a printing system. The method includes imaging  11  a test pattern  10 . The test pattern  10  includes a set of densities. Each density is based on a plurality of exposures which have been randomized thereby forming an imaged test pattern.  
         [0048]     The method further includes scanning  13  the densities of the imaged test pattern thereby forming a dataset  15 . In one embodiment, the density of the test pattern is made from a thermal dye donor. In another embodiment, the density of the test pattern comprises multiple colors.  
         [0049]     Then, the method includes analyzing  17  the dataset  15  to identify outlier data. If outlier data is identified, then the test pattern  10  undergoes a second scan  19 , a second dataset is formed  21  and the second dataset is analyzed  23  for outlier data. If no outlier data is identified, a first density point  28  and a second density point  30  of the dataset are computed  25  to form an array  36   a,    36   b,    36   c  of exposures versus a density between the first and second density points, thereby forming a polynomial calibration curve  38  from the array of exposures and the density array. For example, the first density point can be a toe point. The second density point can be a shoulder point. Alternatively, the second density point can be a desired density point for setting a maximum for a polynomial calibration curve, as described below.  
         [0050]     The method then includes calibrating  25  an adjustable density printing system using the polynomial calibration curve.  
         [0051]     In one embodiment, the method further includes calculating for a threshold of the density data to obtain the toe density point. In another embodiment, the method includes laminating the test pattern onto a paper stock.  
         [0052]     In one embodiment, the imaged test pattern is formed by a printing system comprising a printer, a laminator, a spectrometer or combinations thereof. In yet another embodiment, the printing system is a laser thermal, silver halide, or an inkjet printer. In yet another embodiment, the printing system is a digital printer. In yet another embodiment, the printing system comprises a laser printer.  
         [0053]     In another embodiment, the method further includes removing data outside of three standard deviations from the polynomial calibration curve.  
         [0054]     The method can further include after forming a polynomial calibration curve  38  from the array of exposures and the density array, computing a first slope for density greater than the second density point.  
         [0055]     Alternatively, the method can include after forming a polynomial calibration curve  38  from the array of exposures and the density array, computing a second slope for density less than the first density point.  
         [0056]     In one embodiment, the first and second slopes are extensions of the polynomial calibration curve therein adapting the curve to be usable for extreme density ranges.  
         [0057]     In another embodiment, the method includes subtracting paper density from the dataset enabling the calibration curve to be insensitive to the type of paper stock used in the printing system.  
         [0058]     In yet another embodiment, the test pattern  10  includes an independent row  200  for each set of densities  90 ,  91 ,  92 ,  93  and at least four points within a row  300 ,  302 ,  304 ,  306  that include only paper density.  
         [0000]     Calibration Target or Test Pattern  
         [0059]      FIG. 4  shows a test pattern  10  which is used to measure the output density for a requested exposure. The test pattern preferably includes forty-one sets of density  90 ,  91 ,  92 ,  93 . Up to six colors can be included on the test pattern. A separate row  200  with a unique exposure array per row is used for each thermal dye donor when imaging multiple thermal dye donors on a single calibration target. The patches  40  are arranged in the slow scan direction of the proofer in order to capture the majority of the density variability. Orientating the sets of density data in the direction of the largest noise source is generally necessary to provide an accurate average density response. The number of sets of density patches per row has been maximized in order to generate accuracy in the presence of density noise. The order of the exposures has been randomized to provide stability of the calibration model in the presence of density noise. The order of the sets of density also allows for a minimum patch to patch contrast of 0.5 density, which is required to use a scanning spectrophotometer model number DTP-41 manufactured by X-Rite. Forty-one exposures have been predetermined for each donor based on desired calibration accuracy, thermal media variability, and machine variability. Exposure step rates are not consistent; there exists a smaller exposure step rate at low exposures levels to provide the most accurate low density response. At higher exposure levels, the exposure step rate increases. Very high exposures are added to accommodate machine drift. Four paper points  300 ,  302 ,  304 ,  306  are placed in the calibration target for accurate measurement of the paper stock. The model subtracts the paper density off the measured density to allow for consistent density output across different paper stocks.  
         [0000]     Density Processing  
         [0060]     The test pattern  10  is scanned in using a X_Rite DTP41 spectrophotometer and a custom graphical user interface to allow for density viewing, saving and processing. The results of the density processing can be seen in  FIGS. 5 and 6 .  
         [0061]     The first step in generating a calibration curve  38  is to remove the paper density from the measured density array made of density values  36   a,    36   b,    36   c.  The four paper points  300 ,  302 ,  304 ,  306  are averaged to provide greater statistical accuracy. Removing the paper density allows for consistent calibration using various types of paper.  
         [0062]     The density  36  is checked for the scanning direction of the calibration target or test image. It is possible to scan the target in either direction, therefore, to avoid user introduced errors, the densities are checked for correct order.  
         [0063]     The entered densities  36   a,    36   b,    36   c  are then error checked using the correlation coefficient of an 8 th  degree polynomial fit to the measured density at different exposure request data. This check is to ensure that the entered density data will provide a reasonable model. Spectrophotometer scanning errors and other erroneous data can be caught using this technique and the test image will be required to be re-measure. The entered densities are then checked for outlier data points using the following technique.  
         [0064]     A data point is removed from the measured density and exposure arrays. An eight degree polynomial is fit to the remaining measured density and exposure data. A check is performed to determine if the removed data point lies within +/− three standard deviations from the calculated polynomial. The check is then repeated for the remaining density and exposure data points, as shown in  FIG. 6 . Points exceeding +/− three standard deviations  18  from the calculated polynomial curve are removed.  
         [0065]      FIG. 6  shows in detail the next step in calculating a calibration curve. The next step is to determine where the first density point  28  or toe exposure  38   a  and the second density point  30  or shoulder exposure  38   c  exist. The toe exposure point is the lowest exposure required to produce a density higher than paper density  29   a,    29   b,    29   c,    29   d.  The shoulder exposure  38   c  is the lowest exposure where the maximum density output a thermal dye donor is produced. By evaluating the derivative of density with respect to exposure the rate of change can be determined. A three point moving average is performed on the derivative of density to provide greater statistical accuracy determining the toe exposure and the shoulder exposure. An experimentally calculated threshold is used to locate the toe exposure and the shoulder exposure. Due to thermal donor differences the threshold values are unique to each thermal dye donor.  
         [0066]     The determination of the toe exposure and the shoulder exposure are critical for controlling an eight degree polynomial  38   b.  The polynomial  38   b  is well behaved when using density and exposure data  36   a,    36   b,    36   c  between the toe exposure and the shoulder exposure. Including density and exposure data before the toe exposure and after the shoulder exposure will limit the accuracy of the polynomial  38   b  in the area between the toe exposure and the shoulder exposure as well as provide unpredictable results before  38   a  the toe exposure and after the shoulder exposure  38   c.    
         [0067]     Several thermal dye donors are capable of producing more density than is required. For these thermal donors the polynomial fit is calculated between the toe exposure and the exposure required for the maximum desired density. Limiting the density range for calculation of a polynomial  38   b  allows for increased accuracy for densities between the toe exposure and the maximum required density.  
         [0068]     Densities above the shoulder exposure or the maximum desired density are modeled using a straight line or first slope fit  38   c  to the one valid data point below the shoulder exposure or maximum desired density as well as the shoulder exposure or the exposure for maximum desired density. The post-shoulder slope  38   c  is used to guarantee an increase in exposure will occur for an increased density request.  
         [0069]     Densities below the toe exposure are modeled using a second slope  38   a  fit to the toe exposure and toe density and the paper exposure and paper density.  
         [0070]     The calibration curve  38  is calculated by providing a list of desired densities. From the calculated polynomial  38   b,  the pre-toe slope  38   a,  and the post-shoulder slope  38   c,  the exposures are determined for each density request. The calibration contents are then used as a look up table during imaging.  
         [0000]     Optimizing Exposures  
         [0071]      FIG. 7  show the results of optimizing exposures to produce a calibration target with desired density values  40 . Using the calibration curve  38  for a given thermal donor and a given set of density requests  40  for the corresponding thermal donor, the exposures for the calibration target can be calculated using linear interpolation. This is achieved by selecting exposure values for the density points  36   a,    36   b,    36   c,  etc, that are just higher and lower than the optimized density request  40   a,    40   b,    40   c,  etc. Linear interpolation is then used on the set of density and exposure data points and the density request  40   a,    40   b,    40   c  to calculate the exposure at the given optimized density request  40   a,    40   b,    40   c,  etc. For the lowest and highest optimized density request  40   a,    40   b,    40   c,  etc, linear extrapolation is used based on the two previous density points  36   a,    36   b,    36   c,  etc, and corresponding exposure data points.  
         [0000]     Exposure Adjustment Calibration Process  
         [0072]     Referring to  FIG. 8 , there is an example of a control chart for tracking the calibration performance of an imaging device for a similar set of thermal media and thermal receiver. Calibrated exposures  50   a,    50   b,    50   c,  etc, are plotted for a single density request using a calibration curve  38  for similar emulsions imaged at different points in time. The average exposure  51   d  for a density request is calculated along with the −3, −2, −1, 2 and 3 standard deviation differences from the average exposure  51   a,    51   b,    51   c,    51   e,    51   f,    51   g.  The imaging device is considered in control if the following criteria are met: No single exposure exists beyond 3 standard deviation unit distance from an average exposure line, no 2 out of 3 exposures exist on the same side of an average exposure line and more than 2 standard deviation units beyond the average exposure line, no 4 out of 5 exposures exist on the same side of an average exposure line and more than 1 standard deviation unit beyond the average exposure line, no 8 out of 9 points on the same side of an average exposure line. If the stated specifications are violated then the control imaging device will require servicing.  
         [0073]     Inevitably a control thermal donor set will run out and require replacement. Acceptable alternate thermal donors and thermal receivers can be located by evaluating their calibrated exposure performance. Thermal media and thermal receiver emulsions that test within 1 sigma of the average exposure are good candidates for using as a new control material.  
         [0074]     Due to loss of efficiency the control imaging device will exhibit exposure drift over time and therefore require servicing, however the control imaging device exposure stability may not be guaranteed after servicing.  
         [0075]     When attempting to calibrate thermal donors on a control imaging device and exporting the calibration such that a remote imaging device can use the calibration data to bypass performing calibration on the lot of thermal donor, the stability of the control imaging device becomes critical. As discussed the control imaging device will exhibit drift over time. Media lots calibrated at a significantly different time will therefore inherit drift due to machine drift. A remote machine may image two lots that were characterized at different points in time possibly causing a change in performance. No guarantee can be placed on the order of distribution of media therefore the control imaging device exposures must remain constant if exposures are to be exported to remote imaging devices.  
         [0076]      FIG. 9  shows a method for dealing with control imaging device exposure drift over time. Thermal media emulsions  60   a,    60   b,    60   c,  etc are calibrated on a control imaging device with a control thermal receiver. Calibrated exposures for specified density levels for thermal media emulsions and control thermal receiver  60   a,    60   b,    60   c,  etc are compared to the average calibrated exposure for specified density levels of thermal media emulsions  63  and control thermal receiver to produce exposure adjustment points  62   a,    62   b,    62   c,  etc for each thermal media emulsion. Exposure adjustment points  62   a,    62   b,    62   c,  etc, can be calculated using a moving average for relatively small sample sizes to decouple control imaging device exposure drift from thermal media and thermal receiver calibrated exposures.  
         [0077]     Exposure adjustment points  62   a,    62   b,    62   c,  etc, can be calculated similarly for a thermal receiver. Each thermal receiver is measured using a control thermal media and a control imaging device. Calibrated exposures for specified density levels for control thermal media and test thermal receiver emulsions  60   a,    60   b,    60   c,  etc are compared to the average calibrated exposure for specified density levels of control thermal media and test thermal receiver emulsions  63  to produce exposure adjustment points  62   a,    62   b,    62   c,  etc for each thermal receiver emulsion. Exposure adjustment points  62   a,    62   b,    62   c,  etc, can be calculated using a moving average for relatively small sample sizes to decouple control imaging device exposure drift from thermal media and thermal receiver calibrated exposures.  
         [0078]     Exposure adjustment points  62   a,    62   b,    62   c,  etc, can be calculated at many different density levels. Each thermal media can exhibit different areas of stability. The exposure adjustment points can be used for multiple density levels to quantify the amount of change in exposure for different density ranges for an individual lot of thermal media.  
         [0079]     Exposure adjustment points  62   a,    62   b,    62   c,  etc, or other calibration data can be made available for export to remote locations through internet distribution, hardcopy mail distribution, or coded into an radio frequency ID tag associated with a roll of thermal media.  
         [0080]     Exposure adjustment points  62   a,    62   b,    62   c,  etc, can have their values limited by a predetermined amount to limit the effects of density and exposure noise.  
         [0081]     A remote imaging device can use exposure adjustment points  62   a,    62   b,    62   c,  etc, along with an existing exposure calibration curve  38  to calculate a new exposure calibration curve  38 .  FIG. 10  describes the process to calculate a new exposure calibration curve  38 . The process requires an initial exposure calibration  38  using a lot of thermal media and thermal receiver, the exposure adjustment values  62   a,    62   b,    62   c,  etc, associated with thermal media and thermal receiver used for the initial calibration, and the exposure adjustment values  62   a,    62   b,    62   c,  etc for a set of thermal media and thermal receiver which requires calibration. As a user loads a new set of thermal media and thermal receiver the host software will collect the associated exposure adjustment values  62   a,    62   b,    62   c,  etc. The exposure calibration curve  38  must be sorted by density level  401 . The toe exposure can then be selected from the calibration curve  402 . The exposure adjustment values  62   a,    62   b,    62   c,  etc. for thermal media and thermal receiver for the lowest density are selected  403 . A first exposure adjustment  404  is calculated for specified density levels by adding adjustment values  403  for the initial thermal media to the exposure adjustment value  62   a,    62   b,    62   c,  etc, for the initial thermal receiver, then the exposure adjustment  62   a,    62   b,    62   c,  etc, for the new thermal media and the exposure adjustment  62   a,    62   b,    62   c,  etc, for the new thermal receiver are subtracted.  
         [0082]     The equation to calculate the exposure adjustment  404  can be seen below:
 
Exposure Adjustment 1=Thermal Media Exposure Adjustment 1 Control Imaging Device+Thermal Control Exposure Adjustment 1 Remote Imaging Device−Thermal Media Exposure Adjustment 1 Remote Imaging Device−Thermal Media Receiver Exposure Adjustment 1 Remote Imaging Device
 
         [0083]     The exposure adjustment points  404  are used to pivot a polynomial curve. There can be many defined pivot points for different types of thermal media. The toe exposure  402  is often used as a first pivot point. To pivot the polynomial calibration curve  38 ,  400 , a first scale factor is required. A first scale factor  405  is calculated as follows:
 
Scale Factor 1=(OriginalExposure(Density Request 1)−ToeExposure)/( OriginalExposure(Density Request 1)+Exposure Adjustment 1−ToeExposure)
 
 where Original exposure refers to the initial calibration curve  38 ,  400  and density request  1  refers to the density level of the first exposure adjustment point  403 . 
 
         [0084]     The scale factor is applied from the toe density  402  to the maximum calibration density to the initial calibration curve  38 ,  400  to calculate a set of adjusted exposure values  406  as follows:
 
AdjustedExposure1(Density Request)=(OriginalExposure(Density Request)−Toe Exposure)*Scale Factor 1+Toe Exposure
 
 where Original exposure refers to the initial calibration curve  38 ,  400  and density request refers to the density levels to be calibrated. Typically calibrated density levels range from paper density to the maximum obtainable density by the imaging device. 
 
         [0085]     The process can be extended to include a more than one exposure adjustment point. The polynomial calibration curve  406  will use the first exposure adjustment point  403  as a starting pivot point and a second exposure adjustment point  407  will be used to pivot the polynomial calibration curve  406 . The second calibration curve exposure adjustment point  408  can be obtained through the following equation:
 
Exposure Adjustment 2=Thermal Media Exposure Adjustment 2 Control Imaging Device+Thermal Control Exposure Adjustment 2 Remote Imaging Device−Thermal Media Exposure Adjustment 2 Remote Imaging Device−Thermal Media Receiver Exposure Adjustment 2 Remote Imaging Device
 
 The scale factor for exposure adjustment  2   409  is then needed to pivot the curve and is calculated as follows:
 
Scale Factor2=(AdjustedExposure1(Density Request 2)−AdjustedExposure1(Density Request 1)/(AdjustedExposure1(Density Request 2)−[OriginalExposure(Density Request 2) +Exposure Adjustment 2−AdjustedExposure1(Density Request 2)]−AdjustedExposure1(Density Request 1))
 
 Where AdjustedExposure 1  is the first adjusted calibration curve  406  and density request  2  refers to the density level of the second exposure adjustment point  407 . 
 
         [0086]     The scale factor is applied from the density of the first exposure adjustment point  403  to the maximum calibration density to the first adjusted calibration curve  406  to calculate a second set of adjusted exposure curve  410  as follows:
 
AdjustedExposure2(Drequest)=(AdjustedExposure1(Density Request)−AdjustedExposure1(Density Request 1))*Scale Factor 2−AdjustedExposure1(Density Request 1)
 
 The process can be extended to include multiple exposure adjustment points. 
 
 Calibration Averaging 
 
         [0087]     Thermal media contains variability in sensitometric performance lot to lot. The manufacturing variability is typically large enough to require exposure calibration for each lot of thermal media and thermal receiver. Exposure calibration also exhibits variability that is typically less than the thermal media manufacturing variability.  
         [0088]      FIG. 11  shows a method to reduce the density variability due to variability of sensitometric performance lot to lot for thermal media by averaging multiple exposure calibrations  612 ,  613 ,  614 ,  615 ,  616  of various or individual thermal media lots. Averaging multiple exposure calibrations of individual thermal media lots will increase the amount of data available to build a density and exposure model and may improve calibration accuracy. Averaging multiple exposure calibrations for different thermal media lots will produce an exposure calibration  617  designed for an average thermal media sensitometric performance. Using an average exposure curve  617  will reduce the range of density error due to thermal media lot to lot sensitometric differences. The variability of density accuracy for an average exposure calibration curve  617  will typically be similar to the variability obtained when using unique exposure calibrations  612 ,  613 ,  614 ,  615 ,  616  for individual thermal media lots. The variability of density accuracy for an average exposure calibration curve  617  can be larger if an outlier thermal media lot is used for the calculation of an average exposure calibration curve  617  or if an outlier thermal media is imaged using an average exposure calibration curve.  
         [0089]     Averaging can be used in conjunction with the exposure adjustment calibration calculation described in  FIG. 10 . Multiple exposure calibration curves  400 ,  612 ,  613 ,  614 ,  615 ,  616  can be averaged along with associated exposure adjustment points  62   a,    62   b,    62   c,  etc. The equation to calculate the exposure adjustment  404  for average thermal media can be seen below:
 
Exposure Adjustment 1=Thermal Media Exposure Adjustment 1 Control Imaging Device+Average Thermal Control Exposure Adjustment 1 Remote Imaging Device−Thermal Media Exposure Adjustment 1 Remote Imaging Device−Average Thermal Media Receiver Exposure Adjustment 1 Remote Imaging Device
 
 The first scale factor  405  for average thermal is calculated as follows:
 
Scale Factor 1=(OriginalExposure(Density Request 1)−Average ToeExposure)/(OriginalExposure(Density Request 1)+Exposure Adjustment 1−Average ToeExposure)
 
 where Original exposure refers to the average calibration curve  617  and density request  1  refers to the density level of the first exposure adjustment point  403 . 
 
         [0090]     The scale factor is applied from the toe density  402  to the maximum calibration density to the average calibration curve  617  to calculate a set of adjusted exposure values  406  as follows:
 
AdjustedExposure1(Density Request)=(OriginalExposure(Density Request)−Toe Exposure)*Scale Factor 1+Toe Exposure
 
 where Original exposure refers to the average calibration curve  617  and density request refers to the density levels to be calibrated. 
 
         [0091]     The second average calibration curve  617  exposure adjustment point  408  can be obtained through the following equation:
 
Exposure Adjustment 2=Thermal Media Exposure Adjustment 2 Control Imaging Device+Thermal Control Exposure Adjustment 2 Remote Imaging Device−Average Thermal Media Exposure Adjustment 2 Remote Imaging Device−Average Thermal Media Receiver Exposure Adjustment 2 Remote Imaging Device
 
 The scale factor for exposure adjustment  2   409  is then needed to pivot the average exposure calibration curve  617  and is calculated as follows:
 
Scale Factor 2=(AdjustedExposure1(Density Request 2)−AdjustedExposure1(Density Request 1)/(AdjustedExposure1(Density Request 2)−[OriginalExposure(Density Request 2) +Exposure Adjustment 2−AdjustedExposure1(Density Request 2)]−AdjustedExposure1(Density Request 1))
 
 where AdjustedExposure 1  is the first adjusted calibration curve  406  and density request  2  refers to the density level of the second exposure adjustment point  407 . 
 
         [0092]     The scale factor is applied from the density of the first exposure adjustment point  403  to the maximum calibration density to the first adjusted calibration curve  406  to calculate a second set of adjusted exposure curve  410  as follows:
 
AdjustedExposure2(Drequest)=(AdjustedExposure1(Density Request)−AdjustedExposure1(Density Request 1))*Scale Factor 2−AdjustedExposure1(Density Request 1)
 
 The process can be extended to include multiple exposure adjustment points. 
 
         [0093]     An advantage of using averaging for exposure calibration curves in combination with exposure adjustments is that outlier thermal media sensitometric curves will be characterized and compensated when calculating and adjusting the average calibration curve  617 . The manufacturing process for thermal media is therefore allowed to vary which in turn will reduce waste.  
         [0094]     An advantage of averaging exposure calibration curves is the amount of data used in correlating a control imaging device and a remote imaging device is increased resulting in an increase in accuracy.  
         [0095]     An advantage of averaging exposure calibration curves is to increase repeatability in noisy density areas  618 . By averaging exposure calibration curves the imaging device will reduce the range of density error.  
         [0096]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  test pattern  
           11  test pattern  
           13  first density scan  
           15  first dataset  
           17  density analysis  
           18  standard deviation  
           19  second density scan  
           21  second dataset  
           23  density analysis  
           25  calibration calculation  
           28  first density point  
           29   a  paper density  
           29   b  paper density  
           29   c  paper density  
           29   d  paper density  
           30  second density point  
           36  density  
           38  calibration curve  
           36   a  density  
           36   b  density  
           36   c  density  
           38   a  toe exposure  
           38   b  polynomial  
           38   c  shoulder exposure  
           40  set of density  
           40   a  density request  
           40   b  density request  
           40   c  density request  
           50   a  a calibrated exposure  
           50   b  calibrated exposure  
           50   c  calibrated exposure  
           51   a  control limit  
           51   b  control limit  
           51   c  control limit  
           51   d  control limit  
           51   e  control limit  
           51   f  control limit  
           51   g  control limit  
           60   a  calibrated exposure  
           60   b  calibrated exposure  
           60   c  calibrated exposure  
           62   a  exposure adjustment  
           62   b  exposure adjustment  
           62   c  exposure adjustment  
           63  thermal media emulsions  
           90  sets of density  
           91  sets of density  
           92  sets of density  
           93  sets of density  
           112  image processor housing  
           114  image processor door  
           115  media drive mechanism  
           116  media carousel  
           118   a  donor material  
           118   b  roll media  
           118   c  roll media  
           118   d  roll media  
           118   e  roll media  
           118   f  roll media  
           118   g  thermal print media  
           120  laser assembly  
           122  laser diodes  
           124  fiber optic cables  
           126  distribution block  
           128  printhead  
           130  vacuum imaging drum  
           132  donor ejection chute  
           134  donor waste bin  
           136  transport mechanism  
           137  exit tray  
           140  drive roller  
           142  drive roller  
           144  media knife assembly  
           146  drum load roller  
           148  translation bearing rods  
           150  translation bearing rods  
           152  lead screw  
           154  lathe bed scanning frame  
           156  lead screw drive nut  
           158  movable translation stage member  
           200  separate row  
           300  paper point  
           302  paper point  
           304  paper point  
           306  paper point  
           400  calibration curve  
           401  density level  
           402  find toe exposure  
           403  select first exposure adjustment point  
           404  calculate exposure adjustment  1   
           405  calculate scale factor  1   
           406  calculate adjusted calibration curve  1   
           407  select second exposure adjustment point  
           408  calculate exposure adjustment  2   
           409  calculate scale factor  2   
           410  calculate adjusted exposure  2   
           612  calibration curve  
           613  exposure calibration curve  
           614  exposure calibration curve  
           615  exposure calibration curve  
           615  exposure calibration curve  
           616  exposure calibration curve  
           617  exposure calibration curve  
           618  noise density area