Abstract:
An improvement for non-uniformity correction in a printing apparatus ( 10 ) wherein an image forming assembly ( 22 ) forms an image using a plurality of exposure elements, and the amount of exposure energy at each individual exposure element is capable of being varied. A test print ( 50 ) is generated, having a series of test patches or zones with predetermined density levels. A scanner ( 40 ) scans the test print ( 50 ) to obtain density value readings within each test density zone ( 52 ) for each pixel that corresponds to each exposure element. Density value readings are averaged. Then, difference in measurement from this average is used to compute a correction factor for each individual exposure element. An image data manager ( 12 ) conditions the input data by this correction factor, then sends the conditioned image data to the image forming assembly ( 22 ) for printing.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to an improvement to a printing apparatus that forms an image using a plurality of exposure elements and more particularly relates to a method for improving uniformity of output prints from such a printing apparatus. 
     BACKGROUND OF THE INVENTION 
     The difficulty of achieving uniform density output from a printer is a well-known problem in the printer art. Non-uniformity is particularly noticeable with high-quality color printers, where it is important to be able to faithfully reproduce subtle changes in shading and gradation or flat fields having the same density. Non-uniform response of a printhead causes unacceptable anomalies such as streaking and banding, which can easily render a print useless, or at least disappointing, for its intended audience. 
     Factors that contribute to printer non-uniformity vary, depending on the specific printing technology. With a thermal printhead, for example, where resistive print elements are linearly aligned along a writing surface, slight mechanical irregularities or additive mechanical tolerance variability can cause some elements to be more effective in transferring heat than others. With a printhead that scans optically, such as a CRT printhead, optical aberrations or fringe effects can mean that light power is less effectively distributed at the extreme edges of the scan pattern than it is in the center of a scan line. In a photofinishing system that uses an array of light-emitting elements, such as a Micro Light Valve Array (MLVA) in the Noritsu model QSS-2711 Digital Lab System, manufactured by Noritsu Koki Co., located in Wakayama, Japan, individual elements in the array may vary in the intensity of light emitted. 
     Achieving printer uniformity for high-performance printers used, for example, as photofinishing systems, graphic arts image-setters, and color proofing systems, can be particularly complex. Due to customer expectations for quality, the problem of printing apparatus non-uniformity is especially acute in the photofinishing arts. In photofinishing, the continued development of digital solutions for image scanning and printing of photographic-quality images make the problem of achieving print uniformity particularly important. To complicate the task of achieving uniformity among printers used in photofinishing, these printing apparatus may include components provided by more than one manufacturer. Companies specializing in different aspects of the photofinishing process provide exposure apparatus, development apparatus, scanning devices, film and paper, and consumable development chemicals needed in the process. In order to design a complete photofinishing printing apparatus, a systems integrator may create a system by combining preferred components and consumables from a number of vendors. In many cases, vendor companies providing the various components and consumables may even be, at least in part, competing against one another. From the perspective of a supplier of one or more components, it is advantageous to be able to provide a printing apparatus subsystem that can maintain or improve image quality with minimal dependency on other subsystems. From the alternate perspective of an integrator of components, it is advantageous to be able to purchase a necessary component or consumable from a photofinishing manufacturer as a “black box”, where no proprietary information on internal components or operation is needed or provided. Instead, in order to integrate a component or consumable into a photofinishing printing apparatus, a systems integrator only needs access to information on performance and external operation for those components. 
     As one relatively complex type of printer, a conventional printing apparatus used for digital photofinishing typically comprises the key subsystems shown in FIG.  1 . Here, a printing apparatus is generally numbered  10 . The data path for printing apparatus  10  is represented by solid arrowed line B. A digital image source  12  provides input image data. Digital image source  12  could be, for example, a color scanner. An image data manager  14  performs digital manipulation and processing of the input image data from digital image source  12 . Image data manager  14  is a computer, which may be a Windows or UNIX platform, for example, specially configured for its imaging function. Image data manager  14  comprises the necessary CPU, disk storage, and memory components for processing an image and providing the image data at its output. 
     As the printing engine of printing apparatus  10 , an image forming assembly  22  comprises a printhead  16  and support circuitry, including a transfer element  36 , an optional transport mechanism  28  (where printhead  16  includes moving parts or scanning components), and a drive electronics assembly  26  that controls the amount of energy applied to transfer element  36 . A system controller  30  provides control logic and processing functions for image forming assembly  22  components. Printhead  16  creates an image by applying energy from transfer element  36  onto a receiver substrate  18 . For typical apparatus of this type, receiver substrate  18  is photosensitive print paper. For such a typical system, transfer element  36  applies light energy to expose the paper. Alternate combinations of receiver substrate  18  and transfer element  36  are possible, however, such as using a colorant that is applied directly to receiver substrate  18  (for example, ink) or a colorant donor material. For inkjet printing, transfer element  36  provides colorant directly, where the amount of colorant transfer is modulated by varying the amount of heat exposure energy applied to inkjet nozzles. For printing apparatus  10  using colorant donor imaging technology, transfer element  36  can apply light or heat exposure energy to a donor material (not shown) to transfer colorant to receiver substrate  18 . For any type of printing apparatus  10 , dashed line A represents the travel path of receiver substrate  18  from a receiver supply  24  through image forming assembly  22 . A processor  20  provides any necessary processing of receiver substrate  18  in order to provide a completed output print  38 . For photofinishing printing apparatus  10  that uses photosensitive silver-halide chemistry, processor  20  uses a series of chemicals (for example, bleach, fixer, and developer) that develop the latent image exposed by printhead  16  onto receiver substrate  18 . For printing apparatus  10  using a donor colorant, processor  16  may transfer colorant from a receiver substrate  18  onto paper stock, with optional addition of a lamination layer. 
     Referring again to FIG. 1, it is instructive to note that conventional approaches for non-uniformity correction are directed to internal adjustments that are made to components within image forming assembly  22 . For some types of printing apparatus  10 , a sensor  58  is provided in order to measure a characteristic of transfer element  36 . Sensor  58  feedback then goes to image forming assembly  22  to adjust the behavior of drive electronics assembly  26 . Dotted line C represents this feedback path using sensor  58 . For other types of printing apparatus  10 , a scanning device  60 , such as a scanner or densitometer, is employed to obtain measurements from output print  38 . Data from scanning device  60  is then directed to image forming assembly  22  to adjust the behavior of drive electronics assembly  26 . Dotted line D represents this alternate feedback path using scanning device  60  measurements. It is instructive to note that, when using the feedback path indicated by dotted line D, density data is obtained from output print  38 . Image forming assembly  22  must perform some further conversion of this feedback density data to data values actually used by printhead  16  to control exposure. 
     The disclosures of the following patents illustrate conventional approaches for non-uniformity correction as applied for various types of printheads  16 : 
     U.S. Pat. No. 5,546,165 (Rushing et al.) discloses non-uniformity correction applied in an electrostatic copier, using LED technology in transfer element  36 . Feedback measurements from a scanned, flat field contone test print are obtained in order to calculate adjustments to individual LED drive currents or on-times. Referring to FIG. 1, the approach disclosed in the Rushing et al. patent modifies the behavior of drive electronics assembly  26 . To obtain and adjust non-uniformity data, this approach uses the basic scanning device  60 -based feedback path denoted D in FIG.  1 . 
     U.S. Pat. No. 5,684,568 (Ishikawa et al.) discloses non-uniformity correction applied in a printer used for developing photosensitive media. Light intensity from an exposure source employing an array of lead lanthanum zirconate titanate (PLZT) light valves serves as transfer element  36 . This output light is measured to identify individual light valve elements that require adjustment for non-uniformity. Referring to FIG. 1, the approach disclosed in the Ishikawa et al. patent modifies the behavior of drive electronics assembly  26  for individual light valve elements, either controlling exposure time or light power level. To obtain and adjust non-uniformity data, this approach uses the basic sensor  58 -based feedback path denoted C in FIG.  1 . 
     U.S. Pat. No. 5,997,123 (Takekoshi et al.) discloses non-uniformity correction applied in an inkjet printer, where transfer element  36  comprises an array of nozzles. Control electronics are adjusted to modify dot diameter by controlling the applied nozzle energy or by modulating the number of dots produced. Again referring to FIG. 1, the approach disclosed in the Takekoshi et al. patent modifies the behavior of drive electronics assembly  26  for individual inkjet nozzles in the printhead  16  array. To obtain and adjust non-uniformity data, this approach uses the basic scanning device  60 -based feedback path denoted D in FIG.  1 . 
     U.S. Pat. No. 6,034,710 (Kawabe et al.) discloses non-uniformity correction applied in a photofinishing printing apparatus that employs Vacuum Fluorescent Print Head (VFPH) technology for printheads  16 . Again referring to FIG. 1, the approach disclosed in the Kawabe et al. patent modifies the behavior of drive electronics assembly  26  by adjusting the exposure time of individual elements in the VFPH array. To obtain and adjust non-uniformity data, this approach uses the basic sensor  58 -based feedback path denoted C in FIG.  1 . 
     U.S. Pat. No. 5,946,006 (Tajika et al.) discloses non-uniformity correction applied in an inkjet printer, where transfer element  36  comprises an array of nozzles. Referring to FIG. 1, correction data goes directly to printhead  16 . To obtain and adjust non-uniformity data, this approach uses the basic scanning device  60 -based feedback path denoted D in FIG.  1 . 
     U.S. Pat. No. 5,790,240 (Ishikawa et al.) discloses non-uniformity correction applied in a printer using PLZT (or LED or LCD) printing elements as transfer element  36 . Referring to FIG. 1, a correction voltage is applied directly to drive electronics assembly  26  in order to adjust the output amplitude of an individual PLZT array element. Alternately, duration of the drive signal to an individual PLZT array element is adjusted at drive electronics assembly  26 . To obtain and adjust non-uniformity data, this approach uses the basic scanning device  60 -based feedback path denoted D in FIG.  1 . 
     U.S. Pat. No. 4,827,279 (Lubinsky et al.) discloses non-uniformity correction applied in a printer where printhead  16  uses an array of resistive thermal elements. Density measurements are obtained for each individual thermal element and are used to determine correction factors. Referring to FIG. 1, number of applied pulses or pulse duration at drive electronics assembly  26  are used in order to achieve uniformity. To obtain and adjust non-uniformity data, this approach uses the basic scanning device  60 -based feedback path denoted D in FIG.  1 . 
     With each of the conventional solutions noted above, non-uniformity correction is applied by making adjustments to drive electronics  26  in image forming assembly  22 . This method for non-uniformity correction, however, has disadvantages, making drive electronics design more complex, requiring correction table data to be accessible at an external interface, or requiring the printhead  16  design to accommodate additional control signals. Because correction data must be fed back to image forming assembly  22 , implementation of these conventional solutions requires that an integrator have detailed knowledge of the internal workings of image forming assembly  22 . As noted above, this can complicate and delay commercialization of a printing apparatus, since different manufacturers may be involved. Moreover, image forming assembly  22  may not have a non-uniformity correction scheme, or may have a scheme that must be modified and improved for a specific implementation. Or, a manufacturer of image forming assembly  22  may discontinue production of a specific model, or change the design of printhead  16  components. For these reasons, it can be difficult or impossible to obtain a desired printing apparatus uniformity improvement when using the conventional methods, as illustrated in the prior art patents cited above. Thus it can be seen that conventional approaches, as outlined and illustrated by examples above, present problems that can make it difficult or impossible to obtain uniformity on an output print from a printing apparatus. While there have been methods for compensating for non-uniformity from image forming assembly  22 , there is a long-felt need for a printer improvement and method for achieving uniformity on an output print. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improvement to a printing apparatus for non-uniformity correction and a method for non-uniformity correction. 
     According to an aspect of the present invention, the improvement resides in a printing apparatus that uses an image forming assembly comprising a plurality of exposure elements, where the amount of exposure energy is capable of being varied at each exposure element to provide a corresponding output colorant density, the improvement comprising: 
     (a) an output test print formed by the image forming assembly, the test print having a predetermined set of output colorant density levels; 
     (b) a scanner capable of scanning the output test print and providing, to correspond to each one of the plurality of exposure elements, a plurality of scanned density data values; 
     (c) an image data manager that accepts a plurality of image data values from an image data source and, within each one of the predetermined set of output colorant density levels, is capable of: 
     (1) computing, from the plurality of scanned density data values, an exposure element average density value corresponding to each one of the plurality of exposure elements, to generate a set of exposure element average density values for the plurality of exposure elements; 
     (2) computing, from the set of exposure element average density values for the plurality of exposure elements, a density level average density value corresponding to the predetermined set of output colorant density levels; 
     (3) computing, for each one of the plurality of exposure elements, a non-uniformity correction value based on the difference between the exposure element average density value and the density level average density value; 
     the image data manager further capable of conditioning, corresponding to each one of the plurality of exposure elements, each of the plurality of image data values from the image data source using the non-uniformity correction value to generate a conditioned image density data value and capable of providing the conditioned image density data value to the image forming assembly. 
     A feature of the present invention is the adaptation of the image data manager for non-uniformity compensation. 
     An advantage of the present invention is that it allows an image forming assembly in the printing apparatus to be considered as a modular unit, or “black box”, so that detailed information about internal operation of the image forming assembly is not required for performing non-uniformity correction. No adjustments are made to internal components of the image forming assembly itself. Instead, the image data manager, based on scanner measurements from an output test print, directly modifies input image data provided to the image forming assembly. The present invention measures output density values to obtain a profile of printhead performance. The present invention uses these measured density values to adjust density values in the input image file, without requiring knowledge of how an image forming assembly obtains a desired density. Therefore, the present invention minimizes the need for thorough technical understanding of the particular image forming assembly being used. 
     A further advantage of the present invention is that it minimizes the need for a printing apparatus manufacturer to obtain detailed information about internal operation of an image forming assembly. A printing apparatus manufacturer, when assembling a printing apparatus, can employ an image forming assembly from another supplier, without requiring detailed internal information on the image forming assembly. 
     A further advantage of the present invention is that it allows a printing apparatus manufacturer to obtain non-uniformity correction even if a printing apparatus comprises an image forming assembly that does not already have non-uniformity correction. 
     A further advantage of the present invention is that it improves upon any built-in uniformity correction already applied by an image forming apparatus manufacturer. There is, moreover, no need to interfere with or modify any uniformity correction that is supplied with the image forming apparatus. An improvement to performance can be effected without changing any existing, builtin non-uniformity correction. 
     A further advantage of the present invention is that it provides a method for compensation for non-uniformity that can be used independently from printing apparatus calibration. Uniformity adjustments are separately performed from calibration adjustments. 
     A further advantage of the present invention is that it allows a printing apparatus manufacturer to adapt a printing apparatus to use a different image forming assembly, allowing the design of a printing apparatus that is not constrained to using a specific image forming assembly. 
     Yet a further advantage of the present invention is that it obtains non-uniformity correction without further complicating the design of drive electronics for the printhead. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram showing a prior art printing apparatus; 
     FIG. 2 is a block diagram showing a printing apparatus with the improvement of the present invention; 
     FIG. 3 is a plan view of an output print used to measure uniformity; 
     FIG. 4 is a graph illustrating how typical measured output densities for transfer elements in a printhead may vary from a target density; and, 
     FIG. 5 is a block diagram showing the logic used to compute multiple output adjustment values to correct for non-uniformity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the 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. 
     While the present invention is directed generally to printing apparatus, specific emphasis in the following description is given to photofinishing printing apparatus. Referring again to FIG. 1, this invention relates to printing apparatus where printhead  16  uses a plurality of exposure elements. Each exposure element, in turn, images a discrete “pixel” on receiver substrate  18 , with pixels substantially evenly spaced-apart on receiver substrate  18 . Printhead  16  provides control of colorant density on output print  38 , typically by controlling a voltage applied to drive electronics assembly  26  or by controlling the duration of an applied energizing pulse provided to drive electronics assembly  26 . Transfer element  36  in image forming assembly  22  may create an image on receiver substrate  18  by means of any of the following: 
     Inkjet nozzles (in which exposure energy is generally provided by applying thermal energy within the inkjet nozzle assembly); 
     LED printing, employing one or more focused Light Emitting Diodes; 
     Laser array, such as using diode lasers; 
     Light-valve devices, such as lead lanthanum zirconate titanate (PLZT) light valves, typically in the form of an array with individual array elements separately controllable; 
     Resistive printhead, which applies heat to transfer colorant from a donor material. 
     Other related equipment to which the present invention may be applied also includes apparatus configured solely to develop film negatives or slides or apparatus configured to expose prints onto photosensitive paper. 
     The description that follows describes the present invention primarily as used with minilab apparatus; however, it is to be understood that the methods disclosed in this specification can be applied more broadly to include other types of printing apparatus, including photofinishing apparatus, developers, and other apparatus using the above-mentioned technologies for imaging. 
     Referring again to FIG. 1, the function of processor  20  depends on the imaging technology used. For example, for standard digital minilabs, processor  20  routes exposed receiver substrate  18  through a sequence of chemical baths in which the image is developed, fixed, and stabilized onto paper. 
     It is instructive to note that other types of printing apparatus, using any of the technologies noted above, perform, with variations, one or more similar operations as described for photofinishing minilabs. For example, a digital printer may not provide processor  20 , but may perform only an exposure operation, whereby photosensitive paper, as receiver substrate  18 , is exposed, to be subsequently developed on other equipment. For such equipment, processing takes place by feeding new, unexposed photosensitive paper from a feed roll as receiver supply  24 , exposing the paper, then wrapping the exposed paper about a take-up roll, for development at a later time. 
     Referring to FIG. 2, there is shown a block diagram of an embodiment of the present invention, where printing apparatus  10  prints a uniformity test print  50 . A scanner  40  is used to scan uniformity test print  50  and provides scanned density data to image data manager  14 , over a scanner interface  44 . Scanner interface  44  may be, for example, a SCSI interface connection, well known in the scanner interfacing arts. As indicated by dashed line E, the use of test print  50  and scanner  40  effectively creates a feedback loop to image data manager  14 . Image data manager  14  processes received data from scanner  40  and generates a non-uniformity correction look-up table (NCLUT)  42 , as is described subsequently. 
     Referring to FIG. 3, there is shown a plan view of an exemplary uniformity test print  50  as used in a preferred embodiment. (Necessarily, FIG. 3 is representative only, and not to scale.) Test print  50  comprises a plurality of  32  density patches or density zones  52 , printed in order of increasing optical density. Each density zone  52  is imaged by writing a number of lines of the same density. The height dimension of each density zone  52  is, therefore, dependent on the number of lines written. For a PLZT printhead  16 , the width of each density zone  52  is advantageously equal to the writing width of printhead  16 . For an inkjet or other type of printhead  16 , the width of each density zone  52  can be otherwise suitably specified. Fiducial marks  54  are provided at evenly spaced increments to provide a reference for alignment of scanned points along the writing width of printhead  16 . 
     In a preferred embodiment, density zones  52  are arranged in successive increments of optical density, from 0.04 to 2.52. By a convention used in the description that follows, density zones  52  are denoted by j=1,2,3, . . . 32. Pixel positions along each density zone  52  are then denoted by i=1,2,3, . . . p max , where, in the preferred embodiment for a PLZT printhead  16 , p max  equals the number of pixels that are in transfer element  36  of printhead  16 . For example, for a typical printhead  16  using PLZT technology for transfer element  36 , p max  is in excess of 5,000 pixels. 
     As FIG. 3 shows, the arrangement of test print  50  provides, for each pixel position i, multiple samples of densities j. As will be seen in subsequent description, the most useful measurements are taken from mid-band print lines  56 . This arrangement, sampling readings taken in the middle of a density zone  52 , minimizes stray effects that might be caused by transitions between density levels for density zones  52 . 
     Using test print  50  as shown in FIG. 3, scanner  40  can obtain stable reference data by which to evaluate printhead  16  uniformity. Fiducial marks  54  enable correct alignment of scanned pixel data, to compensate for possible skewing of test print  50  on the scanner  40  platen or for possible mechanical tolerance error inherent to scanner  40 . 
     Generation and Use of Non-Uniformity Correction Look-up Table (NCLUT)  42   
     As noted above, each individual pixel generated for a given target density by transfer element  36  writes to a corresponding coordinate on test print  50 . The goal of non-uniformity correction is to adjust for the differences in each individual pixel at each of the target densities printed on test print  50 . 
     Referring to FIG. 4, there is shown a small portion of an exemplary sampling of density readings  48 . The horizontal dimension of the FIG. 4 graph represents individual pixels, i=1,2,3, . . . p max . The vertical dimension represents the corresponding density reading  48  for a single density zone  52 . An average density  46  (represented in mathematical form as {overscore (D)} j , where subscript j indicates the specific density zone  52 , from 1 to 32 as in the example of test print  50  in FIG. 3) for i=1,2,3, . . . p max  is computed in the normal fashion. Thus, in this example, density readings  48  for pixels i=1 and i=3 are high when compared to average density  46 , while density readings for pixels i=2 and i=4 are low. 
     Referring again to FIG. 4, for each density reading  48 , a density non-uniformity correction value ΔD i,j  is computed. That is, a ΔD i,j  value is computed for each pixel position i for a given density zone j. Non-uniformity correction look-up table  42  stores each computed ΔD i,j  value. 
     Referring to FIG. 5, there is shown an algorithm executed by image data manager  14  for populating non-uniformity correction table  42  with ΔD i,j  values. The steps shown in FIG. 5 are executed once for each color channel. In a preferred embodiment, Steps numbered Step- 1  through Step- 10  execute once for each R, G, and B color printed. 
     Step- 1  prepares the image data scanned from test print  50 , first resampling the scanned image data to the same resolution as provided by image forming assembly  22 . This step rotates and shifts the entire scanned test image so that fiducial marks  54  are precisely located and aligned. Any image outside fiducial marks  54  in extreme corners is cropped. Then, any defective pixel datum, due to dust, for example, is replaced by a neighboring datum. Step- 1  also initializes the density zone index, setting j=1 and the pixel index, setting i=1. 
     Step- 2  compensates for measured scanner  40  response characteristics. As is well known in the imaging arts, any scanner has a characteristic, generally non-linear, response sensitivity within each color channel. Step- 2  corrects for this characteristic, to effectively remove scanner response from interfering with actual scanned data readings. 
     Step- 3  locates thejth density zone  52  on test print  50 . Within jth density zone  52 , an average density D i  is then computed, for each pixel position i from i=1 to i=p max , in Step- 4  and decision Step- 5 . To obtain a stable average density D i , readings are taken from multiple lines, among mid-band print lines  56  within the jth density zone  52 . In a preferred embodiment, Step- 4  uses 24 lines sampled near the middle of jth density zone  52  as mid-band print lines  56 . 
     Step- 6  then computes value {overscore (D j )} that gives average density reading  46  for density zone  52 . As shown in FIG. 5, value {overscore (D j )} is determined in a standard way, by summing all D i  values within the jth density zone  52  and dividing the total sum by the number of pixel positions, p max . Step- 6  then resets pixel index i=1. 
     As was illustrated in FIG. 4, each pixel i may differ in density from value {overscore (D j )}. The amount by which D i  differs from {overscore (D j )} is calculated in Step- 7  to provide a density non-uniformity correction value ΔD i,j . Value ΔD i,j  is stored in non-uniformity correction look-up table  42 . Step- 7  and Step- 8  then loop through to generate a total ofpmax values of ΔD i,j  for thejth density zone  52 . Step- 9  and Step- 10  then loop back through to Step- 3  to repeat the procedure for each density zone  52 . At completion of the algorithm of FIG. 5, two-dimensional non-uniformity correction look-up table  42  is created for each color channel. 
     Once non-uniformity correction look-up table  42  is created and stored by image data manager  14 , image data from digital image source  12  can be modified for non-uniformity correction. The simplest method for non-uniformity correction is, for each color channel, simply to add the appropriate value ΔD i,j  to each corresponding data pixel, for a given target density. If the target density is not one of the  32  density zones  52 , then a correction value will be interpolated between the closest density zones. 
     It can be seen that the method disclosed above provides non-uniformity correction without requiring direct control of individual components of image forming assembly  22 . Instead, the image data provided to image forming assembly  22  is conditioned by image data manager  14  using values from correction look-up table  42 . This arrangement enables the method of the present invention to be used with many types of image forming assembly  22 . This method does not interfere with any built-in non-uniformity correction that is already provided for image forming assembly  22 . Instead, this method is capable of improving upon such built-in non-uniformity correction. It is instructive to note a distinction between the method of the present invention and conventional methods for non-uniformity correction. In the method of the present invention, image data manager  14  operates on image data only in “density space.” That is, only density values need to be measured and used in the computation of NCLUT  42 . This is in contrast to conventional methods described above, in which density measurements are obtained from output print  38 , then must be converted to obtain exposure data values usable by image forming assembly  22 . By performing operations only with image density, the present invention avoids the necessity for a detailed understanding of the inner workings of image forming assembly  22 . 
     While the invention has been described with particular reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements in the preferred embodiments without departing from the scope of the invention. For example, this method can be used for a single-color printer or for a printing apparatus that uses either additive color (red, green, blue) or subtractive color (cyan, magenta, yellow, and optionally black) with any number of channels. This method could be applied for an image forming assembly that uses inkjet, laser thermal, resistive thermal, LED (organic or inorganic), light-valve, or other technologies for image marking or exposure. 
     The preferred embodiment of the present invention scans test print  50  to obtain density data that can be used by image data manager  14 . Alternately, feedback data from sensor  58  disposed to measure the output power of each exposure element of transfer element  36  could be used, as is illustrated for prior art printing apparatus  10  in FIG.  1 . However, use of exposure feedback information would present some drawbacks. For many types of image forming assembly  22 , it can be difficult to obtain exposure power measurements without significant disassembly effort, making such an alternative impractical. Moreover, the data obtained would be exposure data, while image data manager  14  works with density data. Thus, some type of conversion would be required in order to use sensor  58  data. This conversion could be further complicated by considerations of media response from receiver substrate  18  and from chemicals used in processor  20 . As a result, while it might be feasible to use sensor  58  exposure data, the preferred embodiment, using test print  50  readings as described in detail above, appears to present significant advantages over the use of exposure measurements directly. Therefore, what is provided is an improvement to a printing apparatus and a method for improving printer uniformity. 
     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 
       10 . Printing apparatus 
       12 . Digital image source 
       14 . Image data manager 
       16 . Printhead 
       18 . Receiver substrate 
       20 . Processor 
       22 . Image forming assembly 
       24 . Receiver supply 
       26 . Drive electronics assembly 
       28 . Transport mechanism 
       30 . System controller 
       32 . Frame memory 
       36 . Transfer element 
       38 . Output print 
       40 . Scanner 
       42 . Non-uniformity correction look-up table 
       44 . Scanner interface 
       46 . Average density reading 
       48 . Density reading 
       50 . Uniformity test print 
       52 . Density zone 
       54 . Fiducial marks 
       56 . Mid-band print lines 
       58 . Sensor 
       60 . Scanning device