Abstract:
A method is disclosed. The method includes receiving image data from one or more image readers and analyzing the image data to locate and classify artifacts on the medium caused by defective print engine nozzles.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of printing systems. More particularly, the invention relates to maintaining ink jet printing systems. 
     BACKGROUND 
     An ink jet printer is as an example of a printing apparatus that ejects droplets of ink onto a recording medium such as a sheet of paper for printing a specific job. Ink jet printers include one or more print engines having at least one ink jet print head provided with an ink cartridge that accommodates the ink. In operation of the print engine, ink is supplied from the ink cartridge to ejection nozzles in each print head so that a printing operation is performed by ejection of the ink droplets from selected ejection nozzles. 
     Present high speed ink jet printers include wide array print heads capable of printing on wider (e.g., &gt;20 inch) mediums at high resolutions. One major issue with such ink jet print heads is the clogging of nozzles due to evaporation of solvent from ink, resulting in an increase in viscosity, an accumulation of paper dust at the nozzle surface, and an intrusion of air bubbles. Each of these results causes a failure of regular nozzle functionality and degraded print quality. 
     A Print Verification System (PVS) is typically used to immediately capture the printed output exiting the printer and provide feedback on any nozzle dysfunction to a controller. Most current PVSs provide an approximate estimate of the location of defective nozzles, but do not provide any information regarding the type of defect. 
     Accordingly, a PVS system that automatically detects, locates, classifies the type of defect and counts the number of defective nozzles is desired for facilitating efficient corrective/cleaning processes. 
     SUMMARY 
     In one embodiment, a method is disclosed. The method includes receiving image data from one or more image readers and analyzing the image data to locate and classify artifacts on the medium caused by defective print engine nozzles. 
     In another embodiment, a print verification system (PVS) includes one or more image readers to read image data from a print medium and a control unit to receive verification data from the image readers and analyze the verification data to locate and classify artifacts on the medium caused by defective nozzles. 
     In yet a further embodiment, a printer is disclosed. The printer includes a print engine having a plurality of nozzles to apply print data to a medium, a PVS to read the print data applied to the medium and a control unit to receive image data from the PVS and analyze the image data to locate and classify artifacts on the medium caused by defective nozzles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention can be obtained from the following detailed description in conjunction with the following drawings, in which: 
         FIG. 1  illustrates one embodiment of a printing system; 
         FIGS. 2A and 2B  illustrate embodiments of optical density signatures; 
         FIG. 3  illustrate one embodiment of density variations; 
         FIG. 4  is a flow diagram illustrating one embodiment of a process to determine defective nozzles; 
         FIG. 5  illustrate one embodiment of a point spread function; and 
         FIG. 6  illustrates one embodiment of a computer system. 
     
    
    
     DETAILED DESCRIPTION 
     A print verification system is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form to avoid obscuring the underlying principles of the present invention. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
       FIG. 1  illustrates one embodiment of a printing system  100 . Printing system  100  includes a print application  110 , a print server  120  and a printer  130 . Print application  110  makes a request for the printing of a document. In one embodiment, print application  110  provides a print job data stream to print server  120  in a presentation format (e.g., Advanced Function Printing, Post Script, etc.) 
     Print server  120  processes pages of output that mix all of the elements normally found in presentation documents (e.g., text in typographic fonts, electronic forms, graphics, image, lines, boxes, and bar codes). In one embodiment, the data stream is composed of architected, structured fields that describe each of these elements. 
     According to one embodiment, printer  130  includes a control unit  140 , print engine  160  and print verification system (PVS)  180 . In such an embodiment, print server  120  communicates with control unit  140  in order to integrate with the capabilities and command set of printer  130 , and to facilitate interactive dialog between the print server  120  and printer  130 . In one embodiment, the dialog between the print server  120  and printer  130  is provided according to a device-dependent bi-directional command/data stream. 
     Control unit  140  processes and renders objects received from print server  120  and provides sheet maps for printing to print engine  160 . Control unit  140  includes a rasterizer  150  to prepare pages for printing. Particularly, rasterizer  150  includes a raster image processor (RIP) that converts text and images into a matrix of pixels (bitmap) that will be printed on a page at print engine  160 . 
     In one embodiment, print engine  160  includes a fixed, wide-array inkjet print head having one or more nozzles  170  that are implemented to spray droplets of ink onto a sheet of paper in order to execute a print job. However, print engine  160  may include other types of ink jet print heads, as well as a moving print head design. 
     PVS  180  is implemented to read pages printed by print engine  160  in order for any defects on the page to be identified. In one embodiment, PVS  180  includes image line scanners that are positioned to read image data printed on each side of a medium that leaves the print engine  160 . Subsequently, the image data may be forwarded to control unit  140  for analysis. Note that in other embodiments, PVS  180  may include a separate control unit to perform the analysis. 
     In one embodiment, control unit  140  detects printing defects on each page of a print job. In such an embodiment, control unit  140  automatically performs a procedure to mathematically classify, accurately locate and count commonly prevalent artifacts (e.g., deviated/misdirected jets and jet outs) caused by clogged nozzles across the web. 
     According to one embodiment, each artifact is associated with specific optical density (OD) signatures as listed below. For example, jet out signatures have an undershoot characteristic that can be modeled by an inverted Gaussian.  FIG. 2A  illustrates an embodiment of OD signatures for jet out artifacts. 
     Similarly, deviated jet artifacts exhibit an overshoot and undershoot, or vice versa, signature which can be modeled by the derivatives of a single or sum of two Gaussian functions.  FIG. 2B  illustrates an embodiment of OD signatures for deviated jet artifacts. It should be understood that other types of reflectance measurement in any color space can also be used instead of optical density. 
       FIG. 3  illustrates one embodiment of mean density variations for a color target (e.g., cyan) with five tint levels. Also shown in  FIG. 3  is a graph plotting the mean tint densities vs. nozzle number. As shown in  FIG. 3 , the mean tint density for nozzle  200  illustrates the “undershoot” characteristic attributed to jet out artifacts, while nozzles  800  and approximately 1220 exhibit the deviated jet overshoot and undershoot characteristics. Also shown, is an OD signature for a print head overlap characteristic. Print head overlap is inherent to print engine  160 , although its OD signature is to be accounted for in the process for determining defective nozzles. 
       FIG. 4  is a flow diagram illustrating one embodiment of process  400  for determining defective nozzles at print engine  160 . Process  400  accurately determines (or detects) defective nozzles across an entire medium web using mean Optical Density (OD) variations per color of the input target. At processing block  410 , scan data is received subsequent to a print job medium has been read at PVS  180 . 
     At processing block  420 , a characterization of system behavior is performed. Characterization of system behavior measures an amount of blurring of an image attributed to scanners (or any other optical elements) within the PVS  180 . In one embodiment, system behavior is characterized by a mathematical system level characterization of the production printing system in terms of its Point Spread Function (PSF). 
     The PSF of an optical/imaging system is the image of a single point object formed at the image plane and the degree of spreading/blurring of this point at the image plane is typically used as a measure for quantifying the overall quality of an optical/imaging system.  FIG. 5  illustrate one embodiment of a point spread function. As shown in  FIG. 5 , an object point source remains the same in an ideal optical/imaging system. However in practical optical/imaging systems, the object point source experiences significant spreading attributed to PSF. 
     In one embodiment, the PSF of an optical/imaging system may also be determined by computing a Line Spread Function (LSF) derived from a line target at different orientations at the object plane. The LSF at a specific orientation is the one dimensional (1-D) projection of the 2-D PSF along that orientation/direction. In a further embodiment, the PSF may be derived from a corresponding LSF obtained using single pel line targets. 
     Referring back to  FIG. 4 , having characterized the behavior of the system, next an OD computation is performed in processing block  430 . In general, OD is a measure of the degree of darkness of a photographic or semitransparent material or a reflecting surface. More specifically, OD is a measure of how dark a print is relative to the paper. 
     In one embodiment, OD is calculated using Opponent Color Substitutions, such that:
 
 X= 0.4124 R+ 0.3576 G+ 0.1805 B  
 
 Y= 0.2126 R+ 0.7152 G+ 0.0722 B  
 
 Z= 0.0193 R+ 0.1192 G+ 0.9505 B  
 
OD=+log 10 (100/ X ) for Cyan
 
OD=+log 10 (100/ Y ) for Magenta, Black and Paper
 
OD=+log 10 (100/ Z ) for Yellow
 
     In another embodiment, different color conversion mechanism can be employed which can efficiently detect variations in all colors. 
     At processing block  440 , an estimation of original OD values is performed to simulate values at the input end of print engine  160  (e.g., to undo blurring effects). In one embodiment, the estimation is performed by de-convolution and resizing. The output g(x,y) of an imaging system can be represented as 2-D convolution of the input f(x,y) with the PSF h(x,y) (e.g., g(x,y)=f(x,y)*h(x,y)). Typically, most ‘real-world’ imaging/optical systems (including the Human Visual System-HVS) have an effect of “blurring” the input f(x,y) at the image plane (or the retina of the eye). 
     De-convolution is a procedure employed as a solution to the inverse problem of estimating the original input at the object plane, f(x,y) given the effect (or PSF) of the optical/imaging system h(x,y) and the output at the image plane g(x,y). From a mathematical view point de-convolution is represented as: 
     
       
         
           
             
               
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     Blind De-convolution is a de-convolution technique that permits recovery of the input from a single or set of “blurred” output images in the presence of a poorly determined/unknown PSF. Following the de-convolution procedure the data under every print head is resized to match physical nozzle alignment. 
     De-convolution facilitates a mechanism to counteract the effect of the printing system on the data being printed, renders the de-convolved data to be close representations of the effective original/input data values to the printing system. Additionally, de-convolution substantially increases the fidelity and sensitivity of the OD values, enabling accurate capture of defective nozzles and classification of the artifact type without the blurring introduced by the printer and scanner. 
     At processing block  450 , mean OD is computed using OD information. At processing block  460 , a signature analysis of the mean OD is performed. As discussed above with reference to  FIGS. 2 and 3 , signature analysis is performed by monitoring for undershoot and overshoot characteristics at each nozzle  170 . For example in  FIGS. 2A and 2B , lines  210  represent the signatures after de-convolution (without blurring effects), while lines  220  represent the signatures with blurring effects. 
     At processing block  470 , the defective nozzle data is analyzed. For instance, a determination is made of the location and total number of defective nozzles  170 , as well as each artifact type. 
       FIG. 6  illustrates a computer system  600  on which print controller  140  and/or print server  120  may be implemented. Computer system  600  includes a system bus  620  for communicating information, and a processor  610  coupled to bus  620  for processing information. 
     Computer system  600  further comprises a random access memory (RAM) or other dynamic storage device  625  (referred to herein as main memory), coupled to bus  620  for storing information and instructions to be executed by processor  610 . Main memory  625  also may be used for storing temporary variables or other intermediate information during execution of instructions by processor  610 . Computer system  600  also may include a read only memory (ROM) and or other static storage device  626  coupled to bus  620  for storing static information and instructions used by processor  610 . 
     A data storage device  625  such as a magnetic disk or optical disc and its corresponding drive may also be coupled to computer system  600  for storing information and instructions. Computer system  600  can also be coupled to a second I/O bus  650  via an I/O interface  630 . A plurality of I/O devices may be coupled to I/O bus  650 , including a display device  624 , an input device (e.g., an alphanumeric input device  623  and or a cursor control device  622 ). The communication device  621  is for accessing other computers (servers or clients). The communication device  621  may comprise a modem, a network interface card, or other well-known interface device, such as those used for coupling to Ethernet, token ring, or other types of networks. 
     Embodiments of the invention may include various steps as set forth above. The steps may be embodied in machine-executable instructions. The instructions can be used to cause a general-purpose or special-purpose processor to perform certain steps. Alternatively, these steps may be performed by specific hardware components that contain hardwired logic for performing the steps, or by any combination of programmed computer components and custom hardware components. 
     Elements of the present invention may also be provided as a machine-readable medium for storing the machine-executable instructions. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, propagation media or other type of media/machine-readable medium suitable for storing electronic instructions. For example, the present invention may be downloaded as a computer program which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection). 
     Throughout the foregoing description, for the purposes of explanation, numerous specific details were set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without some of these specific details. Accordingly, the scope and spirit of the invention should be judged in terms of the claims which follow.