Abstract:
Small, fast, and inexpensive in-line spectrophotometers can produce in-line spectrums of a substrate before or after printing on the substrate. In-line spectrums are generally far less complete than a reference spectrum produced with a large, slow, and expensive reference spectrophotometer. An in-line spectrum can be mapped to a reference spectrum using a variety of known algorithms. However, the mapping is erroneous when the media substrate type changes. Reference transform matrices and in-line transform matrices can correct the erroneous mapping.

Description:
TECHNICAL FIELD  
       [0001]     Embodiments relate to the areas of printing and xerography. Embodiments also relate to correcting measurements produced by in-line sensors that can be placed inside printing machines or xerographic machines. The sensors can measure the reflected spectrum of a substrate before printing and after printing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Printers, copiers, and xerographic machines are machines that produce a pattern on a substrate. Typically they consist of a means of obtaining the desired pattern and a marking engine that fixes the pattern to a substrate. Paper, cloth, and plastic are examples of substrates. A pattern can be fixed to a substrate using ink, pigment, or a similar material. Precise control is required to ensure that the pattern produced by the marking engine is acceptably similar to the desired pattern. Over time, the marking engine can experience unintentional change because its mechanical components can change, the ink can change, and the substrate can change. To compensate for unintentional change, a controller can make intentional changes to the marking process such that the marking engine produces a consistent product.  
         [0003]     In performing its function, a controller runs control algorithms and needs control inputs and outputs. An ideal control input for a marking engine controller is the reflectance spectrum of printed materials. A spectrophotometer can be used to produce a reflectance spectrum. An in-line spectrophotometer is a device that can be used for monitoring and control of production processes such as printing and copying. A spectrophotometer that can be used as an in-line spectrophotometer is disclosed in U.S. Pat. No. 6,384,918, which is incorporated herein by reference. Utilized as an in-line sensor, an aspect of the disclosed spectrophotometer produces a measurement, called an in-line spectrum that consists of the reflectance measured at eight different wavelengths. Other types of in-line spectrophotometers can also produce in-line spectrums and those in-line spectrums can have a different number of reflectance measurements made at different wavelengths.  
         [0004]     In many applications, a spectrum that includes at least 30 different wavelengths is desired. A well-calibrated and precise spectrophotometer or a similar device, hereinafter called a reference spectrophotometer, can make the desired measurement, hereinafter called a reference spectrum. Reference spectrophotometers typically cannot be used in-line because they tend to be large expensive and slow. A close approximation of a reference spectrum, called a reconstructed spectrum, can be calculated from an in-line spectrum using methods disclosed in U.S. patent application publications numbers 20030050768, 20030055611, and 20030055575. The disclosed methods produce a reconstruction matrix that maps an in-line spectrum to a reconstructed spectrum. The referenced methods use an in-line spectrum and a reference spectrum to produce a reconstruction matrix, but thereafter use in-line spectrums and the reconstruction matrix to produce corrected spectrums.  
         [0005]     Current art teaches systems and methods by which a reconstructed spectrum having the quality of a reference spectrum can be produced from an in-line spectrum and a reconstruction matrix when the substrate does not change. In the real world, substrates do change. For example, a specific type of paper used as a printing substrate can change between manufacturing batches. Another example is that two versions of a publication can be printed, one on a high quality paper and the other on a low quality paper. Substrate changes cause the reconstruction matrix to become inaccurate. The reason is because the reflectance spectrum of printed material depends on the reflectance spectrums of both the substrate and the printed pattern and because different types of substrates can have significantly different reflectance spectrums. Prior methods and systems produce excellent results as long as the substrate does not change.  
         [0006]     Linear algebra, which includes vector and matrix manipulations, is well known. The prior art and the present embodiment are presented via linear algebraic notation. A few of these notations are of particular interest. One is the diag( ) function that transforms an M element vector into an M by M matrix with the vector elements on the main diagonal of the matrix. Another is the matrix inverse. A matrix multiplied by its own matrix inverse results in an identity matrix. Another concept is the distance between two vectors. One common measure is the Euclidean distance. Given two vectors with two elements, (a, b) and (c, d), the Euclidean distance is sqrt((a−c)*(a−c)+(b−d)*(b−d)), where sqrt( ) is the square root function. Other distance measures such as the Mahalinobis distance, or absolute difference are also common.  
         [0007]     The embodiments described herein therefore overcome the aforementioned limitations and flaws of the prior art.  
       BRIEF SUMMARY  
       [0008]     Aspects of the embodiments address limitations and flaws in the prior art by using transform matrices on the reconstruction matrix and thereby correct for substrate changes.  
         [0009]     It is therefore one aspect of the embodiments provide for obtaining a target in-line spectrum from a target substrate and identifying the substrate type of the target substrate. A target substrate is a substrate that can be patterned by a marking engine. A “substrate type” refers to a general class of substrates that have the same properties. For example, if a manufacturer produces printable plastic substrates that are consistent from batch to batch, then all the plastic substrates from that manufacturer are of the same substrate type. If another manufacturer produces significantly similar substrates, then all the plastic substrates from both manufacturers would be of the same substrate type.  
         [0010]     It is another aspect of the embodiments to provide for retrieving data from a storage device wherein the specific data retrieved is based on the substrate type of the target substrate.  
         [0011]     It is a further aspect of the embodiments to provide for producing a corrected spectrum from the retrieved data and the target in-line spectrum.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying figures, in which like reference numerals refer to identical or functionally similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the background of the invention, brief summary of the invention, and detailed description of the invention, serve to explain the principles of the present invention.  
         [0013]     In accordance with one aspect of the embodiments,  FIG. 1  illustrates production of a corrected spectrum;  
         [0014]     In accordance with another aspect of the embodiments,  FIG. 2  illustrates a high level flow diagram;  
         [0015]     In accordance with yet another aspect of the embodiments,  FIG. 3  illustrates production and storage of data that can be used for producing a corrected spectrum; and  
         [0016]     In accordance with a further aspect of the embodiments,  FIG. 4  illustrates components of a marking engine.  
     
    
     DETAILED DESCRIPTION  
       [0017]     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate embodiments that are not intended to limit the scope of the invention.  
         [0018]      FIG. 1  illustrates production of a corrected spectrum. An in-line spectrophotometer  101  produces a target in-line spectrum  103  by measuring the reflectance spectrum of a target substrate  102 . The target in-line spectrum  103  is mathematically represented by w t   v  where t means target and v means in-line. A processor  105  uses the target in-line spectrum  103  and data retrieved from a storage device  104  to produce a corrected spectrum  106 .  
         [0019]      FIG. 2  illustrates a high level flow diagram that can be used in conjunction with the system illustrated in  FIG. 1 . After the start  201 , a target substrate is chosen  202  its target in-line spectrum is obtained  203 . The substrate type of the target substrate is identified  204  and used to retrieve data that can be used to produce a corrected vector  205 . The target in-line spectrum and retrieved data are used to produce a corrected vector is produced  206  that is returned  207 .  
         [0020]      FIG. 3  illustrates production and storage of data that can be used for producing a corrected spectrum. A representative substrate  302  that is typical of the ith substrate type  301  is chosen. A reference spectrophotometer  303  measures the representative substrate&#39;s reflectance spectrum to produce a representative reference spectrum  305 . An in-line spectrophotometer  101  measures the representative substrate&#39;s reflectance spectrum to produce a representative in-line spectrum  304 . 
     
    
       [0021]     The representative reference spectrum  305  is mathematically represented by w i   r  where r means reference and i means the representative reference spectrum is associated with the ith substrate type  301 . The representative in-line spectrum  304  is mathematically represented by w i   v  where v means in-line and i means the in-line reference spectrum is associated with the ith substrate type  301 .  
         [0022]     A processor  306  takes the representative reference spectrum  305  and the representative in-line spectrum  304  and uses them to produce data that is stored on a storage device  104 . The data can be used, along with a target in-line spectrum, to produce a corrected spectrum.  
         [0023]      FIG. 4  illustrates components of a marking engine  401 . Printable substrates  403  can be supplied to the marking engine  401  via an input port  402 . Substrates can be moved from the input port  402  to a marking apparatus  404  that produces patterns on the substrate. The patterned substrates  405  can then be collected in an output port  406 . The substrates in  FIG. 4  are shown as stacked sheets. A substrate can also be presented in an input port  402  in rolled form where hundreds of feet of substrate are rolled into a cylinder. Substrates can also be rolled up at the output port  406 .  
         [0024]     A spectrophotometer measures a spectrum by measuring the spectrum at many different wavelengths. Essentially, the spectrophotometer makes many measurements, each at a different wavelength, and combines the measurements into a vector called a spectrum.  
         [0025]     The number of different wavelengths measured by a reference spectrophotometer is denoted M. The first reference wavelength can be expressed as λ l   r  and the last reference wavelength can be expressed λ M   r . Each individual measurement is a function of wavelength. The measurement of a representative substrate made at the first reference wavelength can be expressed as w i   r (λ l   r ). The measurement of a representative substrate made at the last reference wavelength can be expressed as w i   r (λ M   r ). A representative reference spectrum can be expressed as w i   r =|w i   r (λ l   r )w i   r (λ 2   r ) . . . w i   r (λ M   r )| T  where T indicates transpose. The subscripted i is used to indicate the substrate type of the representative substrate.  
         [0026]     An in-line spectrophotometer makes measurements at N different wavelengths. The first in-line wavelength can be expressed as λ l   v  and the last reference wavelength can be expressed as λ N   v . As such, a representative in-line spectrum can be expressed as w i   v =|w i   v (λ 1   v )w i   v (λ 2   v ) . . . w i   v (λ N   v )| T . Similarly, a target inline spectrum can be expressed as w t   v =w t   v (λ 1   v )w t   v (λ 2   v ) . . . w t   v (λ N   v )| T  where the subscripted t means target.  
         [0027]     A corrected spectrum can be calculated as w r   c =T i   r AT i   v w t   v  where T i   r  is the reference transform matrix, T i   v  is the in-line transform matrix, A is the reconstruction matrix, and w t   c  is the corrected spectrum.  
         [0028]     The reference transform matrix can be calculated as T i   r =diag(w i   r )(diag(w 1   r ) −1  and the inline transform matrix can be calculated as T i   v =diag(w i   r )(diag(w 1   v )) −1  where the superscripted −1 indicates the matrix inverse and diag( ) is the well known function that converts a M point vector into an M by M matrix such that the main diagonal contains the vector elements. There are numerous methods known for obtaining the reconstruction matrix.  
         [0029]     The equation for T i   r  includes the term w 1   r  where the subscripted i is replaced with a subscripted 1 meaning that the representative reference spectrum is associated with the first substrate type. If there were only one substrate type, then there would be no error introduced by changes in substrate. When there is more than one substrate type, then one of them is referred to as the first substrate type. Therefore, w 1   r  refers to the reference spectrum associated with the first substrate type. Similarly w 1   v  that appears in the equation for T i   v  refers to the in-line spectrum associated with the first substrate type. The first substrate type is typically the one used for producing the reconstruction matrix, A.  
         [0030]     A corrected spectrum can also be calculated as w t   c =T i   r AT r   v w t   v  where T t   v  is the target transform matrix. The target transform matrix can be calculated as T t   v =diag(w 1   v )(diag(w t   v )) −1 .  
         [0031]     The storage device  104  of  FIG. 1  and  FIG. 2  stores data that can be used to produce a corrected spectrum. Based on the equations given above, all the storage device needs to store is the reconstruction matrix, the representative reference spectrums associated with every substrate type, and the representative in-line spectrums associated with every substrate type.  
         [0032]     It can be seen from the equations that all the information required for calculating the reference transform matrix is known before any target in-line spectrum is obtained. As such, the reference transform matrix associated with each substrate type can be calculated and stored in the data storage device. This is known as pre-calculation and is a common technique for more quickly obtaining numerical results. As such, the processor  105  can retrieve the reference transform matrix associated with the target substrate&#39;s substrate type instead of retrieving the representative reference spectrum.  
         [0033]     By a similar argument, the in-line transform matrix can also be pre-calculated and stored. However, the representative in-line spectrums must also be stored if they are used for finding the target substrate&#39;s substrate type.  
         [0034]     Another observation is that the reconstruction matrix is also known before any target substrate is measured. From the equations, numerous opportunities for pre-calculation and storage present themselves. In general, a numerical result involving the any combination of a reference transform matrix, the reconstruction matrix, and an in-line transform matrix can be pre-calculated and stored.  
         [0035]     When the in-line spectrum of a target substrate is obtained, the target substrate&#39;s substrate type must be identified. One easy, but unreliable, way to identify it is to simply ask the user. It can also be identified by comparing the target in-line spectrum to every in-line spectrum stored by the storage device and choosing the substrate type associated with the most similar representative spectrum.  
         [0036]     Two spectrums can be compared using any of the known methods that are used to compare two vectors for similarity. Such methods include the Euclidean distance and Mahalinobis distance. Similarly, the known techniques for pattern recognition  
         [0037]     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.