Abstract:
The present invention provides a method and apparatus for determining the presence of Color Shifting Elements (CSE) such as optically variable inks and foils on documents such as bank notes. The invention comprises passing a document past an image sensor such as a line scan camera while sequentially illuminating the document from at least two alternating azimuths. The light source at each azimuth alternates between different colors, producing an image that is interleaved according to color and azimuth of illumination. The invention calculates a reflected color value for each azimuth and compares the color values of the different azimuths to each other. A difference in color between azimuths of illumination indicates the presence of a CSE on the document.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is related to co-pending application Ser. No. 12/277,936 filed Nov. 25, 2008, entitled “Sequenced Illumination,” the technical disclosures of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates generally to currency processing machines, and more specifically to a system and method for detecting optically variable ink on notes by recording images of the notes using multiple modes of illumination that facilitate optimal imaging of specific features. 
         [0004]    2. Description of Related Art 
         [0005]    Automated, high-volume currency processing is a growing international industry affecting numerous aspects of the distribution, collection, and accounting of paper currency. Currency processing presents unique labor task issues that are intertwined with security considerations. It requires numerous individual tasks, for example: the collection of single notes by a cashier or bank teller, the accounting of individual commercial deposits or bank teller pay-in accounts, the assimilation and shipment of individual deposits or accounts to a central processing facility, the handling and accounting of a currency shipment after it arrives at a processing facility, and the processing of individual accounts through automated processing machines. Any step in the process that can be automated, thereby eliminating the need for a human labor task, saves both the labor requirements for processing currency and increases the security of the entire process. Security is increased when instituting automated processes by eliminating opportunities for theft, inadvertent loss, or mishandling of currency and increasing accounting accuracy. 
         [0006]    A highly automated, high-volume processing system is essential to numerous levels of currency distribution and collection networks. Several designs of high-volume processing machines are available in the prior art and used by such varied interests as national central banks, independent currency transporting companies, currency printing facilities, and individual banks. In general, currency processing machines utilize a conveyer system which transports individual notes past a series of detectors. By way of example, a note may be passed through a series of electrical transducers designed to measure the note&#39;s width, length, and thickness. The next set of sensors could be optical sensors recording the note&#39;s color patterns or serial number. Detectors can likewise be used to detect specific magnetic or other physical characteristics of individual notes. 
         [0007]    High volume currency processing machines typically pull individual notes from a stack of currency through a mechanical conveyer past several different detectors in order to facilitate the sorting of the individual notes and the accumulation of data regarding each note fed through the machine. For example, a currency processing machine can perform the simple tasks of processing a stack of currency in order to ensure that it is all of one denomination with proper fitness characteristics while simultaneously counting the stack to confirm a previous accounting. A slightly more complex task of separating a stack of currency into individual denominations while simultaneously counting the currency can be accomplished as well. 
         [0008]    On the more complex end of prior art currency processing machines, a stack of currency consisting of various denominations can be fed into the machine for a processing that results in the separation of each denomination, a rejection of any currency that does not meet fitness specifications, the identification of counterfeit bills, and the tracking of individual notes by serial number. The detection of counterfeit bills in particular is an increasingly complex task as the number of anti-counterfeiting features incorporated into currency notes increase both in number and sophistication. 
         [0009]    Among the most effect security measures in use are color shifting elements (CSE) such as optical variable ink (OVI), color shifting foils, and similar materials. These elements produce different reflective colors (e.g., magenta and green) at different angles of incidence and reflection. CSEs are widely used security features on major currencies such as the US dollar and the Euro and similar documents. CSEs are typically classified as public security features, meaning they are overt and easily recognizable by the members of the general public upon visual inspection of the note, in contrast to esoteric security features like magnetic strips that are only detected by specialized equipment. Because CSEs such as optically variable inks and foils are considered public security features there are no CSE detectors on high-speed currency sorters. 
       SUMMARY OF INVENTION 
       [0010]    The present invention provides a method and apparatus for determining the presence of Color Shifting Elements (CSE) such as optically variable inks and foils on documents such as bank notes. The invention comprises passing a document past an image sensor such as a line scan camera while sequentially illuminating the document from at least two alternating azimuths, wherein a different angle of incidence of illumination is used for each line scanned by the camera, producing an interleaved image of the document. In a preferred embodiment, the light source at each azimuth alternates between different colors, producing an image that is interleaved according to color and azimuth of illumination. A transform is extracted from each azimuth of illumination to produce a reflected color value for that azimuth such as a hue value or red/green ratio. The reflected color values of the two azimuths are compared to each other. A difference in reflected colors between azimuths of illumination indicates the presence of a CSE on the document. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as the preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  shows a currency processing machine embodying the present invention and loaded with a batch feed of currency prior to starting the currency processing cycle; 
           [0013]      FIG. 2  illustrates the operation of sequenced illumination in bank note imaging in accordance with the present invention; 
           [0014]      FIG. 3  shows an example of a raw interleaved image recorded by the line scan camera and its division into separate RGB images in accordance with the present invention; 
           [0015]      FIG. 4  shows an arrangement of light sources capable of implementing the different modes of sequenced illumination in accordance with the present invention; 
           [0016]      FIGS. 5A and 5B  illustrate how the light color reflected by color shifting elements (CSE) differs for the observer depending on the angle of incidence and reflectance; 
           [0017]      FIG. 6  shows an arrangement of light sources for implementing multi-azimuth sequenced illumination to detect CSEs in accordance with the present invention; 
           [0018]      FIG. 7  illustrates an example lookup table used to control sequenced illumination in accordance with the present invention; 
           [0019]      FIG. 8  shows a lookup table used to control multi-azimuth sequenced illumination for CSE detection in accordance with the present invention; 
           [0020]      FIG. 9  is a simplified block diagram of the control system used for sequenced illumination in accordance with the present invention; and 
           [0021]      FIG. 10  is a flowchart that illustrates the overall process of applying sequenced illumination to detect color shifting elements in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows a currency processing machine  10  embodying the present invention and loaded with a batch feed of currency  12  prior to starting the currency processing cycle. This batch feed of currency  12  is fed into the currency processing machine one single note at a time. Single notes then travel on a conveyer past several different detectors before being deposited in one of the sort bins  14 . Typically, a single sort bin is used to accumulate a single denomination of note at the end of the sort process. 
         [0023]    The limitation of the prior art imaging techniques is that they rely on reflectance measurements over a single spectrum of light, producing a one dimensional metric. The present invention replaces the single white light reflectance measurement of the prior art with sequenced illumination using different wavelengths of light (e.g., red, green, blue, UV, IR). Soiling of the note (including ink wear) produces different reflectance effects in each color, which are not visible in a single white light image. 
         [0024]      FIG. 2  illustrates the operation of sequenced illumination in bank note imaging in accordance with the present invention. The invention uses a standard line scan camera  201  to capture an image of a note  202  as the note passes by in the direction indicated by the arrow. A light source (light stick)  203  illuminates the passing note  202  using light emitting diodes (or similar light elements) that emit different wavelengths of light in a variable, sequential manner. 
         [0025]    This sequenced illumination produces an interleaved image in which each line scanned by the camera  201  is recorded under the illumination of a different wavelength of light in a pre-determined sequence (e.g., red, green, blue, UV, red, green, blue, UV, etc) until the entire note  202  is scanned.  FIG. 2  shows the interleaved pattern  212  superimposed on the note  202  to help illustrate this concept. In the present example, the interleaved image can be separated into red  210 , green  220 , blue  230 , and ultraviolet (UV)  240  reflective images. The repeating RGBUV pattern used in  FIG. 2  is a simplified example, but it clearly illustrates the concept. 
         [0026]    At a minimum, the light source  203  uses two different wavelengths. In a preferred embodiment, four wavelengths are used. The illumination switching between the different colors is synchronized with the image capture by the camera  201  and may use a simple repeating pattern such as that described above or a more complex pattern (explained in more detail below). 
         [0027]      FIG. 3  shows an example of a raw interleaved image  301  recorded by the line scan camera. This image includes all of the lines scanned under different wavelengths of light (e.g., RGB) combined together in sequence. The interleaved image  301  is elongated because the image is sampled at a higher rate than single reflectance white light illumination to preserve image resolution. Below the interleaved image  301  are the individual images  310 ,  320 ,  330  that result from separating the scan lines according to color (red, green and blue). The separate RGB images  310 ,  320 ,  330  can be combined into a single composite image  340  equivalent to white light illumination. The composite image can serve as the white light reflectance image against which the individual color reflectance images can be compared. 
         [0028]    It should be emphasized that images  310 ,  320 ,  330  are not color images. All of the scan lines, regardless of the color emitted by the light source, are recorded by the same camera in greyscale. However, the reflection of light will differ according to the color of the light. This is due to the way photons of different wavelengths interact with ink and surface features on the note (including soiling). As a consequence, even though the reflective images produced under different wavelengths of illumination are all recorded in greyscale, each image reveals features not seen in the others, as shown in  FIG. 3 . 
         [0029]    An essential element of the efficacy of the present invention is the recording of the different wavelength images at the same location by the same camera. If the different images were recorded separately at different locations, slight variations in the position of the note relative to each camera would make it more difficult to composite and compare the separate wavelength images, thereby greatly reducing the accuracy of the image analysis. 
         [0030]    Whereas the prior art is limited to merely measuring the reflectance over a fixed spectrum in terms of brighter or darker, the present invention allows the cross referencing of reflected light of different wavelengths and different illumination modes. 
         [0031]    In addition to using different wavelengths of reflected light, sequenced illumination may also alternate between reflective and transmissive illumination, as well as illumination from different angles of incidence to the note (different azimuths). 
         [0032]      FIG. 4  shows an arrangement of light sources capable of implementing the different modes of sequenced illumination in accordance with the present invention. Whereas the example shown in  FIG. 2  only covers the multi-wavelength reflectance mode of sequential illumination, the configuration shown in  FIG. 4  also covers the multi-azimuth and reflective/transmissive modes. 
         [0033]    In this example, the currency note  401  moves along a straight note guide  402  in the currency processor. It should be pointed out that in some embodiments, the note guide  402  may be curved. However, the straight note guide in the present example allows for easier illustration. 
         [0034]    Light sources  410  and  420  are used in the multi-azimuth mode of operation. Similar to the light source shown in  FIG. 2 , light sources  410  and  420  can each illuminate the passing note  401  using alternating wavelengths as described above. Because light sources  410  and  420  are positioned at different azimuths relative to the note  401 , the reflected image recorded by the line scan camera  450  will differ between the two azimuths if the note includes features printed with a color shifting element such as optically variable ink (OVI). Therefore, in addition to interleaving different reflected wavelengths from the same light source (as shown in  FIG. 2 ), the present invention can also interleave reflective images produced by different azimuths of illumination. 
         [0035]    Color Shifting Elements (CSE) produce different reflective colors (e.g., magenta and green) at different angles of incidence and reflection, even if the spectrum of illumination is the same for both angles (e.g., white light). CSEs include optically variable ink (OVI), optically variable foils, and similar types of color shifting materials. After more than a decade in circulation, CSEs have remained a widely used public security feature on bank notes and similar documents, easily identifiable by the general public. However, currently there are no CSE detectors on high-speed currency sorters. 
         [0036]    The multi-azimuth mode of sequenced illumination of the present invention provides an effective means for detecting CSEs without the need for additional specialized equipment, thereby filling a security gap in present high-speed processors. Not only does the present invention allow for the detection of CSEs on notes, it can also be used to evaluate their fitness. 
         [0037]      FIGS. 5A and 5B  illustrate how the light color reflected by CSEs differs for the observer depending on the angle of incidence and observation. In  FIG. 5A , the incidence of illumination to the CSE  511  is approximately perpendicular, producing different reflective colors depending on the position of the observer  530  relative to the CSE. In the present example, the light reflected by the CSE is green in one direction  512  and magenta in the opposite direction  513 , with the observer  530  seeing the magenta reflection as shown in the figure. 
         [0038]    If the light source  520  and observer  530  are held in the same position relative to each other, the only way for the observer in this example to see the green reflectance  512  is to change the angle of the bank note  510  and its CSE  511  relative to the light source and observer, as shown in  FIG. 5B . This presents a problem for high speed sorters in which the observer is a camera at a fixed angle of incidence relative to the note. For obvious practical reasons, the angle of the passing note cannot be changed relative to the camera in the manner shown in  FIGS. 5A and 5B  in order to detect both reflective colors during high speed processing. 
         [0039]    The present invention overcomes this difficulty by manipulating the angle of illumination and reflection while leaving the angle between the camera and the note constant. As shown in  FIG. 6 , the camera  610  maintains a fixed viewing angle relative to the note  640 . To detect the presence of the CSE, illumination is provided by two light sources  620 ,  630  positioned at different azimuths relative to the note and camera. In this example, the first light source  620  is approximately perpendicular to the document  640 . The second light source  630  is positioned at a much shallower angle. The different angles of incidence and reflection of the light sources  620 ,  630  produce different reflective colors from the CSE without having to change the angle of the camera  610 . 
         [0040]    In addition to alternating the azimuth of illumination, each of the light sources  620 ,  630  has multiple LEDs, allowing each light source to illuminate the CSE with different colors in an interleaved pattern as described above. 
         [0041]    The sequenced illumination allows the present invention to interleave the different reflective colors to produce a composite image of the CSE feature on the document. As with the above example, the images produced by the different reflective colors of the CSE are not color images but instead are recorded in greyscale. However, the image of the CSE is slightly different for each reflected wavelength. 
         [0042]    It should be emphasized that CSEs are not limited to reflecting only two different wavelengths. CSEs may reflect three or more different colors at different angles of incidence and reflection. Applying this option to the present invention would therefore entail a separate light source at each azimuth of illumination corresponding to each reflected color. However, for ease of illustration the present example restricts itself to a two-color CSE. 
         [0043]    Furthermore, the wavelengths reflected by CSEs do not have to be in the visible range. Typically CSEs are used as public security features that are overt and easily recognizable by members of the general public upon visual inspection. However, by enabling CSE detection during high speed processing, the present invention permits the introduction of covert CSEs that shift between non-visible wavelengths of light. For example, a currency note might include a CSE that shifts between different IR wavelengths. Obviously such non-visible CSEs are intended only for high speed processing and may complement the visible CSEs used for visual verification by the public. 
         [0044]      FIG. 7  illustrates an example lookup table used to control sequenced illumination in accordance with the present invention. The lookup table  700  is stored in the memory of the control system. There is a separate memory address for each line of the image recorded by the camera, represented by the rows in the table. Each column represents a different source of illumination, which includes all of the LEDs on all of the light sticks in the machine. 
         [0045]    The lookup table shown in  FIG. 7  is a simplified example that only includes five image scan lines and four illumination sources. In this example, the illumination sources are the different color LEDs present in one light stick, which are red, green, blue and infrared (IR). The number at the intersection of each row and column is the control byte applied to each LED array during the recording of that image line. The control bytes determine the intensity of illumination produced by the LED in question. In the present example, a value of 255 represents full intensity, while a value of 0 represents off. In one embodiment, the control system might employ a value of 128, representing half intensity. 
         [0046]    Applying this lookup table to the sequential illumination of a note, scan line  1  would be illuminated by red light, while the remaining LEDs remain off. For line  2 , only the green LED is lit. Similarly, only the blue LED is lit for line  3 , and only the IR LED is lit for line  4 . For line  5 , the red, green and blue LEDs are all lit at full intensity while the IR LED is off, thereby producing a white light reflectance. 
         [0047]      FIG. 8  shows a lookup table used to control multi-azimuth sequenced illumination for CSE detection in accordance with the present invention. Whereas the example lookup table  700  in  FIG. 7  illustrates the generic operation of a single light source with multiple LEDs, the table  800  in  FIG. 8  illustrates one possible control sequence for producing a six-way interleaved image using two light sources positioned at different azimuths relative to the note. 
         [0048]    The illumination sequence shown in  FIG. 8  maintains the RGB sequence but alternates between the light sources for each color, thereby interleaving the high azimuth RGB sequence with the low azimuth RGB sequence. Therefore, the first scan line imaged by the camera is under red illumination provides by the light source at azimuth  1 . The second scan line is also imaged under red illumination but this time from the light source at azimuth  2 . The sequence then returns to azimuth  1  and now uses green illumination for scan line  3 , and so on. The process continues back and forth alternating between the first and second azimuths for each color before moving to the next color, until the specified number of scan lines is imaged. In the present example, only six scan lines are shown in order to illustrate a complete RGB sequence for both azimuths, but the actual number of scan lines imaged will depend on the size of the color shifting element in question. 
         [0049]      FIG. 9  is a simplified block diagram of the control system used for sequenced illumination in accordance with the present invention. 
         [0050]      FIG. 10  is a flowchart that illustrates the overall process of applying sequenced illumination to detect color shifting elements in accordance with the present invention. The process begins with the acquisition of raw data (step  1001 ). This involves the capture of the interleaved image using the methods described above. 
         [0051]    Once the raw image is acquired, the next step is observation extraction (step  1002 ). This is the process of extracting a multi-dimensional observation from the raw data based on the known document type: specific currency (e.g., US dollar or Euro), denomination and series (e.g., 1996 US twenty dollar bill), and specific orientation presented to the camera (e.g., front face left edge leading). Observation extraction is also based on the image geometry, which describes the illumination sequence (mode) that was used to acquire the raw data for this note type, as well as the known location and rotation of the document within the acquired image frame (document skew). 
         [0052]    Following observation extraction, the invention applies a Transformation Function to the data (step  1003 ). This is a mathematical transformation function that converts the multi-dimensional observation data into a three-dimensional vector. This process can be quite complex and may be any linear or non-linear combination of the observation data. For example, the observation data may be a two-dimensional array corresponding to a certain rectangular region of the note, wherein each point in the two-dimensional array is a three-dimensional value containing a red, green, and blue reflectance value. The Transformation Function may convert this into a single three-dimensional measurement that contains a mean hue, saturation, and luminance value for the entire rectangular region. For example, when detecting color shifting elements, the transform may produce a hue value from the RGB values. Alternatively, the transform may produce an RIG ratio. Other methods may be used to determine the color reflected by the surface of the note. 
         [0053]    From the Transformation Function, the system determines the detected color from the azimuth in question (step  1004 ). Steps  1003  and  1004  are performed in parallel for each azimuth of illumination. 
         [0054]    After the system determines the reflected color in step  1004 , it then compares this to the color detected from the other azimuth(s) (step  1005 ). If there is indeed a CSE present on the image region of the note the comparison in step  1005  will reveal a difference in the values between the azimuths, e.g., hue values or RIG ratios. By contrast, the values will not vary between azimuths if there is no CSE present. 
         [0055]    The methods of sequential illumination described above are not limited to use with currency notes. They can also be applied to other types of documents that circulate widely such as checks, bonds, share certificates, etc. 
         [0056]    Although preferred embodiments of the present invention have been described in the foregoing Detailed Description and illustrated in the accompanying drawings, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions of parts and elements without departing from the spirit of the invention. Accordingly, the present invention is intended to encompass such rearrangements, modifications, and substitutions of parts and elements as fall within the scope of the appended claims.