Abstract:
A station optically inspects banded cigarette paper by directing an elongated beam of white light laterally across a web of the cigarette paper. The elongated beam impinges on the surface of the cigarette paper and forms reflections. A plurality of line scan camera containing linear CCD arrays receive the reflections and generate output signals. One or more processing units process the output signals to generate data indicative of the spacing between bands, the width of the bands, and the contrast of the bands. These calculations can be periodically transferred to a separate computer workstation over a network. The workstation generates statistical reports on the basis of the calculations, such as the band width, band spacing and band contrast as a function of lane number, and as a function of time. The statistical reports provide a convenient way of quickly detecting irregularities in application of the bands. According to exemplary embodiments, the inspection station is designed for installation in a cigarette paper manufacturing system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to commonly assigned Ser. No. 08/893,500 (our reference 021238-177/PM-1803) entitled “Bobbin Optical Inspection System” and Ser. No. 08/893,505 (our reference 021238-178/PM-1804) entitled “Cigarette Making Machine Including Inspection of Paper Containing Bands”, both of which were filed on the same date as the present application. Both of these applications are incorporated by reference in their entirety herein. 
    
    
     BACKGROUND 
     The present invention relates generally to an optical inspection system for determining the characteristics of a moving web. More specifically, the present invention relates to an optical inspection system for determining the characteristics of a moving web of cigarette paper containing bands. 
     Commonly assigned U.S. Pat. Nos. 5,417,228 and 5,474,095 disclose cigarette papers comprising a base web and banded regions of add-on material. As illustrated in FIG. 1, an exemplary cigarette  7  might contain two bands  5  of material formed by depositing a layer of cellulosic pulp on base cigarette paper  3 . Cellulon, microcrystalline cellulose, flax or wood pulp, or amylopectin are some of the various preferred substances which have been used to form the bands. 
     Commonly assigned U.S. Pat. No. 5,534,114 discloses that the above described bands can be formed by modifying a conventional Fourdrinier paper making machine to deposit additional layers of cellulose at some stage in the production of the cigarette base paper  3 . To streamline process, the bands are preferably applied while the paper is moving at high speeds, such as 500 feet per minute. At these high speeds, breakdowns and other factors (such as clogged band applicators), can result in the production of a base web having misplaced bands. 
     For example, as illustrated in FIG. 2, common anomalies arise when the width of a band  1  deviates from a desired width  12 , or the band becomes skewed so that it is no longer orthogonal with respect to the edge of the paper (as is the case with band  1 ). Other anomalies arise when the separation  2  between two bands deviates from a desired separation width  10  (also called “band spacing” herein). Moreover, an irregular band applicator may produce a band with gaps or a band having a contrast which is either too high (e.g. as in band  9 ) or too low. 
     The prior art includes web inspection devices for use in the manufacture of fabrics, film, paper and like material. Some of these devices include a light source for projecting electromagnetic radiation on a moving web of material. The light impinges on the surface of the moving web, where it is reflected and received at a detector device. Any anomalies in the moving web can be detected by investigating the nature of the reflected electromagnetic radiation. For instance, a tear, pinhole or blemish in the web will manifest itself in a spike in the signal level from the detector (which is attributed to an increase or decrease in reflected radiation). This spike can be viewed by connecting the detector output to an oscilloscope, as exemplified by U.S. Pat. No. 5,426,509 to Peplinkski. 
     While useful, these devices are ill-suited to the task of detecting the integrity of bands on cigarette paper. Bands formed on cigarette paper often have reflective properties similar to the cigarette paper itself. Often, for instance, the bands are formed of white colored material which is difficult to distinguish from the white colored cigarette paper. Moreover, the basis weight of the cigarette paper may vary along the direction of travel of the paper on the paper making machine (due to the difficulty in maintaining a constant pulp application rate). The variance in basis weight of the paper influences its reflective properties, thereby obfuscating the differences between banded and non-banded regions, which are subtle enough to begin with. The prior art devices do not have the ability to interpret a reflection from a web of this nature. As mentioned, these devices are configured to examine a web surface for tears, pinholes and blemishes which manifest themselves in dramatic spikes in the video camera signal. 
     Also, whether a band width is too long, too short, or separated from its neighboring band by more or less than a desired distance can not be determined by simply observing the properties of a single point on a moving web. Rather, the properties of a band should be gauged by determining the spatial relationship between different elements on the web. 
     Pattern recognition techniques are one way of determining the spatial relationship between different features on a printed web of material. In a common technique, a camera forms a digital image of a portion of a web of material and information printed thereon. The digital image is then compared with a pre-stored template representing an error-free web portion. Discrepancies between the template and the image represent an irregular web. These techniques offer accuracy, but unfortunately entail a great deal of data processing. These techniques are therefore ill-suited to detecting the properties of bands on a web which may be moving at speeds of greater than or equal to 500 feet per minute. 
     Accordingly, it is an exemplary objective of the present invention to provide an inspection system for accurately detecting the properties of bands contained on a moving web of cigarette paper without delaying other stages in the manufacture of the cigarette paper. 
     SUMMARY 
     These and other exemplary objectives are achieved according to the present invention through an inspection station which is mounted over a moving web in a paper making machine, downstream of a band applicator. 
     The paper inspection machine includes a mounting frame including a plurality of light sources. The light sources channel light via a fiber optic cable to a light distribution assembly. The light distribution assembly directs a narrow stripe of light across the web. The stripe of light is reflected at the paper surface and then received by a plurality of cameras, each containing a linear CCD array. 
     The data from the CCD arrays is fed to one of two processor units also mounted on the frame. The processing units divide the data from each array into a plurality of lanes. A single pixel from each lane is then compared with a dynamic threshold to determine whether the lane corresponds to a band region or a non-band region. By monitoring and recording the pixels from successive lanes, the processing units are able to compute the width of bands on the web, the spacing between bands, and the average contrast of the bands. 
     At periodic intervals, the information calculated by the units is assembled into an Ethernet packet and transferred over an Ethernet network to a computer workstation. The computer workstation then aggregates the packet with previously received packets and presents various summary statistical displays for the operator. For instance, the display provides graphs illustrating the band width, band spacing, band contrast, and band anomalies as a function of lane number for the most recent interval. Furthermore, the display presents cumulative statistics by presenting a graph of the band width, band spacing and band contrast as a function of time. 
     Among other advantages, the apparatus accurately assesses major cigarette band paper anomalies, and timely presents the information in a format which can be easily understood at a glance. For instance, the user can be apprised that a particular element in the band applicator is clogged by noting that a particular lane number is producing irregular bands. Further, the user can be apprised of a general trend of degradation in the system by observing the composite graphs discussed above, and thereby take prompt remedial action. 
     According to another particularly advantageous feature, the threshold used to discriminate band regions from non-band regions is dynamically set on the basis of moving averages of immediately preceding band regions and non-band regions. In one embodiment, the threshold represents the moving average of non-band background plus the greater of: (1) a set constant value (such as 10 gray levels) or (2) 50% of the moving average of banded region peak heights (where the “peak heights” correspond to the gray level of the banded region minus the gray level of a neighboring non-banded region). Dynamically setting the threshold in this manner accommodates a wide variety of different types of cigarette paper and band material, and also can account for changes in the basis weight (and other properties, such as chemical composition, opacity, etc.) of the paper along the direction of travel of the paper making machine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, and other, objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which: 
     FIG. 1 shows an exemplary cigarette containing banded regions; 
     FIG. 2 shows an exemplary web of cigarette material including bands, some of which are irregular; 
     FIG. 3 shows an exemplary paper making machine in which the inspection station of the present invention can be employed; 
     FIG. 4 shows an exemplary paper inspection machine of the present invention; 
     FIG. 5 shows another view of the paper inspection machine of FIG. 4; 
     FIG. 6 shows an enlarged view of a camera employed in the paper inspection machine of FIG. 4; 
     FIG. 7 shows an enlarged cross-sectional view of the light distribution assembly employed in the paper inspection machine of FIG. 4; 
     FIGS. 8A and 8B show an exemplary electrical system for use in conjunction with the paper inspection machine of FIG. 4; 
     FIG. 9 shows an exemplary technique for processing data from a line scan camera; 
     FIG. 10 shows an exemplary waveform of pixel gray level as a function of scan line; 
     FIG. 11 shows an exemplary algorithm for determining various properties of the bands imaged by the line scan cameras; and 
     FIGS. 12A and 12B show an exemplary graphical display of various properties of the bands imaged by the line scan cameras. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the invention. However it will be apparent to one skilled in the art that the present invention can be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention with unnecessary detail. In the Figures, like numbers designate like parts. 
     According to exemplary aspects, the inspection system of the present invention is designed to inspect the characteristics of cigarette paper during its manufacture. Thus, before discussing the inspection station itself, it is useful to first describe exemplary aspects of a cigarette paper manufacturing system. 
     FIG. 3 illustrates an exemplary machine for producing a web  17  of fibrous material. As shown there, a central tank  53  of refined pulp (such as refined flax or wood pulp) is delivered to a head box  51  by means of a plurality of conduits  50 . The Fourdrinier wire  49  transports the slurry pulp from the head box  51  in the direction of the arrow  54 . At this point, the pulp has a high moisture content. Water is allowed to drain from the slurry, and is also removed by vacuums (not shown). Reference numeral  48  shows the return loop of the Fourdrinier wire  49 . 
     The band application assembly  99  is located downstream of the vacuums. Assembly  99  generally includes a frame housing an endless perforated steel belt (not shown), which is guided by drive wheel  27 , guide wheel  29 , and follower wheel  46 . The bottom of the assembly  99  includes a chamber box (not shown) containing a reservoir of slurry supplied from day tank  14  via conduits  15 . The flow of slurry through conduits  15  is maintained at appropriate levels by a flow distribution system comprising a series of pumps (not shown) in conjunction with a pressure monitoring system (not shown). 
     Slurry is dispensed through the perforations in the endless steel belt (not shown) as it passes through the bottom portion of the chamber box. The belt is moving as the slurry is dispensed, thereby compensating for the motion of the web moving beneath the chamber box. According to exemplary embodiments, the belt is moved at a rate of 1000 feet per minute to compensate for a Fourdrinier wire moving at a rate of 500 feet per minute. As a result of this compensation, the chamber box applies the bands (e.g. bands  34 ) so that they are orthogonal to the edges of the web  17 . If the bands are not completely orthogonal, the angle of the band application assembly  99  can be adjusted. Alternatively, a non-orthogonal application of bands may be desired. Those interested in further details regarding the band application assembly  99  are referred to commonly assigned U.S. Pat. No. 5,534,114, the entire disclosure of which is incorporated herein by reference. 
     The banded paper then passes through one or more press rollers  24  which squeeze as much water out of the paper as possible through mechanical pressure. The remaining water can then be evaporated out of the paper by passing the paper over the surface of one or more drying rollers  20 . These moisture removal techniques are conventional in the art and thus will not be discussed in further detail. Furthermore, those skilled in the art will appreciate that other moisture-removal techniques can be used to replace or supplement the above-identified techniques, such as the conventional use of a felt web to remove moisture from the paper. 
     According to exemplary aspects of the present invention, the inspection station of the present invention is positioned downstream from the drying rolls  20 , just before the paper is wound on the final paper reel  32 . More specifically, in the exemplary embodiment shown in FIG. 3, the inspection station is positioned over the roller  30 , which follows roller  31 , at a position denoted by the line A—A. Roller  30  can be a stationary stainless steel tube having a diameter of six inches. Those having skill in the art will recognize that the inspection station can be placed at a variety of locations downstream of the band application assembly  99 , or more than one inspection station can be employed to inspect the paper web. 
     An exemplary inspection station  70  for use in connection with the paper making machine of FIG. 3 is shown in FIG.  4 . By way of overview, the inspection station includes a frame  80  which straddles the Fourdrinier wire  49  over the roller  30 . The inspection station  70  includes eight light sources, one of which is denoted by  78 . The light sources are connected by fiberoptic cabling  92  to a light distribution assembly  90 , which spans the lateral width of the roller  30 . The light distribution assembly  90  directs the light onto the paper in a narrow line as the paper passes over the roller  30 . Light is specularly reflected off the paper and received by one or more of sixteen cameras which span the length of the web, one of which is denoted by  84 . Each camera can be individually positioned by means of adjustment mechanism  86 , which adjustably fixes the cameras (e.g.  84 ) to an overhead bar  82  of the frame  80 . Information from the cameras is transferred via electrical lines (not shown) to processing circuitry located in enclosures  72  and  76 . More specifically, enclosure  72  includes processing circuitry which services the left-most four light sources and left-most eight cameras. Enclosure  76  contains processing circuitry which services the remaining right-most four light sources and eight cameras. According to exemplary embodiments, each group of eight cameras monitors a 60 inch lateral segment of the paper on the roller  30 . Thus, the entire station  70  monitors a web having a total width of 120 inches. Moreover, the inspection station is modular in construction; additional sets of light modules and camera can be added to integrate the station into paper making machines having larger lateral widths. 
     FIG. 5 shows a cross-section of the optical inspection system shown in FIG.  4 . In one exemplary embodiment, the light source  78  includes a 200 watt halogen bulb (although other light sources can be used). The white light generated thereby is fed via fiber optic cable  92  to a fiber optic head end  102 , which laterally disperses the white light. The dispersed light is then focused by a rod lens  104  onto the paper  17  passing over the roller  30 . The light is reflected from the paper  17  and received by camera  84 , which includes a linear CCD array. The angle θ which the reflected light forms with respect to the normal of the roller  30  can be chosen to maximize detection of the bands. In one exemplary embodiment, the angle θ equals approximately 55 degrees. Signals from the CCD array are thereafter fed to a computer unit (e.g. units  72  or  76 ) for analysis. 
     A still more detailed depiction of the camera assembly  84  and light distribution assembly  90  can be found in FIGS. 6 and 7, respectively. FIG. 7 shows a cross-section of the light distribution assembly  90 . The assembly includes an optic head end  102 , which laterally disperses the white light. The dispersed light is then focused by a rod lens  104  onto the paper  17  to form a narrow illuminated stripe across the paper. Head end  102  and rod lens  104  are sandwiched between two plates  144  which span the length of the roller. The light distribution assembly  90  is fed light via fiber optic cables (one of which is denoted by  92 ) from light sources (one of which is denoted by  78 ). By way of example, a rod lens assembly produced by Fostec can be used for the light distribution assembly  90 . 
     As shown in FIG. 6, the camera includes a housing containing the linear CCD array. The housing is attached to the adjustment mechanism  86  which allows the operator to adjust both the azimuth and elevation of the camera via elements  130  and  132 , respectively. The adjustment mechanism includes plate  134  which allows the camera assembly  84  to be attached to the overhead member  82  of the frame  80  (as illustrated in FIGS.  4  and  5 ). By way of example, a camera produced by EG&amp;G Reticon can be used for the camera  84 . 
     The electrical signals generated by the CCD arrays of the camera (such as  84 ) are fed to the processing circuitry enclosed by one of the units  76  or  72 . More specifically, as shown in FIG. 8, the unit  76  includes two computer modules  162  and  163 , preferably including Pentium™ processors (not shown). Each computer module includes plural line scan processor boards connected thereto for processing data received from the line scan cameras. In the embodiment shown in FIG. 8, computer module  162  has two processor boards  164  connected thereto and computer module  163  includes another two processor boards  166  connected thereto. Each processor board services two cameras. As will be discussed in further detail below, the units  76  and  72  determine the presence of bands and compute statistical information pertaining to the bands. This statistical information is transmitted at periodic intervals via an Ethernet interface (not shown) over line  199  to a signal junction box  200 . The junction box, in turn, channels the data from the units  76  and  72  to a separate computer workstation  150  (not shown in FIGS. 3 or  4 ). Unit  72  has an identical construction to unit  76 . Unit  72  includes two computer modules,  170  and  171 . Computer module  170  has two line scan processor boards  172  connected thereto, and computer module  171  has another two processor boards  174  connected thereto. 
     As the lamps (e.g.  78 ) and other components of the station  70  can generate heat during their operation, the electronic units  76  and  72  include air conditioning units  190  and  192 , respectively. Alternatively, the electronic units  76  and  72  can be cooled with conditioned air from a separate detached air conditioning system (not shown). An interconnected network of conduits (not shown) can also channel pressured air to the cameras (e.g.  84 ). The pressured air cools the cameras and also helps keep the cameras free of residue which otherwise would settle on the cameras and degrade their performance. Power sources  176  and  178  provide power to the various components of the system  70 . The specific connection of electrical components will be readily apparent to those skilled in the art, and thus need not be discussed in detail. 
     In addition to data from the line scan camera, the junction box routes signals from a paper break sensor  202 , new roll input  204 , and an encoder  206  or tachometer (not shown in FIGS. 3 or  4 ). The paper break sensor  202  includes an infrared detector located adjacent to the moving web at some point along the wire  49  (with reference to FIG.  3 ). As the name suggests, this sensor provides an active high or low signal when the web is discontinued for some reason, such as a breakage. The new roll input  204  is a button which the user depresses to signal the start of a production run. This input can be used to inform the workstation  150  to begin accumulating statistics for a new production run. The button can be physically located on or near the workstation  150 . 
     The encoder is a device which monitors the velocity of the moving web, and thereby provides a frame of reference by which the output of the camera can be correlated with the actual width of bands and band spacings. According to one exemplary embodiment, the encoder includes a collar which is mounted on a roller in the paper making machine, in conjunction with a nearby magnetic sensor. The collar includes magnetic inserts attached thereto. When the collar is rotated, the inserts come in close proximity to the sensor, upon which the sensor generates a pulse. The rate of pulses from the sensor is related to the rate of rotation of the roller, and in turn, the velocity of the web moving over the roller. 
     The workstation  150  includes a CPU  156 , modem  154  and Ethernet interface  152 . The output of the workstation can be channeled to a 3-color beacon  74  (to be described later), to a remote computer via phone line  75 , a printer  77  and/or a display  79 . The transfer of information via modem  154  to a remote computer allows a remote technician to perform diagnostic evaluation from a remote site. An InterColor™ industrial workstation can be used for the workstation  150 . 
     The processing of the data from the line scan cameras by the processing units  76  and  72  can be understood by reference to FIGS. 9-11. As shown in FIG. 9, each camera (e.g.  84 ) includes a linear CCD array  210 . For instance, the camera can employ a 1024×1 CCD array which spans a 7.5 inch portion of the web. The exemplary resolution of the array in the lateral direction across the roller  30  is 0.2 mm. Furthermore, the CCD array is exposed at a rate which allows the computer to sample information at a resolution of 0.2 mm in the longitudinal direction. Thus, the array effectively samples elements having a spatial dimension on the paper of 0.2 mm ×0.2 mm. Accordingly, each element of the CCD array includes a value indicative of the magnitude of the reflection sensed in a 0.2 mm×0.2 mm portion of the moving web. 
     The data from each linear array is thereafter converted from analog to digital form in A/D converter  212  and stored in memory  214  of one of the processing units  76  or  72 . The processing unit then divides the data from each array into a series of contiguous lanes (e.g. a total of 32 lanes total in one embodiment). To facilitate discussion, each lane shown in FIG. 9 comprises 6 contiguous pixel elements, although each lane will typically include many more pixels. The magnitude of each pixel is quantified into one of, for example, 255 different levels. 
     During each exposure, a single pixel from each lane is compared with a dynamic threshold. Pixels above the given threshold are indicative of banded regions of the web, while pixels below the given threshold are marked as non-banded regions. Upon the next exposure, the next contiguous pixel in the lane is exposed, and the comparison is repeated. For example, at an arbitrary time denoted to, the fifth pixel in each lane is compared with the dynamic threshold (e.g. see bottom-most row of lanes denoted as “line t 0 ”). In the next exposure, the sixth element is compared to the threshold (e.g. see the rows of lanes denoted as “line t 1 ”). After this, the system will continue back in the opposite direction, choosing the fifth pixel for comparison with the threshold in line t 2 . Thus, the pixel chosen for comparison with the threshold varies in a serpentine path, as generally denoted by FIG.  9 . 
     According to another embodiment, the inspected pixel is not advanced at each line. Rather, in this embodiment, the processing unit can dwell on each pixel for a prescribed number of lines (e.g. corresponding to 30 mm), after which it will advance to a next adjacent pixel. The comparison of only one pixel from each lane enhances processing speed without significantly degrading performance. 
     The pixel elements marked with an “X” denote a pixel value above the threshold. Thus, it is seen that a band started at line t 3 . 
     According to one exemplary embodiment, the threshold used to detect a band region and a non-band region varies to accommodate changes in the base paper, band material, or measuring environment. For instance, as shown in FIG. 10, an exemplary waveform of pixel gray level as a function of scan line shows local perturbations which represent transitions from background non-banded regions (e.g. as in regions NB 1 , NB 2 , NB 3 , NB 4  and NB 5 ) to banded regions (e.g. as in regions B 1 , B 2 , B 3 , B 4  and B 5 ). The waveform also shows a global change in which the general baseline of these local perturbations slowly undulates. For example, the global undulation is at its lowest point around the scan line 1000, and at its highest point around scan line 2000. This global undulation is primarily due to changes in the basis weight of paper caused by uneven application of pulp by the paper making machine. The present invention takes this phenomenon into account by adjusting the threshold level (T) so that it generally tracks the changing baseline of the waveform. 
     One technique for dynamically varying the threshold level is described as follows. Generally, the threshold at any given moment is a function of the gray levels of the immediately preceding band region or regions, and the gray levels of the immediately preceding non-band region or regions. In one embodiment, the threshold represents a moving average of previous non-band background (e.g. an average of NB 1 , NB 2 , etc.) plus the greater of (1) a set constant (such as 10 gray levels), or (2) 50% of the moving average of peak heights of the banded regions (e.g. an average of the heights of B 1 , B 2 , etc.). For example, consider the band region B 3 . The threshold used to discriminate this band region is determined by first calculating the average background level of the non-band regions NB 2  and NB 3 . Thereafter, an average peak height value is determined by computing the average of the heights of the B 1  and B 2  band regions. The “height” of a band region generally corresponds to the difference in pixel gray level between the band region and a subsequent non-band region. In making this measurement, a single gray level can be used to represent the gray level of the band region (such as the maximum gray level), or an average of gray levels within the band region can be used. Similarly, a single gray level can be used to represent the gray level of a subsequent non-banded region, or an average of gray levels within the subsequent non-banded region can be used. After computing the peak heights in this manner, half of the average peak heights (e.g. from B 1  and B 2 ) is compared with the preset value. The greater of the two is added to the average background level (computed above) to derive the threshold value. For example, the average of the heights of B 1  and B 2  is approximately 30 gray levels, half of which is 15 gray levels. If the preset value is set at 10 gray level values, then the algorithm will select 15 as the value to be added to the average background. However, if a series of shorter peaks (such as B 5 ) are encountered, then the algorithm will rely on the preset value (e.g. of 10 gray levels) to discriminate band regions from non-band regions. The preset value is preferably set at least high enough so that noise in the non-banded region will not be misinterpreted as the start of a band region. 
     It will be readily apparent to those skilled in the art that the window selected for calculating the moving average of peak heights and non-banded region levels need not be restricted to two banded regions and two non-banded regions, respectively. A smoother threshold can be obtained by widening the window. Furthermore, the above discussed threshold levels are dependent on the type of paper and the band material used, as well as the operating environment; the specific values cited above are entirely exemplary. 
     The actual task of determining the characteristics of the bands can be understood with reference to the flowchart shown in FIG.  11 . The analysis commences at step S 2 , followed by a determination whether it is time to report data from the processing units  76  and  72  to the workstation  150  over the Ethernet network  199  (step S 4 ). In an exemplary embodiment, the processing performed by units  76  and  72  is reported every half second. However, having just commenced analysis, the results of this query will be answered in the negative, and the system will advance to step S 6 . In step S 6  it is ascertained whether the pixel in a lane is above the dynamic threshold. To facilitate discussion, step S 6  is framed in the context of a single lane of a single linear array from a single camera.. However, it should be kept in mind that the system includes a plurality, e.g. 16, similarly constituted cameras each with their own linear arrays and the output of each array is divided into a plurality of lanes. Thus the comparison shown in step S 6  is in actuality repeated many times for different lanes and different cameras. Preferably the processing units perform the computations for different cameras in parallel to improve processing speed. 
     If it is determined in step S 6  that the magnitude of the pixel is above a dynamic threshold, then the algorithm advances to step S 8 , where the presence of a banded pixel and its contrast are recorded. If the previous pixel in the previous line was not a band pixel (as determined in step S 10 ), then the current line represents a start of a band. This would correspond to line t 3  shown in FIG. 9, since the previous line at t 2  contained a pixel below the dynamic threshold. It is therefore possible at this time to determine whether the spacing between the present band and the last encountered band (if appropriate) is within prescribed tolerances (steps S 12  and S 14 ). If the band spacing is either too long or too short, this fact is logged in step S 16 , whereupon the algorithm advances to the next line in step S 32 . 
     If, on the other hand, the pixel examined in step S 6  is below the dynamic threshold, then this fact is recorded in step S 18 . It is then determined if the previously examined pixel in the previous line was a band pixel (step S 20 ). If so, this marks the end of a band, and it is possible to determine the average contrast of the band and the width of the band (step S 22 ). It is determined whether these values are outside of prescribed tolerances (steps S 24 -S 30 ). If so, these anomalies are recorded and the algorithm advances to the next line in step S 32 . 
     Supposing, at this time, it is determined that a half of a second has elapsed (in step S 4 ). This causes the processor units  76  and  72  to enter their report mode. As shown in FIG. 11, the units will compute the number of bands in the lane over the last half of a second (step S 34 ), the average and standard deviation for band width, band spacing and band contrast (step S 36 ), the minimum and maximum average background for the lane (step S 40 ) and the total number of anomalies (e.g. out-of-tolerance band width, spacing and contrast) (step S 40 ). This information is assembled into a packet which is forwarded to the workstation  150  (step S 42 ), and then the various counters are reset (in step S 44 ). 
     The workstation then aggregates this information with previously transmitted information to provide a statistical summary of the operation of the band application assembly  99  (of FIG.  3 ). This information is displayed on display panel  300  as illustrated in FIGS. 12A and 12B. The panel  300  includes a first subpanel  302  listing the band width as a function of lane number for the last reporting interval. A subpanel  304  illustrates band spacing as a function of lane number for the last reporting interval. A subpanel  306  illustrates band contrast as a function of lane number for the last reporting interval. Finally, subpanel  308  illustrates the number of band anomalies (aggregate of band spacing, band width, and contrast anomalies) as a function of lane number for the last reporting interval. The subpanels  302 ,  304  and  306  contain a middle line indicating the average values of the band width, band spacing and band contrast over the half second interval of reporting. The two other curves bracketing the middle curves denote the plus and minus 3σ readings. The middle curve can be shown in green, while the 3σ curves are shown in red so that they can be more readily distinguished. 
     In addition to the current lane summary, the workstation  150  provides statistics summarizing the performance of the band application assembly  99  since the start of operation. Notably, subpanel  310  illustrates the composite band width (e.g. the average bandwidth) as a function of time. Subpanel  312  illustrates composite band spacing  312  as a function of time. Subpanel  314  shows composite band contrast as a function of time. And finally, subpanel  320  shows the number of band anomalies as a function of time. Thus, with the right-hand subpanels, it is possible to observe any trends in degradation. With the left-hand subpanels, it is possible to observe specific points in the lateral span of the web which are producing out-of-tolerance bands, band-spacing or band contrasts, which can be caused by clogged pulp applicators. 
     In addition to these graphs, the workstation  150  presents status information  316  regarding the roll length, the velocity of the web (from the encoder or a tachometer) and a sample id (which the user enters in advance to label the run). All of the above data can be stored for further non-real-time analysis. The run is indexed by the ID number. 
     The interface software of the workstation  150  additionally includes routines to monitor system parameters to determine system status. When an anomaly is detected, the operator interface will display a message identifying the most-likely cause of the anomaly. In the panel  317  shown in FIGS. 12A and 12B, the message indicates that lamps are currently functional. The software also controls a 3-color beacon that can be mounted at any of various locations, such as on the workstation  150 . The beacon flashes red to denote a system failure, yellow to denote an inspection inhibited mode, and green to denote an inspection active mode. 
     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. 
     By way of example, the present invention has been described in the context of detecting bands located on cigarette paper. But the present invention extends to the detection of any information formed on sheet-like material. For instance, the present invention can be used to detect bands on other papers, including papers prepared for security purposes, such as paper currency, stock certificates, bearer negotiable bonds, etc.