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1. Background |
| Spec*Scan 2000 was developed in 1995 by Apogee Systems to count and categorize dirt specks and other contaminants in recycled pulp and paper. The enthusiastic feedback from many customers has elicited continuing enhancements to Spec*Scan . It is now the leading program of its type in the world, and is installed in over 275 locations in 15 countries, including Sweden, Finland, Germany, South Africa, Indonesia, People’s Republic of China, Canada, and the USA. |
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Spec*Scan counts dirt by scanning the sample using an HP ScanJet 4C document scanner. The scanner produces an image of the sample at up to 600 dots per inch (236 dots/cm) in each direction. Each of these dots or picture elements (pixels) is assigned a grayscale value between 0 and 255, representing the reflectivity ("whiteness") of the sample at that point. White paper samples typically are about 220 on this scale, while dirt, residual ink, and other black impurities have much lower grayscale values. Thus a matrix of pixels results, each representing the reflectivity of the sample in a small (typically 1/600 inch square) area.
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The pixels which fall below some threshold are declared as constituting dirt. Dirt pixels in the same vicinity are "clustered" together to define a dirt speck. Speck areas are computed by counting how many pixels compose them and dividing by the area of an individual pixel. Likewise, average grayscale values, etc. can be calculated. Histograms of the number of pixels of various sizes are computed and displayed. The expected number of specks per square meter of sample or the expected total area of the specks per square meter are reported as the dirt content of the sample. Additionally, Spec*Scan allows the user to categorize the detected specks by size and average grayscale value. That is, all detected specks within a specified size range and whose average grayscale values are within a range can be counted and the total area computed. Spec*Scan supports up to 10 such categories.
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Recently Spec*Scan users have suggested various ways of discriminating and categorizing different kinds of contaminants. Of particular note are shives and stickies. Shives are generally insufficiently processed particles of wood fiber, especially bark, which often appear as brown specks in the finished paper. Often they have long, slender, or thread-like appearance. Because they are dark in color, shives are often hard to discriminate from ink specks using the threshold and category structure of Spec*Scan.
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2. Discriminating shives based on shape
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Spec*Scan has always treated all specks which are detected identically. That is, round specks are treated the same as long slender specks. Starting in version 1.2.24, an updated eccentricity calculation for each speck is used:
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Eccentricity (ECC) - A measure of the "roundness" of the speck; perfectly round is 1.0, square is 1.27, and the more oblong the occurrence, the higher the number. The roundness of the detected speck is based upon the dimensions of the speck and area.
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where Perimeter = the length around the outside of the speck
Area = the area of the speck, determined by counting the
number of pixels in the detected speck and
multiplying times the area of a pixel
4p = Correction so that a round speck has an ECC = 1.
Note that ECC is a unit-less value and is more sensitive for small specks. The following depicts eccentricity for several pixel patterns:
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The calculated eccentricity value for any detected speck can be displayed in Spec*Scan by using the "right mouse button." The user simply uses the mouse to point at a detected speck, and clicks the right mouse button. Spec*Scan opens a window on the left of the screen which lists the speck location, size, dimensions, and eccentricity. While this means of calculating the eccentricity is not mathematically rigorous, it does seem to work rather well for characterizing the shapes of the specks.
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Eccentricity can be used in the definition of a category. Thus the user can create separate listings of large round specks, long thin specks, small very black specks, etc. More simply, by proper selection of the eccentricity bounds, the user can create separate categories for dirt and shives, based solely on the fact that shives tend to be long and slender in shape. Additionally, the screen display and listing of individual specks (locations screen and printout) has a column which lists the eccentricity.
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3. Separating stickies based on their behavior
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Stickies are small particles of polyester, polystyrene, polyethylene, or other plastic or adhesive. Stickies are particularly troublesome in papermaking because they tend to melt and adhere to dryer drums and rollers, often accumulating on the drums or tearing small holes in the paper. Stickies may come in many colors or shapes, depending upon their chemistry. Basically they fall into two broad categories:
Opaque - most plastics, etc. which tend to be colored and easy to distinguish by eye from residual ink and shives. Since Spec*Scan detects using
grayscale values, the colored plastics may or may not be detected, depending upon how the scanner converts the colors to grayscale values.
Transparent or Translucent - Adhesives and some plastics often appear to transmit all or some of the light striking them. Because of this, they often are the same color or near the same color as the substrate in which they are imbedded. Because they are nearly the same color as the substrate, Spec*Scan usually has difficulty detecting stickies of this type.
Various processes must be used to detect and count the various types of stickies in pulp and paper. The first involves the preparation of the sample, and several techniques have been developed.
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3.1 Separation using a Master Screen
The Pulmac, Inc. Master Screen may be used to separate larger stickies and shives from the wet pulp fiber. Basically the machine uses a slotted screen (typically 100 micron slots) to filter the wet sample. The stickies and shives larger than 100 microns do not pass through the screen and fall onto a filter paper where they are separated from the water. The filter paper is then placed in a clear plastic sleeve to protect it while it is scanned and to keep the wet dirt, stickies, and shives from adhering to the scanner glass.
3.2 Separation by heat
Whether the number of stickies has been concentrated by using a Master Screen of not, the definition of stickies revolves around their behavior when heated. Several methods have been proposed for separating the stickies by heating them until they adhere to a clean transfer or measurement sheet. All of the methods involve laying a transfer sheet over the hand sheet or Pulmac filter paper. Other layers of paper may be needed over the transfer paper or under the sample. The sandwich of sample and transfer sheet are then heated until the stickies bond to the transfer sheet. Heating methods proposed have included a sheet dryer, iron, office laminator, etc. The sheets are then separated, and the transfer sheet is scanned and counted using Spec*Scan.
Specialized dyes have been developed which affect only plastics and adhesives. By mixing this dye with the wet pulp before making the handsheets or before filtering with a Master Screen, the color of the stickies can be changed to a color more suitable for scanning and counting. One of these dyes changes the normally transparent adhesives to a light blue (cyan) color. Once dyed, the stickies can be counted based on their affinity to the dye rather than their behavior when heated.
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| 4. Counting stickies and shives based on their color |
| Both stickies and shives tend to be a different color than residual ink and other impurities. As mentioned, shives tend to be brown and stickies may be a variety of colors, depending upon their natural color and whether they have been dyed. Thus if Spec*Scan could distinguish colors and separate the specks into separate categories based on their color, it could discriminate between residual ink (black or dark gray), shives (brown or dark yellow), and stickies (brighter colors or the dye color). Combined with shape, Spec*Scan could provide a powerful way to separate and measure the three types of contaminant. |
| The HP ScanJet 4C always measures the color of all pixels in three colors (tristimulus), red, green, and blue; however, to improve the speed of the analysis, Spec*Scan uses the scanner’s internal hardware to convert the tristimulus colors to a grayscale value. Internally the scanner performs the following for each pixel:
The a, b, and c factors are referred to as the "color gain" values. By default, these factors are set at the values defined in the National Television Standards Committee standards:
  
Note that these settings favor green over red and blue. In Spec*Scan , this means that green pixels appear as a light gray, red as a medium gray and blue as a dark gray, while white remains white and black scans as black. Unfortunately, for samples which contain both shives and light blue stickies, both scan as dark gray, and until recently could not be counted separately. |
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Spec*Scan Version 1.2.24 has added the ability to modify the factors above. That is, the user can specify how the scanner converts various colors into gray. The following table may clarify how this helps discriminate between residual ink, shives and blue stickies.
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Scanner Setting
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Ink speck:
Red= 20
Green= 20
Blue= 20
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Blue sticky:
Red= 20
Green= 20
Blue= 75
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Brown Shive:
Red= 75
Green= 75
Blue= 20
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Paper fiber:
Red= 220
Green= 220
Blue= 220
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Default
a=0.29
b=0.59
c=0.09
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Optimize Cyan
a= 1.74
b=-0.17
c=-0.17
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Optimize Yellow
a=-0.17
b=-0.17
c= 1.72
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| The four columns represent the color values of an ink speck, sticky, shive, and fiber. As stated before, the scanner measures each pixel in red, green and blue. Typical red, green, and blue values are shown. Each row represents different scanner settings. The numbers in the matrix are the grayscale value resulting from using equation 2. Note that using the default color settings, it is impossible to discriminate between the blue sticky and the brown shive based on the grayscale value; however, by setting Spec*Scan to optimize Cyan (light blue) and Yellow (brown is a shade of yellow), the differences between the sticky and the shive can be maximized. |
Note also that in all cases, the black ink speck remains dark. Thus while this technique can be used to distinguish between shives and stickies, it cannot be used to distinguish between residual ink specks and other imperfections.
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| 5. Scanning and selection in color |
Obviously the next step in this progression is to implement color scanning in Spec*Scan . Rather than instruct the scanner to convert the colors to grayscale and send the grayscale across the SCSI interface into Spec*Scan , the scanner will scan in color and will pass the tristimulus color values into Spec*Scan . Running in this mode will triple the amount of data that Spec*Scan must process, but will allow implementation of logic which selects only blue pixels for counting in one category, and only yellow pixels for counting in another.
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6. Summary and Roadmap to Spec*Scan’s Future
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Apogee recognizes the importance of separately counting residual ink, stickies, shives, and other impurities. We are committed to working with the industry to implement these functions in the best possible way. To that end, we have already
- Developed the concept of speck categories, including control of the category settings and printouts.
- Instituted computation of a speck shape parameter called eccentricity.
- Worked extensively with several mills to develop sample preparation methods to isolate and concentrate stickies.
- Implemented color gain control of the scanner.
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Through this work and many discussions with researchers and mill operators, we have learned more about the requirements of shive and sticky counting. Given this understanding, we are moving to implement the following:
- Color scanning and separation of specks based on their specific color.
With this added feature, we fully expect that the forthcoming version of Spec*Scan will be able to completely separate residual ink, stickies, and shives into individual categories with count, area, and PPM for each, and separate listings of the specks and their locations, colors, and shapes.
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© Copyright 1999 - All Rights Reserved. Apogee Systems, Inc.
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