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Standard: |
TAPPI |
Method: |
TAPPI T 563 |
Title: |
Equivalent Black Area (EBA) and count of visible dirt
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The level of visible dirt present in pulp, paper or paperboard can impact its usefulness in a specific end use application. In such cases, the presence of visible dirt specks which are high in number, easily noticed in visual examination, or both, may detract more from the apparent usefulness of the paper material than does a lower number of specks, or specks which are less easily noticed by the eye. Both the number of dirt specks and their visual impact may be important. For someone controlling or monitoring the paper making process, the absolute physical area of dirt, or the number of dirt specks present in an inspection area may be of greatest importance. For the end user of the paper material, the overall visual impression may be the critical parameter.
The TAPPI T 563 Visible dirt test uses image analysis to determine the level of dirt in pulp, paper, and paperboard in terms of Equivalent Black Area (EBA) of dirt specks within the physical area range of 0.02 to 3.0 mm2 reported in parts per million as well as the number of dirt specks per square meter of sample. Using the algorithm prescribed in this method, the maximum dirt size is limited to 3.0 mm2. Extension to other speck sizes (for example those greater than 3.0 mm2 in physical area), may require changes in equipment, calculation procedures, or both, and is not covered in this test method. This method cannot be used for physical area measurements since it does not correctly measure the dirt specks for that mode of measurement.
The specimen to be evaluated should have a brightness, as determined by TAPPI T 452 “Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm),” of 30% or greater. It may be necessary to reform some pulp sheets into handsheets if the surface is too rough or textured.
This method, and corresponding precision statement, was developed using laboratory analyzers and cannot be considered applicable to on-line systems.
The visual impact of a dark speck on a light background varies as a function of the speck size. The calculation of this visual impact is done based on Ricco’s law in this test method, and reported in terms of equivalent black area. As the dirt speck becomes sufficiently large, the equivalent black area of a black speck on a white background will approach the speck’s physical area. The use of the equivalent black area calculation procedure in this test method is highly significant, as it provides data continuity of dirt measurement with procedures such as TAPPI T 437 which have been in use for over 50 years.
Dirt is usually found on the surface of the sheet, however dirt particles which are embedded may also be discernible by eye in papers that are transparent or translucent. The level of visible dirt present in papers used in printing, writing, and other forms of communication may be both an aesthetic and performance requirement. For example, bond and writing papers may be subjected to close visual inspection in their intended use, but may also be required to perform on scanners, bar code readers, or other automated optical recognition devices where dirt above some critical level could impact performance.
Similar considerations may be appropriate for papers used in wrapping and packaging. In addition, special considerations may be required where such materials come in contact with foodstuffs, and where visible specks may be considered unacceptable.
Dirt in the form of gritty materials embedded in the sheet is a serious defect in many printing papers used in contact printing because of the pitting and wear that they can cause in printing plates. Such dirt can also occur in paperboard such as linerboard, and when present may cause significant reduction in bursting strength. Dirt of this type is not easily detected using this procedure, because there is little color contrast between the dirt speck and the background sheet.
Please note:
SGS-IPS cannot sell or otherwise provide standards, specifications, or test procedures to third parties.