This method 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. 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. 

 

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 (a Yc/2 value of 2.4%) on a white background (a Yc/2 value of 81.5%) 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.

 

To perform adequate testing, a minimum area of 2000 square centimeters should be tested. Paper or paperboard would require four sheets that are 8.5” x 11.” If pulp sheets, slush pulp, or flashed dried pulps are tested, enough sample is required to make at least ten TAPPI handsheets.  We would like the equivalent of 30 oven dried grams to make the handsheets.