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Paper brightness is a gauge of how well the lignin component of paer has been removed from the paper pulp. Papers with large amounts of residual lignin will have lower brightness. Paper mills use fillers such as, calcium carbonate, chalk, and titanium dioxide to increase the brightness of a paper.
Measuring Paper Brightness
Paper brightness can be tested using 2 different methods. In the US, the TAPPI T 452 standard is used. The international community uses ISO standards, the ISO brightness tester. The TAPPI T 452 standard calls for a pad of paper to be illuminated at 45o and the resulting reflected light read at the 457 nm. – Which falls in the blue portion of the visible light spectrum.
The test is sensitive to the grain direction of the paper, so measurements must paper direction must remain consistent during the test. The brightness scale is based on the 100% reflectance of a standard magnesium oxide tile.