When the TAPPI directional (45 degree illumination/0 degree viewing) geometry was introduced in the 1930's, it included the effects of fluorescence in its measurements. However, it did not have the ability to measure the effectiveness of these additives. As optical brighteners and fluorescing agents became more common and more expensive, they needed to be measured and controlled. It wasn't until the 1980's that the TAPPI geometry brightness tester was equipped with the ability to measure the fluorescent component of brightness. Fluorescent component is the additional brightness obtained from the use of optical brighteners. It is determined by measuring the sample with and then without the effect of UV energy on the sample.
Once this capability was added to the directional geometry instrument, the TAPPI Test Method T452 was updated with this information. Even today this can be found in Appendix C.
As the use, effectiveness and cost of OBA and FWA continue to increase, it has become more important to accurately measure samples with very high levels of these additives. The round robin within TAPPI will help establish:
- A procedure for this process
- A way to verify the calibration
- Tolerances of brightness at these very high levels of OBA and/or FWA