TAPPI T 452 om-18 is for the determination of the brightness of white, near-white, and naturally colored pulp, paper, and paperboard. Brightness is a commonly used industry term for the numerical value of the reflectance factor of a sample with respect to blue light of specific spectral and geometric characteristics. This method requires an instrument employing 45° illumination and 0° viewing geometry with the illuminating and viewing beams adjusted so that translucent materials are evaluated on an arbitrary but specific scale. The cone of light (see A.3.2 and A.3.5) used by this method is wider than that specified for the CIE Standard 45/0 geometry.

Brightness and whiteness are not the same measurement. Brightness and whiteness are not interchangeable, and there is no known mathematical relationship between the two values.

This procedure is applicable to all naturally-colored pulps, and papers and board made therefrom and is suitable for optically brightened pulp and paper. The measurement is not suitable for paper or paperboard containing added coloring matter (such as yellow or green dyestuff) which appreciably absorbs light in that part of the spectrum extending from about 400 to 500 nm. This brightness method is not applicable to colored papers which must be measured either spectrophotometrically or colorimetrically in order to obtain meaningful results (1).

This method gives:

A procedure for using the test instrument to measure the directional reflectance factor at 457 nm of a sample of paper or paperboard (2-5).