This method describes a procedure for determining the brightness of clay and other mineral pigment that has been pulverized under controlled conditions and made into uniformly compacted pigment plaques. This method is for use with minerals normally used in the manufacture of paper and is not intended for highly colored pigments.

The instrument employed has the same spectral, geometric, and photometric characteristics as that described in TAPPI T 525 “Diffuse Brightness of Pulp.” The brightness scale applicable to this method is the same as the brightness scale described in T 525.

In contrast to TAPPI T 646 “Brightness of Clay and Other Mineral Pigments (45°/0°),” which uses 45° illumination and perpendicular viewing, this method utilizes an instrument with an integrating sphere to provide hemispherical (diffuse) illumination and perpendicular observation. Thus, the specimen surface structure and azimuthal orientation have negligible influence on the brightness results.

NOTE 1: Brightness values obtained using this method will not agree with those obtained using T 646.

Precautions are taken to exclude specularly reflected light from smooth, glossy surfaces by requiring installation of a gloss trap in the upper hemisphere of the instrument’s integrating sphere. Gloss exclusion provides measurements of the intrinsic reflectance of the pigment, which are less dependent upon specimen preparation techniques than gloss inclusion measurements.

The specimens must be prepared with close adherence to the instructions found in the Appendices.

This method utilizes sample preparation apparatus which is identical to that required for TAPPI T 646. The measurement procedure is also similar.