1.1
Absolute brightness is defined as the reflectance of blue light with a
specified spectral distribution peaking at 457 nm compared to that of a
perfectly reflecting, perfectly diffusing surface. Although no physical
material has such a surface, the discrepancies between a reference material and
the perfect diffuser can be rigorously determined. Reference materials thus
calibrated establish the scale of absolute brightness. 1.2
Bleaching agents strongly influence the reflectance of pulp in this portion of
the spectrum. Absolute brightness is thus used to monitor bleaching action.
The method is not intended to be solely definitive in the case of materials
having a pronounced color (Note 1) or containing optical brighteners. 1.3
Since the method utilizes an integrating sphere geometry, it may be used with
materials having relatively rough surfaces. The specular component is
excluded. The use of a relatively large sample aperture averages out point to
point differences and minimizes the number of measurements necessary.