Every paper optical number rests on this ratio: the instrument compares the sample with an ideal white diffuser, using reference standards to carry the scale. Because the value depends on how the sample is lit and viewed, a reflectance factor always comes with its geometry, wavelength band and, for fluorescent samples, its UV content. Brightness, opacity and color coordinates are all calculated from reflectance factors.
Reflectance factor at 457 is dead on for the tile, so the drift is in the samples, not the instrument.
Related terms
R457
Notation for diffuse blue reflectance factor at an effective wavelength of 457 nm, the quantity behind brightness.
R-infinity
Reflectance factor of a pad so thick that adding more sheets no longer changes the reading.
Reference standard
Certified material with assigned reflectance values that transfers a measurement scale to a working instrument.
d/0 geometry
Measurement setup with diffuse illumination of the sample and viewing perpendicular to its surface.