Brightness (paper)

Brightness is a single-wavelength measurement — blue light around 457 nm — chosen because the human eye is particularly sensitive to yellowness in that band. A paper that reflects more blue light reads as 'whiter' even though full-spectrum whiteness is a different (related) metric.

TAPPI and ISO scales differ slightly in instrument geometry and produce numbers a few points apart for the same paper. Specifications usually identify which scale is in use; mixing the two without conversion introduces a real measurement error.

High-brightness grades use optical brightening agents (OBAs) — fluorescent compounds that absorb UV light and re-emit it in the visible blue range. OBAs can push apparent brightness above 100 on some scales, which sounds physically impossible but reflects the way the measurement is defined.