From Online Dictionary of Crystallography

سطوع (Ar). Luminosité (Fr). Helligkeit (Ge). Luminosità (It). 明度 (Ja). Яркость (Ru). Luminosidad (Sp).

Brightness is defined as the flux per unit of solid angle of the radiation cone. It takes thus the beam divergence into account. The unit is p.p.s. [math]\times[/math]mra2. Brightness is an appropriate parameter to use when comparing two X-ray sources with identical focal spot size, as the definition does not contain a unit area.