From Online Dictionary of Crystallography
Brillance (Fr). Brillanz, Glanz (Ge). Brillo (Sp).
In contrast to brightness, brilliance additionally takes the beam dimensions into account and is defined as brightness per mm2. The unit is p.p.s. [math]\times[/math]mm2 [math]\times[/math]mrad2. Brilliance is maximized by making the beam size and divergence as small as possible, and the photon flux as large as possible. Two X-ray beams may have the same flux density but different brilliance if the two beams have different beam divergence. Brilliance is thus an appropriate parameter to use when comparing two X-ray sources with different focal spot sizes.