From Online Dictionary of Crystallography

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Brillance (Fr).

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.