# Brilliance

### From Online Dictionary of Crystallography

Brillance (*Fr*).

In contrast to brightness, **brilliance** additionally takes the beam dimensions into account and is defined as brightness per mm^{2}. The unit is p.p.s. [math]\times[/math]mm^{2} [math]\times[/math]mrad^{2}. 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.