# Brilliance

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. $\times$mm2 $\times$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.