Flux and brilliance are benchmarks of x-ray beam quality. Both are based on a measure of the number of photons per second in a narrow energy band-width and in a unit of solid angle in the horizontal and vertical directions.
Brilliance is a measure of the intensity and directionality of an x-ray beam.
It determines the smallest spot onto which an x-ray beam can be focused. The figure below shows APS and ALS spectral brilliance compared to a variety of conventional ultraviolet and x-ray tube sources.