  In many active galaxies, the X-ray reflection features from the innermost
regions of the accretion disc are relativistically distorted. This distortion
allows us to measure parameters of the black hole such as its spin. The ratio
in flux between the direct and the reflected radiation, the so-called
reflection fraction, is determined directly from the geometry and location of
primary source of radiation. We calculate the reflection fraction in the lamp
post geometry in order to determine its maximal possible value for a given
value of black hole spin. We show that high reflection fractions in excess of 2
are only possible for rapidly rotating black holes, suggesting that the high
spin sources produce the strongest relativistic reflection features. Using
simulations we show that taking this constraint into account does significantly
improve the determination of the spin values. We make software routines for the
most popular X-ray data analysis packages available that incorporate these
additional constraints.
