  We describe a new code which can accurately calculate the relativistic
effects which distort the emission from an accretion disc around a black hole.
We compare our results for a disk which extends from the innermost stable orbit
to $20r_{g}$ in both Schwarzschild and maximal ($a=0.998$) Kerr spacetimes with
the two line profile codes which are on general release in the XSPEC spectral
fitting package. These models generally give a very good description of the
relativistic smearing of the line for this range of radii. However, these
models have some limitations. In particular we show that the assumed form of
the {\em angular} emissivity law (limb darkening or brightening) can make
significant changes to the derived line profile where lightbending is
important. This is {\em always} the case for extreme Kerr spacetimes or high
inclination systems, where the observed line is produced from a very large
range of different emitted angles. In these situations the assumed angular
emissivity can affect the derived {\em radial} emissivity. The line profile is
not simply determined by the well defined (but numerically difficult) physical
effects of strong gravity, but is also dependent on the poorly known
astrophysics of the disc emission.

Astrophysics