  The accretion flow in the disk dominated state of black hole binaries has
peak temperature and luminosity which vary together in such a way as to
indicate an approximately constant emitting area. The association of this with
the last stable orbit gives one of the few ways to estimate spin when the mass
of the black hole is known. However, deriving this radius requires knowledge of
how the disk spectrum is modified by radiative transfer through the vertical
structure of the disk, as well as special and general relativistic effects on
the propagation of this radiation. Here we investigate the extent to which
differences in vertical structure change the derived disk spectra by
calculating these for a range of different stress prescriptions. We find that
at a given mass accretion rate the spectra are almost identical for accretion
rates of L/L_Edd <~ 0.1. The spectra are remarkably similar even up to the
highest luminosities considered (L/L_Edd ~ 0.6) as long as the stresses do not
dissipate more than about 10 per cent of the gravitational energy above the
effective photosphere. This is exceeded only by classic alpha disks with alpha
>~ 0.1, but these models give spectral variation which is incompatible with
existing data. Therefore, we conclude that disk spectral modelling can place
interesting constraints on angular momentum transport, but still provide a
robust estimate of the spin of the black hole.
