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 } \lesssim 0.1 .
The spectra are remarkably similar even up to the highest luminosities considered ( L / L _ { Edd } \sim 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 \gtrsim 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 .