  We build a simple model of the optical/UV emission from irradiation of the
outer disc by the inner disc and coronal emission in black hole binaries. We
apply this to the broadband Swift data from the outburst of the black hole
binary XTE J1817--330 to confirm previous results that the optical/UV emission
in the soft state is consistent with a reprocessing a constant fraction of the
bolometric X-ray luminosity. However, this is very surprising as the disc
temperature drops by more than a factor 3 in the soft state, which should
produce a marked change in the reprocessing efficiency. The easiest way to
match the observed constant reprocessed fraction is for the disc skin to be
highly ionized (as suggested 30 years ago by van Paradijs), so that the bulk of
the disc flux is reflected and only the hardest X-rays heat the disc. The
constant reprocessed fraction also favours direct illumination of the disc over
a scattering origin as the optical depth/solid angle of any scattering material
(wind/corona) over the disc should decrease as the source luminosity declines.
By contrast, the reprocessed fraction increases very significantly (by a factor
~6) as the source enters the hard state. This dramatic change is not evident
from X-ray/UV flux correlations as it is masked by bandpass effects. However,
it does not necessarily signal a change in emission e.g. the emergence of the
jet dominating the optical/UV flux as the reflection albedo must change with
the dramatic change in spectral shape.
