  Spectral Energy Distribution (SED) of the broad-band continuum emission from
black-hole accretion discs can serve as a tool to measure parameters of the
central body and constrain the geometry of the inner accretion flow. We focus
on the case of an active galactic nucleus (AGN), with an accretion disc
dominating the UV/optical bands. We parameterize the changes in the thermal and
power-law components, which can reveal the diminution of the emissivity. To
this end we explore the effects of gaps in the accretion disc and the emerging
SED that can be caused by the presence of either (i) the inner, optically thin,
radiatively inefficient hot flow; (ii) a secondary black hole embedded within
the accretion disc; or (iii) a combination of both components. We suggest that
the resulting changes in the SED of the underlying continuum can help us to
understand some departures from the standard-disc scenario. We estimate that
the data required for such a project must be sampled in detail over the far-UV
to soft X-ray bands during the interval of about a month corresponding to the
characteristic variability timescale of an AGN. Detecting a gap at intermediate
radii of a few 100 gravitational radii would require quality photometry with
uncertainties up to $\sim$ 1%. The presence of the central cavity in the
standard disc can be recovered in UV photometric data with an accuracy of 5%
and better. We show the effect of the intrinsic reddening of the source and
demonstrate when it can be disentangled.
