  We introduce a new scheme to study the nature of the central engine in a
lensed QSO. The compact emission regions could have different sizes in
different optical wavelengths, and our framework permits to obtain the source
size ratios when a microlensing special high-magnification event (e.g., a
caustic crossing event, a two-dimensional maximum crossing event and so on) is
produced in one of the QSO components. To infer the source size ratios, only
cross-correlations between the brightness records in different optical bands
are required. While the deconvolution method leads to a richer information (1D
intrinsic luminosity profiles), the new approach is free of the technical
problems with complex inversion procedures. Using simulations related to recent
VR data of Q2237+0305A, we discuss the ability of the scheme in the
determination of the visible-to-red ratio q = $R_V/R_R$. We conclude that
extremely accurate fluxes (with a few microJy uncertainties, or equivalently, a
few milli-magnitudes errors) can lead to ~10% measurements of q. Taking into
account the errors in the fluxes of Q2237+0305A from a normal ground-based
telescope, ~10 microJy (~10 mmag), it must be possible the achievement of
smaller errors from the current superb-telescopes, and thus, an accurate
determination of q. Obviously, to measure the visible-to-red ratio, the light
curves cannot be contaminated by an intrinsic event or an important
high-frequency intrinsic signal, i.e., exceeding the microJy (mmag) level. For
an arbitrary lensed QSO, we finally remark that the framework seems to work
better with very fast microlensing events.
