  The variation of the specific intensity across the stellar disc is essential
input parameter in surface brightness reconstruction techniques such as Doppler
imaging, where the relative intensity contributions of different surface
elements are important in detecting starspots. We use PHOENIX and ATLAS model
atmospheres to model lightcurves derived from high precision (S/N ~ 5000) HST
data of the eclipsing binary SV Cam (F9V + K4V), where the variation of
specific intensity across the stellar disc will determine the contact points of
the binary system lightcurve. For the first time we use chi^2 comparison fits
to the first derivative profiles to determine the best-fitting model
atmosphere. We show the wavelength dependence of the limb darkening and that
the first derivative profile is sensitive to the limb-darkening profile very
close to the limb of the primary star. It is concluded that there is only a
marginal difference (< 1sigma) between the chi^2 comparison fits of the two
model atmospheres to the HST lightcurve at all wavelengths. The usefulness of
the second derivative of the light-curve for measuring the sharpness of the
primary's limb is investigated, but we find that the data are too noisy to
permit a quantitative analysis.
