On time-scales of the solar rotation most of the solar irradiance variation
s are caused by the changing distribution of solar surface magnetic feature
s. We model these short-term irradiance variations using calculations of su
nspot and facular contrasts as a function of wavelength and limb angle on t
he Sun. The position of active regions on the solar disc is derived from th
e MDI magnetograms. The reconstructed irradiance variations are compared wi
th total and spectral irradiance measurements obtained by the VIRGO experim
ent on SOHO.