We present a reflectance validation method, using a polarized radiative transfer model as a reference. The method involves calculating the complete radiative transfer in the ultraviolet for a carefully selected, natural, cloud-free Earth target. An extensive sensitivity study shows that in the ultraviolet, the ozone profile (mainly below 330 nm) and the surface albedo (mainly above 330 nm) are the important parameters that affect the reflectance and are therefore the major sources of error in the modeled reflectances. The accuracy with which the reflectance in the UV range of 240--400 nm can be calculated, because of uncertainty in input parameters, is 2--6%, depending on wavelength. The method is applied to Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), using a cloud-free scene over the Sahara desert. The reflectance measured by SCIAMACHY turns out to show a scan-angle-independent calibration error of 10--25% for this particular Sahara target. The result is confirmed for ocean and vegetation targets.