UV attenuation factor f_UV as a function of the predicted ratio of the IR/UV luminosity. A tight relation, fitted by Eq. (1) and shown by the dashed line, is obtained independently of the assumed SF history and of the extinction law. For comparison the relation determined by Boquien et al. (2012) is shown by the dotted line. The corresponding attenuation A_V assuming the Calzetti law is shown on the right scale.