Measurements of BRDF allow to compare spectral signatures obtained in different laboratories in an optimal way. However its measure require well calibrated sources and quite expensive laboratory equipments. The concept of BRF (Bidirectional Reflectance Factor) allows a more simple, indirect, measurement of BRDF by using a reference sample (highly reflective so usually named "white reference WR") of known BRDF and two subsequent measurements of reflected radiance (one from the WR, one from the sample) obtained under identical illumination conditions. In these conditions results BRDF(sample)=BRF(sample)xBRDF(WR)