Radiometric normalization is a vital stage in the pre-processing of multi-temporal imagery. It aims to insure a reliable exploitation of images acquired under different imaging conditions. In this study, we investigate whether a relative normalization can replace atmospheric correction. The investigation was done using a time series of eighteen SPOT 5 images acquired over Reunion Island and intended to be used for sugarcane monitoring. An automatic method for relative normalization is introduced, and its results are compared to atmospherically corrected data. The relative method is based on the reflectances of invariant targets (IT) that are selected automatically. The atmospheric correction is carried out by the 6S code. The comparison was performed a) by using a set of manually selected invariant targets (MSIT), and b) by assessing the NDVI behavior of a set of sugarcane fields. An excellent correlation is obtained between relatively and atmospherically corrected data: the coefficient of determination (R²) is higher than 0.96 for all spectral bands and for the NDVI. Moreover, a comparable impact is observed on the temporal profiles of MSIT and on the NDVI trajectories of sugarcane field.