Method and a polarimetric measurement device of a planar object carrying patterns repeated regularly and forming the lines of a grid. A first measurement is carried out at zero order, under an angle of incidence θ1 and for a first azimuthal angle φ1, a second measurement at least is carried out at zero order, under an angle of incidence θ2 and for a second azimuthal angle φ2, the polarization of the incident beam is modulated and the polarization of the reflected beam is analyzed for each measurement, theoretical polarimetric data is calculated for a model object of the real object, the model object including parameters adjustable using a formalism of electromagnetism. An iterative comparison of the measurements is conducted with the theoretical polarimetric data for different values of the adjustable parameters.