The knowledge of solar irradiance and its temporal variations is an important issue for climat modeling and space weather. However, mesurements of TSI and SSI are difficult and suffer from weaknesses (as, for example, insufficient temporal and spectral coverage) ; it is therefore necessary to model the TSI and SSI to obtain complete time series and to validate the observations. Irradiance models use magnetograms to distinguish the different magnetic structures of the Sun. The contribution of each structure is determined using a theoretical spectrum for each of these structures. The goal of our work is to put observational constraints on these theorical spectra at UV wavelengths. To do so, we use magnetograms from the HMI instrument onboard SDO and solar images at 30.4 nm, 160 nm and 170 nm from the AIA instrument, also onboard SDO. These data enable us to study the contrast of magnetic structures in function of their position on the Sun and of the value of the magnetic field. We also take advantage of this work to study the evolution in time of the noise of the solar magnetogram.