A method for determining a level of a substance in a material is disclosed. The method comprises: obtaining a first measured spectrum from a first noninvasive sensor, the first measured spectrum relating to a first characteristic of the material obtaining a second measured spectrum from a second noninvasive sensor, the second measured spectrum relating to a second characteristic of the material calculating a first intermediate estimate ŷx of the concentration of a chemical constituent in the material based on the first measured spectrum calculating a second intermediate estimate ŷ2 of the concentration of a chemical constituent in the material based on the second measured spectrum determining a first and second residual spectrum r1, r2, the residual spectra defined as the deviation between the measured spectrum and an estimated spectrum, the estimated spectrum being the component of the measured spectrum corresponding to the intermediate estimate of the concentration of the chemical constituent and calculating an improved estimated concentration of the chemical constituent in the material based on the first intermediate estimate ŷ1, the second intermediate estimate ŷ2 and the residual spectra r1, r2.Linvention porte sur un procédé pour déterminer un taux dune substance dans un matériel. Le procédé comprend les étapes consistant : à obtenir un premier spectre mesuré à partir dun premier capteur non invasif, le premier spectre mesuré se rapportant à une première caractéristique du matériel à obtenir un second spectre mesuré à partir dun second capteur non invasif, le second spectre mesuré se rapportant à une seconde caractéristique du matériel à calculer une première estimation intermédiaire ŷx de la concentration dun constituant chimique dans le matériel sur la base du premier spectre mesuré à calculer une seconde estimation intermédiaire ŷ2 de la concentration dun constituant chimique dans le matériel sur la base du second spectre mesuré à déterminer un premier
