An efficient selection by means of Field Radiometry, of a targets set of homogenous spectral response, named endmembers, is going to optimize the methodology of the mixed spectral response modelling, obtained in the registration process of an image from remote sensors in satellites. Additionally, the information obtained through the analysis of its spectral profiles can contribute to facilitate the task of labelling in a thematic classification process in remote sensing. There is little bibliography about the selection process of endmembers from Field Radiometry with the application of statistical models to this process. In this work, the main objective is the establishment of an efficient selection protocol that allows a selection of the "endmember", based on the different types of land covers placed in a selected geographical area. This established methodology will be sustained by the application of statistical models basically applied to the phase of sampling. Specifically, a new statistic related to the sampling process and based on t-Student distribution will be shown. Its use will allow to determine the optimal number of samples in each case.