The sensors of spectral imaging devices have often large sizes of pixel in which numerous materials contribute to the spectrum obtained at a single pixel. In most of applications we need to identify the constituent materials present along with their proportions in such mixed pixels. The process by which the mixed pixel is decomposed into a collection of constituent spectra (endmembers) and a set of corresponding fractions (abundance, indicate the proportion of each endmember present in the pixel). It is not an easy task because of its ill-posed inverse problem nature due to model inaccuracies, noise in the observation, environmental conditions, data size and endmember variability.