1. A method comprising: determining an unknown amplitude A of noise that contaminates at least some of the pixels of a digital image T of an arbitrary target, the amplitude being possibly different in different digital images, the determination being based on the digital image T and on a reference digital image that is representative of the contaminating noise and obtained by the same sensor, the determination being made from a decorrelation condition: Correlation (T−A*{the reference digital image}, {the reference digital image})=0, the determination being made over pixels in some regions of T and the reference digital image, to estimate the amplitude of the noise for each of at least some pixels of T.