1. A method of determining an applicable threshold for determining the critical dimension of at least one category of patterns imaged by atomic force scanning electron microscopy, said method comprising: from a plurality of patterns, acquiring a pair of images for each pattern, the acquiring comprising obtaining a first image by an imaging instrumentation implementing an atomic force scanning electron microscopy technique which one seeks to determine the threshold; obtaining a second image by a reference imaging instrumentation implementing a different technique from said scanning electron microscopy technique; for each pair of images obtained for each of the patterns: determining a reference critical dimension via the image obtained by said reference imaging instrumentation; determining an empirical threshold applicable to the image obtained by said atomic force scanning electron microscopy instrumentation such that said empirical threshold substantially corresponds to said reference critical dimension; determining a threshold applicable to at least one category of patterns, said threshold being determined from a plurality of empirical thresholds, each empirical threshold corresponding to a pair of images.