A system configured to detect defects in an inspection image generated by collecting signals arriving from an article, the system comprising a tangible processor which includes: (i) a distribution acquisition module, configured to acquire a distribution of comparison values, each of the comparison values being indicative of a relationship between a value associated with a pixel of the inspection image and a corresponding reference value; (ii) a fitting module, configured to fit to the distribution an approximation function out of a predefined group of functions; and (iii) a defect detection module, configured to: (a) set a defect detection criterion based on a result of the fitting; and to (b) determine a presence of a defect in the inspection image, based on the defect detection criterion.