1. A method of operating an inspection system adapted to acquire an array of measurements, each measurement representative of a region of a plurality of regions of an item under inspection, the method comprising: a) moving the item under inspection relative to an X-ray data acquisition system to acquire the array of measurements; b) forming an array of computed values, each computed value representative of a region of the plurality of regions, the array formed according to a method: i) comprising, for each measurement, computing a plurality of weighting coefficients, the weighting coefficients computed so that the measurement is represented by a combination of a plurality of basis functions, each of the basis functions being weighted in the combination by a respective weighting coefficient of the plurality of weighting coefficients; ii) wherein: A. the plurality of basis functions comprises at least three basis functions, each basis function defining a relationship between a measured parameter and a parameter of operation of the X-ray data acquisition system; and B. each computed value in the array of computed values comprises a weighting coefficient computed for a measurement in the array of measurements, the weighting coefficients in the array being computed for the same basis function of the plurality of basis functions; c) making an inspection decision concerning the item under inspection based at least in part on the at least one array of computed values.