1. A calibration method for an imager having an adjustable filter arranged between a radiation source of the imager and a detector of the imager, the filter suitable to effect, by adjustment of the filter's setting, different non-homogenous energy intensity profiles of a radiation beam generated by the radiation source before passage of said beam through an object to be imaged, the method comprising: adjusting the filter for a plurality of different basis filter settings, one at a time; acquiring at the imager's detector a plurality of basis gain images, at least one gain image for each of the different filter settings; and synthesizing a target gain image from the one or more of the plurality of gain images, wherein the synthesizing step includes forming a linear combination from one or more of the plurality of basis gain images, wherein the linear combination involves weights each weight representing an individual attenuation strength of a respective ones of filter elements of the filter.