A calibration method and corresponding object for calibrating an X-ray imaging system with respect to dark field imaging is disclosed. The calibration object (1) generically comprises a plurality of sections (10, 20), wherein at least a part of the sections comprises two different materials, respectively. One material (101, 201) in a corresponding section leads to attenuation of passing X-ray beams and the other material (102, 202) is a dark field active material leading to small-angle scattering signals of incident X-rays. The ratio of the two materials in one section varies from section to section. The calibration object can be used to calibrate an X-ray imaging system with respect to a non-linear behavior of the dark field visibility.
