1. A method of estimating background radiation for each of a plurality of regions of a target object comprising: providing an estimate of background radiation detected by a detector; measuring radiation scattered by the target object at the detector for each of a plurality of positions of the object with respect to the incident radiation; calculating, for each of the positions, an estimate of a wavefront at the detector; and determining, for each position, a coherent contribution to the wavefront from radiation scattered by the target object and a background contribution, wherein said background contribution is at least partly incoherent with the radiation scattered by the target object.