Method of investigating the wavefront of a charged-particle beam

A method of investigating a wavefront of a charged-particle beam that is directed from a source through an illuminator so as to traverse a sample plane and land upon a detector, an output of the detector being used in combination with a mathematical reconstruction technique so as to calculate at least one of phase information and amplitude information for the wavefront at a pre-defined location along its path to the detector, in which method: Said beam is caused to traverse a particle-optical lens system disposed between said sample plane and said detector; At a selected location in the path from said source to said detector, a modulator is used to locally produce a given modulation of the wavefront; In a series of measurement sessions, different such modulations are employed, and the associated detector outputs are collectively used in said mathematical reconstruction.