Reconstruction of an object by which radiation is diffracted is disclosed. Specifically, the reconstruction includes: estimating the object shape; deriving a model diffraction pattern from the estimated shape; illuminating the object with radiation; detecting a diffraction pattern of radiation diffracted by the object; comparing the model diffraction pattern and the detected diffraction pattern; and determining the actual object shape from the difference between the model diffraction pattern and the detected diffraction pattern, wherein the model diffraction pattern is determined using Bloch Mode Expansion.