Scatterometry analysis for a patterned structure, in which a patterned structure model is provided having a selected number of virtual segment data pieces indicative of a respective number of segments of the patterned structure along Z-axis through the structure, the segment data pieces processed for determining a matrix comprising Z-axis derivatives of electromagnetic elds' response of the segment to incident eld based on Maxwell's equations' solution, transforming this matrix into an approximated response matrix corresponding to the electromagnetic eld interaction between two different points spaced along the Z-axis, the transformation preferably carried out by a GPU, and comprises embedding the matrix in a series expansion of the matrix exponential term, the approximated response matrices for all the segment data pieces are multiplied for determining a general propagation matrix utilized to determine a scattering matrix for the patterned structure.