. . . "phase zones of the optical element, [0020] a binary amplitude transmittance of the masks being defined as: T M = t ??? { sin ??? ??? ( P ??? ??? Mod ??? [ ?? 2 ??? ?? ] ??? d d 0 ) } [0021] where P is a parameter which is defined by a serial number of a mask, i.e. P=P(M). and which determines a number of boundaries of phase levels provided in each phase zone by this mask, and d() is a maximal achi" .