A multilevel diffractive optical element comprising a base and a plurality of phase zones having phase levels of a substantially identical height h, each phase zone being defined by a local modulation depth d and a local number of phase levels &xgr;=d/h, the local number of phase levels &xgr; per phase zone being a real number, an integer of which defines the number of complete levels in the phase zone, which complete levels have identical widths, and a fraction of which, in at least one phase zone, defines an incomplete level which is narrower than the complete levels, said local number of phase levels &xgr;=&xgr;(x, y) varying among different phase zones so as to provide corresponding variation of said local modulation depth d=d(x, y) whereby local diffraction efficiencies and consequently an overall diffraction efficiency of the optical element is arbitrarily controlled.