Modeling of polychromatic MTF losses due to secondary effects in diffractive lenses

Although hybrid ( i.e. refractive-diffractive) surfaces are in common use in optical design there are several phenomena which affect design MTF that are not routinely modeled in current commercial versions of optical design software. Typically the details of the diffractive structure are not taken into account and rays are traced through the hybrid surface employing a vector grating equation which uses the phase gradient associated with the diffractive definition to calculate a local grating spacing and orientation and from this grating information a ’diffracted ray ’ angle. This geometrical-optics based procedure has limitations; (1)it considers only the design diffraction order, (2)it does not take into account the sub-aperturing effect whereby color correction is reduced along with zone count, and (3) the model used does not generate an exact blaze profile. In this paper we discuss progress in application of diffraction-based beam propagation tools in combination with a physical definition of the diffractive structure to more accurately model these secondary effects on design MTF. Results are given for some simple lenses and also the effects to be expected for a more complex zoom lens