How to replace diffractive optical elements for color correction by refractive lenses from specific materials

Axial as well as lateral color aberrations in broadband optical systems can efficiently be corrected by using diffractive optical elements (DOEs). However, DOEs such as kinoforms consisting of only one material are not suitable for high-quality optics because of straylight in spurious diffraction orders. The amount of stray light can significantly be reduced by using so-called efficiency-achromatized DOEs (EA-DOEs), which consist of a material pair whose refractive indices fulfill a specific material condition. Unfortunately, manufacturing of EA-DOEs is very challenging because the grating structures are subject to tight fabrication tolerances. Therefore, only few broadband optical systems with EA-DOEs are on the market. Here we show that DOEs in broadband optical systems can surprisingly be replaced by refractive doublets made of materials that fulfill the material condition for EA-DOEs. As opposed to the EA-DOEs themselves, these purely refractive replacements do not suffer from stray light. In addition, from a theoretical point of view, our result allows for understanding the effect of DOEs in optical designs by classical refractive optical design theory.