Nearly index-matched optical elements are described that have application to aspherical, diffractive, and hybrid refractive–diffractive elements. Owing to the small difference in index, tolerances on the interfacial surface profile are much looser than for conventional aspherical and diffractive elements. A condition on both the index and the dispersion is described that results in an achromatic phase, thereby allowing for diffractive elements with high efficiency over broad spectral bands. Properties of the elements are described and compared with those of conventional diffractive elements. Analytical results and experimental measurements with an inexpensive element with 40 waves of asphericity are presented.
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