Design of achromatic diffractive lenses

Diffractive lenses can be very thin and light. They usually suffer from chromatic aberration and work only over a narrow range of wavelengths but so-called achromatic diffractive lenses have recently attracted attention. Ways in which the profile of such lenses can be chosen to optimize either the Strehl ratio or the efficiency are compared and the extent to which the performance of the resulting lens designs approaches theoretical limits is investigated. Simple rules are given for the average Strehl ratio and efficiency expected in certain conditions. In other cases they provide approximate guidelines. Some reported simulated and measured efficiencies greatly exceed those that appear credible. This is attributed to failure to take into account radiation scattered to large off-axis angles or to inadequate sampling of the radial profile.