Machining raw material by a lathing operation is the most common method for making the optical part of a polymethyl-methacrylate (PMMA) IOL. Spherical surfaces have been used for many years to achieve this goal using the radioscope for in-process control. For a few years, advanced control techniques such as Talysurf or Fizeau interferometer have been used to achieve improved surface quality and ideal lathe setup. Machining an optimized conic surface would appear to be more difficult to control. CAD drawings of a conic surface of an optimized IOL versus a spherical surface show a difference in vault of a few microns, which can be seen easily on interferometer display fringes. It is then easier to control the asphericity of a lathed surface by achieving tolerance in fringes when the dome is within 1 to 4 fringes of a perfectly spherical surface. This method has been validated with several thousand lenses using the USAF target method as a routine control and has been shown to produce increased accuracy of lens power and resolution efficiency.
Patrick Meunier, Patrick Meunier,
"Achievement in machining intraocular lenses by interferometry control", Proc. SPIE 2127, Ophthalmic Lens Design and Fabrication II, (26 May 1994); doi: 10.1117/12.176841; https://doi.org/10.1117/12.176841