What's it about?

Magnetorheological Finishing (MRF) offers an effective way for correcting transmitted wavefront of lenses. Traditional feedback based methods of polishing and testing a lens until it is tolerance made direct correction of transmitted wavefront difficult if not impossible. Being a feedforward, deterministic process, MRF makes direct targeting and correction of transmitted wavefront possible. The lens must have low spherical aberration, and full aperture metrology is required. The metrology will show the sum of all errors, and all errors are corrected simultaneously by fixing deterministically the transmitted wavefront. Optimax Systems uses this MRF process to correct aspheric lenses in transmission to subwavelength errors.

If surface form error is tested in reflection, in-process metrology is rather simple. Using a test plate or a tower interferometer, the polisher can rapidly assess the progress of the polishing process. Base the test on Transmitted Wavefront Error (TWE) and things become less direct. Transmission testing requires a more specialized setup, and this setup is often in a different location than the polishing equipment. Since TWE testing is not as fast, the manufacturer will often "hedge bets" by making the parts to demanding and therefore expensive reflected tolerances and buy higher quality and therefore expensive material...