NMF™ Application: High point-density mid-spatial frequency measurement on polished and ground surfaces

Next to polished surfaces, the optical probe of the NMF machines can also measure ground surfaces.

This brings the versatility, speed and accuracy to surfaces that can normally only be measured after a first polishing step. The high measurement speed associated with the motion platform and the optical probe, translate in the ability to acquire very high-density data which is ideal for characterizing mid-spatial frequency errors in aspherical and freeform surfaces.

Figure (1) shows the mid-spatial content of a  Ø200 mm polished aspherical mirror, with Z36 Zernike coefficients subtracted. The pointspacing is 0.1 mm, it has 2k x 2k points (3.3M datapoints) and takes 18 minutes. Although only a few nm rms, the grinding residue radiating outwards from the center is still visible, as is the center defect.

The machine can be used to zoom without loss of resolution, contrary to an interferometer. In Figure (2) a subsequent measurement is shown where the measurement area was reduced to the central  Ø40 mm. This measurement was done with 0.025 mm pointspacing, yields 1.6k x 1.6k points (2M datapoints) and takes less than 10 minutes. 

Figure (3) shows a similar measurement with 0.1 mm pointspacing on a  Ø180 mm concave sphere, but this time in ground condition with Ra = 1-2 um. With Z36 removed, it shows residual grinding cusps of 36 nm rms. Being able to measure this without having to first polish the surface is obviously a valuable advantage for process optimization.