Microstepping Improves Ronchigrams

A team from the Universitat Politècnica de Catalunya in Terrassa, Spain, has developed a Ronchi test that increases the number of sampling points in a ronchigram and offers higher accuracy without increasing diffraction effects. In an experiment that they described in the Sept. 1 issue of Applied Optics, the researchers profiled a 1.76-sq-cm spherical surface with 7583 sampling points using the new technique. The standard test offers 72 sampling points for a 2.04-sq-cm spherical surface.

The improved test, which the researchers call microstepping, superimposes multiple ronchigrams, rather than increasing the frequency of the ruling lines on the Ronchi grating. The grating is rotated by 90° between measurements, and encoder motors move the grating with respect to the surface from one pair of measurements to the next.

The researchers note in a mathematical comparison that the conventional method remains sufficient for determining a surface's radius of curvature.