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Excerpt from Field Guide to Interferometric Optical Testing
A Ronchi test uses a low-frequency grating, called a Ronchi ruling, in place of the knife edge in a Foucault test or the wire in the wire test. For a perfect lens, the pattern observed is straight lines, with fewer lines closer to focus.
The Ronchi ruling is usually illuminated by a diffuse source from behind and acts as a multiple slit source. A white light source may be used. The Ronchi ruling acts as a diffraction grating, creating multiple images of the test surface. The orders from coarse gratings (fewer than 10 lines/mm) will overlap and cause only a slight perturbation of the shadow pattern. Orders from high-frequency gratings (more than 100 lines/mm) will likely be separated. The patterns obtained from using middle-frequency gratings are difficult to analyze.