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Ronchi Test

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.

For a wavefront with third-order spherical aberration (W₀₄₀), the Ronchi patterns are as follows. These examples neglect diffraction patterns.

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.

The advantages of the Ronchi test are its simplicity and the fact that white light can be used. Unfortunately, the diffraction effects are very troublesome, limiting the accuracy of this test.