A Ronchi grating consists of alternate dark and clear stripes. One design is a small frame with several evenly spaced fine wires attached.
Light is emitted through the Ronchi grating (or a single slit), reflected by the mirror being tested, then passes through the Ronchi grating again and is observed by the person doing the test. The observer's eye is placed close to the centre of curvature of the mirror under test looking at the mirror through the grating. The Ronchi grating is a short distance (less than 2 cm) closer to the mirror.[citation needed]
The observer sees the mirror covered in a pattern of stripes that reveal the shape of the mirror. The pattern is compared to a mathematically generated diagram (usually done on a computer today) of what it should look like for a given figure. Inputs to the program are line frequency of the Ronchi grating, focal length and diameter of the mirror, and the figure required. If the mirror is spherical, the pattern consists of straight lines.
The Ronchi test differs from the knife-edge test, requiring a specialized target (the Ronchi grating, which amounts to a periodic series of knife edges) and being more difficult to interpret. This procedure offers a quick evaluation of the mirror's shape and condition. It readily identifies a 'turned edge' (rolled down outer diameter of the mirror), a common fault that can develop in objective mirror making.
The figure quality of a convex lens may be visually tested using a similar principle. The grating is moved around the focal point of the lens while viewing the virtual image through the opposite side. Distortions in the lens surface figure then appear as asymmetries in the periodic grating image.
^V. Ronchi (1923) "Le frange di combinazioni nello studio delle superficie e dei sistemi ottici" [Combination fringes in the study of surfaces and optical systems], Rivista d'Ottica e Meccanica di precisione [Journal of Optics and Precision Mechanics], vol. 2, pages 9-35.
^Vasco Ronchi (1927) "Due nuovi metodi per lo studio delle superficie e dei sistemi ottici" (Two new methods for the study of surfaces and optical systems), Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Series 1, vol. 15, pages 1-50. [Note: This citation is somewhat misleading. Ronchi first published this article in 1923, but it was included in volume 15 of Annali della Scuola..., which wasn't issued until 1927.] Available on-line (in Italian): http://archive.numdam.org/ARCHIVE/ASNSP/ASNSP_1927_1_15_/ASNSP_1927_1_15__A1_0/ASNSP_1927_1_15__A1_0.pdf .
