A Ronchi ruling, Ronchi grating, or Ronchi mask, named after the Italian physicist Vasco Ronchi,[1] is a constant-interval bar and space square-wave optical target or mask. The design produces a precisely patterned light source by reflection or illumination, or a stop pattern by transmission, with precise uniformity, spatial frequency, sharp edge definition, and high contrast ratio.
Ronchi rulings are typically manufactured through photolithographic deposition of metallic chromium on a substrate, which yields a precise, nearly 100% contrast pattern. For a reflective or illuminated type, dark stripes are printed on a diffusely reflecting or translucent substrate, such as a square of white ceramic material or opal glass. For a transmissive type, opaque stripes are printed on a transparent glass substrate. A transmissive type may be readily modified to act as an illuminated type by stacking a reflective object behind it.
A test target in the Ronchi pattern provides a precise signal for testing resolution, contrast, distortion, aberrations, and diffraction in optical imaging systems.[2] In constructing telescope mirrors, the Ronchi test procedure evaluates the quality of the surface figure and provides guidance on refining the figure. The ruled divisions in a linear encoder are in the form of Ronchi ruling, which provides the basic precision of the measurements.
A single Ronchi device provides a pattern that is periodic in one dimension. Stacking a pair of Ronchi transmission elements produces a two-dimensional array of periodic rectangular or rhomboidal apertures. By varying the translation, rotation, and chosen frequency of one element against the other, a wide assortment of laboratory test patterns can be generated with the paired arrangement.
Telescopic focusing aids such as the Bahtinov mask consist of multiple adjacent Ronchi transmission stop patterns. Inserting a Ronchi mask in the entrance aperture produces diffraction effects in the image, which visibly change with the focusing condition.