Ronchi ruling

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.

A Ronchi ruling

Manufacturing

Ronchi rulings are typically manufactured through photolithographic deposition of metallic chromium on a substrate, which yields a precise, near-100 percent 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.

Applications

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.

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See also

References
  1. http://www.plicht.de/chris/22ronchi.htm
  2. "Testing and Targets". Edmund Optics Imaging Resource Guide. Retrieved October 2, 2018.
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