Mirau

The Mirau interferometer is a type of interferometric objective used in optical profilometry to measure surface topography with high vertical resolution. It is commonly integrated into a microscope objective, enabling non-contact, high-precision measurements on small areas such as microstructured or polished surfaces. The key feature of the Mirau design is a reference mirror and a beam-splitting element located at the distal end of the objective, forming a compact interferometric element within the lens. Light is reflected from the sample surface and from the reference surface inside the objective, producing an interference signal that can be analyzed to determine surface height.

In operation, illumination passes through the objective to the sample. Light reflected from the sample and from the reference surface interferes as it returns through the objective to the detector. White-light (broadband) illumination is often used to produce short coherence fringes, which helps isolate a single interference location corresponding to the surface under test. The collected interferograms are processed by phase-shifting or Fourier techniques to extract a quantitative height map of the surface, typically with sub-miqurometer vertical resolution depending on the system.

The Mirau configuration is valued for its compactness, ease of integration with standard microscopy setups, and suitability for inspecting small-area, smooth to moderately rough surfaces, including metals, semiconductors, coatings, and micro-electromechanical systems. It is commonly compared with other interferometer objectives, such as Linnik or Michelson configurations, which place reference or test optics outside the objective.

See also: Linnik interferometer, Michelson interferometer, Wyant interferometer.