etalon

Etalon is a term used in science with two principal meanings. In general, it denotes a reference standard or benchmark used for calibration and traceability. In optics, an etalon (often called a Fabry-Pérot etalon) is a specific type of optical resonator made of two parallel, partially reflecting surfaces that enclose a transparent cavity. Light entering the cavity undergoes multiple reflections and interference, producing a spectrum of sharp transmission peaks. The transmitted wavelengths satisfy the condition mλ = 2nd cos θ for normal incidence, where m is an integer, n is the refractive index, d is the cavity spacing, and θ is the angle of incidence. An etalon behaves as a narrow-band filter, with a free spectral range roughly given by λ²/(2nd cos θ) and a finesse that increases with mirror reflectivity. It is widely used for high-resolution spectroscopy, laser frequency stabilization, and selective wavelength filtering in optical instruments.

As a reference standard, etalon signifies a physical artifact or material used to verify the accuracy or calibration of measurement systems. In metrology, such reference samples or calibration artifacts establish traceability and ensure consistent performance across instruments and laboratories. In some contexts, etalon also describes a benchmark form or dataset that serves as a canonical reference within a given domain.

Etymology and usage: the word derives from the French étalon, meaning standard or benchmark, and has been adopted in English with the two related senses described above. In addition to optics and metrology, the term appears in other fields to denote a gold standard or canonical reference. See also Fabry-Pérot interferometer and reference standard.