etalon

An etalon is an optical device with two partially-reflective surfaces spaced a specific distance apart that cancels EMR at some wavelengths and strengthens it at others. It can consist of partially-transparent mirrors with an air or vacuum gap between them, or the two surfaces of a single transparent plate. The term is essentially another name for a Fabry-Pérot interferometer (FPI), or is a name for an FPI's inner optics.