Filters, Optical

Abstract This article reviews the performance of optical filters for the 0.005 to 1000 µm spectral region. The filters described operate in transmitted or reflected light and are intended for use in free‐space optics. The theory of two or more filters placed in series or in parallel is presented. The effect of surface reflections on the performance of the filters is discussed. Optical filters can be based on many different physical principles. Brief explanations are given of the modes of operation of filters based on absorption, reflection, interference in thin films, holography, scattering, diffraction, and interference of polarized light. The principal advantages and disadvantages of filters based on these phenomena are discussed. Filters can also be classified according to the functions that they are intended to perform. In this article the following generic filter types are considered: antireflection coatings, neutral attenuators, narrow band, medium‐band and wideband reflectors, short‐ and long‐wavelength cutoff filters, narrowband transmission filters, rejection filters, neutral‐ and color‐selective beam splitters and correction or gain flattening filters. For each of the above filter types, spectral transmittance or reflectance curves are presented that correspond to representative filters based on a number of the above‐mentioned physical principles. Information is provided on how to specify the performance of the various filter types.