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8.1 Introduction Optical coatings are essential to control the radiation throughput of an optical element. Referring to the basic relation τ+ρ+α=1, where τ = transmittance, ρ = reflectance, and α = absorption, we are primarily interested in the first two contributors. Most of the time, we like to see either a high transmittance for refractive elements (lenses) or a high reflectance for reflectors (mirrors). In the applications for beamsplitters, we like to maximize the efficiency between the transmitted and the reflected energy, which means the absorption must be kept low. Since the subject of optical coatings is complex and extensive, the coverage will be limited to some fundamental behaviors of thin film coatings and to application remarks. Of special interest for the infrared spectrum are antireflection coatings and bandpass filters. Without these, the entire field of IR optics would be very much restricted. An important breakthrough came in 1935, when Alexander Smakula from Carl Zeiss in Jena, Germany, received his patent for “a method to increase the transmissivity of optical elements, by reducing the index of refraction on the boundary layers of these optical elements.” A few years later, in 1939, Walter Geffcken received his patent for transmission-type interference filters. 8.2 Effects at a Single Surface For radiation at normal incidence, the reflectance for a single surface is expressed by the fundamental Fresnel equation ρ s =(N−1 N+1 ) 2 , which remains a good approximation for tilt angles of up to 45°. Neglecting absorption, the transmitted portion is then τ s =1−ρ s =4N (N+1) 2 .
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