High-Reflection Coatings
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High-reflection coatings may be applied to the outside of a component, such as a flat piece of glass, to produce a first-surface mirror. Alternately, they may be applied to an internal surface to produce a second-surface mirror, such as a prism. High-reflection coatings can be categorized as either metallic or dielectric coatings. |
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Metallic Coatings Metallic coatings are used primarily for mirrors and are not classified as thin films in the strictest sense. They do not rely on principles of interference, but rather on the optical properties of the coating material. However, metallic coatings are often over-coated with thin dielectric films to increase reflectance over a desired range of wavelengths or angles of incidence. In these cases, the metallic coating is said to be “enhanced.” Overcoating metallic coatings with a hard, single, dielectric layer of half-wave optical thickness improves abrasion and tarnish resistance but only marginally affects optical properties. Depending on the dielectric used, such overcoated metals are referred to as durable, protected, or hard coated. The main advantages of metallic coatings are broadband spectral performance, insensitivity to angle of incidence and polarization, and low cost. Their primary disadvantages are lower durability, lower reflectance, and lower damage threshold. Dielectric Coatings High-reflectance dielectric layers work on the same principles as dielectric antireflection coatings. Quarter-wave thicknesses of alternately high- and low-refractive index materials are applied to the substrate to form a dielectric multilayer. By choosing materials of appropriate refractive indices, the various reflected wavefronts can be made to interfere constructively in order to produce a highly efficient reflector. The peak reflectance value is dependent upon the ratio of refractive indices of the two materials, as well as the number of layer pairs. Increasing either increases the reflectance. Reflectance of such films can easily be made to exceed the highest metallic reflectances over limited wavelength intervals. Such films are effective for both s- and p-polarization components and over a wide angle-of-incidence range. At oblique incidence, reflectance is markedly reduced. Because of the materials chosen for the multilayer, durability and abrasion resistance of such films are normally superior to those of metallic films. |
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