Optical Specifications

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Almost every lens receives an antireflective coating to maximize transmission or image brightness, and to minimize ghost images. In fact, complex lens designs involving six or more elements could not realize their maximum potential if it were not for antireflective coatings.

For mirrors, coatings have replaced solid castings of polished metal in all but a few specialized applications. Mirror coatings perform with better reflectivity than solid metal mirrors, are lighter in weight and cost less to produce.

Mirror coatings reflect light, and antireflective coatings transmit light by reducing reflection. It is easy to forget that optical coatings are components because they always work with lenses, prisms, windows or solid mirror substrates, whose imaging properties occupy most of the systems-design effort.

From the perspective of development and manufacturing, coatings can be classified as either metallic, dielectric or hybrid and as single-layer or multi-layer. Metallic coatings are usually deposited by evaporating a metal, such as aluminum or gold, in a chamber so that the vapor condenses upon the substrate. Other methods include ion-beam-assisted deposition, sputtering and electrolytic deposition.

Dielectric coatings are made of dielectric materials (electrically non-conductive) such as magnesium fluoride (MgF2). Hybrid coatings consist of dielectric layers deposited upon a metallic base layer. Purely dielectric coatings may be single layer, or they may be stacked to form multi-layer coatings with improved characteristics. Hybrid coatings are, of course, always multi-layered.

Applications engineers usually classify coatings as reflective or antireflective and broadband or narrowband. Broadband coatings handle many colors, i.e., a broad range of wavelengths, whereas narrowband coatings are designed for one color, i.e., a narrow range of wavelengths.

A high-performance coating is not just one coating but several thin films deposited on top of each other. Any one of the layers might exhibit modest performance. Working together, however, a reflective stack of layers can achieve very high reflectivity (99.9%) and an antireflective stack can achieve very low reflectivity (0.1%).

Each layer is very thin, typically one-quarter to one-half the wavelength of light, or about 10 to 20 millionths of an inch. Design and manufacture of these multi-layer coatings is complex and difficult, but computers and vacuum deposition techniques make them cost effective.

Choice of a coating is most influenced by the reflectivity or transmission required at certain wavelengths, but altogether there are seven issues involved in the design and manufacture of a high-quality coating. These issues are:
1. Wavelength
2. Reflectivity or transmission
3. Polarization of light
4. Angle of incidence
5. Substrate
6. Intensity or power of light
7. Environmental conditions

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