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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.
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