Editing Optical coating (section)

===High-reflection coatings===<!-- This section is linked from [[Laser construction]] -->
{{see also|Dielectric mirror}}
[[File:Woman wearing reflective sunglasses.jpg|left|thumb|A woman wears sunglasses featuring a highly reflective optical coating]]
[[image:Dielectric mirror diagram.svg|thumb|right|Diagram of a dielectric mirror. Thin layers with a high refractive index ''n''<sub>1</sub> are interleaved with thicker layers with a lower refractive index ''n''<sub>2</sub>. The path lengths ''l''<sub>A</sub> and ''l''<sub>B</sub> differ by exactly one wavelength, which leads to constructive interference.]]
High-reflection (HR) coatings work the opposite way to antireflection coatings. The general idea is usually based on the periodic layer system composed from two materials, one with a high index, such as [[zinc sulfide]] (''n''=2.32) or [[titanium dioxide]] (''n''=2.4), and one with a low index, such as [[magnesium fluoride]] (''n''=1.38) or [[silicon dioxide]] (''n''=1.49). This periodic system significantly enhances the reflectivity of the surface in the certain wavelength range called [[band-stop]], whose width is determined by the ratio of the two used indices only (for quarter-wave systems), while the maximum reflectivity increases up to almost 100% with a number of layers in the ''stack''. The thicknesses of the layers are generally quarter-wave (then they yield to the broadest high reflection band in comparison to the non-quarter-wave systems composed from the same materials), this time designed such that reflected beams ''constructively'' interfere with one another to maximize reflection and minimize transmission. The best of these coatings built-up from deposited dielectric lossless materials on perfectly smooth surfaces can reach reflectivities greater than 99.999% (over a fairly narrow range of wavelengths). Common HR coatings can achieve 99.9% reflectivity over a broad wavelength range (tens of nanometers in the visible spectrum range).

As for AR coatings, HR coatings are affected by the incidence angle of the light. When used away from normal incidence, the reflective range shifts to shorter wavelengths, and becomes polarization dependent. This effect can be exploited to produce coatings that polarize a light beam.

By manipulating the exact thickness and composition of the layers in the reflective stack, the reflection characteristics can be tuned to a particular application, and may incorporate both high-reflective and anti-reflective wavelength regions. The coating can be designed as a long- or short-pass filter, a bandpass or notch filter, or a mirror with a specific reflectivity (useful in lasers). For example, the [[dichroic prism]] assembly used in some [[camera]]s requires two dielectric coatings, one long-wavelength pass filter reflecting light below 500&nbsp;nm (to separate the blue component of the light), and one short-pass filter to reflect red light, above 600&nbsp;nm wavelength. The remaining transmitted light is the green component.

====Extreme ultraviolet coatings====
In the [[ultraviolet|EUV]] portion of the spectrum (wavelengths shorter than about 30&nbsp;nm) nearly all materials absorb strongly, making it difficult to focus or otherwise manipulate light in this wavelength range.  Telescopes such as [[TRACE]] or [[Extreme ultraviolet Imaging Telescope|EIT]] that form images with EUV light use multilayer mirrors that are constructed of hundreds of alternating layers of a high-mass metal such as [[molybdenum]] or [[tungsten]], and a low-mass spacer such as [[silicon]], [[vacuum deposition|vacuum deposited]] onto a substrate such as [[glass]].  Each layer pair is designed to have a thickness equal to half the wavelength of light to be reflected.  [[Constructive interference]] between scattered light from each layer causes the mirror to reflect EUV light of the desired wavelength as would a normal metal mirror in visible light.  Using multilayer optics it is possible to reflect up to 70% of incident EUV light (at a particular wavelength chosen when the mirror is constructed).