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Encyclopedia > Optical coating
Optically coated mirrors and lenses

An optical coating is a thin layer of material placed on an optical component such as a lens or mirror which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and photographic lenses. Another type is the high-reflector coating which can be used to produce mirrors which reflect greater than 99% of the light which falls on them. More complex optical coatings exhibit high-reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film optical filters. Various optically coated mirrors and lenses File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Thin-film optics is the branch of optics which deals with very thin structured layers of different materials. ... A lens is a device for either concentrating or diverging light, usually formed from a piece of shaped glass. ... A mirror is a reflective surface that is smooth enough to form an image. ... The word reflection (also spelt reflexion in British English) can refer to several different concepts: In mathematics, reflection is the transformation of a space. ... In communications, transmission is the act of transmitting electrical messages (and the associated phenonomena of radiant energy that pass through media). ... Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the human eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. ... Photographic lens A photographic lens (or more correctly, objective) is an integrated system comprising one or more simple optical lens elements, used for an optical telescope, camera or microscope. ... The wavelength is the distance between repeating units of a wave pattern. ... In optics, the term dichroic has two related but distinct meanings. ... Thin-film optical filters are optical filters which allow light of specific wavelengths to pass through unencumbered, while blocking other light from passing (to some extent). ...

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Types of coating

The simplest optical coatings are thin polished layers of metals, such as aluminium, which are deposited on glass substrates to make mirror surfaces. The metal used determines the reflection characteristics of the mirror; aluminium is cheapest and most common coating, and yields a reflectivity of around 88%-92% over the visible wavelength range. More expensive is silver, which has a reflectivity of 95%-99% even into the far infrared, but suffers from decreasing reflectivity (<90%) in the blue and ultraviolet spectral regions. Most expensive is gold, which gives excellent (98%-99%) reflectivity throughout the infrared, but limited reflectivity below 550 nm wavelength, resulting in the typical gold colour. Metal - Wikipedia, the free encyclopedia /**/ @import /skins-1. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... General Name, Symbol, Number silver, Ag, 47 Chemical series transition metals Group, Period, Block 11, 5, d Appearance lustrous white metal Atomic mass 107. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... A nanometre (American spelling: nanometer, symbol: nm) is 1. ...


By controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the surface, resulting in a half-silvered mirror.


The other major type of optical coating is the dielectric coating. These are thin layers of materials such as magnesium fluoride and calcium fluoride which are deposited onto the optical substrate. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmitivity of the coating to produce almost any desired characteristic. Reflection coefficients of surfaces can be reduced to less than 0.2%, producing an antireflection (AR) coating. Conversely, the reflectivity can be increase to greater than 99.99%, producing a high-reflector (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% and transmits 10% of the light that falls on it; such mirrors are often used in lasers. Alternately, the coating can be designed such that the mirror reflects light only in a narrow band of wavelengths, producing an optical filter. The electrons in the molecules shift toward the positively charged left plate. ... Magnesium fluoride (MgF2) is a white crystalline salt composed of one magnesium ion and two fluoride ions. ... Calcium fluoride (CaF2) is an insoluble ionic compound of calcium and fluorine. ... Laser (US Air Force) A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical device which uses a quantum mechanical effect called stimulated emission (discovered by Einstein while researching the photoelectric effect) in order to generate a coherent beam of light from a lasing medium of controlled purity... Coloured and Neutral Density filters An optical filter is a device which selectively transmits light having certain properties (often, a particular range of wavelengths, that is, range of colours of light, or polarizations), while blocking the remainder. ...


The versatility of dielectric coatings leads to their use in many scientific optical instruments (such as lasers, optical microscopes, refracting telescopes, and interferometers) as well as consumer devices such as binoculars, spectacles, and photographic lenses. 1852 microscope Compound microscope made by John Cuff in 1750 A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... The 50 cm refractor at Nice Observatory. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... Binoculars A set of binoculars (from Latin, bi-, two-, and oculus, eye) is a hand-held tool used to make distant objects appear closer by passing the image through two adjacent series of lenses, and erecting prisms. ...


Antireflection coatings

Antireflection coatings are useful because of an particular property of optics. Whenever a ray of light moves from one medium to another (e.g., when light enters a sheet of glass after travelling through air), some portion of the light is reflected from the surface (known as the interface) between the two media. This can be observed when looking through a window, for instance, where a (weak) reflection from the front and back surfaces of the window glass can be seen. The strength of the reflection depends on the refractive indices of the two media as well as the angle of the surface to the beam of light. The exact value can be calculated using the Fresnel equations. See also list of optical topics. ... The word medium has a number of uses: Medium is an average or mean in a range of sizes or conditions. ... The materials definition of a glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form. ... Air is a name for the mixture of gases present in the Earths atmosphere. ... Highly decorative Window in a Japanese Onsen in Hakone A window is an opening in an otherwise solid, opaque surface through which light and air can pass. ... The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ... The Fresnel equations, deduced by Augustin-Jean Fresnel, describe the behaviour of light when moving between media of differing refractive indices. ...


When the light meets the interface at normal incidence (i.e. perpendicularly to the surface), the amount of light reflected is given by the reflection coefficient or reflectance, R:

,

where n0 and nS are the refractive indices of the first and second media, respectively. The value of R varies from 0.0 (no reflection) to 1.0 (all light reflected) and is usually quoted as a percentage. Complimentary to R is the transmission coefficient or transmitance, T. If the effects of absorption are neglected, then the value T is always 1-R. Thus if a beam of light with intensity I is incident on the surface, a beam of intensity RI is reflected, and a beam with intensity TI is transmitted into the medium. A percentage is a way of expressing a proportion, a ratio or a fraction as a whole number, by using 100 as the denominator. ... Absorption has a number of meanings: In physics, absorption is a process in which particles of some sort encounter another material and are taken up by or even disappear in it. ... In physics, intensity is a measure of the time-averaged energy flux. ...

Reflection and transmission of an uncoated and coated surface

For a typical situation with visible light travelling from air (n0≈1.0) into common glass (nS≈1.5), value of R is 0.04, or 4%. Thus only 96% of the light (T=1-R=0.96) actually enters the glass, and the rest is reflected from the surface. The amount of light reflected is known as the reflection loss. (Note in a real situation with light travelling through a window, light is reflected both when going from air to glass and at the other side of the window when going from glass back to air. The size of the loss is the same in both cases. Light also may bounce from one surface to another multiple times, being partially reflected and partially transmitted each time it does so. In all, the combined reflection coefficient is given by 2R/(1+R). For glass in air, this is about 7.7%.) Image File history File links Reflection and transmission coefficients of an optical coating File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...


The simplest form of antireflection coating was observed by Lord Rayleigh in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces. This effect can be explained by envisioning a thin layer of material with refractive index n1 between the air (index n0) and the glass (index nS). The light ray now reflects twice, once from the surface between air and the thin layer, and once from the layer-to-glass interface. See also Rayleigh fading Rayleigh scattering Rayleigh number Rayleigh waves Rayleigh-Jeans law External links Nobel website bio of Rayleigh About John William Strutt MacTutor biography of Lord Rayleigh Categories: People stubs | 1842 births | 1919 deaths | Nobel Prize in Physics winners | Peers | British physicists | Discoverer of a chemical element ... 1886 is a common year starting on Friday (click on link to calendar) // Events January 18 - Modern field hockey is born with the formation of The Hockey Association in England. ...


From the equation above, and the known refractive indices, reflectivities for both interfaces can be calculated, and denoted R01 and R1S, respectively. The transmission at each interface is therefore T01 = 1-R01 and T1S = 1-R1S. The total transmitance into the glass is thus T1ST01. Calculating this value for various values of n1, it can be found that at one particular value of optimum refractive index of the layer, the transmittance of both interfaces is equal, and this corresponds to the maximum total transmittance into the glass.


This optimum value is given by the geometric mean of the two surrounding indices, i.e.: The geometric mean of a set of positive data is defined as the product of all the members of the set, raised to a power equal to the reciprocal of the number of members. ...

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For the example of glass (nS≈1.5) in air (n0≈1.0), this optimum refractive index is n1≈1.225. The reflection loss of each interface is approximately 1.0% (with a combined loss of 2.0%), and an overall transmission T1ST01 of approximately 98%. Therefore an intermediate coating between the air and glass can reduce the reflection loss by half of its normal (uncoated) value.


The use of an intermediate layer to form an antireflection coating can be thought of as analoguous to the technique of impedance matching of electrical signals. (A similar method is used in fiber optic research where an index matching oil is sometimes used to temporarily defeat total internal reflection so that light may be coupled into or out of a fiber.) Further reduced reflection could in theory be made by extending the process to several layers of material, gradually blending the refractive index of each layer between the index of the air and the index of the substate. Some factual claims in this article or section need to be verified. ... Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ... The larger the angle to the normal, the smaller is the fraction of light transmitted, until the angle when total internal reflection occurs. ...


Practical antireflection coatings, however, rely on a intermediate layer not only for its direct reduction of reflection coefficient, but also use the interference effect of a thin layer. Assume the layer thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light deep (λ/4), forming a quarter-wave coating. If this is the case, the incident beam I, when reflected from the second interface will travel exactly half its own wavelength further than the beam reflected from the first surface. If the intensities of the two beams, R1 and R2, are exactly equal, then since they are exactly out of phase, they will destructively interfer and cancel each other. Therefore, there is no reflection from the surface, and all the energy of the beam must be in the transmitted ray, T. Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... Waves with the same phase Waves with different phases The phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). ...

Interference in a quarter-wave antireflection coating

Real coatings do not reach perfect performance, though they are capable of reducing a surface's reflection coefficient to less than 0.1%. Practical details include correct calculation of the layer thickness; since the wavelength of the light is reduced inside a medium, this thickness will be λ0 / 4n1, where λ0 is the vacuum wavelength. Also, the layer will be the ideal thickness for only one distinct wavelength of light. Other difficulties include finding suitable materials, since few useful substances have the required refractive index (n≈1.23) which will make both reflected rays exactly equal in intensity. Magnesium fluoride (MgF2) is often used, since this is hard-wearing and can be easily applied to substates using physical vapor deposition, even though its index is higher than desirable (n=1.38). Image File history File links Interference in a λ/4 coating File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Magnesium fluoride (MgF2) is a white crystalline salt composed of one magnesium ion and two fluoride ions. ... Physical vapor deposition (PVD) is a technique used to deposit thin films of various materials onto various surfaces (e. ...


Further reduction is possible by using multiple coating layers, designed such that relections from the surfaces undergo maximum destructive interference. One way to do this is to add a second quarter-wave thick higher-index layer between the low-index layer and the substrate. The reflection from all three interfaces produces destructive interference and antireflection. Other techniques use varying thicknesses of the coatings. By using two or more layers, each of a material chosen to give the best possible match of the desired refractive index and dispersion, broadband antireflection coatings which cover the visible range (400-700 nm) with maximum reflectivities of less than 0.5% are commonly achievable. In optics, dispersion is a phenomenon that causes the separation of a wave into spectral components with different frequencies, due to a dependence of the waves speed on its frequency. ...


The exact nature of the coating determines the appearance of the coated optic; common AR coatings on eyeglasses and photographic lenses often look somewhat bluish (since they reflect slightly more blue light than other visible wavelengths), though green and pink tinged coatings are also used.


If the coated optic is used at non-normal incidence (i.e. with light rays not perpendicular to the surface), the antireflection capabilities are degraded somewhat. This occurs because a beam travelling through the layer at an angle "sees" a greater apparent thickness of the layer. The effect of this is that the anti-reflection band of the coating tends to move to longer wavelengths as the optic is tilted. Coatings can also be designed to work at a particular angle; beam splitter coatings are usually optimised for 45° angles. Non-normal incidence angles also usually cause the reflection to be polarization dependent. A beam splitter is an optical device, that splits a beam of light in two. ... This article treats polarization in electrodynamics. ...


High-reflection coatings

High-reflection (HR) coatings work the opposite way to antireflection coatings. Instead of a low refractive index layer, a material with a high index, such as zinc sulfide (n=2.32) or titanium dioxide (n=2.4) is used. This increased the reflection coefficient of the surface. By using several such layers, interspersed with low-index layers, a coating stack is built up which gives maximum reflectance. The thicknesses of the layers are generally quarter-wave, this time designed such that reflected beams constructively interfere with one another to maximise reflection and minimise transmission. The best of these coatings 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 (hundreds of nanometres). Zinc sulfide is a chemical compound with the formula ZnS. Zinc sulfide is a white to yellow colored powder or crystal. ... Titanium dioxide, also known as titanium(IV) oxide or titania, is the naturally occurring oxide of titanium, chemical formula TiO2. ...


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 longer 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 cameras requires two dielectic coatings, one long-wavelength pass filter reflecting light below 500 nm (to separate the blue component of the light), and one short-pass filter to reflect red light, above 600 nm wavelength. The remaining transmitted light is the green component. A dichroic prism is a prism that splits light into two beams of differing wavelength (colour). ... A camera is a device used to take pictures (usually photographs), either singly or in sequence, with or without sound, such as with video cameras. ...


External Links

  • Java demonstration of anti-reflection coating

 
 

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