Anti-reflective coating

Anti-reflective (AR) coatings are a type of optical coating applied to lenses and other devices to reduce reflection from optical surfaces. This often improves the efficiency of the system; this is especially important if the light wasted by reflections is difficult to obtain, i.e. from a large telescope, an obstacle on a dark roadway, or an intricate optics experiment. They also have qualitative benefits like making the eyes of a glasses-wearer more visible and reducing the glint from a sniper's scope.

Related Topics:
Optical coating - Lens - Reflection - Telescope - Optics - Glasses - Sniper

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Often, they are composed of transparent thin film structures, with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the many interfaces, and constructive interference in the corresponding transmitted beams. This makes the structure's performance change with wavelength and incident angle (as in diffraction), so that color effects often appear at oblique angles. A wavelength range must be specified when designing or ordering such coatings, but good performance can often be achieved for a relatively wide range of frequencies: usually a choice of IR, visible, or UV is offered.

Related Topics:
Thin film - Refractive index - Destructive interference - Diffraction - IR - UV

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The simplest AR coating consists of a single quarter-wave layer of transparent material whose refractive index is the square root of the substrate's refractive index. This gives a (theoretically) zero reflectance at the center wavelength and a higher reflectance at higher and lower wavelengths.

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This comes directly from the expression for Fresnel reflectance, the amplitude or intensity of reflected light at the interface between two media. The reflectance amplitude is (n1-n2)/(n1+n2) and the intensity is the square of the amplitude.

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For example the use of magnesium fluoride film (n = 1.38) on crown glass (n = 1.52) forms a near-perfect AR having a reflectance of ((1.38-1.52)/(1.38+1.52))^2 = 0.02% compared to the reflectance of glass in air which is (((1.0-1.52)/(1.0+1.52))^2 = 4.26%. In this example, if the coating were designed for use in the mid-visible range at 550 nm, the MgF2 film would have a physical thickness of (550*nm)/4/1.38 = 99.6 nm.

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Multiple layers can be used to broaden the band over which the AR operates, to provide near-zero reflectance at multiple wavelengths, or to optimize for performance at desired angles of incidence.

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