Anti-reflective coating: Encyclopedia - Anti-reflective coating

Anti-reflective coating

Anti-reflective or antireflection (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.

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.

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 theoretically gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broad band around the center.

The most common type of optical glass is crown glass, which has an index of refraction of about 1.52. An optimum single layer coating would have to be made of a material with an index equal to about 1.23. Unfortunately, there is no material with such an index that has good physical properties for an optical coating. The closest 'good' material available is magnesium fluoride, MgF₂, with an index of 1.38. On crown glass, this gives a reflectance of about 1%, which is much better than the 4% reflection from bare glass. MgF₂ coatings perform much better on higher-index glasses, especially those with index of refraction close to 1.9. MgF₂ coatings are commonly used because they are cheap, and when they are designed for a wavelength in the middle of the visible band they give decent antireflection over the entire band.

By using multiple layers, it is possible to obtain reflectivities as low as 0.1% at a single wavelength. Coatings that give very low reflectivity over a broad band can also be made, although these are complex and relatively expensive. Coatings can also be made with special characteristics, such as near-zero reflectance at multiple wavelengths, or optimum performance at angles of incidence other than 0°.

See also

Optical coating

Category: Optics

Adapted from the Wikipedia article "Anti-reflective coating", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki