Anti-reflective coating
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Anti-reflective or antireflection (AR) coatings are a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also improves the contrast of the image by elimination of stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating on eyeglass lenses that makes the eyes of the wearer more visible, or a coating to reduce 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 alternating layers of a low-index material like silica and a higher-index material, 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°.
An additional category of antireflection coatings is the so-called "absorbing AR". These coatings are useful in situations where high transmission through a surface is unimportant or undesirable, but low reflectivity from the surface is desired. They can produce very low reflectance with a small layer count and can often be produced more cheaply, or at greater scale, than standard non-absorbing AR coatings. (See, for example, US Patent 5,091,244.[1]) The absorbing ARs often make use of unusual optical properties exhibited in compound thin films produced by sputtering. For example titanium nitride and niobium nitride are used in absorbing ARs. These can be useful in applications requiring contrast enhancement or as a replacement for tinted glass (for example, that used in a CRT display).
