The reflection of light incident on an optical window or optical component can degrade the performance of the window and the optical system associated with the window over its operating spectral range, whether it is the human eye or an optical device. It can also cause unwanted glare from a window, flat panel display, picture, automobile windshield, or television screen. Thin film antireflection (AR) coatings are used to reduce the reflectance of an optical window and increase its transmission in a narrow or broad wavelength band. Recall that T + R + A = 1, and if the reflectance (R) is decreased, the transmission (T) will be increased (ignoring absorption A). Thin film AR coatings are commonly applied to ophthalmic (eyeglass) lenses, camera lenses, solar cells, sensor window, lenses in optical instruments (e.g., telescopes), fiber optic components, and flat panel displays. AR coatings are also used on more exotic applications, including increasing the transmission of aircraft windows, electro-optic (EO) sensors, missile guidance optical systems, light detection and ranging (LIDAR) systems, and night vision apparatus. In many cases, they serve two or more functions: improvement of abrasion resistance, electromagnetic shielding, and gas and water permeation barriers, in addition to increase in transmission. AR coatings are required to operate over a narrow wavelength band to a very broad band of wavelengths from the ultraviolet (UV) to the long wavelength infrared (LWIR). They are even used to increase the transmission of semitransparent metal films. They can have either discrete or graded layers with different refractive indices. The reflectance of the surface of an untreated optical window depends primarily on its refractive index and can vary from 0.03 (reflects 3%) for magnesium fluoride (MgF₂) to 0.36 (reflects 36%) for germanium (Ge). Because an optical widow reflects light from both its surfaces, optimal performance will be achieved if an AR coating is applied to both faces. The reflectance from one face is given by the well-known relation: where n is the refractive index of the window material, n₀ is the refractive index of the incident medium (usually air and = 1), and k is the extinction coefficient of the window. Figure 1 shows the dependence of the reflectance from one surface of an optical substrate with its refractive index. The reflection of light from each surface can be reduced to very low values (often < 0.0025) by application of an AR coating. Fig. 1. Dependence of the reflectance from the surface of an optical substrate with its refractive index.

The objective of this article is to present a useful guide, based on experience, for the design, deposition, and utilization of AR coatings by: 1) describing the design tools available; 2) providing an understanding of how AR coatings are used and insights into the design and use of materials; and 3) providing guidelines for deposition of AR coatings. The theory of optical design is quite complex and tedious, and has been presented in books, technical literature, and articles. Although it is useful for those with extensive expertise in coating design, this theory will be used very little here.