An antireflective or anti-reflection(AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection.In typical imaging systems,this improves the eff -iciency since less light is lost due to reflection.In complex systems such as talescopes and microscope -s the reduction in reflections also improves the contrast of the image by elimination of stray light.Thi -s is especially important in planetary astronomy.In other applications,the primary benefit is the elimin -ation of the reflection itself,such as a coating on eyeglass lenses that makes the eyes of the wearer more visible to others,or a coating to reduce the glint from a covert viewer's binoculars or telescopic sight.
Anti-reflective coatings are used in a wide variety of applications where light passes through an opt -ical surface,and low loss or low reflection is desired.Examples include anti-glare coatings on correctiv -e lenses and camera lens elements,and antireflective coatings on solar cells.
The simplest form of anti-reflective coating was discovered by Lord Rayleigh in 1886.The optical gla -ss available at the time tended to develop a tarnish on its surface with age,due to chemical reactions with the environment.Rayleigh tested some old,slightly tarnished pieces of glass,and found to his surp rise that they transmitted more light than new,clean pieces.The tarnish replaces the air-glass interface with two interfaces:an air-tarnish interface and a tarnish glass interface.Because the tarnish has a refra -ctive index between those of glass and air,each of these interfaces exhibits less reflection than the air -glass interface,as can be calculated from the Fresnel equations.
One approach is to use graded-indes(GRIN) anti-reflective coatings,that is,ones with nearly continu -ously varying index of refraction.With these,it is possible to curtail reflection for a broad brand of fre -quencies and incidence angles.
