Anti-reflective coatings are applied to lens surfaces to reduce reflections, and increases lens transparency. The coating is made up of a thin invisible layer of metal on the lens surfaces which also provides the lens with superior protection, and scratch resistance. The coating almost eliminates all reflections from the lens and increases lens transparency and provides the best possible vision, by up to an additional eight percent. It also reduces glare and helps reduce fatigue. This coating is recommended for all lens types, but is essential for those who drive at night, or work with computers.
An antireflective or anti-reflection (AR) coating improves the efficiency of the lenses since less light is lost. In complex systems such as a telescope, the reduction in reflections also improves the contrast of the image by the elimination of stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself. On spectacle lenses, the coating makes the eyes of the wearer more visible to others. The same coatings are applied to reduce the glint from binoculars or telescopic sights.
Many anti-reflection lenses include an additional coating that repels water and grease, making them easier to keep clean. Anti-reflection coatings are particularly suited to high-index lenses, as these reflect more light without the coating than a lower-index lens.
AR is made of a very hard thin film that is layered on the lens. It is made of material that has an index of refraction that is somewhere between air and glass. This causes the intensity of the light reflected from the inner surface and the light reflected from the outer surface of the film to be nearly equal. When applied in a thickness of about a quarter of light's wavelength, the two reflections from each side of the film basically cancel each other out through destructive interference, minimizing the glare you see. AR coatings are also applied to the front of prescription eyewear and some sunglasses to eliminate the "hot spot" glare that reflects off the lens.
Moths' eyes have an unusual property: their surfaces are covered with a natural nanostructured film, which eliminates reflections. This allows the moth to see well in the dark, without reflections to give its location away to predators. The structure consists of a hexagonal pattern of bumps, each roughly 200 nm high and spaced on 300 nm centers. This kind of antireflective coating works because the bumps are smaller than the wavelength of visible light, so the light sees the surface as having a continuous refractive index gradient between the air and the medium, which decreases reflection by effectively removing the air-lens interface. Practical anti-reflective films have been made by humans using this effect, this is a form of biomimicry.
Such structures are also used in photonic devices, for example, moth-eye structures grown from tungsten oxide and iron oxide can be used as photoelectrodes for splitting water to produce hydrogen. The structure consists of tungsten oxide spheroids of several 100 micrometer size coated with a few nanometers thin iron-oxide layer.
As mentioned above, natural index-matching "coatings" were discovered by Lord Rayleigh in 1886. Harold Dennis Taylor of Cooke company developed a chemical method for producing such coatings in 1904.
Interference-based coatings were invented and developed in 1935 by Alexander Smakula, who was working for the Carl Zeiss optics company. Anti-reflection coatings were a German military secret until the early stages of World War II. Katharine Burr Blodgett and Irving Langmuir developed organic anti-reflection coatings known as Langmuir–Blodgett films in the late 1930s.
Anti-reflective coatings are heavily promoted by eye care practitioners. The coating does just what the name implies, that is, it decreases reflections of bright lights from the surfaces of the lenses. Anti-reflective coatings also increase the light transmission through the lenses. If you are someone who makes public appearances wherein spotlights or other bright lights shine on your face, anti-reflective coatings will make you more photogenic. Be aware that most of the lower-cost anti-reflective coatings make the spectacle lenses difficult to keep clean.
An anti-reflective coating is essential if one does a lot of night driving. One of the most common problems with night driving is Glare, a light source that interferes with your vision, which may slow your reaction time. Night time glare is a result of both bright and dim lights. Although trying to see something in the presence of a light that’s too bright can cause your eyes to squint and become teary, a reduction in the contrast of images brought on by dim lighting can impair your vision as well. The inclusion of an anti-reflective coating will definitely reduce these negative effects.
AR coating is essential when used on high-index lenses, which reflect more light than regular plastic lenses. For example, regular plastic lenses reflect roughly 8 percent of light hitting the lenses, so only 92 percent of available light enters the eye for vision. High index plastic lenses can reflect up to 50 percent more light than regular plastic lenses (approximately 12 percent of available light), so even less light is available to the eye for vision. This is particularly troublesome in low-light conditions Generally, the higher the index of refraction of the lens material, the more light that will be reflected from the surface of the lenses. It also will reduce the effect of halos around lights at night.
The visual benefits of lenses with anti-reflective coating include sharper vision with less glare and greater comfort during prolonged computer use. First prize while working with a computer all day, is an anti-reflective coating with an added blue light control filter. Research has shown this to reduce the fatigue associated with prolonged computer use, which is a reality we are faced with now in any modern office environment.
Anti-reflective coating also is a good idea for sunglasses, because it eliminates glare from sunlight reflecting into your eyes from the back surface of tinted lenses when the sun is behind you. (Generally, AR coating is applied only to the back surface of sunglass lenses because there are no cosmetic or visual benefits to eliminating reflections from the front surface of dark-tinted lenses.)
Most premium AR lenses include a surface treatment that seals the anti-reflective layers and makes the lenses easier to clean. "Hydrophobic" surface treatments repel water, preventing the formation of water spots. Some anti-reflective lenses have surface treatments that are both hydrophobic and "oleophobic" (also called lipophobic), which means they repel both water and oil. These combination treatments typically contain fluorinated materials that give the lenses properties that are very similar to those of nonstick cookware.
Some eyeglass lenses have factory-applied AR coating on both lens surfaces. Other lenses, particularly progressive lenses and other multifocal lenses have the coating applied after the lenses have been customized to your eyeglass prescription by an optical lab.