An optical coating known as an AR coating is applied to a surface to lessen the quantity of light reflected off it. It is frequently employed in optical applications when the coating is placed in front of the air-glass barrier, lens, or mirror interface. Light loss due to reflection is minimized with AR coatings by maximizing the quantity of light that transmits or enters the surface. The coatings boost contrast in imaging equipment, increase the optical efficiency of telescopes, cameras, and binoculars, and lessen the glare on eyeglasses. They also diminish scattered light that might interfere with the optical performance of these devices.
How an antireflective coating functions and behaves is determined by the physics of how light moves through a material and reacts at interfaces between two different mediums. AR coatings increase transmittance by utilizing the electromagnetic wave characteristics of light.
Types of Anti-reflective Coatings
1. Single-layer AR Coatings
A single, thin layer of transparent material makes up the most basic interference anti-reflective coating. For visible light applications such as crown glass, magnesium fluoride (MgF2) is frequently utilized as a Broadband AR (BBAR) coating.
A magnesium fluoride coating is used on AR lenses for eyeglasses, cameras, and other visible light optical applications. Due to the coating's hardness and relative ease of application, it is perfect for these applications. Magnesium fluoride may be cleaned with gentle solvents and has a modest resistance to abrasion as well as good resistance to humidity.
Other dielectric coating materials may be employed as a single-layer coating for applications that target wavelength ranges outside of the visible spectrum.
2. Multi-layer AR Coatings
The optical performance of an AR coating is frequently enhanced by multi-layer coatings. As the name suggests, a multi-layer coating employs multiple thin film coating layers to gradually cut down on reflected light. It is feasible to lower reflection to less than 0.1% of the incident light with a multi-layer coating.
Each layer of the coating is made of a substance and is a specific thickness in order to optimize the destructive interference of the reflected light and maximize transmission.
For a multi-layer coating to work optically well, different substrate materials, incidence angles, and wavelength ranges can be combined. Optical substrate materials include things such as glass, sapphire, germanium, zinc selenide, zinc sulphide, calcium fluoride, and chalcogenides.
Multi-layer coatings are typically only employed in high-performance optical equipment used for planetary astronomy, photolithography, and aerospace telemetry since they are more expensive to produce and need more careful design.
3. “V” Coatings
The primary objective of designing “V” coatings is to maximize performance across a very narrow range of wavelengths.
The shape of the index of the refraction curve over a range of wavelengths is where "V" coats get their name. The index of refraction of "V" coatings is set to be high everywhere but at the intended wavelength (DWL). The resulting curve has a shape that resembles a "V," and it is centered on the DWL.
For specific uses such as lasers that utilize light sources tuned to a single frequency, "V" coatings are created.
Properties of Antireflective Coatings
Thin-film coatings called AR coatings are used to cover a substrate. Due to the mechanical and chemical differences between the thin film and the substrate, the longevity of AR coatings heavily depends on both their bond with one another and the bond between the layers of coatings in multilayer coatings. Therefore, solvents that weaken the binding, temperature cycling that stresses the bond, abrasion, and adhesive pulls that remove the coating, are the most damaging to AR coatings.
The longevity of an AR coating is significantly influenced by the hardness, strength, and durability of the coating itself. The type of coating used determines how resistant to solvents and scratches an AR coating is.
The Bottom Line
For optical materials, AR coatings are a great technique to lower light reflection and boost light transmission. In the case of "V" coatings, they can be designed for a very narrow and precise target wavelength or for specialized purposes to operate over a wide range of wavelengths. Both commonplace objects such as eyeglasses and high-tech equipment including infrared imaging systems use AR coatings.
