The optical coating can be simply be defined as one or more than one thin layer of material deposited on an optical component such as a lens or mirror, which eventually keeps changing the way in which the optic reflects and transmits light. Anti-reflective coating is one of the types of optical coating, which tends to reduce the unwanted reflection from the surface and is commonly used in camera lenses and spectacle. In order to produce mirrors that reflect more than 99.99% of the light that falls on them we can use high-reflective coating is another type of optical coating. The optical coating which is more complex and difficult usually displays high reflection over some range of wavelengths, and anti-reflection over another range, which allows the production of dichroic thin-film filters.
The metals which have extremely thin layers are the simplest optical coatings, such as aluminium, which gets deposited on the glass substrate to make mirror surfaces, through a process known as silvering. The reflective characteristics of the mirror can be determined with the metals; Aluminium is the cheapest and most common coating and produces a reflectivity of about 88%–92% over the visible spectrum. It is highly important to maintain the extinction ratio polarization in the optical beamsplitter and optical mirror lens.
The silver coating is expensive in comparison to others, which could reflect up to 95-99% of light even into the far-infrared, but suffers from decreasing reflectivity in the ultraviolet and blue spectral regions. Gold is the most expensive, which results in an excellent (98%–99%) reflectance at full infrared, but has limited reflectance at wavelengths less than 550 nm, resulting in the distinctive gold color.
It is possible to reduce the reflectivity and increase the transmission of the surface by adjusting the density and thickness of the metal coatings, which eventually results in a half-silvered mirror. The use of “one-way mirrors” is done in some cases.
Dielectric coating is also another major type of coating. Materials such as magnesium fluoride, calcium fluoride, and various metal oxides are present in these thin layers, which are deposited on optical substrates. By carefully choosing the thickness, exact composition, and a number of these days, it becomes possible to tailor the transmissivity and reflectivity of the coating to produce almost any required feature. We can reduce the reflectance coefficient of surfaces up to less than 0.2% which eventually creates an antireflection (AR) coating. On the other hand, we can increase the reflectivity to more than 99.99%, producing a high-reflectivity (HR) coating.