An optical coating is the application of one (or several) metallic (or dielectric), films to an optical part's surface. A coating applied to the optical part's surface is used to increase or decrease the intensity of the light, such as reflection, beam separation, color separation, and filtering. Among the most popular coatings are vacuum-coating and electronicless coating.

Principle of optocoating:
Vacuum coating:
Vacuum coating refers to coatings that must be done in higher vacuums, such as vacuum ion, magnetron, magnetron, molecular beam epitaxy and PLD laser-sputtering. A substrate is formed from the plated materials and electroplated material. The target and substrate are in the same vacuum.

The evaporation coating is usually the target of heating so that the surface components evaporate in the form of free radicals or ions and are deposited on the surface of the substrate by film-forming method (scattering island structure-trapezoidal structure-layered growth).

Sputtered coat
You can easily understand the process of sputtered coating. It involves bombarding the target material with electrons or high energy lasers. Finally, the surface components are sputtered under the form free radicals orions. This forms a thin film.

An optical film has a smooth top, a geometrically-segmented interface between layers, and a refractive Index that can jump at interfaces. But it's continuous within the film.

The absorption medium can be either uniformly normal or not. Practical application of the film can be more difficult than an ideal one. Because of its optical and physical characteristics, it is different from bulk material. Its surface and interface are also rough which causes diffuse scattering. Due to mutual penetration, the films form a diffusion interface. The anisotroy is formed by the film's growth, structure, and stress.

Common optical coating materials include the following:
1. Metal (alloy): germanium, chromium, aluminum, silver, gold, etc.
Germanium.
This rare metal is safe and non-radioactive. It has a light transmission range of 2000NM to 14000NM.

Chrome.
Sometimes found in spectroscopes, and often used as a "colloid" layer to improve adhesion. It may range from 550NM to 30NM. But, under the guidance of aluminum mirror, 30NM is an acceptable value to boost adhesion.

Aluminium.
The film has the greatest ultraviolet reflectivity of any common metal, with a thickness greater than 50NM.

Silver.
When the evaporation rate and substrate temperature are fast enough, silver will have the same high reflectivity of aluminum. This is because there is more absorption due to the massive accumulation that occurs at very low speed.

Gold.
Material with highest reflectivity of all known materials is above infrared 100nm wavelength

2. Oxides.
Yttrium trioxide.
By electron gun evaporation the property of the material changes with film thickness. The refractive index, which is approximately 1., can be used. 8 % at 500nm. It's very well-known as an aluminum protective coating, in particular due to its high incident angle at 800-12000nm.

Cerium dioxide.
High-density tungsten boat evaporates cerium dioxide on a substrate of 200°C to get a refractive Index of about 2.2. The absorption band is located at about 3000nm, and its refractive indice changes with temperature. Oxygen ion plating can produce nylon 2.35 (500nm), which is a low absorption thin film.

titania.
The refractive Index is defined as the light transmission range between 2.21500 and 3.15500nm. People love this material because of its high refractive and relative firmness. It is used for anti-reflection films, splitter films, cold light films, filter, high reflective film, glasses film and thermal mirror.

Silicon dioxide.
It is a transparent colorless crystal with a high melting point, hardness, and chemical stability. Use it for high-quality Si02 coatings. It has a very pure crystal with no melting point. You can divide it into infrared, visible and ultraviolet depending on your use. A film with too much pressure can cause pores to form and be fragile. Conversely, a film with too little pressure will absorb more light and its refractive Index will rise.

Zirconium dioxide.
High refractive index, high resistance to temperature and chemical stability make the white heavy crystal very pure. You can use it for high quality zirconia coatings. Because of its roughness, the incident light will be diffused and the transmittance of your lens is reduced. Additionally, optical rotation will cause certain incident light sources to dissipate particularly strongly. One example is that a material which absorbs red light appears green. But, poor processing can still be avoided.

Hafnium oxide.
If the substrate is heated to 150C by an electron gun, its refractiveindex is approximately 2.0C. Oxygen ion assisted plating can achieve a stable refractiveindex of 2.5-2.1. In the region of 8000-12000NM, HFO2 works better than SiO2 for the protective outer layer.

3. Fluoride.
Magnesium fluoride.
This antireflective coating has a 1x4 wave thickness and is used widely as an optical glass. It transmits about 120NM true ultraviolet radiation to the middle-infrared region at about 7000nm.

Calcium fluoride or barium fluoride.
The only problem with their compactness is that they are not completely solid. Transmittance shifting at higher temperatures results in longer wavelengths. They can therefore only be used as infrared films.

Lead fluoride.
This material can also be used in UV. If it's used in 300nm it will have a lower refractive value when it comes into direct contact with molybdenum or tantalum. Therefore, platinum and ceramic dishes are required.

4. Additional compounds
Zinc sulfide.
Light transmission ranges with refractive indices of 2.35400-13000m have good durability and stress. Mainly used to make spectroscopic and cold light films, decorative films, filters, high reflective films, and infrared movies.

Lead Telluride.
It's an IR material of high refractive. Because it's a thin-film material, transparency is possible in between 300 and 4000NM. The material is sublimated in the infrared. Substrat temperature is 250C. Prevention is essential. The maximum range of 40000NM is sufficient to make it work. Many other materials can be used to create a 14000NM edge.

Silicon dioxide Price
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