A proper definition of an optical coating is one or more thin layers of material creates an effect and interaction between light and a certain optical component- such as a lens or mirror. To understand optical coatings, you have to know how light interacts with transparent media. The layers’ effect alters how the optic reflects and transmits light.
The design of optical coatings is ideal for the enhancement of performance, transmission, reflection or polarization properties of an optical component for a specific angle of incidence and polarization of light. These include S-Polarization, P-Polarization, or random polarization. The composition of an optical coating consists of a combination of thin layers of materials (these vary, including oxides, metals or rare earth materials). The factors that determine an optical coating’s performance are the number of layers, their thickness and the refractive index difference in-between.
Optical Coating Theory
In order to truly comprehend optical coatings, one must understand The Fresnel Equations of Refraction and Reflection.
Snell’s Law of Refraction displays the change in the direction of a wave’s propagation in its passage from one optical medium to another.
The Law of Reflection shows that the angle of a reflected ray, with respect to the surface normal, holds equal magnitude to the angle of incidence, but opposite direction with respect to the surface normal.
Different Types of Optical Coatings
Anti-Reflective Coating is effective in reducing the unwanted reflections from surfaces, and its most common use is on spectacle and camera lenses.
High-Reflective Coating produces mirrors that reflect greater than 99.99% of the light that falls on them.
Additional complex optical coatings demonstrate high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film filters.
Low Reflective – High Emissivity – Optical Black Coatings are used where it is desired to suppress any reflections from internal parts of an optical system or device. Typical applications for low reflectance are in the area of stray light suppression for signal-to-noise enhancement. High emissivity is often a requirement in passive thermal control applications. Acktar optical black coatings are available in various versions tailored for optical wavelength ranges of interest.
Edge Blackened Lens. Coating type: Acktar Vacuum Black TM
Optical Coatings can be applied to various commonly used physical vapor deposition technologies. None of them is the ideal choice for all applications, because each technology has its own unique strengths that make it the most suitable for specific and overlapping use cases.
Ion-Assisted Electron-Bean (IAD E-Beam) Evaporative Deposition is a coating technique that consists of the bombardization and vaporization of source materials by an electron gun in a vacuum chamber. This technique offers the most flexibility for coating design out of any other method, since it can apply the widest scope of useable materials, the lowest cost compared to all the other methods, and accommodate larger coating chamber sizes. When the top priorities are not high performance, but flexibility and cost are- it is the ultimate ideal method.
Ion Beam Sputtering (IBS) is a highly-repeatable coating technology that produces very high optical quality and stability of coatings. Among the main advantages of IBS, some of the notable ones are the precise monitoring and control of parameters, which includes layer growth rate, oxidation level, and energy input, all of which results in highly repeatable coatings. Contrary to other coating technologies, environmental factors such as temperature and humidity has a lesser affect of IBS’ performance. Naturally, IBS coatings also has some disadvantages, like higher stress and loss in the UV spectrum, as well as lower growth rates and chamber sizes, and a much higher relative cost than the rest of the methods.
Advanced Plasma Sputtering (APS) is basically an alternate version of IAD E-Beam evaporative deposition that utilize from the advanced automated processing capabilities. The outcome of APS is smooth, dense and hard coatings that offer more stable optical properties than IAD E-Beam while keeping IAD E-Beam’s high level of multilaterality. In many ways, APS, along with magnetron sputtering, can be considered as interim solutions for many parameters between IAD E-Beam evaporative deposition and IBS.
Plasma Assisted Reactive Magnetron Sputtering (PARMS) is yet another plasma generation-based coating technology. Glow discharge plasma is generated similarly to APS, however it is “confined” by a magnetic field near the target instead of filling the entire coating chamber. PARMS has higher turnout, making it an attractive middle ground between the high price and performance of IBS with more economical coating technologies such as IAD E-Beam evaporative deposition. PARMS is often used to manufacture fluorescence filters because of the technology’s relatively balanced high optical performance and relatively high-volume turnout.