Optical coatings encompass a variety of technologies, including reflective and dielectric coatings, which can enhance the performance of optical components.
They have many applications, such as creating glasses for those with light-sensitive eyes and improving camera lenses.
Contemporary coating technologies include ion beam sputtering (IBS) and plasma-assisted reactive magnetron sputtering (PARMS), offering specific benefits for optical applications.
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Optical Coating Explained
An optical coating is a very thin layer of film applied to the surface of an optical component to improve transmission, modify reflective qualities, or alter the polarization of light emitted by the component.
Such a coating can be as basic as a sheet of metal, e.g. aluminum, or as complex as a dielectric coating comprising several thin layers of material, with precisely regulated parameters for the number of layers, thickness and composition.
Optical coatings have been actively contributing to the optical industry since 1935. Alexander Smakula patented their innovation and the initial method, which used a single layer of an anti-reflective coat.
Since then, the Smakula company has relied on vacuum technology to incorporate a fluoride component into glass lenses, but this method results in undesirable reflection.
Since then, new techniques have been established, and the coating operations for optical components have been improved and altered, increasing affordability and productivity.
Types of Optical Coatings
Different optical coatings have various functions to produce results based on use.
- Filter Coatings: By applying one or more thin layers to a substrate, filter coatings can advance or degrade the appearance of an image. They can also reflect specific wavelengths during picture transmission.
- Beam-Splitter Coatings: These coatings can split a single light beam into two separate beams, usually with an equal or varying transmission-to-reflectance ratio. Depending on their use, they can also manage multiple beam pathways, recombine those routes, and control the combined beams’ polarization.
- High-Reflective Coatings (HRC): The surface on which an HRC is placed reflects all or part of the light that hits it. For example, an uncoated glass optic would have a reflection of 4 %. However, metal coatings will fundamentally change the features of the optic.
For example, using an aluminum coating on the same optic will cause it to reflect around 88 to 92 % of visible light. Silver coatings, combined with the appropriate dielectric coating, can potentially increase reflectivity in the far infrared band to 99.9 % (apart from in the UV and some visible spectrum regions).
Dielectric mirrors often have coatings of two distinct repeating layers: one layer with a high index (such as TiO2 or ZnS) and one with a low index. These two levels are typically separated by an intermediate layer (such as MgF or SiO2).
These highly reflective dielectric coatings have a reflectance that greatly exceeds the band stop, which is a relatively narrow range of wavelengths. In summary, HRC coatings are essential for properly operating laser equipment because they reduce the power required for various applications.
- Anti-Reflective Coating (ARC): It is common practice to apply ARC to surfaces of lenses and other optical elements because they help to decrease reflection. Less reflection leads to significantly enhanced image capture results.
Equipment, including cameras, telescopes, binoculars, and microscopes, can all benefit from ARC. Eliminating stray light improves visual contrast and decreases light reflection.
Opticians can apply ARC to glasses lenses to ensure that people around them can see the person wearing the glasses. Furthermore, the antiglare coating can reduce glint from binoculars or telescopic lights used by concealed observers.
Coating Technologies
Optical coatings can be used in various popular processes, including physical vapor deposition. Each coating has unique benefits, making it the most suitable alternative for a given set of use cases (although these can overlap).
However, no single technology is best suited for all applications. The following are currently the most commonly used optical coating technologies.
- Ion-Assisted Electron-Beam (IAD E-Beam) Evaporative Deposition electron-bean assisted ionization involves bombarding and vaporizing source materials in a vacuum chamber with an electron gun.
This technique provides the most design flexibility of any method on this list because it can utilize the greatest variety of materials, is the most affordable of all the processes, and can unite substantially larger coating chamber sizes.
- Ion Beam Sputtering (IBS) is very repeatable, and the coatings created using it are of exceptional optical quality and stability.
The method's ability to accurately monitor and manage elements such as layer development rate, oxidation level, and energy input is an important benefit that enables the generation of extremely reproducible and durable coatings.
IBS is more resistant to the impacts of temperature and humidity than other technologies, but it does have a few negatives to consider, such as a significantly higher relative cost compared to the different techniques. They also have smaller chamber sizes, slower rates of growth and more UV spectrum stress and loss.
- Plasma-Assisted Reactive Magnetron Sputtering (PARMS) uses a glow discharge plasma to accelerate positive ions onto a target. The plasma is restricted by a magnetic field to the area local to the target, causing atoms to be ejected and spread out to cover the optical surface.
This procedure can function smoothly even with a low chamber pressure, so there is no need for any setup. PARMS does not have quite the same level of repetition as IBS, but it has a high throughput and is significantly more repeatable than evaporative deposition.
Furthermore, the coatings in PARMS are typically tough and dense. The method is well-suited for application as a fluorescent optical filter due to its perfect equilibrium between optical performance and volume throughput.
- Advanced Plasma Sputtering (APS) uses the same principle as IAD E-Beam but benefits from more sophisticated forms of automation in the processing stage. APS provides hard, smooth, and dense coatings, which provide more consistent optical qualities than the other evaporative deposition methods.
APS and magnetron sputtering are intermediate solutions for some characteristics that vary between IBS and IAD E-Beam evaporative deposition.
Conclusion
There is a growing market for optical coatings due to contemporary technological developments and an increase in the consumption of electronic goods and semiconductors. With a huge range of applications and benefits, optical coating is a technology that should not be overlooked.
This information has been sourced, reviewed and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.