Optics form a crucial aspect of numerous military applications. These include the likes of vision systems and target designators, as well as a range of vision enhancing tools used by infantry on the ground.
Maintaining good visibility for these systems is of critical importance, but it is also an engineering challenge. That’s because optics can be subject to a huge variety of conditions, including various ambient temperatures, humidity, abrasive materials, and more. One piece of military hardware might be forced to reach the upper limits of the Earth’s atmosphere (perhaps even leave it), and some might need to be used in desert storms. Another might need to be left out in the rain for days on end.
The role of thin film coatings is a highly important one to protect these optics while also maintaining optimal visibility – allowing only the most important wavelengths to pass through, while blocking the unuseful ‘noise’. In these applications, failure could mean the loss of lives, so it’s a hugely delicate balance that needs to be navigated with the utmost caution and attention to detail.
Optical thin film coatings are extremely thin films that can be applied to lenses, monitors, windshields, and other optical elements in order to protect them while also improving visibility. These materials need to be extremely resilient against corrosive elements and harsh conditions, while also being thin enough so as not to obscure vision.
Most optical thin film coatings will therefore only be a few microns or even a nanometers thick. This allows the use of much stronger materials – such as metals – without becoming opaque.
But there is more to a successful application than simply selecting the correct material to begin with. Equally important is thinking about the uniformity of the application (it cannot range in thickness), the adhesion (will it stay in place), the sequence, the refraction indices, and more.
Acceptable levels might vary on all these metrics depending on the application, budget, and more.
Generally, there are three primary technologies that are used for these applications. They are:
Each attaches the material to the substrate in distinct methods, and can be operated slightly differently depending on the vendor.
These methods are each very different and thereby evade direct comparison. However, they each have their own strengths and weaknesses, which should be considered by clients before making a selection.
As mentioned, optical coatings need to meet a number of set requirements in most applications and situations. We can therefore judge the different approaches by looking at their outcomes.
To focus a little more on these requirements: the first is hardness. Hardness refers to the ability of the coating to resist damage not only to itself but also to the material it has been applied to. The evaporation method provides the softest films, while sputtering and vapor deposition result in much harder surfaces.
This also has the added benefit of preventing water molecules from entering the film when in areas of high humidity. Moisture absorption can alter the refractive index, which means that choosing a harder coating can also improve the performance.
Internal film stress: Coatings will show signs of compressive residual stress/tensile stress. The higher the stress, the more durability you can expect from the product. In other words, it will be likely to last longer before a newer coat is needed.
Once again, evaporation appears to have the worst performance in this regard, resulting in coatings with the least stress. Higher energy methods like deposition result in greater stress. It is worth noting however that there is a point at which high stress is no longer desirable. Too much stress for a very thick coating can potentially result in a catastrophic failure. For these reasons, very thick coatings using less opaque materials might benefit from evaporation more so.
Surface roughness: As the name suggests, this refers to the feel of the surface. Roughness and ‘bulk scatter characteristics’ can have an impact on various types of application (if a surface needs to slide beneath another surface for instance) and can also have a negative impact on things like the refraction. If a surface is rough, then it is likely to impact the signal to noise ratio.
Keep in mind too that surface roughness might result in more grime and dirt attaching to substrate. This can then in turn make it more difficult to keep clean and have an indirect effect on the quality of an image.
Scatter creates stray light, and can make it more difficult to correctly identify targets and other important subjects. Sputtering tends to be the best option for improved optical efficiency and better contrast/lower noise.
When making a solution, it is very important to weigh up these different factors while also considering the cost. Clients may conduct a performance cost analysis in order to analyse the potential expense of the product/solution versus its performance. Certain features may be surplus to requirement, meaning that there is no reason to spend additional money in order to achieve results that aren’t needed by the end user.
A coating specifier’s role is to correctly choose the best solution to meet performance targets, while avoiding driving costs up unnecessarily.
An example might be when considering narrow and wide optical bandpass filters and edge filters. These are important elements in military applications that have precise specifications that need to be met (the half power point and center wavelength/full-width half-maximum).
While there is a lot to consider, ultimately the variety of technologies and varying performance provides the versatility and flexibility for clients to find the precise solutions they need.