Optical Coatings and Cost

Hello and I am back with my second blog post, not quite by popular demand, but I did receive great feedback from my last blog post, Optical Design for Manufacturability. All through my career, I have fielded many, many questions about today’s topic, optical coatings. Specifically, questions involving cost and specifications, and how they relate to one another. 

At Esco, we refer to ourselves as a hybrid optical manufacturer. What that means is we fabricate both custom and catalog optics. When we calculate a quote for a simple custom part, we start with pricing out the material and the processes it takes to grind, polish, inspect and measure the final product. If the custom part requires a coating, then that creates a whole new set of variables. Optical manufacturing is one thing but then adding a thin-film coating is a science of itself. For catalog parts, we can quickly calculate a fixed price but to add a coating, while at face value it might seem easy, there are significant nuances to consider. I hope I can demystify the process to give some insight into what variables come into play. 

Let's take a step back for a moment and talk about what an optical coating is. An optical coating is a layer or multiple layers of a specific formula that adheres to an optics surface which will alter the reflectance or transmittance of the optic. There are many options within the coating world, but the two main paths are physical vapor deposition process (PVD) and chemical vapor deposition (CVD). The type of coating one wants to achieve can dictate which process is used. I have found the physical deposition tends to be more popular. Within PVD, you have multiple methods such as ion-assisted electron-beam deposition and ion beam sputtering, advanced plasma sputtering and plasma-assisted reactive magnetron sputtering. While I am not going to go in-depth on all of these (because frankly, it gets quite heady) it is important to ascertain the specific method required for your application. It is critical to point that out to your sales representative. 

As I mentioned, the coating process is a science, as it is chemistry and physics working together. Unfortunately, there is no simple Easy-Bake^® oven for coatings. We have several partners that we work with to hit tight specifications whether that is for AR, Dichroic, hyperbolic or metallic coatings. Coatings are like the Baskin Robbins of optics, in that while there are 31 flavors, you can mix them up and create something that satisfies your palette.  

Transparency of pricing 

The cost will vary depending on the specific coating, layers needed, and the number of parts that can fit in a coating chamber. For example, consider a simple AR (anti-reflective) coating for 100 pieces of 25.4 mm diameter fused silica windows that may cost $750. That's is an additional $7.50 to each unit cost of the optic itself. Now, let's  say that you have only two pieces of a prototype window that are 25.4 mm in diameter. The cost to run the chamber is still a fixed price at $750. Therefore, the coating for these windows is an additional $375 per piece. Another part of the equation is inspection and metrology. While a coated part may look good, does it perform as it is supposed to? Inspection time and compiling data for a given run can add to the unit cost. Complex multi-layer coatings can therefore cost a few thousand dollars as the coatings are doing the heavy lifting of the performance of the part. If one needs to transmit or reflect a specific wavelength of light through a system, the optical material may not do it on its own. The coatings then become your MVP and each of the layers becomes the foundation of the final product. 

Specifications

Additionally, while I have had customers state they just need an AR coating, whatever is standard, that can be a bit tricky. A standard AR coating generally means (400-700 nanometers), but within that range you have narrowband AR, broadband AR, MgF2 AR and dual-band AR. For example, if one is using an precision optic in a detection system, and that system is pushing a fair amount of laser light through it, eventually that coating will wear and the coatings will fail, and trust me, no one is happy about that. Your vendor should be asking each time you need a coating, what is the end-use of the part and is there a Laser-Damage Threshold (LDT)?  LDT is the amount an optical component can perform dependent on the wavelength, pulse duration, and beam diameter.  LDT is a complex calculation but there are some useful online calculators to arrive at your intended solution. 

In conclusion, to get the best pricing for coated optics overall quantity matters. You want to optimize the most amount of parts you can get in a coating run. For any laser application, you always want to include a laser damage threshold to be sure that your coating will perform within its intended specifications. Lastly, it can be hard not to have sticker shock at times when you have the cost of the optic in hand, and you'd like to add-on a coating. What you have to remember is not just a topping, it's a main ingredient that will make or break your optics success. 

If you would to like discuss further please feel free to reach out to me Mike@EscoOptics.com.