So you’ve got a device and you need to make it get from point A to a lot of other points with a minimal loss and as diffusely as possible. What does one do? Well, obviously, you paint the reflecting surfaces white, with a surface coating that is ‘white’ and matte- that’s simple enough! Or is it?
Well, not really. There are a number of factors that must weighed before we start coating things. How much heat is your part going to take? Will it be exposed to high humidity? What wavelength range is the device being used over? The answer to each of these questions will determine the ideal coating for your device.**
There are a number of solutions to this problem. The traditional solution is to use a barium sulfate-based coating. The old war-horse is Eastman 6080, white reflectance coating, developed in the early 1970’s. There are a number of iterations of this coating out there (many of them developed by yours truly). Our version is called Avian-B™ white reflectance coating (the –B, cleverly enough, is for barium sulfate). This is a water/alcohol-based coating with barium sulfate and a binder. It adheres well to most metal and plastic surfaces (with a primer coat) and is both highly reflective and highly diffuse. The coating is also easily applied with a spray gun or even an air-brush for small parts. It is really the standard (and traditional) white reflectance coating. The coating is quite effective over a wide wavelength range (250- about 1300 nm, although it can be pushed out to about 2000 nm) and is 98+% reflective over a good bit of that range.
The downside of barium sulfate coating is that it is quite fragile and the binder is water-soluble. In addition, humidity affects the reflectance, especially in the NIR (near infrared). So while barium sulfate is good, it’s not perfect for every application. There are some alternatives. One is our Avian-D™ (-D for durable) white reflectance coating. Avian-D™ is also a water-borne coating, but with a polyurethane binder and a different pigment system. This coating was originally developed for outdoor use but we’ve found that it has numerous uses in place of barium sulfate. It is much more tenacious and is harder to rub off. And the reflectance is essentially equivalent to barium sulfate. The major drawback is a shorter wavelength range (really from about 320-1250 nm). And it is not totally water-proof; it’s water resistant (in other words, it wets, but when it dries it retains its original properties).
If one needs truly waterproof white reflectance coatings, there are some options. Labsphere’s Duraflect™ is a solvent based coating with a dual pigment system, which is waterproof, but has a fairly narrow wavelength range of usage. A similar coating is available from the former Optronic Laboratory (now Gooch and Housego) known as Optilon II™. Neither of the aforementioned coatings are available for customer application, primarily due to their hazardous nature.
Now, suppose you have a system where you can’t use a coating. Maybe it’s a small part that you can’t get inside to spray the coating (these coatings all need to be applied in multiple- usually 15-20 coats) or the device will be seeing fairly high temperatures (say extended time at >100°C). What can you do? The solution is not a coating but a material, sintered PTFE, known variously as Spectralon™, Fluorilon™, and other trade names. They are all derived from work done by Vic Weidner at NIST in the early 1980’s but extended upon and patented by the author in the late 1980’s. We’ll cover sintered PTFE in the next blog offering. It’s a long and fascinating story…
** by devices, we mean integrating spheres, boxes, cavities, and any myriad of other optical components- baffles, port plugs, port frames, etc.
Summary Table of Diffuse White Materials Properties
(All trade names are trademarked to their respective owners.)