There are several factors which must be considered when you’re selecting a coating for an integrating sphere. The most significant of these factors are durability and reflectance, and which of these two is most important depends on the environment in which your equipment is to be used. Consider the conditions to which you’ll subject your apparatus and you’ll be able to choose the correct surface coating.
Choosing the Right Coating for an Integrating Sphere
There are several simple scenarios that can be described in order to help you come to a basic understanding of when certain materials are more appropriate for covering the surface of an integrating sphere.
If your setup permits the presence of a significant amount of light but your equipment will have to survive the dangers of contaminating substances, even such as the simple buildup of dust, you would be best off choosing a coating that sacrifices reflectance for a greater degree of durability.
One of the majors concerns associated with the use of an integrating sphere is the reduced throughput caused by the absorption of energy by objects found within the apparatus itself. Absorption can occur due to the presence of internal parts like baffles, lamp sockets and the lamps themselves. This absorption and subsequent decrease in throughput can be minimized with the use of a more reflective coating that may or may not sacrifice some of its durability to achieve this great degree of reflectance.
Typical Coating Materials Used
There are several different kinds of coating materials that you can use, some of which are generic substances that can be sourced from a number of places and others which are manufactured by individual companies or laboratories. The coating material you need to use will depend on several factors including the reflectance and durability you require for your integrating sphere application to succeed.
Barium Sulfate
This was the first commercial coating developed and it remains the typical choice for most applications. Barium sulfate powder is combined with a PVA binder, water and ethanol before it is sprayed on the surface of the sphere. This substance is highly reflective and can be applied and removed rather easily, and when used properly it is highly lambertian.
These advantages also have a flip side as they mean that barium sulfate is easily damaged owing to its solubility in water. As such, even well-formulated barium sulfate-based coatings like Spectraflect should not be used in humid or rapidly changing environments.
PTFE Coating
Another kind of coating often used is made by packing PTFE powder with a base of silicone vacuum grease to create a substance with a consistent approximate density of 1 g cm-3. The advantages of such a material are that it offers a very high reflectance and is particularly lambertian in nature. However, it’s high fragility is a problem and the difficult manufacturing process is not conducive to applications that warrant the repeat covering of surfaces.
These two major categories constitute the main types of coating choices for any given integrating sphere, but there are others available, including electrochemically plated options like Infragold. You must carefully consider the application for which your sphere will be used and then pick a coating material that satisfies all of your requirements.
