The accuracy of an integrating sphere largely depends on the type of diffuse coating and the lining material that is used. Although different coatings are recommended for different optical power measurement applications, which are based on the wavelength of the light, it is important that the coating materials have properties that allow perfect and non-selective diffusing. This prevents the change in the color of light that may be caused due to repeated reflection inside the sphere. Read more about integrating sphere >>
Properties of an Ideal Integrating Sphere Coating
In general, the coating has to possess the following physical properties, as mentioned in the Principle of Opticshandbook (c. 1934):
- Absence of fluorescence as such materials tend to elongate the wavelength of light at output, giving skewed power measurements
- High diffuse reflection (Lambertian reflectance)
- High flat reflectance
- Easier to apply as well as remove (as a single sphere may be used for different applications)
Most modern coating materials have these properties, with some of the most widely used materials being Magnesium Oxide, Barium Sulfate, and Polytetrafluoroethylene compounds. For infrared measurements, gold deposits are also used.
Coating Type is Based on Wavelength of Light
The primary application of an integrating sphere is optical power measurement where light is passed through the holes of the instrument, which is then attached to a detector to measure the scattering. Since wavelength plays a big role in scattering and diffusing, it is an important element to be considered when it comes power measurement. In that regard, following are the main sections of the spectrum for which a sphere is used:
- Visible light
- Infrared light
- Ultraviolet light
It is important that compatible coating material is used for different wavelengths of light, as high luminosity can chafe the coating and can even damage the integrating sphere. The rating of the instrument is measured based on the coating and its corresponding wavelength limit. For example, Barium Sulfate sphere coating is one of the most common materials and is used for visible light measurements.
How are Spheres Coated?
The most common way to coat an integrating sphere is by spraying. For example, for white-spray painting, the internal walls of the sphere is first painted white after which the coating material is “smoked” on the surface. One interesting thing to note here why the material is smoked i.e. sprayed using small amounts is because there is a tendency for the material in question to undergo chemical reaction during the process. In order to avoid such reactions, it is recommended that the operator use a small pile of the material and smoke it on the sphere surface using a blow torch. This is the traditional method that is being employed since early 1950s. Modern coating methods have emerged over the decades where the sphere is coated when it is fully packed. This prevents flow of impurities and provides more accurate results.
Conclusion
This is the basic overview of coatings used for integrating spheres. The key point is to relate the coating material with the wavelength of light to be measured.