Materials
The optical properties of the lining of the sphere greatly affect its accuracy. Different coatings must be used at visible, infrared and ultraviolet wavelengths. High-powered illumination sources may heat or damage the coating, so an integrating sphere will be rated for a maximum level of incident power. Various coating materials are used. Early experimenters used a deposit of magnesium oxide. Barium sulfate has a usefully flat reflectance over the visible spectrum. Finely-deposited gold is used for infrared measurements. Various proprietary PTFE compounds are also used for visible light measurements.
The theory of the integrating sphere assumes a uniform inside surface. Openings where light can exit or enter, normally called ports, that are used for detectors and sources must be small, less than about 5% of the surface area of the sphere, for the theoretical assumptions to be valid. Unused ports should therefor have matching plugs, with the interior surface of the plug coated with the same material as the rest of the sphere. Baffles are normally inserted in the sphere to block the direct path of light from a source to a detector, since this light will have non-uniform distribution.
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