Due to the simplifications assumed for an ideal integrating sphere, the relations derived in “Theory of the ideal integrating sphere” cannot be applied directly in practical applications. They have to be altered for the following reasons:
Etotal = Φ0 × ρ = Φ0 × K Asphere 1 - ρ (1 - a) Asphere
K = p 1 - ρ (1 - a)
In these relations, α denotes the relative share of the area of all ports and other non-reflecting areas on the sphere’s total inner surface:
a = sum of all non-reflecting areas Asphere
The figure below shows the dependence of the sphere multiplier on reflectance ρ for different values of a. It can be noted that even a small variation of reflectance might cause significant change in the sphere multiplier.
Fig. 1: Dependence of the sphere multiplier K on reflectance ρ for different values of the share of nonreflecting areas on the sphere’s total inner surface
For this reason, a slight wavelength dependency of ρ may result in a strong wavelength dependency of the sphere multiplier.
Apart from these factors, integrating spheres are also subject to temporal variations of their optical properties, which are primarily caused by degradation of their coating material. In particular, the traditional coating material, Barium sulphate (BaSO4), ages significantly when exposed to UV radiation.
Optically diffuse material (OP.DI.MA) is an optical grade plastic that is specially designed to work as a volume reflector. It has been designed to replace barium sulfate as a coating for integrating spheres in UV and high temperature applications. Its reflective properties depend on its thickness, generally specified at 10 mm which is the recommended minimum thickness for lighting engineering.
Apart from its temporal stability, OP.DI.MA offers additional advantages. Using different additives, its reflection factor can be adjusted to any value between 3 % (deep black) and 99 % (brilliant white), whereby uniform reflectance over a wide spectral range and over large geometrical areas can be achieved. Like other plastics, it can be processed by turning, drilling, sawing and milling and is available in raw blocks, plates and foils in various sizes for this purpose.