Light-loss when measuring transmittance of thick scattering samples with an integrating sphere

Light scattering materials have several uses in solar energy applications, ranging from a purely aesthetic function as a cover glass to a way of increasing the path-length of photons inside a semiconductor. Knowing the transmittance of such elements is of essence to properly model, simulate, and design a solar energy system. The traditional method for obtaining the transmittance is to use a spectrophotometer fitted with an integrating sphere detector. However, it is well-known that most commercial integrating spheres underestimate the true transmittance of thick scattering samples. This study investigates a method to obtain quantitative values of the losses associated with measuring a scattering sample. The International Commission on Glass (ICG TC-10) is conducting an inter-laboratory comparison (ILC) on scattering samples to improve the methodology for characterizing such samples. A fritted glass sample similar to one in the ILC was used as an example. One side of a clear glass sample has a highly scattering layer. The bi-directional transmittance distribution function (BTDF) for the sample was obtained using a goniophotometer and then used as scattering function in a ray-tracing simulation. The ray-tracer was configured to report the amount of light exiting all six surfaces of the sample as well as through various ports defined by the integrating sphere geometry. The sample was then measured with a commercial integrating sphere in several different configurations, verifying the accuracy of the model.