Alternative measurement configurations for extracting bulk optical properties using an integrating sphere setup

The usual approach for estimating bulk optical properties using an integrating sphere measurement setup is by acquiring spectra from three measurement modes namely collimated transmittance (Tc), total transmittance (Td), and total diffuse reflectance (Rd), followed by the inversion of these measurements using the adding–doubling method. At high scattering levels, accurate acquisition of Tc becomes problematic due to the presence of significant amounts of forward-scattered light in this measurement which is supposed to contain only unscattered light. In this paper, we propose and investigate the effectiveness of using alternative sets of integrating sphere measurements that avoid the use of Tc and could potentially increase the upper limit of concentrations of suspensions at which bulk optical property measurements can be obtained in the visible–near-infrared (Vis-NIR) region of the spectrum. We examine the possibility of replacing Tc with one or more reflectance measurements at different sample thicknesses. We also examine the possibility of replacing both the collimated (Tc) and total transmittance (Td) measurements with reflectance measurements taken from different sample thicknesses. The analysis presented here indicates that replacing Tc with a reflectance measurement can reduce the errors in the bulk scattering properties when scattering levels are high. When only multiple reflectance measurements are used, good estimates of the bulk optical properties can be obtained when the absorption levels are low. In addition, we examine whether there is any advantage in using three measurements instead of two to obtain the reduced bulk scattering coefficient and the bulk absorption coefficient. This investigation is made in the context of chemical and biological suspensions which have a much larger range of optical properties compared to those encountered with tissue.