Inversion of two-flux and four-flux radiative transfer models for determining scattering and absorption coefficients for a suspended particle device

Intrinsic and extrinsic scattering and absorption coefficients of a suspended particle device (SPD) smart window sample at dark and clear appearance states-without and with applied electrical voltage, respectively-were determined by means of the Maheu, Letoulouzan, and Gouesbet four-flux (intrinsic) and Kubelka-Munk two-flux (extrinsic) radiative transfer models, respectively. Extrinsic values were obtained from fitting to the two-flux model taking into account the predominantly forward scattering of the SPD. As an approximation, the Fresnel reflection coefficients were integrated out to the critical angle of total internal reflection in order to compute diffuse interface reflectances. Intrinsic coefficients were retrieved by adding a new proposed approximation for the average crossing parameter based on the collimated and diffuse light intensities at each interface. This approximation, although an improvement of previous approaches, is not entirely consistent with the two-flux model results. However, it paves the way for further development of methods to solve the inverse problem of the four-flux model.