Measuring the Optical Properties of Highly Diffuse Materials

Measuring the optical properties of highly diffuse materials is a challenge as it could be related to the white colour or an oversaturation of pixels in the acquisition system. We used a spatially resolved method and adapted a nonlinear trust-region algorithm to the fit Farrell diffusion theory model. We established an inversion method to estimate two optical properties of a material through a single reflectance measurement: the absorption and the reduced scattering coefficient. We demonstrate the validity of our method by comparing results obtained on milk samples, with a good fitting and a retrieval of linear correlations with the fat content, given by R-2 scores over 0.94 with low p-values. The values of absorption coefficients retrieved vary between 1 x 10-3 and 8 x 10(-3) mm(-1), whilst the values of the scattering coefficients obtained from our method are between 3 and 8 mm-1 depending on the percentage of fat in the milk sample, and under the assumption of the anisotropy factor g>0.8. We also measured and analyzed the results on white paint and paper, although the paper results were difficult to relate to indicators. Thus, the method designed works for highly diffuse isotropic materials.

Automated summary

We developed a spatially resolved method and a nonlinear trust-region algorithm to estimate the optical properties of highly diffuse materials. Our method successfully estimated the absorption and scattering coefficients of milk samples, showing a linear correlation with the fat content. The absorption coefficients ranged from 1 x 10-3 to 8 x 10(-3) mm(-1), while the scattering coefficients ranged from 3 to 8 mm-1, depending on the fat percentage and assuming an anisotropy factor g>0.8.