Measuring Mueller Matrices of ‘Zesy002’ Kiwifruit Peel and Pericarp Slices

A transmission-based polarized light system was used to make polarized light measurements on 20 eating ripe, yellow-fleshed kiwifruit (Actinidia chinensis var. Chinensis ‘Zesy002’) in the 650–1000 nm Near-Infrared (NIR) region. Mueller matrices of kiwifruit outer pericarp slices and skin peel were calculated from measurements made using a rotating retarder fixed polarizer (RRFP) polarimeter. The Lu-Chipman decomposition was used to decompose the filtered experimental matrices into depolarizer, retarder and diattenuation matrices, with the depolarization coefficient, linear retardance and circular retardance extracted from those decomposed matrices. A strong linear relationship was observed between the de-polarization coefficient of the pericarp slices and the penetrometer firmness of the fruit, the firmer kiwifruit showing a higher depolarization coefficient than softer fruit. The depolarization coefficients were spectrally of only moderate or low slope, lower at higher wavelengths, and those for the skin peel were generally higher than those of the pericarp slices. The latter observation is likely due to the more complex light scattering morphology of the skin peel. The linear retardance was spectrally flat and varied greatly by sample, possibly due to variation in the presence of birefringent material such as cellulose. Although circular retardance was negligible for the pericarp slices, spectrally flat around zero, that was not the case for skin peel, which was significantly non-zero and with a strong spectral trend of decreasing circular retardance with wavelength. These results provide some new insights into understanding the measurable responses of polarized light in fruit.