A stepwise method for estimating optical properties of two-layer turbid media from spatial-frequency domain reflectance

Authors: HU, DONG - Zhejiang University; Lu, Renfu; YING, YIBIN - Zhejiang University; FU, XIAPING - Zhejiang Sci-Tech University

Submitted to: Optics Express
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2018
Publication Date: 1/14/2019
Citation: Hu, D., Lu, R., Ying, Y., Fu, X. 2019. A stepwise method for estimating optical properties of two-layer turbid media from spatial-frequency domain reflectance. Optics Express. 27(2):1124-1141. https://doi.org/10.1364/OE.27.001124.
DOI: https://doi.org/10.1364/OE.27.001124

Interpretive Summary: Many food and horticultural products comprise two or more homogeneous layers (i.e. peel and flesh), whose physiochemical properties are distinctly different. Hence measurement of optical properties of tissues in each layer is needed for better assessment of quality and condition of food products. In this research, a stepwise method was proposed for estimating optical properties of each homogenous layer for two-layer samples based on spatial-frequency domain reflectance, which is an emerging technique for optical property measurement of biological and food materials. The efficacy and accuracy of the proposed method were evaluated and compared with the conventional one-step method, for 66 simulation samples generated using a computer program. A sample-based calibration procedure was proposed to correct for the simulated (or actually measured data in real applications) to improve parameter estimation accuracy. Furthermore, the research also determined the constraining conditions on the thickness of the first layer, under which optical properties of each layer can be estimated with acceptable accuracy. Results showed that regardless of the data correction, the stepwise method resulted in significantly better estimations of the optical properties of both layers, compare with the conventional one-step method. Moreover, the data correction effectively improved the estimations of the optical properties of two layers. To accurately estimate the optical properties of the first layer, its thickness should be no smaller than 0.2 mm, while good estimations of the second layer can be obtained when the first layer is not larger than 2 mm. Implementation of the proposed parameter estimation method can greatly improve the measurement of optical properties of food and horticultural products by spatial-frequency domain reflectance technique, thus enhancing food quality assessment.

Technical Abstract: This research was aimed at estimating the optical absorption and reduced scattering coefficients of two-layer turbid media using a stepwise method from the spatial-frequency domain reflectance generated by Monte Carlo (MC) simulation. The feasibility of a stepwise method for optical property estimations was first investigated by comparing the reflectance generated by the diffusion model and MC simulation for one-layer and two-layer turbid media. The results showed that with proper selection of frequencies, the one-layer model could be used for estimating the optical properties of the first layer of the two-layer turbid media. A sample-based calibration method was proposed for correcting discrepancies of the reflectance between the diffusion model and MC simulation, which significantly improved the parameter estimation accuracy. Results showed that the parameter estimation efficacy and accuracy of the stepwise method were much better than that for the one-step method, which was especially true for estimating the absorption coefficient. Absolute error contour maps were generated for determining the constraining conditions for the first-layer thickness. It was found that the minimum thickness of the first-layer could be as small as 0.2 mm, when estimating the optical properties of the first layer. To have acceptable estimations of the optical properties of the second layer, the thickness of the first layer cannot be greater than 2 mm.