Use of steady-state imaging setup for assessing the internal optical properties of non-spherical apple samples

•The effect of the local boundary curvature of a non-spherical apple is considered.•Optical coefficients are retrieved using a spatially-resolved imaging technique.•Optical diffusion and firmness measurements are observed for Royal Gala cultivars.•Monte Carlo simulation results and experimental measurements are compared.•Correlation between firmness and estimated reduced scattering coefficient is plotted. The aim of this paper was to retrieve the absorption (µa) and reduced scattering (µ's) coefficients of whole apples which exhibit a complex shape. The effect of the local boundary curvature on the retrieved optical properties was investigated by means of numerical simulations and measurements carried out at the wavelength of 633 nm. A first attempt was made by performing Monte Carlo simulations on an apple-like spheroid model covered with a thin skin layer of thickness 80 µm. Monte Carlo data were then analyzed to depict the changes of photon densities, diffusively reflected images and optical properties as a function of the light source location over the surface of such target. Second, spatially-resolved backscattered images were acquired from 207 ‘Royal Gala’, and the values of µa and µ's were retrieved using an inverse algorithm to fit the scattering profiles with a diffusion theory model, in a selected fitting range of 2.8–10 mm. The results confirm the theoretical prediction and show that the absorption coefficient µa may be overestimated, while the reduced scattering coefficient µ's is slowly changed when the measurements are performed on these apple species. Finally, experiments carried out on 200 apples still show that µ's is negatively correlated to the fruit firmness with a correlation coefficient (r) of 0.63. The spatially-resolved technique provides an efficient means for measuring the optical properties of fruits, and may be also useful for assessing the apple firmness.