Visible/near-infrared spectroscopy is a well-established method to measure optical properties of tissues,
assuming that a light propagation model can be used to recover absorption and reduced scattering
coefficients from non-invasive probing. Spectroscopic measurements have achieved success in
non-destructive assessment of apple optical properties and quality attributes. However, the spectroscopy
of apples must consider the size of the fruit and the presence of the thin skin layer that surrounds the
flesh, to correctly read the signals acquired on the boundary. In this research, the fruit was modelled
as a two layer spherical structure with various radii and finite thickness of the upper skin layer. Monte
Carlo computations were performed to generate time-resolved reflectance and spatially-resolved reflectance
measurements. Simulated data were then fitted using a procedure based on Levenberg–Marquardt
algorithm with specific semi-infinite models. The errors in the retrieved optical properties of the flesh
(absorption coefficient la, and reduced scattering coefficient l0
s) were studied as functions of apple
radius, skin thickness, and source–detector distance, for given optical parameter sets assigned to the flesh
and the skin. The results suggest that the time-resolved reflectance spectroscopy may probe optical properties
of the flesh regardless of the skin layer, when a sufficient source–detector distance (15 mm) is used
for the measurements. Similar results were found in case of using the spatially resolved spectroscopy,
because measurements extend up to 15–29 mm by steps of 1 mm or 2 mm. The computations also show
that the curvature of the boundary has noticeable effect on the errors in the retrieved optical coefficients
of the flesh. However, results from time-resolved spectroscopy are more influenced by the size of apples,
compared with the spatially-resolved spectroscopy.