Light distribution inside mandarin fruit during internal quality assessment by NIR spectroscopy

A custom fibre optic probe has been built for the assessment of light levels inside a mandarin (Citrus reticulata), which has been illuminated with 808 nm laser light. The probe has minimal effect on the light being measured and enables the direct measurement of light levels at any location in the fruit. Results show there is a rapid reduction in light level across the thick illuminated skin, a less rapid but still high reduction as the light continues to diffuse into the flesh, a generally exponential decay through the tissue regions (with some perturbation through the central pith and air) and finally a rapid drop across the distal skin. Measurements supported by Monte Carlo simulations show that the skin has significant internal reflective properties, which increases the light level measured inside the flesh. For subjects of this size and light transmission ability, boundary conditions are critical to the internal light distribution. We suggest that separate scattering and absorption parameters for skin, flesh, and air spaces of the core would enable the development of a more accurate model for the light levels in a mandarin. (C) 2002 Elsevier Science B.V. All rights reserved.