## 5 Spatially-Resolved Spectroscopic Technique for Optical Property Measurement
### Quality Evaluation of Horticultural Products  
The OPA was used for measuring the absorption and reduced scattering coefficients for a variety of horticultural products and for quality evaluation. Figure 23 shows the absorption and scattering coefficient spectra for five fruit and vegetable samples. The \(\mu_{\mathrm{a}}\) spectra of all the samples except the tomato have absorption peaks at 675 nm, which is associated with chlorophyll absorption, and the \(\mu_{\mathrm{a}}\) values at this wavelength range from 0.10 to 0.48 cm\({}^{-1}\). The 'Granny Smith' apple has the highest chlorophyll absorption due to its greenish skin and flesh, while the ripened tomato does not exhibit the absorption peak, because chlorophyll in fully ripened tomatoes decreases dramatically or even disappears completely. In fully ripened tomatoes, anthocyanin becomes the dominant pigment, thus causing an absorption peak at 535 nm. The \(\mu_{\mathrm{a}}\) values of these samples are relatively small and consistent over the spectral range of 720-900 nm, and they increase dramatically above 900 nm and peak at 970 nm due to the water absorption. The \(\mu_{\mathrm{s}}^{\prime}\) spectra of these samples are generally more than 10 times greater in magnitude than the \(\mu_{\mathrm{a}}\) spectra due to the dominant scattering effect, and they exhibit a steady downward trend as the wavelength increases, which is in agreement with the Mie scattering theory and other reported studies [116; 117]. Apple samples have higher \(\mu_{\mathrm{s}}^{\prime}\) values (9.0-17.0 cm\({}^{-1}\)) over the entire spectral region of 500-1000 nm, while the tomato has the lowest \(\mu_{\mathrm{s}}^{\prime}\) values (4.5-6.0 cm\({}^{-1}\)). The ability of a scattering particles to scatter light depends on their density and size, according to the equation \(\mu_{\mathrm{s}}^{\prime}=a\lambda^{-b}\) where a is proportional to the density of the scattering particles and b depends on the particle size [110; 115]. Hence, knowledge of \(\mu_{\mathrm{s}}^{\prime}\) and its pattern of change with wavelength can provide useful information about the structural and physical characteristics of these samples, such as firmness in fruit. Figure 22: (**a**) A hyperspectral scattering image of a peach sample; (**b**) a reflectance spectrum corresponding to the vertical line in (**a**); and (**c**) a spatially-resolved reflectance profile corresponding to the horizontal line (**a**). In measuring the \(\mu_{\text{a}}\) and \(\mu_{\text{s}}^{\prime}\) of normal and bruised apples, [PERSON] et al. (2018) reported the different effects of bruising on the two parameters. While no consistent trend of changes in the \(\mu_{\text{a}}\) spectra was observed between bruised and normal apples, the values of \(\mu_{\text{s}}^{\prime}\) for normal apples were considerably larger than those for bruised apples, and they decreased consistently over time for a period of up to 23 days after the apples had been bruised. In a later study, [PERSON] et al. (2019) investigated the effect of mechanical damage on pickling cucumbers. As shown in Figure 24, one day after the damage, \(\mu_{\text{a}}\) increased significantly at 700-920 nm, whereas an opposite pattern of changes was found for \(\mu_{\text{s}}^{\prime}\). Mechanical damage had a greater impact on \(\mu_{\text{s}}^{\prime}\) than \(\mu_{\text{a}}\) in terms of absolute value. These two studies suggest that the optical properties, especially \(\mu_{\text{s}}^{\prime}\), are useful for evaluating horticultural products for defects due to mechanical damage. Effective defect detection can be achieved by means of enhancing scattering feature measurements. The concept was used in laser scattering detection of internal defect of pickling cucumbers ([PERSON] et al., 2015). [PERSON] et al. (2017) reported on the use of \(\mu_{\text{s}}^{\prime}\) and \(\mu_{\text{a}}\) spectra for predicting firmness and SSC of 'Golden Delicious' apples.