## 5 Spatially-Resolved Spectroscopic Technique for Optical Property Measurement  
Optical properties, according to the radiation transfer theory, mainly refer to two fundamental parameters, i.e., absorption (\(\mu_{\mathrm{a}}\)) and scattering (\(\mu_{\mathrm{s}}\)) or reduced scattering (\(\mu_{\mathrm{s}}^{\prime}\), a lumped parameter incorporating the anisotropy (\(g\)) and the scattering coefficient). The two parameters, along with the refractive index, largely determine how light transfers or propagates in turbid or diffusive media. The absorption coefficient measures the amount of the light being converted into other forms of energy, which is related to the chemical composition of the product. The scattering or reduced scattering coefficient, on the other hand, represents the probability of photons being scattered during travelling, which is closely associated with the cellular structures and density [92]. Hence, measurement of the optical properties can help us better understand light interactions with agro-food products and also provide an effective means for better evaluation of the physiological condition and quality attributes.  
The optical absorption and scattering properties can be determined by quantifying the diffuse light remitted from a turbid medium in the spatially- or time-resolved domain (or frequency domain, which may be viewed as a special case for the time-resolved domain, or vice versa), and then using inverse algorithm for an appropriate light transfer model (e.g., diffusion approximation model) to fit the measured diffuse reflectance data. Because absorption and scattering are interwoven during light transfer in turbid media, fast and accurate measurement of the absorption and scattering  
Figure 18: Examples of pest infestation in pickling cucumbers: (**a**) normal; (**b**) slight; (**c**) mild; and (**d**) severe infestation. Reproduced with permission from [86], Copyright Elsevier, 2013.  
coefficients still remains a challenging task. Over the years, researchers in the field of biomedical optics have developed several techniques for measuring the optical properties of biological tissues, which, according to their measurement principles, can be categorized into: time-resolved [93; 94], frequency domain [95; 96], spatially-resolved [56; 97], and spatial frequency domain [98; 99]. While time-resolved and frequency domain techniques are able to interrogate the tissues at a greater depth, they generally require more expensive and sophisticated instrumentation and have fewer wavelength selection options. On the other hand, spatially-resolved and frequency domain techniques are simpler in instrumentation and easier to implement with a broader spectral range; however, the measurement is usually confined to the superficial region of tissues. While these techniques provide a new approach for optical characterization of food and agricultural products [93; 100; 101], they are still short of meeting the expectations for food quality evaluation, because of the difficulty or inconvenience in carrying out measurements, sophistication in instrumentation, slow measurement speed and lack of robustness or measurement accuracy.  
In recent years, our group has been working on a new technique based on the spatially-resolved principle and hyperspectral imaging for measuring optical properties of horticultural products [102; 103; 104; 55]. The use of hyperspectral imaging technique provides a fast and more effective means for estimating the absorption and reduced scattering coefficients of horticultural and food products over a wide spectral range. In the following, we provide an overview on the development and application of this hyperspectral imaging-based spatially-resolved spectroscopic technique for optical property measurement and quality evaluation of horticultural products.