*2.2. Optical Properties*

The automatic integrating sphere system with IAD was employed to measure the μ*<sup>a</sup>* and μ- *s*. The diffuse reflectance and diffuse transmittance of the samples, reference and dark noise were needed to calculate μ*<sup>a</sup>* and μ- *<sup>s</sup>*, and the spectral signals (905–1650 nm) were collected by a spectrometer (SW2520, Isuzu Optics, Taiwan, China). Different measurement configurations were presented in Figure 1, while specific components of instruments, and measurements detailed descriptions, are presented by Wei et al. [36] and Prahl [37].

**Figure 1.** Illustration of the setup of automatic single integrating sphere (**a**), and the schematics for measurements on the reflectance of dark noise*Rd* (**b**), reference*Rr* (**c**) and sample*Rs* (**d**), the transmittance of dark noise *Td* (**e**), reference *Tr* (**f**) and sample *Ts* (**g**).

The accuracy of the estimated μ*<sup>a</sup>* and μ- *<sup>s</sup>* was validated with both pure water and 1% Intralipid® (Sigma Aldrich, St. Louis, MO, USA). The main ingredients were soybean oil, glycerin and egg yolk phospholipids. In addition, the particle size distribution in the solution was relatively uniform and stable, also very close to the main scattering particles (chylomicrons) in biological tissues. Thus, it was taken as standard scattering resolution. The scattering resolution was established by diluting 20% stock solution with water. The reference μ*<sup>a</sup>* and μ- *<sup>s</sup>* values were described by Van et al. [38] and Deng et al. [39]. The performance of this methodology was validated and the results were reported by Wei et al. [36].

According to the Monte Carlo simulation of Ma et al. [35], the penetration depth of light at 1198 nm is 3.3 mm in the whole tissue of Fuji apple. To ensure the penetration effect of light and the sample volume needed for chemical detection, two 2.5 mm thick flesh slices were cut from each sample apple in the opposite side. The samples were then cut into slices with 2.5 mm thickness × 30 mm width × 35 mm length and the thickness of each slice was measured by using digital calipers. The sample slices were then sandwiched with two pieces of quartz glasses to measure the spectral signals. The specific description of measurements can be found in Wei et al. [36] and Ma et al. [40].
