3.2.2. Experiment 2: Investigating Light Penetration Depth

The second experiment was to quantify the light penetration depth in apple tissue under sinusoidal illumination. 'Golden Cream Delicious' apples, which were free of visual blemishes or defects and grown in Shandong, China, were purchased from a fruit market. Both simulation and practical experiments were carried out. In the first, the simulation methodology proposed by Hayakawa et al. [33] was adopted to roughly determine the optical sampling depth in apple tissues. In this method, Monte Carlo (MC), which has been widely applied to simulate light propagation in single- and multiple-layered biological tissues, was employed for the simulation experiment [8,29,34]. Optical property parameters (*μa*, *μ <sup>s</sup>*) of apple tissues at the six wavelengths were measured by our integrating sphere system [35], which were taken as the inputs in the simulation.

**Figure 3.** Schematic of relative positions and sizes of holes in the nylon sample filled with absorbing ink solution.

For the practical experiment for validating the results of the simulations, there were two types of samples, apple slices and a USAF-1951 target. A slicer was used to produce apple slices with different thicknesses with peel [0.9, 1.0–1.1, 1.3, 1.5–1.6, 1.6–1.7, 1.8, 2.1–2.2, 2.4–2.5, 2.6, 3.0, 3.4, 3.8, 4.0] mm and without peel [0.8, 1.0, 1.2–1.3, 1.5, 1.6, 1.8, 2.0, 2.3, 2.5–2.6, 2.7–2.8, 3.0, 3.2, 3.6, 4.0] mm. These varying-thickness apple slices were used to cover the USAF-1951 target, which was made of fiber material with high scattering properties, and their combination was imaged by the SFDI system, illuminated with a sequence of frequencies of 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 mm<sup>−</sup>1. It is apparent that the light has completely penetrated though the apple slice when the black horizontal bars of the USAF-1951 are recognized, as shown in the region of interest (ROI) in Figure 4. Two parameters in the ROI, i.e., image contrast and PVR, as mentioned above, were calculated to resolve the image details. Image contrast was evaluated based on the Michelson contrast metric (CM) [36]:

$$\mathcal{C}\_M = (I\_{\max} - I\_{\min}) / (I\_{\max} + I\_{\min}) \tag{9}$$

where *Imax* and *Imin* denote the maximum and minimum intensities of the image in ROI, respectively. Another evaluating parameter, PVR, defined as the ratio of peak and valley's intensity [37], was also determined from the captured images:

$$PVR = I\_{p\text{cak}} / I\_{\text{valalty}} \tag{10}$$

where *Ipeak* and *Ivalley* denote the intensities of the peak and valley in the ROI, respectively.
