*2.2. REP Extraction Methods*

The first-order differential of the spectral reflectance is the most commonly used method to extract the REP values. Since the spectral profiles are sampled with Δλ, Equation (2) is employed for calculating the first differential of the spectral reflectance. The equation is given in detail as:

$$\rho'(\lambda\_i) = \left[\rho(\lambda\_{i+1}) - \rho(\lambda\_{i-1})\right] / 2\Delta\lambda\_\prime \tag{2}$$

where, λ*<sup>i</sup>* is the corresponding spectral wavelength, ρ-(λ*i*) is the first-order differential of the spectral reflectance, ρ(λ*i*+1) and ρ(λ*i*−1) are reflectance values at spectral wavelength of λ*i*+<sup>1</sup> and λ*i*−<sup>1</sup> respectively, 2 × Δλ is the spectral increment between λ*i*+<sup>1</sup> and λ*i*−1. In this AOTF-HSL system, 10 nm spectral resolution is selected, thereby λ*<sup>i</sup>* is sampled as 670, 680, 690, 700, 710, 720, 730, 740 and 750 nm in the spectral band.

Apart from this, another two RE parameters, RE slope and RE area, are also included. REP refers to the position of the RE in spectral wavelength, and REP slope is the spectral reflectance slope of the REP. REA refers to the area surrounded by the first derivative of the spectral reflectance, and it is calculated by the accumulating the reflectance slope with the spectral range from 680 nm to 750 nm [32–37]. Illustrative figures of these RE related parameters can be found in [32–37].

In addition, another two methods, Linear Four-point Interpolation Technology (LFPIT) and Linear Extrapolation Technology (LET), are employed for REP determination. Firstly, LFPIT is based on Equations (3) and (4). In this experiment, the method employs four wavelength data for calculating the REP. As illustrated, the 670 nm and 780 nm spectral information is used to calculate the reflectance at the REP, and the 700 nm and 740 nm spectral data are for determining the REP. In Equations (3) and (4), *R*<sup>670</sup> and *R*<sup>780</sup> are the corresponding reflectance values at 670 nm and 780 nm respectively. *R*<sup>700</sup> and *R*<sup>740</sup> are the corresponding reflectance values at 700 nm and 740 nm respectively:

$$R\_{REP} = \frac{(R\_{670} + R\_{780})}{2},\tag{3}$$

$$
\lambda\_{REP} = 700 + 40 \frac{(R\_{REP} - R\_{700})}{R\_{740} - R\_{700}},
\tag{4}
$$

Secondly, the LET method can be represented by Equations (5)–(7), and the REP is determined using the two extrapolation equations. Equation (5) is the extrapolation of the reflectance for spectral wavelengths ranging from 680 nm to 700 nm. Equation (6) is the extrapolation of the reflectance for spectral wavelengths ranging from 725 nm to 760 nm. Then, the REP is determined using the parameters (*m*1, *m*2, *c*<sup>1</sup> and *c*2) from Equations (5) and (6). In the following equations, the *FDR*<sup>1</sup> and *FDR*<sup>2</sup> are the spectral reflectance slope of the spectral ranging 680 nm and 700 nm and 725 nm and 760 nm. *m*1, *m*2, *c*<sup>1</sup> and *c*<sup>2</sup> are the parameters for describing the spectral reflectance slope and determining the REP. The REP determination using this method is as Equation (7):

$$FDR\_1 = m\_1 \lambda + c\_1 \tag{5}$$

$$FDR\_2 = m\_2 \lambda + c\_{2\prime} \tag{6}$$

$$
\lambda\_{REP} = \frac{-(c\_1 - c\_2)}{m\_1 - m\_2},
\tag{7}
$$

#### **3. Results**

As aforementioned, four different plants with green or yellow leaves were used in the laboratory experiments for the AOTF-HSL testing. Figure 6 shows these measured plants, namely dracaena (Figure 6a), aloe (Figure 6b), rubber plant (Figure 6c) and radermachera (Figure 6d). The AOTF-HSL and the SVC spectrometer are employed to measure the REP, the corresponding reflectance REP slope and REA. As aforementioned in Section 2, the LCTF-HSL is not sufficient for determining the REA parameters according to the LCTF parameters.

(**a**) Dracaena (**b**) Aloe

(**c**) Rubber plant (**d**) Radermachera

**Figure 6.** Four different plants employed in lab experiment. (**a**) Dracaena, (**b**) Aloe, (**c**) Rubber plant, (**d**) Radermachera.
