*3.2. Comparison of REP Results from Di*ff*ernent Calculating Methods*

In Section 3.1, the REP is calculated using the FRS technique. Actually, there are other methodologies for interpolating the reflectance REP, for instance, Linear Four-point Interpolation Technology (LFPIT), and Linear Extrapolation Technology (LET) [37,38]. For further validating and evaluating the proposed active remote sensing method, the AOTF-HSL spectral information is employed to calculate the REP based on LFPIT and LET.

With the parameters listed in Table 1, the selected wavelength spectral reflectance in LFPIT and LET beyond the wavelength scope of the LCTF-HSL, AOTF-HSL is more applicable than LCTF-HSL. In this section, only the spectral profiles collected by AOTF-HSL are used for extraction of these parameters. Tables 5–10 give the results of the four REP determination methods. Among them, four tables (Tables 5, 7, 9 and 10) are the REP results from the green leaves, and the other two (Tables 6 and 8) are the REP results from yellow leaves of dracaena and dloe.

Firstly, in the aspect of the green leaves results, the AOTF-HSL and SVC give a similar REP using the four different results, with differences all below 1% except for the rubber plant REP using the LFPIT method. The lower spectral resolution of HSL may account for the minor differences between AOTF-HSL and SVC. In this paper, the AOTF-HSL spectral resolution is 10 nm, and the SVC spectral resolution is better than 2 nm.


**Table 5.** The three different methods' "Red edge" position results for dracaena green leaf.

**Table 6.** The three different methods' "Red edge" position results for dracaena yellow leaf.


**Table 7.** The three different methods' "Red edge" position results for aloe green leaf.



**Table 8.** The three different methods' "Red edge" position results for aloe yellow leaf.

**Table 9.** The three different methods' "Red edge" position results for rubber plant.


**Table 10.** The three different methods' "Red edge" position results for radermachera green leaf.


Secondly, in aspect of the yellow leaves (Tables 6 and 8), the differences of the results between AOTF-HSL and SVC calculated by the LFPIT and LET methods are all more than 5%, and the figures are slightly higher than the FPS-derived REP results. Compared with the green leaves, the differences between AOTF-HSL and SVC are larger in the yellow leaf cases. In addition, the three REP-derived methods give similar results in the green leaf cases, including AOTF-HSL and SVC, however, there is an obvious difference in the yellow leaves results. Especially, in the AOTF-HSL REP of the dracaena yellow leaf case, the LEPIT and LET methods afford similar results, but they are different from the FRS results. As aforementioned, the spectral resolution has an influence on the operation of LET and LEPIT, and in this experiment, the spectral resolution of AOTF-HSL is selected as 10 nm, which might affect the calculation of the results.
