*3.1. REP, RE Slope and REA Measurement Results and Analysis*

The Dracaena leaves spectral measurement results are shown in Figures 7–10. In Figure 7, the first-order derivative spectral profiles of green leaf and yellow leaf from AOTF-HSL and SVC are presented. Figure 8 shows the results of aloe green and yellow leaf, and the remaining Figures 9 and 10 are the spectral slope curves for rubber plant and radermachera green leaf, respectively.

**Figure 7.** First derivative of the spectral reflectance versus spectral values of dracaena green and yellow leaf measured by the AOTF-HSL and the SVC spectrometer. (**a**) Dracaena green leaf, (**b**) Dracaena yellow leaf.

**Figure 8.** First derivative of the spectral reflectance versus spectral values of Aloe green and yellow leaf measured by AOTF-HSL and SVC. (**a**) Aloe green leaf, (**b**) Aloe yellow leaf.

**Figure 9.** Rubber plant green leaf.

Among these figures, it can be seen from Figures 7a, 8a, 9 and 10 that spectral slope curves of green leaves of the selected plants extracted from AOTF-HSL coincide with its corresponding referenced measurements from the SVC spectrometer. Tables 2–4 list the quantitative results of the corresponding vegetation parameter results and the differences between the active and passive methods; the percentages are calculated using difference values dividing the corresponding referenced results from SVC. The differences of REP of all green leaf test cases are below 1%, however, for the REP slope, only the aloe green leaf REP slope difference percentage is below 1%. Specifically, the REP slope of the dracaena green leaf is large than 10%. For green leaf REA results, dracaena green leaf difference is also the largest, and it is more than 5%.

**Figure 10.** Radermachera green leaf.

Figures 7b and 8b present the spectral slope curves representing Dracaena yellow leaf and aloe yellow leaf, respectively. Compared with green leaf curves, both the yellow leaf curves from AOTF-HSL measurements are distinctive from that of SVC. Further analysis is also given in Tables 2–4, where the REP differences are 9.45 nm and 24.1 nm, respectively, and the corresponding difference is considerably higher than in the green leaf cases.

In addition, from comparing the REP between green and yellow leaves from the same plant, it can be seen by comparing the curves of Figures 7a, 7b, 8a and 8b that REP has an obvious shift towards a shorter wavelength, named "blue shift", since the yellow leaves have lower chlorophyll content and this tendency or REP behavior is consistent with previous research results.


**Table 2.** "Red edge" position measuring results based on FRS.

**Table 3.** "Red edge" slope measuring results.



**Table 4.** "Red edge" area measuring results.
