**4. Discussion**

The development of active measuring methods to obtain spectral information is of great significance. The AOTF-HSL technique presented in this paper has the unique characteristic of collecting a continuous spectrum in the visible and near infrared (VNIR) regions with 10 nm spectral resolution, which is the best spectral resolution in any published paper. However, limited by the HSL hardware configuration and the data processing capacity, compared with SVC spectrometer, the spectral resolution is still slightly restricted. We presume this is the major reason contributing to the slight differences between AOTF-HSL and SVC in green leaves measuring results. In addition, for REP slope and REA, these results are quite distinctive and the percentages are all larger than 30%. We think that these following reasons might account for this phenomenon:


1 cm in this experiment with a field of view (FOV) of 0.2 mill radian. The sampled area of the spectrometer is larger (resulting in a 5.5 cm radius footprint with a 25◦ field of view). Area coverage by the laser pulse has different reflectivity due to the non-uniformity of the yellow leaves.

For the FRS method, the spectral resolution affects the calculation results. Due to the higher spectral resolution of the SVC, the spectral profile collected by SVC could contribute to more detailed descriptions of the reflectance slope changes. In the LET and LPFIT methods, the parameters are calculated using the selected reflectance at several specific spectral wavelengths, and some of them are not consistent with the selected wavelength of the AOTF-HSL. Some spectral reflectance values are calculated using the average values of the intensities from the nearby spectral wavelength. For instance, in the LET method, the 725 nm reflectance is calculated through averaging that of 720 nm and 730 nm, which might slightly affect the results. Moreover, in the LET method, only three spectral reflectance values from AOTF-HSL are employed to fit the FDR1 (680, 690 and 700 nm), which might bias the final results. Thus, we think following future works are necessary for extending the HSL enabling vegetation index estimation and determination:

