2nd. April

Recently, we met one problem about negative SiF value when there is no vegetation.

In the beginning, we suspected it was caused by longer optical path for the downward observation. But it turned out our ground experiment dose not support this idea.

So the another possible reason is the different fractions of diffuse and direct light for incoming and outgoing measurements. But it is hard to assess it with field measurements. And sadly, this paper proved this point.

Damm A, Guanter L, Verhoef W, et al. Impact of varying irradiance on vegetation indices and chlorophyll fluorescence derived from spectroscopy data[J]. Remote Sensing of Environment, 2015, 156: 202-215.