.

Barreto, M. A. P., Johansen, K., Angel, Y., & McCabe, M. F. (2019). Radiometric Assessment of a UAV-Based Push-Broom Hyperspectral Camera.  Sensors ,  19 (21), 4699. As for the dark reference, the standard deviation was higher at the edge position. Also the position would be overlapped when UAV flying. It is important to put the interesting objects to be the center of view.

Hovi, Aarne, et al. "Evaluation of accuracy and practical applicability of methods for measuring leaf reflectance and transmittance spectra."  Remote Sensing  10.1 (2017): 25. Leaf clip can sense only certain angular light. The reflectance significantly differs from integrating spheres which can sense hemisperical radiant flux, as leaf surface is not assumed as a lambertian surface.