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Féret, J.B. et al., 2019. Estimating leaf mass per area and equivalent water thickness based on leaf optical properties: Potential and limitations of physical modeling and machine learning. Remote Sensing of Environment, 231. Physical model has more ability to generalize itself then empirical or statistical models. But the calibration and the inversion algorithm can be essential and complicated step.

Dechant, B., et al. (2017). "Estimation of photosynthesis traits from leaf reflectance spectra: Correlation to nitrogen content as the dominant mechanism." Remote Sensing of Environment 196: 279-292. Photosynthetic traits has its mechanism by leaf internal components. Comparing the spectroscopic absorption of causing components and their traits, they can be detected and correlation. In the case of Vcmax, N dominantly controls it.

Brodrick, P. G., Davies, A. B., & Asner, G. P. (2019). Uncovering Ecological Patterns with Convolutional Neural Networks.  Trends in ecology & evolution . When extracting foliar information from high resolution image, it would be quite hard work to detect the leaf, which one wants to know in the map. In the map, the pixel information varies depending on individual leaf angle. And it can change depending on solar zenith angle, which changes every observation. T hough one is interested in just the information of a certain leaf, s patial pattern have to be considered.

Asner, Gregory P., and Roberta E. Martin. Spectral and chemical analysis of tropical forests Scaling from leaf to canopy levels. Remote Sensing of Environment 112.10 (2008) 3958-3970. When up-scaling leaf level to canopy level, canopy structure may obstruct leaf traits to fully appear as the form of spectra. Except LAI, however, most canopy structural features had a negligible effect on the spectra created by canopy modeling.

Gara, Tawanda W., et al. Leaf to canopy upscaling approach affects the estimation of canopy traits. GIScience & remote sensing 56.4 (2019) 554-575. In remotely sensing, upper-most leaves are dominantly detected so they are usually measured for upscaling, assuming that sunlit leaves are representing the whole canopy. Short plants or plants with sparse leaves, however, cannot be seen dominated by sunlit leaves because of canopy vertical diversity. In that case, it is significantly better to measure sunlit and mid-layer leaves(approach B,D and E) more than to measure only sunlit leaves(C). Though r square in remotely sensing canopy chlorophyll was higher at C, there was no significant difference between 2-leaf and 1-leaf approach.