.

  Zarco-Tejada, P. J., Miller, J. R., Noland, T. L., Mohammed, G. H., & Sampson, P. H. (2001). Scaling-up and model inversion methods with narrowband optical indices for chlorophyll content estimation in closed forest canopies with hyperspectral data.  IEEE Transactions on Geoscience and Remote Sensing ,  39 (7), 1491-1507. In this paper, the author uses vegetation index as a merit function. The method is more stable than using all channels. Integrating empirical method, physics based model inversion could be improved. 

Bousquet, L., Lachérade, S., Jacquemoud, S. and Moya, I., 2005. Leaf BRDF measurements and model for specular and diffuse components differentiation. Remote Sensing of Environment, 98(2-3): 201-211. Specular reflectance originates from leaf surface. On the other hand, diffuse reflectance originates from leaf intrastructure. To focus on the target traits, the two reflectance needs to be separated.

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.