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A sensitivity analysis is performed for both crop parameters and external factors, using the SAIL canopy reflectance model and the PROSPECT leaf reflectance model, towards the possibilities of using vegetation indices (VIs) in estimating the fraction of absorbed photosynthetically active radiation (FPAR). Results from this theoretical study show that a linear relationship may be assumed between WDVI (Weighted Difference VI) or NDVI (Normalized Difference VI) and FPAR as an approximation. External factors (soil background, ratio diffuse/total irradiation, solar zenith angle) do not have a large influence on the relationship between FPAR and the WDVI. Moreover, leaf parameters (such as leaf chlorophyll content, leaf mesophyll structure and hot spot size-parameter) also have quite a small influence for green leaves, as concluded from simulations with the combined PROSPECT-SAIL model. The main crop parameter influencing the relationship between WDVI and FPAR is the leaf angle distribution (LAD). So, for different LADs different regression functions must be used. Although the relationship between FPAR and NDVI is slightly less influenced by LAD and solar angle, important disturbing factors are the soil background and the leaf chlorophyll content.