The canopy radiation model EXTRAD was used to quantify the accuracy of leaf area index (LAI) estimations from vegetation indices (Vis), derived from green and infra-red crop reflectance. The Vis were the infra-red/ green (IR/GR) ratio, the normalized difference vegetation index (NDVI), the perpendicular vegetation index (PVI) and the weighted difference vegetation index (WDVI). The accuracy of LAI estimation was calculated in relation to variation in leaf green and infra-red colour, leaf angle distribution, soil background and illumination conditions. The theoretical calculations were supported with a field experiment on sugar-beet.