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Validation  of  Leaf  Area  Index  (LAI)  derived  from  moderate  resolution  remote  sensing observations generally involves optical technique to measure ground LAI. As the current validation datasets are derived using multiple optical retrieval techniques, assessment of the consistency between these techniques is required. In this study the effective Plant Area Index (PAI_eff ) retrievals by three major optical instruments, LAI-2000, AccuPAR, and Digital Hemispherical  Photographs  (DHPs),  were  analyzed  over  10 crops  (soybean,  corn,  alfalfa, sorghum, peanut and pasture) at Manfredi site in Cordoba province, Argentina. The focus of research was on quantifying PAI_eff sensitivity to the type of instrument, retrieval parameters and gap fraction inversion methods as well as environmental conditions (canopy heterogeneity, senescent vegetation, illumination conditions). Results indicate that sensitivity of DHP method to illumination conditions is low (14% compared to 28% and 86% for LAI-2000 and  AccuPAR,  respectively).  The  intercomparison  of  PAI_eff retrievals  indicates  large  discrepancies between optical techniques for short canopy over which downward-pointing DHP technique performs better than LAI-2000 and AccuPAR. Better agreement was found for tall canopy without  senescent vegetation and low spatial heterogeneity. Overall,  discrepancies  in  PAI_eff between  instruments  are  mainly  explained  by  differences  in  spatial sampling of transmittance between instruments (over short and heterogeneous canopies) caused  by  variations  in  instrument  footprint,  azimuthal  range,  and zenith angle  spatial resolution (coarser for LAI-2000 than DHP). Our results indicate that DHP is the most robust technique in terms of low sensitivity to illumination conditions, accurate spatial sampling of transmittance, ability to capture gap fraction over short canopy using downward-looking photographs,  independence  from  canopy  optical  ancillary  information,  and  potential  to derive clumping index. It can thus be applied to a large range of canopy structures, and environmental conditions as required by validation protocols.