Small-scale variations in radiative transfer through forest canopies are strongly linked to canopy-structural heterogeneity. To date, upscaling of radiative transfer parametrizations developed at the point scale is hampered by (i) poor representation of canopy structure variability and (ii) limited spatially-explicit sub-canopy irradiance data to assess the performance of radiative transfer schemes at typical resolutions of land surface models. This study presents a novel approach for efficient in-situ characterization of canopy structure and sub-canopy irradiance over large spatial extents. The method involves a handheld radiometer assembly mounted on a motorized gimbal developed for non-stationary continuous measurements of shortwave and longwave radiation along forest transects. In combination with radiation and temperature data from a stationary reference station, spatially-resolved estimates of sky-view fraction, canopy transmissivity and longwave enhancement could be obtained. Under favourable meteorological conditions, validation against sky-view fraction data from hemispherical photographs yielded a RMSE of 0.03 (i.e. 3%). Irradiance measurements under heterogeneous canopy cover revealed strong spatial coherence between longwave radiation enhancement, shortwave radiation attenuation, and sky-view fraction on overcast days. Under clear-sky conditions however, sun flecks caused highly variable shortwave radiation transmissivity patterns. This study demonstrates the potential of handheld radiometer surveys to deliver valuable spatially-distributed datasets of co-located canopy structure and sub-canopy irradiance which can be used (i) as reference data for alternative approaches to derive canopy structure parameters, (ii) to improve modelling of sub-canopy radiation across a wide range of canopy distributions, and (iii) to support respective model upscaling efforts.