Measured spectral bidirectional reflection properties of three mature hemiboreal forests

• We measured BRF of hemiboreal forests with high angular resolution.
• We engineered a special BRF-sensor for this task.
• We report 3D and 2D BRF-profiles for a pine, birch, and spruce stand.

A combined spectrometer-BRF sensor was developed and built at Tartu Observatory, Estonia. The BRF sensor records angular distribution of target radiance with high angular resolution in red or near-infrared spectral band in a plain selected with sensor orientation up to 70° zenith angles. Bidirectional reflectance factor (BRF) measurements on board a low flying helicopter over three mature forest stands in South-East Estonia – a birch stand, a spruce stand, and a pine stand – are reported. Airborne measurements are supported by extensive ground truth data which have been reported in separate publications. The measurements revealed that both the BRF values and the BRF shape depend on the forest type and wavelength. The asymmetry of reflectance factor in the principal plane is strong and the hot-spot shape is very sharp in coniferous stands in the red spectral band. Multiple scattering of radiation in the NIR spectral band smooths the angular distribution of forest BRF. The collected data allow to validate forest radiative transfer models.