A Laboratory System for Multiangular Spectral Measurements of Vegetation

Accurate characterization of the directional spectral properties of vegetation is important for interpreting optical remote sensing observations and parameterizing radiative transfer models. We present a newly developed laboratory goniospectrometer for acquiring spectrally resolved bidirectional reflectance factor (BRF) measurements to characterize the bidirectional reflectance distribution functions (BRDFs) of vegetation. The system was designed to be versatile and easily configurable to accommodate various needs in vegetation remote sensing. For small targets, such as leaves and branches, it uses Köhler illumination to produce a small, homogeneous illumination footprint within the receiver field of view (FOV). By changing the illumination and receiver optics, the system is also configurable for measurement of larger targets, such as patches of short-statured vegetation. In this study we demonstrate its capabilities by using the configuration for small targets. Measurements of a Spectralon white reference panel demonstrate that the system is geometrically precise and spectrally stable, provides a usable signal over 400–2300 nm, and reproduces previously reported directional reflectance patterns. Measurements of Scots pine and silver birch branches further demonstrate the applicability of the system to small and structurally complex vegetation elements. The system provides a flexible laboratory platform for characterizing the directional spectral properties of vegetation and generating reference measurements for physically based interpretation and modeling of optical remote sensing observations.