A Laboratory Goniometer Facility for Measuring Reflectance Anisotropy in Support of the Sentinel Missions
Roosjen, P.P.J.; Clevers, J.G.P.W.; Bartholomeus, H.M.
Laboratory of Geo-Information Science and Remote Sensing / Wageningen University, NETHERLANDS
Anisotropic reflectance behaviour of natural surfaces affects all remotely sensed imagery and should therefore be included in the interpretation of remote sensing data. Reflectance anisotropy is a wavelength dependent effect and is caused by the illumination and viewing geometry and the optical and structural characteristics of the observed surface. Especially sensors with a large field of view (FOV) are strongly influenced by these effects. Wageningen University has built a laboratory goniometer facility for performing multi-angular reflectance measurements under controlled illumination conditions to investigate anisotropic reflectance effects. The core of the system is a programmable Kawasaki FS10E industrial robot arm on which an ASD FieldSpec 3 spectrometer, which measures the spectral reflectance in the range of 350-2500 nm, is mounted. The robot arm enables fast acquisition of biconical reflectance factors at a large number of positions and thereby describes the anisotropic reflectance behaviour of the sampled surface. The obtained multi-angular reflectance measurements can be used for exploration of the bidirectional reflectance distribution function (BRDF), for validation of BRDF models and for the development of novel methods for retrieving canopy biophysical and biochemical parameters by including BRDF information. This is in particular interesting for the upcoming Sentinel-2 and -3 missions, which will both have a relatively wide swath width. In future an improved BRDF correction could be included in their processing chain. The objective of the system is to determine BRDF correction models for different types of surface cover, but moreover, to use the full potential of BRDF data to derive biophysical and biochemical parameters from these future missions. So far, several soil and vegetation targets have been measured and the results indicate that the goniometer system is very well capable of capturing anisotropic reflectance behaviour. In future research, upscaling the laboratory measurements to airborne and satellite level will be studied.
Figure 1. a) The goniometer setup: the spectrometer is mounted on the robot arm. b) The laboratory during the measurements: the room is completely covered by highly absorbing black materials in order to avoid unwanted scattering. c) Interpolated polar plot of nadir normalized reflectance factors of a soil target, observed at 550 nm at an illumination angle of 45°. The black dots indicate the measurement positions and the circle indicates the hot-spot.