Recent development in BTDF/BRDF metrology on large-scale Lambertian-like diffusers: application to on-board calibration units in space instrumentation

The Centre Spatial of Liege (CSL) is involved from more than 10 years in the BTDF/BRRDF metrology of large Lambertian diffusers used for on-board calibration on space instrumentation. In this context, a dedicated automatized BSDF calibration facility has been developed, suitable for calibration of all types of diffusers (from industry to space applications). Accurate calibration of such systems induces constraining requirements on the calibration bench: manipulate large diffusers and mechanisms in class ISO5 environment; incidence beam divergence close to the Sun divergence; large spectral range coverage; measurement of very low diffused signals (highly stable, with low noise, straylight free set-up), high accurate measurement of attenuator and a high knowledge of all calibration properties for BSDF modelling. Based on previous heritage, recent improvements have been implemented in order to correct some defaults like mechanical stabilities (complete new design), specimen alignment (new iterative procedure), absolute error (super stable light bulb, improved filters wheel repeatability, optics to limit diffraction effects in UV, incidence beam non-uniformity compensation …). This paper presents the design of the improved bench, highlights critical parameters for BTDF/BRDF calibration and relates main improvements to reach todays performances. The current bench performances are illustrated by calibration campaigns results performed at CSL for Sentinel 2, Sentinel 3 and Sentinel 4 calibration assemblies.