A new method to determine multi-angular reflectance factor from
lightweight multispectral cameras with sky sensor in a target-less workflow
applicable to UAV
A new physically based method to estimate hemispheric-directional reflectance
factor (HDRF) from lightweight multispectral cameras that have a downwelling
irradiance sensor is presented. It combines radiometry with photogrammetric
computer vision to derive geometrically and radiometrically accurate data
purely from the images, without requiring reflectance targets or any other
additional information apart from the imagery. The sky sensor orientation is
initially computed using photogrammetric computer vision and revised with a
non-linear regression comprising radiometric and photogrammetry-derived
information. It works for both clear sky and overcast conditions. A
ground-based test acquisition of a Spectralon target observed from different
viewing directions and with different sun positions using a typical
multispectral sensor configuration for clear sky and overcast showed that both
the overall value and the directionality of the reflectance factor as reported
in the literature were well retrieved. An RMSE of 3% for clear sky and up to 5%
for overcast sky was observed
