Bi-directional reflectance is dependent on the orientation of illumination source, the view point and the level of the surface that is being illuminated. This group conducted research observing nadir and off-nadir measurements in two different modes. (Click on image to enlarge.)
This section includes a graphical definition for Bi-Directional Reflectance and It also includes examples of visual and numerical data for different crops and bare soil surfaces.
This type of mast places the instrument on a pole that pivots at the ground surface. The spot the instrument views is relatively the same spot for each view zenith angle, but the spot size increases with increasing view zenith angle.