Radiance factor (I∕F) as measured by VIR at 0.55 μm against the modeled values (I∕F_m) at the corresponding observation geometry, for different solutions of the Hapke model discussed in Sect. 3.3: S2 (a), S3 (b), and S4 (c). The black line in each plot indicates the ideal behavior I∕F = I∕F_m. The color in the contour plots is proportional to the square root of the point density after normalization of the maximum value to unity.