  Metasurfaces are promising tools towards novel designs for flat optics
applications. As such their quality and tolerance to fabrication imperfections
need to be evaluated with specific tools. However, most such tools rely on the
geometrical optics approximation and are not straightforwardly applicable to
metasurfaces. In this Letter, we introduce and evaluate, for metasurfaces,
parameters such as the intercept factor and the slope error usually defined for
solar concentrators in the realm of ray-optics. After proposing definitions
valid in physical optics, we put forward an approach to calculate them. As
examples, we design three different concentrators based on three specific unit
cells and assess them numerically. The concept allows for the comparison of the
efficiency of the metasurfaces, their sensitivities to fabrication
imperfections and will be critical for practical systems.
