The photoreceptor response is significantly lower than expected by the reduction in the photoreceptor acceptance angle of light entering near the edge of the pupil.
This demand of a large acceptance angle, however, is in conflict with the requirement of optimum depth resolution dependency on the detection geometry.
For example, the parabolic-trough concentration gives about 1/3 of the theoretical maximum for the design acceptance angle, that is, for the same overall tolerances for the system.
For a given concentration, nonimaging optics combine the widest possible acceptance angles with high efficiency and, therefore, are the most appropriate for use in solar concentration.