Inverse of the singular values of the pseudo inverse of the observation matrix, H, at each pixel of the DH (9–3 λ/D), for all three sets of probe shapes described in Sect. 4. High values represent poorly estimated pixels. All maps are plotted on the same scale. The condition numbers for the respective matrices are noted in their respective panels. Top: modulation maps for sets of two probe pairs, for all three probe shapes. All of them leave a strip of poorly modulated pixels along the central detector axis. Bottom: modulation maps for sets of three probe pairs, for all three probe shapes. In all three cases, using three probe pairs renders these maps very uniform, which fulfills the necessary condition in Eq. (10) and qualifies the probes for efficient electric field estimation.