
Holographic SHG Imaging 221
in the axial direction. In fact, the precision of this method was evaluated at roughly 10–50 nm for
hologram-to-hologram comparison and 10 nm for comparison within the same hologram (Shaer et
al., 2010a). On the other hand, it also raises the problem of determining the position of two or more
particles located more than one fundamental wavelength apart in the axial direction. Even worst, for
discrete SHG scatterers, it is impossible to rely on unwrapping algorithms, because the wavefront is
discontinuous. Without a priori knowledge, recourse to the rst method presented here is the only way
to li the 2
π
phase a