Figure 2

(a) Graphical representation of the operating principle of the HP filter: light rays propagating through a weakly scattering medium are focused on the plane of the HP filter. Only light that experiences few scattering events is blocked by the filter, the remaining contributes to the speckle at the camera plane. (b) Full Width at Half Maximum of foci obtained at the end of the optimization process plotted as a function of the optical length of the samples. The green triangle represents the beam waist that impinges onto the sample. A comparison between the ordinary approach¹⁴ (no filter, blue circles) and the case with the 0.8 mm diameter HP filter (red squares) is shown. The three insets, starting from the top, correspond to foci obtained at , 0.81 and 6.3. White scale bars correspond to 30 μm.