Figure 3.

Images of a Platinum surface taken all at a scan frequency of 1 line/s, but at different settings for the feedback loop. From left to right, feedback parameters settings are (in arbitrary units): 90/45, 45/22.5, 15/7.5, 5/2.5 and 1/0.5. The images and graphs in the same column correspond to a common data set, since the corresponding images have been acquired simultaneously at a fixed values of the feedback loop. The upper row shows topographic images, the middle row amplitude images and the lower row graphs of the PSD curves of surface roughness. In each graph, the PSD of surface roughness has been calculated for the topography and amplitude image shown in the corresponding column, as well as for the difference z_true = z_fb - ΔI_err (corresponding image not shown). Lateral size of the larger images is 10 μm, smaller images show a zoom of 1 μm. The total grey scale of all images is 5 nm (large and small scale images as well as topographic and amplitude images). Lower row: PSD curves calculated from the topography and amplitude images as well as the difference data (image not shown). The grid lines for the PSD graphs are Δlog[κ] = 1 (horizontal axis) and Δlog[PSD] = 1 (vertical axis). Green lines correspond to the PSD curves of topography plus error signal, blue curves to the topography and red curves to the error signal.