Topographic images of a glass cover slide acquired with the same imaging speed. Top row: topography images of the surface of a commercial glass cover slide acquired at a scan frequency of 1 line/s, but at different settings for the feedback loop. Feedback response was decreased from left to right: proportional/integral parameters are 125/25, 80/16, 40/8, and 20/4 (in arbitrary units). Image size in points is 1024 × 1024 and the total acquisition time was about 18 minutes. The insets shows enlarged regions of each topographic image with an amplified grey scale. Lateral size of the larger images is 25 μ m, smaller images show a zoom of 5 μ m. The total grey scale of all large scale images is 8 nm and 1 nm for all insets. Bottom row: PSD curves calculated from the topographic images; the graphs show the logarithm of the PSD of surface roughness plotted versus the logarithm of the inverse length scale. For each graph the thinner yellow lines show the PSD curves of all the other images, while the thicker blue line shows the PSD curve for the correspondent topographic image shown in the same column. The grid lines for the PSD graphs are Δlog[κ] = 1 (horizontal axis) and Δlog[PSD] = 1 (vertical axis).