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(a) The four different regions at the structures (in side view): the ridges (the red region), the backward-facing steps (the yellow region), the valleys (the green region), and the forward-facing steps (the purple region); (b) the average of ten images of the structures (in top view), taken using the transmission wide field illumination rather than the TIRFM illumination; (c) the gradient (along the x-axis) of the image shown in (b).
