Scale used: the information bar of input.tif (1280x1024) contains a scale bar labelled "1 µm" that is 53 px long (cols 1202-1254), giving about 18.9 nm per pixel (I did not rely on the magnification text). The image area is rows 0-~955.

Method: background-subtracted, Hann-windowed 2D FFT of the image area (zero-padded), plus a column-profile autocorrelation as a check. The strongest peak is at a spatial period of about 47.4-47.6 px (autocorrelation: 47 px), i.e. about 895-900 nm (about 0.9 µm). The peak is sharp and its second harmonic region is consistent.

Orientation: the wave vector lies almost along the image x axis (about -2 degrees from horizontal), so the periodic structures are stripes/ripples running essentially vertically in the image (about 2 degrees off vertical), repeating horizontally with a period of about 0.9 µm.

RESULT: period = 895 nm
