Scale used: the information bar reads "SEI 15.0 kV, x5,000, WD 10.0 mm" and shows a scale bar. I measured the bar in pixels (rows 968-973, columns 1202-1254, 53 px wide). I read its label as 1 um, so the scale is about 18.9 nm/px (about 1000/53). The image is 1280 px wide, which is about 24 um across the field. I analysed the SEM area only (rows 0-959, excluding the info bar).

Method: background-subtracted, Hann-windowed 2D FFT of the SEM area. The dominant spectral peak is at a period of about 47.5 px (range 46-49 px across sub-regions), which is about 900 nm. The 2D spectrum peaks near the horizontal frequency axis, and the angular power in the 20-130 nm-scale band is concentrated within about +/-20 degrees of it, mostly 170-180 degrees and 0-10 degrees. The wave-vector is therefore roughly horizontal (about 178 degrees, i.e. about 2 degrees off the x-axis). The periodic structures (ridges/lines) therefore run roughly vertically in the image, tilted a few degrees from vertical, and they repeat along the horizontal direction.

Dominant period of about 900 nm (about 0.9 um), with a spread of about 860-930 nm from sub-regions.

RESULT: period = 900 nm
