Scale used: the information bar reads X8,000 (magnification), WD 10.0 mm, and a scale bar labelled "1 um". I measured the scale bar in pixels: it is a solid white bar 53 px long (x = 1202..1254, rows 968-973), so 1 um = 53 px, i.e. 18.9 nm per pixel (image 1280 x 1024 px, the SEM area is the top ~955 rows; the info bar was excluded from the analysis).

Method: background-subtracted (Gaussian sigma 40), Hann-windowed 2D FFT of the SEM area, plus a column-averaged profile. The strongest spectral peak is at a spatial period of about 47 px (45-48 px across the neighbouring peaks, power-weighted mean about 46 px), i.e. about 890 nm (range roughly 860-910 nm). The mean x-profile gives 47.4 px = 894 nm.

Orientation: the spectral power is concentrated around the horizontal frequency axis (wave vector along x, spread about +/-20 degrees, secondary peaks at about +/-17 degrees). So the structures are periodic stripes/ripples whose wave vector is along the horizontal image axis, with the ridges/grooves running approximately vertically (top-to-bottom) in the image, with a spread/tilt of up to about 20 degrees from the vertical.

RESULT: period = 890 nm
