Scale used: the information bar at the bottom (rows ~965-1024) contains a scale bar labelled "1 um" (read by OCR and by inspecting the glyph pixels). The bar is 53 px long (x = 1202-1254, rows 968-973), so 1 um / 53 px = about 18.9 nm per pixel. The image is 1280 x 1024 px and the 960 rows above the bar are the SEM image. The magnification text in the bar was only partly legible by OCR (about "X10,000"), so I relied on the scale bar rather than on the magnification.

Method: the image area (rows 0-959) was background-flattened and Hann-windowed, then a zero-padded 2D FFT was taken. There is one dominant pair of peaks at spatial frequency of about 0.0210 cycles/px along the x axis, i.e. a period of about 47.6 px (the 1D x-profile gives 48.2 px). Weaker side peaks at about +/-18 to 22 degrees from the x axis come from slight curvature and irregularity of the lines.

Period: 47.6 px x 18.87 nm/px = about 900 nm (roughly 0.9 um, estimated uncertainty about +/-30 nm).

Orientation: the wave vector points along the horizontal image axis (about 2 degrees from horizontal), so the periodic structures are line-like ripples (LIPSS-type) running vertically, i.e. parallel to the image y axis, within a few degrees, with the periodicity measured along the x direction.

RESULT: period = 900 nm
