Scale used: the information bar reads SEI 15.0 kV, x5,000, WD 10.0 mm, with a scale bar labelled 1 um. I measured the bar in the image as 53 px long (columns 1202-1254), so 1 um / 53 px = about 18.9 nm per pixel (1280 px is about 24 um wide). The structured area is rows 0-960; the info bar below was excluded.

Method: I removed the background with a Gaussian high-pass and applied a Hann window. A 2D FFT then shows a strong peak at a spatial period of about 47 px (45.5-49 px across the peak and the neighbouring peaks), which is about 885 nm (860-900 nm). The 1D spectrum along x gives about 49 px (about 930 nm). The mean radial frequency in the 32-70 px band gives about 45.7 px (about 865 nm).

Orientation: the dominant wave vector lies close to the horizontal image axis (about 175-177 degrees, within roughly +/-15-20 degrees). The lines/ridges therefore run roughly vertically in the image (about 85-90 degrees from the x axis, slightly tilted). The period is measured along the horizontal direction, perpendicular to the ridges. The spread in angle suggests some waviness or irregularity in the ripples.

RESULT: period = 885 nm
