Scale used: the information bar reads "SEI 15.0kV x5,000 WD10.0mm" with a 1 um scale bar (OCR). The bar measures 53 px (x = 1202 to 1254), so 1 um = 53 px, or about 18.9 nm per pixel. The image is 1280 x 1024 px, with a 960 px image area above the bar.

Method: I removed the background (Gaussian high-pass), applied a Hanning window, and took the 2D FFT of the image area. The dominant peak is a symmetric pair at spatial frequency about 0.0211 cycles/px (period about 47 px). The wave vector lies along the horizontal image axis, within about 3 degrees. A 1D FFT of the column-averaged profile gives 48.3 px, and the autocorrelation shows peaks at 47, 93 and 137 px (about 46 px per period). The mean is about 47.5 px, or about 900 nm, with a spread of roughly +/-30 nm.

Orientation: the periodic ripples (laser-induced periodic surface structures) run vertically in the image, i.e. the ridges are parallel to the image y-axis and are repeated along the x-axis, within a few degrees of vertical.

RESULT: period = 900 nm
