Scale used: the information bar reads SEI, 15.0 kV, x5,000, WD 10.0 mm, with a scale bar labelled 1 um (OCR read it as "1m"; I took 1 um from the x5,000 magnification). The bar is 53 px long (x = 1202-1254), so the scale is about 18.9 nm per pixel (1 um / 53 px). Image is 1280 x 1024 px; the bottom ~65 rows are the information bar and were excluded from the analysis.

Method: removed the background (Gaussian high-pass, sigma 40 px), applied a Hann window and took the 2D FFT of the image area. The strongest peak is at a spatial frequency of about 0.021 cycles/px, a period of about 47 px (46-48 px across sub-regions), which is about 890 nm (875-900 nm). A very weak low-frequency contribution near 140 px (~2.7 um) appears only in the top half and is not the dominant structure.

Orientation: the FFT peak lies on the horizontal frequency axis (wavevector angle about 0 deg, within +/-5 deg), so the periodic modulation runs along the image x direction. The structures are therefore ripples/lines that run vertically (parallel to the image y axis, within a few degrees), repeating horizontally with a period of about 0.9 um.

RESULT: period = 890 nm
