Scale used: the information bar reads SEI, 15.0 kV, X5,000, WD 10.0 mm, with a scale bar labelled 1 um (OCR of the label was weak; it read "1m", taken as 1 um). The bar measured 53 px wide (columns 1202-1254), so 1 um / 53 px = 18.9 nm per pixel. The image is 1280 x 1024 px, with the data area above row ~964.

Method: I subtracted the background (Gaussian, sigma 40), applied a Hann window, and took the 2D FFT of the data area. The strongest peaks sit on the horizontal frequency axis (wave vector within about 3 degrees of the x axis) at a period of 47-49 px. A 1D autocorrelation of the column-averaged profile has peaks at 47, 93, 142, 191, 241 and 290 px, giving 48.3 px per period. Rotation-scanned profile spectra give 47-49 px as well.

Result: the dominant period is about 48 px, which is about 0.91 um (about 910 nm; the spread across methods is roughly 860-930 nm).

Orientation: the periodicity runs along the horizontal (x) direction, so the ripples or lines are elongated nearly vertically in the image. They are within about 3 degrees of the vertical axis, with a slight tilt.

RESULT: period = 910 nm
