Scale used: the information bar reads "SEI 15.0kV, x5,000, WD 10.0 mm" with a scale bar labelled 1 um. The bar is 53 px long in the image (measured in code), so the scale is 1000 nm / 53 px = about 18.9 nm per pixel. The OCR dropped the micro sign and read the label as "1m", so I took it as 1 um, which is consistent with x5,000.

Method: I cropped off the information bar (rows below about 955), subtracted a Gaussian background (sigma 60 px), applied a Hann window and took the 2D FFT. The strongest peaks sit at a spatial period of about 47.3 px (neighbouring peaks at 45.7 to 49.2 px; the radial-profile maximum is at about 46.5 px). That gives 47.4 px x 18.9 nm/px = about 895 nm.

Orientation: the FFT peaks lie along the horizontal frequency axis, tilted about 3 degrees. The wave vector therefore points horizontally, and the periodic ridges or lines run approximately vertically in the image. They are tilted about 3 degrees from the vertical, and the period is measured perpendicular to them, along x.

RESULT: period = 895 nm
