Scale used: the information bar (OCR) reads SEI, 15.0 kV, x5,000, WD 10.0 mm, with a 1 um scale bar. The bar measures 53 px in the image, so 1 um / 53 px = 18.9 nm per pixel (field of view about 24 um wide). The TIFF has no usable calibration metadata, so the scale bar was used.

Method: 2D FFT of the image area above the info bar (Hann window). The power spectrum has a single dominant peak pair at spatial frequency about 0.021 cycles/px, i.e. a period of about 47.5 px (radial profile maximum 47.6 px, FFT peak 47.3 px, spread 45-49 px). That is 47.5 px x 18.87 nm/px = about 896 nm.

Orientation: the FFT wave vector lies along the horizontal image axis (about -2 degrees), so the periodic structures (ripple-like lines/grooves) run vertically in the image, tilted by about 2 degrees from the vertical, with the spacing measured along the horizontal direction.

RESULT: period = 896 nm
