Scale used: the information bar at the bottom of the image (rows ~965-1023) contains a scale bar, drawn at rows 968-973, x = 1202-1254 (53 px long), labelled "1 µm". So the scale is 1000 nm / 53 px = 18.9 nm per pixel (the magnification text in the bar was not clearly readable, so I relied on the scale bar). The image area above the bar (1280 x ~960 px) was analysed.

Method: background-subtracted, Hann-windowed 2D FFT of the image area (zero-padded to 4096 for sub-bin peak refinement). The dominant peak (strongest by a wide margin, with weaker neighbouring peaks at about 860-920 nm) lies at a spatial period of about 47.6 px, i.e. about 898 nm (~0.9 µm). A band-weighted spectral centroid gives about 870 nm, consistent with the peak within a few percent.

Orientation: the frequency vector lies almost along the image x axis (about 2-3 degrees off), so the periodic structures are line-like ridges/grooves (LIPSS-type) running nearly vertically in the image, tilted about 2-3 degrees from the vertical axis; the periodicity is measured across them, along the horizontal direction.

RESULT: period = 900 nm
