Scale used: the image has no calibration in its metadata (72 dpi only), so I used the information bar. The scale bar in the bottom-right is 53 px long (columns 1202-1254, rows 968-973), and its label reads "1 um" (read from the rendered glyphs). That gives 1000 nm / 53 px = about 18.9 nm per pixel. I did not decode the magnification text. The image area is rows 0-949 of 1280x1024 px; the info bar starts at row ~960.

Method: background-subtracted the image area (Gaussian sigma 60), applied a Hann window and a zero-padded 2D FFT. The strongest peak is at a period of 47.6 px (the column-averaged profile gives 48.2 px). Neighbouring peaks span 45.7-49.2 px, so the structures are slightly irregular. At 18.9 nm/px this is about 47.6-48 px = 900 nm (range about 860-930 nm).

Orientation: the wave vector is almost horizontal (about 2-3 degrees off the x axis), so the surface structures are periodic ridges/stripes running nearly vertically in the image, tilted by about 3 degrees from the vertical. They repeat along the horizontal (x) direction.

RESULT: period = 900 nm
