Scale used: the scale bar in the information bar (bottom right, JEOL-style bar: "SEI 15.0 kV, x5,000 (OCR reading, not fully certain), WD 10.0 mm, 1 um"). The bar is 54 px long (columns 1202-1255 of the 1280 px wide image) and labelled 1 um, so 1000 nm / 54 px = 18.5 nm per pixel (field of view about 23.7 um wide). The TIFF itself has no calibrated metadata, so the scale comes from the bar.

Method: I cropped off the information bar (rows 0-954), subtracted a Gaussian background and applied a Hann window. I then took the 2D FFT with zero padding and found the strongest peak in the 15-150 px period range. The peak is at a period of about 47.4 px, which is about 877 nm. The spectral peak is a single, strong, narrow line pair. Quadrant sub-images give 846-878 nm (one quadrant was too featureless to give a clear peak). The angular distribution of spectral power is concentrated at k directions of 0 +/- 10 degrees (horizontal).

Orientation: the spatial-frequency vector is nearly horizontal (about 178 degrees from the x axis, i.e. within about 2 degrees of horizontal). The periodic structures (ripples / lines) therefore run vertically in the image (perpendicular to the horizontal x axis), repeating along the horizontal direction with a period of about 0.87 um.

RESULT: period = 877 nm
