Scale used: the information bar reads "[lab] - SEI 15.0kV x5,000 WD10.0mm" with a scale bar labelled 1 um (the label was read by OCR; the "um" is partly garbled). The scale bar measured 53 px in the image (x = 1202-1254), so 1 um = 53 px, about 18.9 nm/px. This is consistent with x5,000 on a 1280 px wide image (field of view about 24 um). The TIFF has no calibrated pixel size in its metadata.

Method: I subtracted the background (Gaussian, sigma 40 px), cropped away the information bar, applied a Hann window, and took the 2D FFT. I cross-checked with zero-padded 1D spectra of column profiles averaged over row blocks.

Result: the dominant spectral peak is at a spatial period of about 47-49 px. Three of the four row blocks give 46.0, 47.5 and 47.9 px. The fourth block, at the top of the cropped region, gave a spurious 110 px and was discarded. The mean of the three is about 47.5 px, which is about 0.9 um (about 900 nm). Estimated uncertainty is roughly +/-5% (about +/-40 nm). The peak is quite broad, with a secondary peak at about 44-53 px.

Orientation: the wavevector points along the image x axis (horizontal), within a few degrees of horizontal (peak at about -3 deg). The stripes/ripples therefore run roughly vertically in the image, tilted only a few degrees from vertical, with the period measured along the horizontal direction.

RESULT: period = 900 nm
