Scale used: the information bar reads "15.0 kV, X5,000, WD 10.0 mm" with a scale bar labelled "1 um". The scale bar is 53 px long (x = 1202 to 1254, rows 968-973), so 1 um = 53 px, i.e. about 18.9 nm per pixel. The image is 1280 x 1024 px, and I analysed only the image area above the information bar.

Method: I removed the background with a Gaussian high-pass filter, applied a Hann window and took the 2D FFT. I cross-checked with a rotated 1D column-profile spectrum.

Result: the dominant spectral peak sits at a spatial frequency of about 0.0211 cycles/px, a period of about 47 px (46.5-49 px depending on the peak, with a broad peak, so the structure is slightly irregular). That is 47 px x 18.9 nm/px, about 890 nm (roughly 0.9 um, uncertainty about +/-5%).

Orientation: the wave vector lies almost along the image x-axis (about 3 degrees off horizontal). The periodic structures (laser-induced ripples/lines) therefore run almost vertically in the image, tilted about 3 degrees from the vertical, and are repeated horizontally at the period above.

RESULT: period = 890 nm
