Scale used: the information bar reads "SEI 15.0kV x5,000 WD10.0mm" with a 1 um scale bar. The bar is 54 px long (columns 1202-1255 in the 1280x1024 image), so 1000 nm / 54 px = 18.5 nm per pixel. The image area is rows 0-~960; the bar is below it.

Method: I subtracted the background, applied a Hann window and took the 2D FFT of the image area. The strongest peaks sit at a spatial frequency of about 1/47.4 px, with the wave vector almost exactly along the image x axis (within about 3 degrees). A 1D profile averaged along y gives the same period, 47-49 px depending on the small tilt applied.

Result: the dominant period is about 47.4 px x 18.5 nm/px = about 880 nm (range about 850-910 nm). The structures are periodic ridges (laser-induced periodic surface structures) running vertically in the image, i.e. along the y axis, within a few degrees. The periodicity is along the horizontal (x) direction.

RESULT: period = 880 nm
