Scale used: the information bar reads "[lab] SEI, 15.0 kV (OCR approx.), x5,000, WD 10.0 mm, 1 um". The scale bar (rows 968-973, columns 1202-1254) is 53 px long and is labelled 1 um, so the scale is about 1000 nm / 53 px = 18.9 nm per pixel. The image is 1280 px wide, so the field of view is about 24 um.

Method: I cropped off the information bar, removed the background with a high-pass filter, applied a Hann window and took a 2D FFT. The dominant peak is at spatial frequency 0.0212 cycles per pixel, a period of about 47 px, along the horizontal axis (k-vector angle about 2.6 deg from horizontal). The autocorrelation of the column-averaged profile has peaks at 47, 93 and 139 px, which gives 46.3 to 47 px per period. At 18.9 nm per pixel this is about 885 nm (FFT 888 nm; autocorrelation about 875 nm).

Orientation: the periodic structures (laser-induced ripples or grooves) are stripes running vertically in the image, i.e. parallel to the image columns. The period is measured horizontally, perpendicular to the stripes. The vertical profile has no comparable periodicity.

RESULT: period = 885 nm
