Scale used: the info bar at the bottom of the image (rows ~962-1023) contains a scale bar of 53 px length (x = 1202-1254, rows 968-973) labelled 1 um (the label was read from a pixel rendering of the text, so this reading is by code, not by eye). That gives 1000 nm / 53 px = 18.9 nm per pixel (image 1280 x 1024 px; field of view about 24 um wide). I did not use a separately read magnification value; the scale bar was the basis.

Analysis: I excluded the info bar (analysed rows 0-954), subtracted a Gaussian background, applied a Hann window and took the 2D FFT (4x zero-padded). The dominant peak is at a spatial period of 47.4 px (about 47 px in the autocorrelation as well, with a second peak at 93 px), i.e. 47.4 px x 18.9 nm/px = about 894 nm (~0.9 um). A weak second harmonic appears at about 24 px.

Orientation: the FFT peak lies on the horizontal frequency axis (about 2 deg from it), so the periodic structure varies along the image x direction. The ridges/lines (LIPSS-like) therefore run essentially vertically in the image (about 2-3 deg off vertical), with the period measured horizontally, perpendicular to the lines.

RESULT: period = 894 nm
