Scale used: the information bar at the bottom of the 1280x1024 image (rows ~962-1023) contains a scale bar at x = 1202-1254 (53 px long) labelled "1 um" (I read the label by rendering the bar's glyphs as ASCII; the magnification text was not needed). So the scale is 1000 nm / 53 px = ~18.9 nm per pixel. The TIFF metadata has no usable calibration (72 dpi only).

Analysis: I cropped the image area above the info bar (rows 0-955), removed the background with a Gaussian high-pass, windowed it and took the 2D FFT. The strongest peaks sit on the horizontal frequency axis at about +/-27 cycles per image width (kx ~ 27, ky ~ 0-1), i.e. a period of about 47.5-48 px. A 1D column-averaged spectrum gives 48.3 px, and sub-blocks of the image give 47.3-49.8 px. That corresponds to 47.5-48.3 px x 18.9 nm/px = about 900 nm (range ~895-911 nm).

Orientation: the periodicity is along the horizontal (x) axis, so the periodic structures are stripes/ripples whose lines run vertically in the image (perpendicular to the horizontal wave vector, tilt within a few degrees of vertical).

RESULT: period = 900 nm
