Scale used: the scale bar in the information bar at the bottom of the image (image area is rows 0-963 of the 1280x1024 frame). The bar is 53 px long (x = 1202-1254) and is labelled "1 um", so 1 px = 1000/53 = 18.9 nm. I read the label from the rendered pixels; I did not read the magnification text.

Method: I removed the info bar, subtracted the background (Gaussian, sigma 60), applied a Hann window and took a zero-padded 2D FFT. The strongest peak is a single pair at a spatial frequency of about 0.0211 cycles/px, which is a period of about 47.4 px (about 47-48 px in the neighbouring peaks). Quadrant-wise FFTs of the image gave 44.5-46.5 px with the same orientation (one quadrant had no clear peak). Period = 47.4 px x 18.9 nm/px, which is about 890 nm (range about 840-900 nm).

Orientation: the FFT wave vector lies about 2.7 degrees from the image x axis. The stripes (ripples) are therefore nearly vertical in the image, tilted about 3 degrees from vertical, with the period measured horizontally. Weaker peaks, which are harmonics or small local variations, lie within about 10-20 degrees of this direction.

RESULT: period = 890 nm
