2D FFT Wavelength Contour

Hi,

I've been working on image processing with 2D fft, and there are some points I'm struggling to understand due to lack of knowledge. The image below shows a contour of a surface -height map, and a fft of that surface. The figure on the right is the result of fft of the image with x and y axes indicating wavelength, while z (colour map) indicating the wave number, as the paper suggests.

What I normally use 2D fft for is to see spectral density figure. However, this time I want to achieve excatly same figure with wavelengths on x/y axes, and wavenumber on z axis. I attach the sample surface data I am working on, the resolution on both x and y is deltax=deltay = 55 (microns).

Any help/idea is much appreciated!

Best.