Two-dimensional spatial frequencies
Surface metrology

The notion of spatial frequencies is very useful for understanding the geometric structure of a surface. This notion is complicated by the fact that we are in two dimensions, and two frequencies of the same period can differ in direction (figure 2). The representation of a two-dimensional function by its spectrum is less intuitive than that of a temporal signal, but the mathematical formalism of the Fourier transform is quite comparable.

The notions of autocorrelation function and power spectral density, well known...