Basis functions in spatial domain of Fourier coefficients , with order , 1, and 2 for even spectral function, being finite in spectral domain with centre frequency = 1873.7 GHz and bandwidth . For this domain, the sinc functions, contributing to the basis functions, are narrower than for = 500 GHz and = 1000 GHz (see Figure 1(a)). As an example, the basis function for (dashed-black) is given. Since holds, approximately 4 times more Fourier coefficients locate inside a given spatial domain. was chosen to equal the Nyquist frequency , having set the spatial sampling increment μm (black dots).