Upper panels: logarithmic fit to the spiral arms in the de-projected 3.6 μm image of NGC 5194 (left) (only central parts considered) (18–23 μ_3.6μm (AB) magnitude scale) and NGC 3992 (right) (21–25 μ_3.6μm (AB) magnitude scale), using 2D Fourier transform spectral analysis (see text). Lower panels: logarithm of the m = 2 Fourier amplitude as a function of the pitch angle for the two galaxies shown above. The vertical line denotes the maximum value, that determines the global pitch angle assigned to the galaxy.