Scattering scanning near-field optical microscopy (s-SNOM) is an imaging method in\nwhich a laser is focused onto the tip of an atomic force microscope (AFM) in the proximity\nof a sample. Due to a near-field coupling between the tip and the sample, the scattered\nradiation from this system contains super-resolution optical information about the sample.\nThis is called the near-field signal. Additionally, the radiation contains background\ncontributions due to secondary light-matter interactions with the tip and the sample. These\ndo not contain near-field information. This work investigates the interference between the\nnear-field and background contributions as a function of various system parameters. To\nthis end, a direct-detection s-SNOM configuration with a higher-harmonic demodulation\nscheme is considered. Here, the AFM probe harmonically oscillates at its resonance\nfrequency. Firstly, the detected s-SNOM signals are investigated from a theoretical point\nof view via Fourier analysis. It can be concluded that the interference between the near-\nfield and background contributions induces different effects, including spatial interference\nfringes and a background-related amplification of the near-field contrast. In addition to the\ntheoretical investigation, s-SNOM signals are experimentally analysed on gold and silicon\ndioxide samples. A wavelength of 10.6 μm is considered. In the experiments, different\nparameters are varied such as the polarisation, the distance between the tip and sample,\nradiation angles and the overall geometry of the AFM tip. These parameters influence\nthe individual contributions that constitute the detected s-SNOM signals. In general, the\nexperimental results confirm the theoretical predictions. Consequently, this work paves a\nway towards being able to optimise s-SNOM signals as a function of the different system\nparameters.