The principal idea of scanning near-field optical microscopy (SNOM) is based on scanning an arbitrarily small aperture which is illuminated from the backside at a close but constant distance across a sample surface and recording optical information pixel-by-pixel collecting either transmitted, reflected, or fluorescence light to form an image. The resolution of the optical image is solely defined by the size of the aperture due to the strong localization of the light to the aperture size at close distances, which is called the near-field.