What is apertureless of scattering NSOM?

Apertureless (or scattering) NSOM is an optical method that overcomes optical diffraction limits  to get optical images with the nanoscale resolution. Sharp metal coated AFM probes are illuminated with a far-field source. The electrical field is enhanced at the sharp metallized tip (as in the lightning rod effect) and thus the tip acts as the "hot spot".  The optical signal scattered from the probe is extracted from the far-field background using probe modulation methods.  This technique is used mostly for the opaque samples in reflection mode and has limitations for collection mode.