The key element in NSOM is a fiber optic cable that has been sharpened such that it ends in a sub-wavelength diameter tip. This tip is coated with aluminum such that it leaves only a sub-wavelength aperture uncoated at the very tip.
Light is seen emanating from the tip in the image shown below.
This image was taken under a conventional optical microscope. The response of the sample to the light emanating from the tip is recorded as a function of the tip position over the sample. In this manner the sample is imaged with very high resolution in the NSOM experiment. The high spatial resolution makes NSOM a very general advancement in optical microscopy. As such, NSOM can, in theory, be combined with any spectroscopic technique to gather spectra from small regions of a sample.