Super-resolution microscopy, in light microscopy, is a term that gathers several techniques, which allow images to be taken with a higher resolution than the one imposed by the diffraction limit. Due to the diffraction of light, the resolution in conventional light microscopy is limited, as stated (for the special case of widefield illumination) by Ernst Abbe in 1873. In this context, a diffraction-limited microscope with numerical aperture N.A. and light with wavelength λ reaches a lateral resolution of d = λ/ (2 N.A.) - a similar formalism can be followed for the axial resolution (along the optical axis, z-resolution, depth resolution). The resolution for a standard optical microscope in the visible light spectrum is about 200 nm laterally and 600 nm axially. Experimentally, the attained resolution can be measured from the full width at half maximum (FWHM) of the point spread function (PSF) using images of point-like objects. Although the resolving power of a microscope is not well defined, it is generally considered that a super-resolution microscopy technique offers a resolution better than the one stipulated by Abbe.
Super-resolution imaging techniques include single-molecule localization methods, photon tunneling microscopy as well as those that utilize the Pendry Superlens and near field scanning optical microscopy, the 4Pi Microscope, confocal microscope (with closed pinhole), or confocal microscopy aided with computational methods such as deconvolution or detector-based pixel reassignment (e.g. re-scan microscopy, pixel reassignment), and also structured illumination microscopy technologies like SIM and SMI.