B to A can be either spontaneous or driven by light of another wavelength. The molecules have to be switchable several times in order to be present in state A or B at different times during scanning the sample. The method also works if the bright and the dark state are reversed, one then obtains a negative image. Resolution below the diffraction-limit In RESOLFT the area where molecules reside in state A (bright state) can be made arbitrarily small despite the diffraction-limit. One has to illuminate the sample inhomogeneously so that an isolated zero intensity point is created. This can be achieved e.g. by interference. At low intensities (lower than the blue line in the image) most marker molecules are in the bright state, if the intensity is above, most markers are in the dark state. Upon weak illumination we see that the area where molecules remain in state A is still quite large because the illumination is so low that most molecules reside in state A. The shape of the illumination profile does not need to be altered. Increasing the illumination brightness already results in a smaller area where the intensity is below the amount for efficient switching to the dark state. Consequently, also the area where molecules can reside in state A is diminished. The (fluorescence) signal during a following readout originates from a very small spot and one can obtain very sharp images. In the RESOLFT concept, the resolution can be approximated by , whereby is the characteristic intensity required for saturating the transition (half of the molecules remain in state A and half in state B), and denotes the intensity applied. If the minima are produced by focusing optics with a numerical aperture , the minimal distance at which two identical objects can be discerned is which can be regarded as an extension of Abbe’s equation. The diffraction-unlimited nature of the RESOLFT family of concepts is reflected by the fact that the minimal resolvable distance can be continuously decreased by increasing . Hence the quest for nanoscale resolution comes down to maximizing this quantity. This is possible by increasing or by lowering . Variants Different processes are used when switching the molecular states. However, all have in common that at least two distinguishable states are used. Typically the fluorescence property used marks the distinction of the states, however this is not essential, as absorption or scattering