RESOLFT

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RESOLFT is an acronym for REversible Saturable OpticaL Fluorescence Transitions.

In a RESOLFT concept the resolution can be approximated by $\Delta d = \frac{\lambda}{\pi \cdot n\cdot\sqrt{\frac{I}{Isat}}}$ , whereby Isat is the characteristic intensity required for saturating the transition, and I denotes the intensity applied. If the minima are produced by focusing optics with a numerical aperture nsin α, the minimal distance at which two identical objects can be discerned is $\Delta d = \frac{\lambda}{2n\cdot\sin\alpha\cdot\sqrt{1+\frac{I}{Isat}}}$ 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 $\varsigma = \frac{I}{Isat}$ . Hence the quest for nanoscale resolution comes down to maximizing $\varsigma = \frac{I}{Isat}$ . This is possible by increasing I or by lowering Isat.^[1]