properties could also be exploited. STED Microscopy (Main article STED microscopy) Within the STED microscopy (STimulated Emission Depletion microscopy) a fluorescent dye molecule is driven between its electronic ground state and its excited state while sending out fluorescence photons. This is the standard operation mode in fluorescence microscopy and depicts state A. In state B the dye is permanently kept in its electronic ground state through stimulated emission. If the dye can fluoresce in state A and not in state B, the RESOLFT concept applies. GSD microscopy (Main article GSD microscopy) GSD microscopy (Ground State Depletion microscopy) also uses fluorescent markers. In state A, the molecule can freely be driven between the ground and the first excited state and fluorescence can be sent out. In the dark state B the ground state of the molecule is depopulated, a transition to a long lived excited state takes place from which fluorescence is not emitted. As long as the molecule is in the dark state, it's not available for cycling between ground and excited state, fluorescence is hence turned off. SPEM and SSIM SPEM (Saturated Pattern Excitation Microscopy) and SSIM (Saturated Structured Illumination Microscopy) are exploiting the RESOLFT concept using saturated excitation to produce "negative" images, i.e. fluorescence occurs from everywhere except at a very small region around the geometrical focus of the microscope. Also non point-like patterns are used for illumination. Mathematical image reconstruction is necessary to obtain positive images again. RESOLFT with switchable proteins Some fluorescent proteins can be switched on and off by light of an appropriate wavelength. They can be used in a RESOLFT-type microscope. During illumination with light, these proteins change their conformation. In the process they gain or lose their ability to emit fluorescence. The fluorescing state corresponds to state A, the non-fluorescing to state B and the RESOLFT concept applies again. The reversible transition (e.g. from B back to A) takes place either spontaneously or again driven by light. Inducing conformational changes in proteins can be achieved already at much lower switching light intensities as compared to stimulated emission or ground state depletion (some W/cm²). In combination with 4Pi microscopy images with isotropic resolution below 40 nm have been taken of living cells at low light levels. RESOLFT with switchable organic dyes Just as with proteins, also some organic dyes can change their structure upon illumination. The ability to fluoresce of such organic dyes can