4Pi microscopy

On the same note, 4Pi microscopy features the largest aperture and hence the smallest focal spot. Yet it does not break the diffraction barrier it just pushes diffraction to its limits. 

All diffraction-unlimited nanoscopy methods can provide improved axial resolution even when implemented with a single lens [77,83,120,131,132[. However, because it starts out from less favorable values, the -resolution usually remains worse than its focal plane counterpart. The coherent use of opposing lenses pioneered in 4Pi microscopy and FM, however, facilitates an independent resolution improvement factor by 3-7-fold along the optic axis as has already been demonstrated with STED [83,84,87]. In the stochastic single molecule switching modalities, a similar gain in resolution will take place by the coherent use of opposing lenses [133]. Thus, while 4Pi microscopy and FM did not break the diffraction barrier, they remain cornerstones of far-field fluorescence nanoscopy in the future. 

A. Egner et al., 4Pi-microscopy of the Golgi apparatus in live mammalian cells. J. Struct. Biol. 147(1), 70-76 (2004)... 

J. Bewersdorf, B.T. Bennett, K.L. Knight, H2AX chromatin structures and their response to DNA damage revealed by 4Pi microscopy. Proc. Natl. Acad Sci. USA 103, 18137-18142 (2006)... 

M. Lang et al., 4Pi microscopy of type A with 1-photon excitation in biological fluorescence imaging. Opt. Expr. 15(5), 2459-2467 (2007)... 

For many fluorescence-based imaging techniques, such as confocal laser scanning microscopy, spinning disk confocal microscopy, 4pi microscopy, stimulated emission depletion (STED) microscopy, and other superresolution microscopy techniques, we refer to the indicated literature. 

Hell S W and Nagorni M 1998 4Pi confocal microscopy with alternate interference Opt. Lett. 23 1567-9... 

The practical z-resolution increase from 400-800 nm down to 70-150 nm, i.e., by 3-7-fold, constituted the hrst substantial resolution improvement in far-held optical microscopy in many decades [48,51-53,63]. As a result, 4Pi and I M imaging most visibly challenged the notion of the time that far-held resolution was a closed matter and near-held optics the only way to go. 

M. Nagorni, S.W. HeU, 4Pi-confocal microscopy provides three-dimensional images of the microtubule network with 100- to 150-nm resolution. J. Struct. Biol. 123, 236-247 (1998)... 

H. Gugel et al.. Cooperative 4Pi excitation and detection yields 7-fold sharper optical sections in live cell microscopy. Biophys. J. 87, 4146-4152 (2004)... 

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