Hell, S. W., Schader, M. and Van der Voort, H. T. M.
Far-field fluorescence microscopy with three-dimensional resolution in the 100-nm range. Journal of Microscopy 187: 1-7 (1997).

We report three-dimensional (3D) microscopy with nearly isotropic resolution in the λ/5 - λ/10 range. Our approach combines 4Pi-confocal two-photon fluorescence microscopy with image restoration. The 3D resolution is demonstrated with densely clustered beads as well as with F-actin fibers in mouse fibroblast cells. A comparison with unrestored two-photon confocal images reveals a total reduction of the uncertainty volume up to a factor of 15.