The 4Pi-Microscope (in its commercially available version) uses two high resolution objective lenses to illuminate the sample (specimen) from both, the back and the front side. Using a single lens, even of the largest numerical aperture possible, only a segment of a spherical wavefront can be "imaged". As a result, the focal spot is longer (z-direction, axial) than wide (x,y-direction, lateral).
Object structures which are smaller than half the wavelength (250 nanometers for green light) can no longer be resolved, because of the blurred image.
Due to the two objective-lenses of the 4Pi microscope, this problem is partially solved; Both focal light spots are superimposed, and the blur can be removed (at least to a certain extent).
The optical resolution of the 4Pi microscope is determined mainly by the extension of the effective focussed 4Pi spot, and its axial extention is approximately 5-7 times sharper than the spot of a conventional Confocal Laser-Scanning Microscope.
The commercial 4Pi Microscope is available since spring 2005 at the KIP (further information see talk und press conference from 25.Apr.2005). Detailed information (also concerning the physical background) see further reading below.
Two-color 4Pi-Microscopy Image of PML and SUMO
Lang et al. 2010, J. Cell Science 123