Measurement of the 4Pi-confocal point spread function proves 75 nm axial resolution

In a 4Pi-confocal microscope the specimen is illuminated and observed coherently from above and below such that the numerical aperture is increased [S. W. Hell, European Patent Application 91121368.4 (filed 1990, published 1992), S. W. Hell and E. H. K. Stelzer, J. Opt. Soc. Am. A 9, 2159 (1992)]. The point spread functions of 4Pi-confocal and confocal microscopes were measured. Our measurements prove a three- to seven-fold increase of axial resolution, thus opening the prospect for a powerful three-dimensional imaging technique with an axial resolution down to 75 nm.