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 in creased [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 confoc al microscopes were measured. Our measurements prove a three- to seven -fold increase of axial resolution, thus opening the prospect for a po werful three-dimensional imaging technique with an axial resolution do wn to 75 nm.