van Oijen, A. M., Kohler, J., Schmidt, J., Muller, M. and Brakenhoff, G. J.
3-dimensional super-resolution by spectrally selective imaging. Chemical Physics Letters 292: 183-187 (1998).

The mutual spatial positions of individual pentacene molecules embedded in a p-terphenyl host crystal, residing within the same diffraction-limited volume, are determined with far-field optics with an accuracy far below the Rayleigh distance. This is achieved by spectrally selective imaging, an approach based on the combination of confocal microscopy, single-molecule detection and position-sensitive imaging. It is demonstrated that the molecules can be localized in three dimensions with a precision of 40 nm in the lateral and 100 nm in the axial dimension which represents an enhancement by a factor of 20 and 65, respectively, compared to the Rayleigh distance at the used numerical aperture of the microscope.