Confocal microscopy with an increased detection aperture: type-B 4Pi confocal microscopy.

We show the point-spread function of a confocal microscope with an increased detection aperture. The increase in aperture is accomplished by coherent collection of the light from the specimen with two opposing objective lenses, i.e., type-B 4Pi confocal microscopy. We demonstrate experimentally its feasibility for detecting scattered or fluorescently emitted light. The 4Pi confocal point-spread functions are shown for constructive and destructive interference of the collected wave fronts and are compared with the point-spread functions of comparable confocal microscopes.