A new high-aperture glycerol immersion objective lens and its application to 3D-fluorescence microscopy

High-resolution light microscopy of glycerol-mounted biological specimens is performed almost exclusively with oil immersion lenses. The reason is that the index of refraction of the oil and the cover slip of approximately 1.51 is close to that of approximately 1.45 of the glycerol mountant, so that refractive index mismatch-induced spherical aberrations are tolerable to some extent. Here we report the application of novel cover glass-corrected glycerol immersion lenses of high numerical aperture (NA) and the avoidance of these aberrations. The new lenses feature a semi-aperture angle of 68.5 degrees, which is slightly larger than that of the diffraction-limited 1.4 NA oil immersion lenses. The glycerol lenses are corrected for a quartz cover glass of 220 microm thickness and for a 80% glycerol-water immersion solution. Featuring an aberration correction collar, the lens can adapt to glycerol concentrations ranging between 72% and 88%, to slight variations of the temperature, and to the cover glass thickness. As the refractive index mismatch-induced aberrations are particularly important to quantitative confocal fluorescence microscopy, we investigated the axial sectioning ability and the axial chromatic aberrations in such a microscope as well as the image brightness as a function of the penetration depth. Whereas there is a significant decrease in image brightness associated with oil immersion, this decrease is absent with the glycerol immersion system. In addition, we show directly the compression of the optic axis in the case of oil immersion and its absence in the glycerol system. The unique advantages of these new lenses in high-resolution microscopy with two coherently used opposing lenses, such as 4 Pi-microscopy, are discussed.