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In all conventional illumination techniques, mentioned above, resolving power is dependent on the wave lengthes of the visible light spectra and the numerical aperture of the microscope“s lens. When the numerical aperture is 1.40, the resolving power is about 0.20 µm when the specimen is illuminated by monochromatic green light (wave length 550 nm, fig. 1). This value can be regarded as the usual limit of light microscopic resolution (4). Submicroscopic structures, smaller than 0.20 µm, can only be recognized in flourescence microscopy when a light microscope is used.
Fig. 1: Aperture and lateral resolution (modified from 4) NA 0.9 and 1.3 are marked In light microscopy, the depth of focus is determined by the respective magnification and the numerical aperture (fig. 2) . The lower the numerical aperture, the higher is the focal depth when the magnification remains constant.
Fig. 2: Aperture, magnification and focal depth (modified from 3)
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