On upright microscopes, the condenser is located beneath the stage and serves to gather wavefronts from the microscope light source and concentrate them into a cone of light that illuminates the specimen with uniform intensity over the entire viewfield. Inverted (tissue culture style) microscopes mount the condenser above the stage and specimen on a frame pillar. It is critical that the condenser light cone be properly adjusted to optimize the intensity and angle of light entering the objective front lens. Each time the objective is changed, a corresponding adjustment must be performed on the condenser to provide the proper light cone to match the numerical aperture of the new objective. The size and numerical aperture of the light cone produced by the condenser is determined by adjustment of the aperture diaphragm. After passing through the specimen, the light diverges into an inverted cone with the proper angle to fill the front lens of the objective.
The tutorial initializes with an image appearing in the Specimen Image field on the left-hand side of the tutorial, along with a cartoon of the condenser aperture diaphragm size adjacent to the specimen as it appears when viewing the objective rear focal plane through a Bertrand lens or by removing one of the eyepieces and peering into the observation tube. In order to operate the tutorial, use the Aperture Size slider to adjust the aperture opening diameter, and thus modulate the contrast level of the image. A new specimen can be selected from the Choose A Specimen pull-down menu.
Aperture adjustment and proper focusing of the condenser (with regard to height in relation to the objective) are of critical importance in realizing the full potential of the objective. Specifically, appropriate use of the adjustable aperture iris diaphragm (incorporated into the condenser or just below it) is of significant importance in securing correct illumination, contrast, and depth of field. The opening and closing of this iris diaphragm controls the angle of illuminating wavefronts that bathe the specimen (and thus the aperture size). Condenser height is controlled by a rack and pinion gear system that allows the condenser focus to be adjusted for proper illumination of the specimen. Correct positioning of the condenser with relation to the cone of illumination and focus (a step in establishing Köhler illumination) is critical to quantitative microscopy and to ensure the best digital images.
Contributing Authors
Rudi Rottenfusser - Zeiss Microscopy Consultant, 46 Landfall, Falmouth, Massachusetts, 02540.
Sunita Martini and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.