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Köhler Illumination: Interactive Tutorials

Condenser Aperture
Diaphragm Adjustment

Correct adjustment of the condenser aperture diaphragm is one of the most critical aspects for obtaining superior image quality, because it controls the numerical aperture, resolving power, depth of field, and the overall image character. Detailed instructions for use of this tutorial appear below the applet window.

To operate this tutorial, use the radio buttons to toggle the Bertrand lens in and out of the optical pathway (a phase telescope in place of a regular eyepiece could similarly be used). The initial state of the tutorial is without a Bertrand lens present and the specimen is visible in the viewfield under 10x magnification. When the Bertrand lens in toggled into position with a radio button, the viewfield displays how the back focal plane of the objective appears when viewed through the microscope eyepieces (Note: the viewfield only shows the specimen at 10x magnification). The slider is used to adjust the size of the condenser aperture diaphragm until it is somewhere between two-thirds and four-fifths the diameter of the fully illuminated back focal plane. This is the optimum position for the diaphragm to give the best compromise between resolution and contrast. When the objective is changed via the radio buttons, a new image will appear in the viewfield that approximates the back focal plane view of the new objective. Students should practice using this tutorial until they are comfortable trying this with a real microscope.

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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