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| Illumination in the transmitted light |
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It is no exaggeration to say that almost the entire art of microscopy – if specimen preparation is not taken into account – consists in the correct use of the luminous field and aperture diaphragms. Thankfully, there are simple rules for this. The following pages describe in detail how to correctly set the microscope for Köhler illumination. It will be much easier for you to understand the Köhler principle if you made yourself familiar with the meaning of the special optical planes mentioned before. Their relationship with each other can be simplified as follows:
Planes
"A" to "D" |
Fields |
For fields of view, illumination, intermediate images.
Components: reticles and scales.
Setting is made via the luminous-field diaphragm. |
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Planes
"1" to "4" |
Pupils |
For resolving power, contrasting techniques, brightfield.
Components: light filter and contrasting.
Setting is made via the aperture diaphragm. |
The fact that the beams are superimposed may also have negative consequences. The best example of this are dust particles on a reticle in the eyepiece: these particles will be sharply imaged together with the microscope image and do not exactly embellish the microphoto.
The above makes it clear that the condenser – which concentrates the illuminating light ray on the specimen – plays a vital part in microscopy: it is as important as objectives and eyepieces. The condenser makes the specimen appear in the right light.
If you only want to get optimum intensity in the illumination, select the critical illumination system, where the light source is not imaged on the pupils, but on the object. The homogeneity of the illumination is lost, however. This type of illumination is obtained by the deliberate adjustment of the lamp collector (fluorescence).
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