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Putting the specimen in the right light:
the condenser |
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Let us follow the path of the light in the microscope and begin with the condenser. As mentioned in detail before, the condenser illuminates the sample and images the luminous-field diaphragm on the specimen and the aperture diaphragm on the objective pupil. Without condensers, the microscope cannot achieve its maximum resolving power, and homogeneous illumination of the images is barely possible either.
The standard version of a simple condenser is designed as an “Abbe condenser”. This condenser type displays good quality in the imaging of the diaphragms and has one major benefit: one fixed front lens suffices to supply the objectives 4x to 100x with light – in compliance with the Köhler principle. Even with the simple version, the homogeneously illuminated fields are large enough to allow the use of eyepieces for fields of view with diameter 20 mm in the intermediate image. For objectives with a magnification of 2.5x or lower, an auxiliary lens can be used below the condenser which helps to illuminate the then very large fields in the specimen.
This condenser, for its part, is available in two versions; the simple brightfield version is shown on the left of above-mentioned illustration, the version with turret disk for phase contrast and darkfield diaphragms on the right. The rotatable turret disk features two precise stops to enable fast changeover between the contrasting techniques.
The dual condenser – also called pathology condenser – is not shown in the illustration; it features a sturdy switchover lever to allow the fast changeover between low-power observation and the viewing of details. However, the dual condenser has only been designed for use in brightfield.
Optimum quality is achieved with the aplanatic-achromatic condenser systems which are used mainly in research.
The traditional term “aplanatic” is often mistaken as a description of the image flatness (“plane”). In fact, however, the ancient Greek verb
meaning “to wander around”, is its root. The negating prefix “a” expresses exactly the opposite. The aplanatic condenser makes the light beams intersect exactly in the object plane. (The light beams do not “wander around”). An image of the luminous-field diaphragm is thus produced with edge-to-edge sharpness.
Achromatic means “free from color”
and indicates the state of correction which is high for a condenser.
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