Condensers

For a high image brightness, a condenser should send as much as possible of the available illumination light to the specimen, and this under conditions where the light is usable for image formation. In bright field transmission microscopy, for example, one should create a light cone where the range of propagation angles approximately matches the numerical aperture of the used objective (or is slightly smaller):

• Obviously, light which either transmits the sample plane at locations outside the observed object or cannot enter the objective would not be useful for image formation, and partially even disturbing (e.g. through glare from stray light).
• On the other hand, the numerical aperture of the condenser should not be lower than that of the objective, because otherwise the full image quality of the objective in terms of resolution, contrast and depth of field could not be realized.

The maximum numerical aperture (or its range) can often be read from an engraving on the condenser housing. There may also be a scale which indicates the related numerical aperture, so that it can be correctly chosen corresponding to the used objective.

A condenser should provide uniform illumination of the object. It should be avoided, for example, to image the filament of an illumination lamp (typically a tungsten halogen lamp) onto the specimen, resulting into a severe modulation of illumination intensity. The common method of Köhler illumination avoids that problem by creating illumination conjugate planes which are well separated from the image-forming conjugate planes.

Substantial chromatic effects, resulting from chromatic dispersion of the optics, should be avoided for reaching optimum image quality. Therefore, achromatic condensers e.g. with three or four lenses have been developed.

Due to the high angular range of the illumination light to be handled, it can also be advantageous to have an aplanatic condenser or a condenser with an aspheric lens which minimizes spherical aberrations. Aspheric condensers allow for the realization of the largest angular opening, i.e., smallest f-numbers.

With more sophisticated condenser designs, it is also possible to correct for spherical and chromatic aberrations at the same time; one then has an achromatic compound condenser, also called aplanatic–achromatic condenser.

In simple microscopes, one still often uses the simple Abbe condenser with two lenses, which is not corrected for chromatic or spherical aberrations.

The applied correction (e.g. aplanatic or achromatic) can often be read from an engraving on the condenser housing.

Additional items are required for modified microscopy techniques. For example, a dark field stop is required for dark field microscopy, and phase rings are applied for phase contrast microscopy. Such optical elements may be inserted in a slot between the illuminator and the condenser.

Obviously, the condenser system should usually be readjusted when the microscope objective is exchanged with one having a different magnification and numerical aperture. Thus, the axial position of the condenser and the diaphragm aperture diameter is often made adjustable. In particular, a condenser should allow some range of numerical apertures, usually adjusted with a variable diaphragm. For the highest numerical apertures (more than 0.95), where oil immersion objectives are used, immersion oil may also be required for the condenser. It is applied between the upper lens of the condenser and the lower surface of the specimen slide.

In some cases, where objectives with a wide range of magnifications can be used, it is even necessary to exchange condensers or at least to insert or remove the top lens, since the required condenser parameter ranges in terms of illuminated area and NA otherwise cannot be spanned.

For illumination systems with high power, there may be substantial heating effects in the condenser, which can exclude the use of plastic optics and even of cemented optics.

Modern illumination systems increasingly make use of light emitting diodes (white LEDs), which produce far less heat than halogen lamps. Besides, their small dimensions also create opportunities for substantial simplification of the illumination optics (leading to entirely different condenser designs) and for various variations of illumination type. For example, one can realize diffuse illumination which minimizes detrimental effects of reflections, particularly on shiny samples.

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