When you first start out using a microscope, or in the routine examination of slides, you might only adjust the eye pieces, objectives, and the focus controls. However, you shouldn’t overlook the microscope condensers as they are an important (if not essential) part of the whole optical system of a microscope.
The adjustment of the condenser is especially important in setting up for correct Koehler Illumination, which is a technique used to provide even illumination and produce a sharper image.
Admittedly, not all condensers are found under the stage, it depends on the configuration of the microscope you use—on an inverted microscope, you’ll find the condenser is situated above the stage and your specimen.
However, the purpose of a condenser is the same in any configuration, to focus the light coming from the source and illuminate a specimen with an even intensity and contrast.
A brief history
In 1665, the English scientist, Robert Hooke, published a book called ‘Micrographia’ which became one of the most important works on microscopy at the time (and included in its pages was the first recorded use of the word ‘cell’). The microscope which Hooke used in his work was not only elaborately and beautifully decorated around the main barrel, but he also used an oil lamp which shone through a salt-water filled globe as his condenser.1
The development of condensers somewhat lagged behind that of the new corrected objectives which were appearing throughout the 18th and 19th Centuries. Although achromatic lenses were being produced in the 18th Century (these are lenses corrected for ‘chromatic aberration’—for more information on aberrations and corrections, see this Bitesize Bio article), it wasn’t until 1837 that similarly corrected condensers were first developed in France.2 It was another four years until the achromatic condensers were commissioned in Britain and it took until 1870 until they became widely used throughout Europe.2
The new condenser design
The most commonly used condenser today is known as the ‘Abbe Condenser’ and was invented by Ernst Karl Abbe in 1870. Abbe is more commonly known as one of the co-founders of the Zeiss Optical Workshop, and he also co-founded the Schott AG, the makers of specialist and laboratory glassware.3
The Abbe condenser contains two or three lenses, and the focusing of the light is controlled by focus wheels similar to those of the fine/coarse focus on the main body of the microscope. The other components consist of an iris diaphragm to adjust the overall diameter of the illuminating light and centering screws. Some Abbe condensers also have an additional swing-out top lenses which are used to provide full illumination over the field of view, depending on the magnification of objectives used (swung in for high power, swung out for low power).
Numerical aperture adjustment and considerations
As well as adjusting the diameter of the illuminating light, the integrated iris diaphragm also serves to change the numerical aperture (NA) of the condenser.
The distance between the condenser and the specimen will vary according to the objective that is used (see the link to the Koehler Illumination article above). Once the correct distance has been determined, the iris diaphragm is opened or closed, which changes the angle of the illuminating light cone and subsequently the NA of the condenser.
High-end Abbe condensers can have an NA of up to 1.4, but the lower-end versions can only achieve an NA of around 0.6. The NA of the condenser should match that of the objective which is in use. The maximum theoretical NA of a dry lens (i.e., non-immersion lens) is 1.0, but in practice, an immersion medium is required to fully realize an NA which is greater than 0.95. Therefore, when using an oil immersion lens with an NA greater than 1.0, an oil immersion condenser should be used.
Optical corrections in condensers
Condensers can be corrected for optical aberrations much in the same way as objectives. Abbe condensers are not corrected for optical aberrations, but there are two levels of optical correction available with other condensers; aplanatic (or spherical) and achromatic (or chromatic).
Aplanatic condensers are corrected for spherical aberration and bring all of the parallel light waves passing through the condenser to the same focal point (instead of being spread along the optical axis). Achromatic condensers are corrected to bring the red and blue wavelengths of light to the approximate same focal point as green light. The aplanatic-achromatic condensers offer the highest level of correction and contain up to eight lenses to correct for chromatic and spherical aberrations.4
The condenser is an integral part of the light path through a microscope and should be correctly adjusted to get the best from the objectives and to achieve optimal images. So don’t forget about the parts underneath (or above) the stage!
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