Condenser

A condenser is one of the main components of the optical system of many transmitted light compound microscopes. A condenser is a lens that serves to concentrate light from the illumination source that is in turn focused through the object and magnified by the objective lens. It is a basic component of almost all compound light microscopes manufactured since the 19th Century. An equivalent condenser, which focuses an electron beam, is a basic component of both transmission and scanning electron microscopes.

Description

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Early condensers

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Most condenser lens systems for research grade microscopes (1000X) have an N.A. value of 1.25. The 100X objective lens is also rated at 1.25. The medium between the 100X lens and the slide can be air or oil. Without getting too technical, the only way to get a Numerical Aperture greater than 1.0 is to use a material with a refractive index greater than 1.0. Oil (1.5) is such a material. So to get the best resolution at 1000X (N.A. 1.25), a drop of this special oil is placed between the lens and the slide. This is why most 100XR objective lenses are called oil immersion lenses. They work fine without oil but will give a sharper image with it (see image comparisons below). The oil can get messy and must be wiped up when you are finished. As a result, most people prefer to use their 100X objectives dry. If you try using immersion oil, clean up with lens tissue or a KimWipe on the lens. Avoid solvents.

Abbe condenser

The Abbe condenser is named for its inventor, Ernst Karl Abbe, who developed it in 1870. The Abbe condenser is mounted below the stage of the microscope, and concentrates and controls the light that passes through the specimen and enters the objective. It has two controls, one which moves the Abbe condenser closer to or further from the stage, and another, the iris diaphragm, which controls the diameter of the beam of light. The controls can be used to optimize brightness, evenness of illumination, and contrast. Abbe condensers are particularly important for magnifications of above 400X.

The Abbe condenser is the basis for most modern light microscope condenser designs.

Aplanatic and achromatic condensers

Several types of condenser represent improvements on the optical design of a basic Abbe condenser. An aplanatic condenser corrects for spherical aberration in the concentrated light path, while an achromatic condenser corrects for chromatic aberration. Even more highly improved is an achromatic-aplanatic condenser, which corrects for both.

Specialized condensers

Dark field and phase contrast setups are based on an Abbe, aplanatic, or achromatic condenser, but to the light path add a dark field stop or various size phase rings. These additional elements are housed in various ways. In most modern microscope (ca. 1990s–), such elements are housed in sliders that fit into a slot between the illuminator and the condenser lens. Many older microscopes house these elements in a turret-type condenser, these elements are housed in a turret below the condenser lens and rotated into place.

In epifluorescence microscopy, the objective lens acts not only as a magnifier for the light emitted by the fluorescing object, but also as a condenser for the incident light.

Condensers and numerical aperture

Like objective lenses, condensers can vary in their numerical aperture (NA). Different condensers vary in their maximum and minimum numerical aperture, and a the numerical aperture of a single condenser varies depending on the where the diameter of the condenser aperture is set. In order for the maximum numerical aperture (and therefore resolution) of an objective lens to be realized, it must be matched with condenser set to like numerical aperture. The technique most commonly used in microscopy to optimize the light pathway between the condenser (and other illumination components of the microscope) and the objective lens is known as Köhler illumination.

As with objective lenses, a condenser lens with a maximum numerical aperture of greater than 0.95 is designed to be used under oil immersion (or, more rarely, under water immersion), with a layer of immersion oil placed between the bottom of the slide and the top lens of the condenser. Without this oil layer, not only is maximum numerical aperture not realized, but the condenser is not able to precisely focus light on the object. Condensers with a numerical aperture of 0.95 or less are designed to be used without oil or other fluid on the top lens and are termed dry condensers. Dual dry/immersion condensers are basically oil immersion condensers that can nonetheless focus light with some degree of precision even without oil between the top lens and the slide. However, a dedicated dry condenser with an NA of 0.95 or less will focus light more precisely than a dry/immersion condenser used without oil immersion.

Condensers in electron microscopy

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References

• "Abbe condenser", Photonics Dictionary (abridged online edition), Pittsfield MA: Laurin Publishing, 2006.
• "Abbe, Ernst", Encyclopædia Britannica.
• "Glossary of microscope terms", Microbus (website), 2003.
• "Anatomy of the Microscope: Substage Condenser" by Mortimer Abramowitz and Michael W. Davidson, Olympus Microscopy Resource Center, 2006.

External links

• "Anatomy of the Microscope: Substage Condenser" by Mortimer Abramowitz and Michael W. Davidson, Molecular Expressions. (Slightly different than the version found at Olympus site.)
• "The Condenser" by Paul James, Micscape Magazine (online publication), February 2002.