The Condenser – Its Use, Types and Applications

Discover different types of condensers, their optical performance and how they are used. Learn how to properly adjust the condenser aperture stop for optimized contrast modulation in transmitted light brightfield microscopy.

Key Learnings:

• The condenser is an optical system used to illuminate the specimen plane homogeneously, with good contrast and low stray light.
  
• Different types of condensers, such as Abbe, aplanatic, achromatic, and aplanatic-achromatic, have varying levels of optical performance.
  
• The free working distance and the color correction of a condenser is important in determining its compatibility with different sample types.

The transmitted light microscope – whether upright or inverted – uses an optical system to illuminate the specimen plane homogeneously, with good contrast and low stray light: the condenser. The optimum illumination method to achieve this is called Koehler Illumination. The Koehler Illumination requires a light source, collector optics, luminous field stop and an adjustable condenser with aperture stop. The condenser usually emits a cone of light. The cone angle depends on the actual aperture of the condenser. It can be modulated by the aperture diameter of the condenser iris: a large aperture diameter produces a large light cone, a small aperture diameter results in a small light cone angle. The light cone angle is matched to the objective´s numerical aperture (NA) and the sample details to be imaged. Depth of field can also be influenced by the diameter of the aperture stop:

Large diameter aperture stop – shallow depth of field
Small diameter aperture stop – strong depth of field

The tutorial shows a condenser with an adjustable aperture stop diaphragm. Details on the right show the actual values for the illumination angle and numerical aperture (NA).

To operate the tutorial, use the Numerical Aperture Control slider to adjust the NA of the condenser. As you move the slider from left to right, observe the changes in the cone of illumination as the numerical aperture is increased.

Condensers are also described by the amount of color correction of the field stop image and/or the geometric correction of the field and/or aperture stop image.

Technically, there are four condenser types:

• Abbe
• aplanatic
• achromatic
• achromatic-aplanatic

The most common ones today are the Abbe type and the achromatic-aplanatic type.

In 1869, Prof. Ernst Abbe developed the first condenser in Jena, Germany, to provide more even and controlled sample illumination. Since then, everyone calls a simple condenser an “Abbe condenser” in honor of this great man.

Today, the Abbe condenser is predestined for routine work and can be used dry or with immersion between the front lens and the slide underside. When used dry, the optical performance is lower. Abbe condensers have a simple optical design and often do not allow the field stop edge to be imaged at 100x objective magnification. The field stop edge is imaged with fairly strong blue or purple color fringes, making them less favorable for color critical work (e.g. hematology). The image of the aperture stop – when viewed in the objective´s back focal plane – is not as sharp and contrasty as with the more advanced types. Due to their simpler optical design, Abbe condensers are well suited for work with transmitted polarized light.

The aplanatism (aberration-free) or aplanatic behavior of optical elements describes their geometric performance (distortion, coma, spherical aberration, astigmatism) when imaging larger objects away from their optical axis. It was first described in 1870 by Prof. Ernst Abbe in Jena with the sine condition. In modern condensers, the image quality of the field stop is usually optimized, while thenimage quality of the aperture stop is of secondary importance, as it also depends on the optical properties of the microscope objective.

Aplanatic condensers produce sharp images of the field stop. Often, they have a high UV- transmission for the UV-excitation in transmitted light fluorescence microscopy of the past. They are no longer commonly manufactured.

The better the color correction of the field stop image, the less amounts of false light will subdue the overall image contrast, especially with high NA objectives or when working with phase contrast or DIC. Color correction of the field stop image is important for color-critical work such as high-resolution brightfield microscopy of blood smears (e.g. malaria diagnosis).

Achromatic condensers have a good color correction of the field stop image. The overall correction of the aperture stop image may be less. Few models of the achromatic types are still in production today.

The most advanced optical performance is provided by the aplanatic achromatic condenser (“achr. apl.”) type: The field stop image should have very few color fringes, if any, and its edges should be visible at all appropriate objective magnifications. Also, with a given objective, the aperture stop image is of maximum geometrical image quality. The achromatic aplanatic condenser is the choice for all upright research microscopes. The highest correction is found in the NA of 1.4 achr. apl. Immersion-1.4-type condensers.

Dedicated LD condensers for inverted microscopes usually have a correction grade between aplanatic and aplanatic-achromatic.

Many of the above-mentioned condenser types, when manufactured as dry condensers, have a front lens that can be mechanically swung out. This is required to illuminate larger object fields without the front lens at all objective magnifications below 10x. Immersion condensers usually have a non-removable and encapsulated front lens element to protect them from contamination by the immersion medium. Therefore, they are usually not compatible with low magnification objectives.

The Abbe condenser usually also has a fixed front lens. For objective magnifications below 10x, an additional auxiliary lens is used below the Abbe condenser for large object fields.

All condensers are labeled with their maximum NA. Often, their suitability for DIC (red engraving) or polarized light work (red engraving and/or “Pol”) is also indicated.

The free working distance of a condenser is sometimes important. It is usually not marked on the condenser itself, except for LD condensers. It is defined as the distance between the bottom of a standard slide (thickness ~ 1.0-1.1 mm) and the front lens mount of the condenser when an object mounted directly on top of the slide is in focus and Koehler illumination has been used.