What Is Microscope Condenser And Its Function?

What Is Microscope Condenser?

On upright microscopes, the condenser is located beneath the stage and serves to gather wavefronts from the microscope light source and concentrate them into a cone of light that illuminates the specimen with uniform intensity over the entire view field.

Condensers are located above the light source and under the sample in an upright microscope, and above the stage and below the light source in an inverted microscope.

They act to gather light from the microscope’s light source and concentrate it into a cone of light that illuminates the specimen.

The aperture and angle of the light cone must be adjusted (via the size of the diaphragm) for each different objective lens with different numerical apertures.

What is a Microscope Condenser and What is it Used For?

Condensers typically consist of a variable-aperture diaphragm and one or more lenses. Light from the illumination source of the microscope passes through the diaphragm and is focused by the lens(es) onto the specimen.

After passing through the specimen the light diverges into an inverted cone to fill the front lens of the objective.

How has the Microscope Condenser Evolved over Time?

The earliest microscope condensers were used during the 17th century. Robert Hooke used a combination of salt water and a Plano-convex lens.

Today microscope condensers usually consist of a variable-aperture diaphragm and one (sometimes more) lens.

Whether you have an electric microscope that uses 110v or battery power to illuminate your specimen, or your microscope uses ambient light with a mirror, a modern microscope condenser focuses available light through its lenses and onto the specimen, illuminating it from below for study.

When the light passes through the specimen you’re observing, the light diverges into an inverted cone, filling the front lens of the objective to display your specimen properly.

Types Of Microscope Condensers

There are three main types of microscope condensers:

  1. The chromatic condenser, such as the Abbe where no attempt is made to correct for spherical or chromatic aberration. It contains two lenses that produce an image of the light source that is surrounded by a blue and red color at its edges.
  2. The aplanatic condenser is corrected for spherical aberration.
  3. The compound achromatic condenser is corrected for both spherical and chromatic aberrations.

1. Abbe Condenser

The Abbe Condenser, named for Ernst Abbe who invented it back in the 1800s, was the original one. This is essentially the condenser we talked about in the above section.

These are used in simpler and less expensive microscopes and are usually the default condenser shipped by the manufacturer.

These types of condensers do not correct for spherical or chromatic aberrations but can have numerical aperture settings, in more expensive models, up to 1.40 numerical aperture. If you are not sure what numerical aperture is take a look at this post.

2. Aplanatic And Achromatic Condensers

An aplanatic condenser corrects for spherical lens aberration. This means they are able to correct for the aberration that occurs due to the surface of a lens being spherical in shape. The edges of the lens will usually have a different focal point from the center, leading to blurring.

To address this, an aplanatic condenser has many layers of lenses inside it which effectively flatten out this effect, allowing the light to concentrate on one central location.

However, aplanatic condensers are not necessarily good at color correction. You may still find there is chromatic lens aberration, where you get ‘color fringing’, as discussed earlier.

3. Specialized Condenser

The final category is rather broad! There are a ton of specialized applications in microscopes – whether you are looking at transparent (see-through) specimens, dark or light specimens, using different kinds of light sources (using LED, OLED, or a halogen light source), etc.

There are different kinds of condensers like darkfield or phase contrast which help create images with more contrast, or if you are probing materials that are dark, you need corrections to address how light interacts with the sample.

How To Adjust Microscope Condenser?

When looking through the microscope if you are having trouble with the light of your microscopy sample, or perhaps the image looks dark, chances are you may need to make an adjustment with your microscope condenser.

When installing the microscope condenser, rotate the coarse focus knob to move the stage to its highest position. Most compound light microscopes have a small knob to raise and lower the condenser holder. Lower this holder so the condenser can slide into the holder below the stage.

Once you have inserted the condenser, tighten the set screw to hold the condenser in place. Finally, raise the condenser back up to its highest position so the light is just beneath the microscope slide.

Most condensers will also have two screws on either side of the condenser holder. These are condenser-centering screws. While looking through the microscope with the iris partway open, adjust each screw so the cone of light is centered in the microscope field of view.

Adjusting the Iris Diaphragm on the Microscope Condenser

The condenser has a lever on the front of it that can be moved to the far right or left. This lever adjusts the iris diaphragm. Some condensers will have corresponding objective values printed on the condenser, while others will not.

When using the 4x microscope objective lens, the iris diaphragm lever should be pushed all the way to the right. When this lever is moved to the right, less light is sent through the condenser, resulting in an image that is not too bright.

When using the 100x objective, move the lever all the way to the left to open the iris diaphragm and allow more light to pass through the condenser, resulting in a better image.

Adjusting the Field Iris for Koehler Illumination

Biological microscopes with Koehler illumination will have a field iris. The field iris is adjusted by rotating the light housing. Koehler illumination is a method of specimen illumination that evenly spreads the light across the sample and ensures that an image of the light source is not visible in the resulting microscopy image.

Koehler illumination is found in more advanced biological microscopes and is typically not found in basic high school microscopes.

In order to obtain Koehler illumination, the microscope must have a collector lens and/or field lens, a field iris diaphragm, a condenser iris diaphragm, and a condenser lens.

When looking through the microscope the field iris should be adjusted when the magnification of the microscope is changed to ensure an evenly lit sample.

Using Filters

Microscope filters are typically placed (above the light source) on a compound light microscope. Filters are used both for observation and photomicroscopy.

Adjusting Light Intensity

A microscope condenser will operate optimally when the light intensity of the microscope is also set accordingly. A good rule of thumb to remember is that lower magnifications require less light.

Additionally, depending on the type of light you are using (LED is a much brighter light than fluorescent), you may need to adjust the light intensity control on your microscope.

What Is The Function Of a Microscope Condenser?

On upright microscopes, the condenser is located beneath the stage and serves to gather wavefronts from the microscope light source and concentrate them into a cone of light that illuminates the specimen with uniform intensity over the entire microscope field of view.

The condenser performs several key functions:

  1. Distributing light evenly over the specimen to remove lighting imperfections;
  2. Aberration correction;
  3. Controlling the angle of the cone of light projected into the objective lens above.

1. Light Distribution

Before the condenser was invented, scientists would often see imperfections in the light source that harmed image quality.

One common issue was that the glowing filament in a halogen light bulb was visible under the specimen, which significantly distorted the image that reaches the eye. Ernst Abbe solved that issue with his Abbe condenser, which he invented in 1870.

2. Aberration Correction

Some higher-end condensers are correct spherical and chromatic aberration of light.

Condensers that correct chromatic aberration prevent the rainbow effect (‘color fringing’) where it looks like there is a colored outline around an image.

Condensers that correct spherical aberration prevent blurry edges around images that occur when the light from the edge of a spherical lens does not reach the same focal point as the rest of the lens.

3. Light Angle Correction

The condenser itself is rarely adjusted for beginner microscopy activities. However, you can raise it and lower it so the cone of light is closer or farther from the specimen.

This affects the angle of the light as it enters the objective lens above. At 1000x, you’d want it very close to the specimen, while at lower magnifications it can be farther away.

The thing you will adjust is the aperture diaphragm which sits above and works with the condenser.

Numerical Apertures and Condensers

As alluded to earlier and worth mentioning here, condensers are a system of two lenses, and every lens has a numerical aperture (NA) setting that along with other factors ultimately determines the resolution of the image you will get. 

This is where things can get tricky with microscope design – how do you combine multiple lenses with different numerical apertures?

Well, the trick is to match the numerical aperture. If your objective has a numerical aperture of 0.80, then your condenser setting should be set to 0.80 numerical aperture.

There are a lot of different techniques to maximize the objective-condenser combinations. As described in the article Microscope Numerical Aperture: A Layman’s Explanation, optimizing the numerical aperture of lenses is a highly sought-after solution for scientists and engineers.

Methods like oil immersion are often used in condensers to improve their numerical aperture and ultimately the resolution of the image.

Condensers with numerical apertures above 0.95 perform best when a drop of oil is applied to their upper lens in contact with the undersurface of the specimen slide.

This ensures that oblique light rays emanating from the condenser are not reflected from underneath the slide, but are directed into the specimen.

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