Microscope Condensers

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When sending a collimated beam into the condenser the light will be focused onto the sample plane (green ray). In this case the sample plane will be conjugate to the light source, which can lead to forming an image of the light source rather than the sample.

To correct for this, an input beam that is focused (i.e., Köhler Illumination) onto the aperture stop of the condenser can be used (blue ray). In this case, conjugate planes are created at the field stop, condenser aperture stop, and the objective's back focal plane. A collimated beam will then be present at the sample.

Features

• Aperture Stop for Optimizing Illumination Conditions at the Sample
• Male D3N Dovetail for Mounting in a Condenser Holder or CSA2001 Dovetail Adapter
• Options Available with Slots for DIC Prisms, Illumination Masks, Ø1" Optics, or Ø32 mm Optics
• Adapters to Connect DIY Light Conditioning Setups
  • CSA2001: For Mounting Condensers to DIY Setups that Use SM2 Lens Tubes
  • LCPN1 and LCPN5: For Attaching Custom-Built Condensers to Cerna or Nikon Microscopes 
• Other Nikon Condensers Available Upon Request

To support home-built Cerna^® microscope systems, Thorlabs offers four achromatic condensers. Designed for upright microscopes, these condensers collect light emitted by an illumination source to illuminate transmissive samples from beneath the objective. They are used in several transmitted light imaging modalities, including brightfield illumination, Dodt contrast, and differential interference contrast (DIC) imaging, and have an internal turret or tray to mount one or more condenser prisms, illumination masks, and/or other optics.

Aperture Stop Diaphragm
Each condenser is equipped with an adjustable aperture stop diaphragm that is controlled by a lever on the side. For the brightest illumination, the condenser's NA should be equal to or slightly smaller than that of the highest-NA objective that will be used with the microscope. By opening and closing the diaphragm, the effective numerical aperture (NA) of the condenser can be adjusted, allowing it to match the NA of the objective. Note that closing the diaphragm will reduce the illumination intensity.

Condenser Mounting with D3N Dovetail
These condensers can be mounted to a condenser holder using the male D3N dovetail on the bottom. D3N is Thorlabs' designation for the dovetail used by the majority of Nikon condensers for upright microscopes. See the Microscope Dovetails tab for more information.

Condenser Trays and Turrets
Each condenser is equipped with either an internal turret (Item #'s CSC1001 and CSC1002) or a removable tray (Item # CSC2001) to allow for the addition of DIC condenser prisms or other optics. See below for details on the options available for each condenser.

Adapters for DIY Light Conditioning Setups
For custom light conditioning setups, Thorlabs offers the CSA2001, LCPN1, and LCPN5 condenser adapters. The CSA2001 adapter features a female D3N dovetail and external SM2 threads. It can mount condensers with a male D3N dovetail to a DIY optical assembly that uses Thorlabs' SM2 lens tubes. The LCPN1 and LCPN5 adapters feature the same male D3N dovetail as the condensers on this page, allowing a user-constructed condenser to mount onto a condenser holder. The LCPN1 adapter has internal SM30 (M30.5 x 0.5) threading for Ø30 mm lens tubes, and the LCPN5 adapter has internal SM2 (2.035"-40) threading for SM2 lens tubes. Both adapters have cage rod through holes for our 60 mm cage systems. The LCPN1 adapter also has 4-40 tapped holes for our 30 mm cage systems. See the DIY Cerna Interfaces tab for a comprehensive list of dovetail and cage compatibility for the Cerna product line.

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This photo shows the male D1N dovetail on the trinoculars next to the female D1N dovetail on the epi-illumination arm.

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This photo shows the male 95 mm dovetail on the microscope body and the female 95 mm dovetail on the CSA1002 Fixed Arm.

Introduction to Microscope Dovetails

Dovetails are used for mechanical mating and optical port alignment of microscope components. Components are connected by inserting one dovetail into another, then tightening one or more locking setscrews on the female dovetail. Dovetails come in two shapes: linear and circular. Linear dovetails allow the mating components to slide before being locked down, providing flexible positioning options while limiting unneeded degrees of freedom. Circular dovetails align optical ports on different components, maintaining a single optical axis with minimal user intervention.

Thorlabs manufactures many components which use dovetails to mate with our own components or those of other manufacturers. To make it easier to identify dovetail compatibility, we have developed a set of dovetail designations. The naming convention of these designations is used only by Thorlabs and not other microscope manufacturers. The table to the right lists all the dovetails Thorlabs makes, along with their key dimensions.

In the case of Thorlabs’ Cerna^® microscopes, different dovetail types are used on different sections of the microscope to ensure that only compatible components can be mated. For example, our WFA2002 Epi-Illuminator Module has a male D1N dovetail that mates with the female D1N dovetail on the microscope body's epi-illumination arm, while the CSS2001 XY Microscopy Stage has a female D1Y dovetail that mates with the male D1Y dovetail on the CSA1051 Mounting Arm.

To learn which dovetail type(s) are on a particular component, consult its mechanical drawing, available by clicking on the red Docs icon () below. For adapters with a female dovetail, the drawing also indicates the size of the hex key needed for the locking setscrew(s). It is important to note that mechanical compatibility does not ensure optical compatibility. Information on optical compatibility is available from Thorlabs' web presentations.

For customers interested in machining their own dovetails, the table to the right gives the outer diameter and angle (as defined by the drawings below) of each Thorlabs dovetail designation. However, the dovetail's height must be determined by the user, and for circular dovetails, the user must also determine the inner diameter and bore diameter. These quantities can vary for dovetails of the same type. One can use the intended mating part to verify compatibility.

In order to reduce wear and simplify connections, dovetails are often machined with chamfers, recesses, and other mechanical features. Some examples of these variations are shown by the drawings below.

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Two examples of how circular male dovetails can be manufactured.

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Two examples of how circular female dovetails can be manufactured.

The Condenser Lens and Köhler Illumination in Microscopy Systems

The resolution of microscopy images is affected by several factors, including the numerical aperture (NA) of the condenser lens and the uniformity of the sample plane illumination. The NA of the condenser lens should be at least as large as the NA of the objective lens. Under these conditions, the condenser lens illuminates the sample over a range of angles that is at least as wide as the range of angles over which the objective lens collects light from the sample. In order to provide uniform illumination of the sample plane, a technique called Köhler illumination is often used. An important result of this illumination approach is that the source and sample are never imaged to the same plane. The following include more information about the relationship between the condenser and objective lenses, as well as Köhler illumination:

• Does the condenser's NA affect the microscope's resolution?
• How does Köhler illumination work in microscopy?

Click here for more Insights about lab practices and equipment.

Does the condenser's NA affect the microscope's resolution?

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Figure 2: In epi-illumination microscopes, the objective provides the light that illuminates the sample. It also collects the light reflected and scattered from the sample. Due to this, and in contrast to the case illustrated in Figure 1, both the illumination and imaging angles depend only on the objective.

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Figure 1: In transmitted light microscopes, light from the light source is directed to the sample by the condenser optical system. The objective lens is used to collect the transmitted light. This collected light is then routed to create an image at a camera or eyepiece.

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Figure 3: Cones describe the light incident on a sample point from the condenser (left, gold), the light transmitted through the sample (right, yellow), and the range of light the objective can collect (right, orange). The cones' angles are measured from the optical axis. The angular ranges of the light cones incident on and transmitted by the sample are approximately the same (θ_cd ), since light that is not absorbed or scattered by the sample travels in an approximately straight line. The angular range (θ_obj ) accepted by the objective lens is can be different. The numerical apertures of the condenser (NA_cd ) and objective (NA_obj ) are often used to compare the angular ranges of the transmitted light to the light the objective lens can gather.

The condenser's numerical aperture (NA) strongly impacts a microscope's resolution, since the angular range of the light incident on the sample affects the angular range of light transmitted or reflected by the sample. According to a general rule for optimizing resolution, the condenser NA should be as least as large as the objective NA. In other words, the cone of light provided by the condenser should have an angular range that matches or exceeds that accepted by the objective lens.

Condenser and Objective
In transmitted light microscopy, the condenser collects light from the source and illuminates the sample (Figure 1). The condenser optica system typically includes several optical elements, which can be aligned to provide uniform illumination of the sample plane. The objective lens is located on the opposite side of the sample plane and collects the light that is transmitted through the sample. This light is then routed to create an image at an eyepiece or camera.

Alternately, the microscope can be configured so the objective both illuminates and collects light from the sample (Figure 2). In this case, there is no separate condenser lens system.

Numerical Aperture
The condenser provides light to the sample plane over a range of different angles (Figure 3). A cone, drawn with its tip at a point on the sample and its base encircling the light from the condenser, can be used to quantify the range of incident angles (θ_cd ). The light transmitted by this point on the sample has approximately the same angular range. A different cone can be used to depict the angular range of light (θ_obj ) the objective lens is capable of gathering.

These angular ranges can be quantified by each lens' numerical aperture (NA),

NA = n sin(θ ),

which depends on the half-angle (θ ) of the cone, as well as the surrounding medium's refractive index (n ). The higher the NA, the wider the cone describing the angular range. This angle is measured from the optical axis.

As an example, when the objective lens has a 0.7 NA with air (n = 1) between the lens and sample, the lens' angle of acceptance is θ = θ_obj = 44.43°. For the system illustrated in Figure 2, the NA of the illumination and the NA of the collected light are the same, since both light paths pass through the objective lens.

Resolution
The resolution (δ ) of the microscope describes its ability to image two closely spaced points as a separable pair, instead of as a single point. A common equation,

used to estimate this minimum separation includes only the wavelength () and the NA of the objective (NA_obj ). While this equation seems to suggest the NA of the condenser (NA_cd ) does not affect resolution, this is not the case. This equationactually assumes that NA_cd ≥ NA_obj .

When NA_cd ≤ NA_obj , a modified version of the equation,

provides a better estimate and illustrates the importance of the condenser's NA to the resolution.

How does Köhler illumination work in microscopy?

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Figure 4: An image of the source includes the structure of the light emitting elements (left). Köhler illumination avoids imaging the source to the sample or sensor planes and provides uniform illumination to the sample plane (right).

A multi-element microscopy system can be aligned to provide Köhler (Koehler) illumination, in which the light collected from a light source like a lamp or light emitting diode (LED) is imaged differently than the light collected from the sample. The light source is intentionally never imaged to the sample (object) plane or to the camera sensor. The sample plane is instead uniformly illuminated, typically over the broad range of angles required for high-resolution imaging. As a result, Köhler illumination prevents superimposing an image of the lamp or LED structure onto the camera sensor.

Image of the Light Source
An image of the light source at the sample plane would not uniformly illuminate the sample (Figure 4, left, for example), since the light-emitting structures are clearly visible in images of the source. Köhler illumination instead uniformly illuminates the sample plane (Figure 4, right) by providing the light to the sample plane as bundles of parallel rays. In addition, aligning the system to provide Köhler illumination prevents the light source from being imaged at the camera sensor, which would superimpose an image of the light-emitting structures onto the image of the sample.

Illumination Light Path
The illumination path begins with the light source and passes through the sample to the camera sensor. The video animation (Video 1, below) traces rays from an extended source, like a lamp or LED, through this light path.

Each of the multiple light-emitting points on the source radiate light over a range of angles. The collector lens gathers this light and transmits a beam whose maximum diameter is limited by the field stop. The light is then incident on the field lens, which forms an image of each source point at the aperture stop. The alignment of the source image at the aperture stop is critical, since the aperture stop is positioned at the front focal plane of the condenser lens.

The image of the source at the aperture stop provides light to the condenser lens, which transmits the light as bundles of parallel rays. Each bundle of rays originates from a unique point on the source image. The angle at which a particular bundle of rays is incident on the sample plane is larger when the source point is displaced farther from the optical axis. In other words, closing the aperture stop would reduce the range of illumination angles, as well as the illumination intensity at the sample plane.

Since an image of the source is at the front focal plane of the condenser lens, only bundles of parallel rays are incident on the sample plane. No source image is formed at the sample and the illumination is uniform.

Source light transmitted through the sample plane is imaged to the objective back focal plane, which is located between the objective and tube lenses. No image of the light source is formed on the camera sensor, which preserves image quality.

Imaging Light Path
The imaging path begins at the sample plane and ends at the camera sensor, and the video animation also traces rays through this light path. Each point on the sample is imaged to a point on the camera sensor.