Abbe condenser
Encyclopedia
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.
. They act to gather light from the microscope's light source and concentrate it into a cone of light that illuminates the specimen. The intensity and angle of the light cone must be adjusted (via the size of the diaphragm) for each different objective lens with different numerical apertures.
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 focussed 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.
There are four types of condenser:
, who developed it in 1870. The Abbe condenser, which was originally designed for Zeiss, is mounted below the stage of the microscope. The condenser concentrates and controls the light that passes through the specimen prior to entering 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.
This condenser is composed of two lenses a plano-convex lens somewhat larger than a hemisphere and a large bi-convex lens serving as a collecting lens to the first. The focus of the first lens is traditionally about 2mm away from the plane face coinciding with the sample plane. A pinhole cap can be used to align the optical axis of the condenser with that of the microscope. The Abbe condenser is the basis for most modern light microscope condenser designs.
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.
Specialised condensers are also used as part of Differential Interference Contrast
and Hoffman Modulation Contrast
systems, which aim to improve contrast and visibility of transparent specimens.
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.
(NA). It is NA that determines optical resolution
, in combination with the NA of the objective. Different condensers vary in their maximum and minimum numerical aperture, and the numerical aperture of a single condenser varies depending on the diameter setting of the condenser aperture
. In order for the maximum numerical aperture (and therefore resolution) of an objective lens to be realized, the numerical aperture of the condenser must be matched to the numerical aperture of the used objective. 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
.
The maximum NA is limited by the refractive index of the medium between the lens and the sample. 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 in contact with both the slide/coverslip and the lens of the condenser. An oil immersion condenser may typically have NA of up to 1.25. 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.
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
compound microscopes. A condenser is a lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
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
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 microscopeInverted microscope
An inverted microscope is a microscope with its light source and condenser on the top, above the stage pointing down, while the objectives and turret are below the stage pointing up. It was invented in 1850 by J. Lawrence Smith, a faculty member of Tulane University...
. They act to gather light from the microscope's light source and concentrate it into a cone of light that illuminates the specimen. The intensity and angle of the light cone must be adjusted (via the size of the diaphragm) for each different objective lens with different numerical apertures.
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 focussed 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.
There are four types of condenser:
- Abbe – The simplest of condensers is the Abbe condenser. It contains two lenses that produce an image of the light source that is surrounded by a blue and red colour at its edges.
- Aplanatic – Aplanatic condensers are corrected for spherical aberration
- Achromatic – Achromatic condensers are corrected for chromatic aberrations
- Aplanatic and achromatic condensers are corrected for both spherical and chromatic aberrations.
Abbe condenser
The Abbe condenser is named for its inventor, Ernst Karl AbbeErnst Karl Abbe
- See also :*Abbe prism*Abbe refractometer*Abbe error*Aberration in optical systems*Calculation of glass properties* German inventors and discoverers-External links:*...
, who developed it in 1870. The Abbe condenser, which was originally designed for Zeiss, is mounted below the stage of the microscope. The condenser concentrates and controls the light that passes through the specimen prior to entering 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.
This condenser is composed of two lenses a plano-convex lens somewhat larger than a hemisphere and a large bi-convex lens serving as a collecting lens to the first. The focus of the first lens is traditionally about 2mm away from the plane face coinciding with the sample plane. A pinhole cap can be used to align the optical axis of the condenser with that of the microscope. The Abbe condenser is the basis for most modern light microscope condenser designs.
Aplanatic and achromatic condensers
Several types of condensers represent improvements on the optical design of a basic Abbe condenser. An aplanatic condenser corrects for spherical aberrationSpherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
in the concentrated light path, while an achromatic
Achromatic lens
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths into focus in the same plane....
condenser corrects for chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
. 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.Specialised condensers are also used as part of Differential Interference Contrast
Differential interference contrast microscopy
Differential interference contrast microscopy , also known as Nomarski Interference Contrast or Nomarski microscopy, is an optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples...
and Hoffman Modulation Contrast
Hoffman Modulation Contrast
In optical microscopy, Hoffman modulation contrast is a technique for enhancing the contrast in unstained transparent biological specimens, invented by Dr Robert Hoffman in 1975. This is achieved by using components in the light path which convert phase gradients into variations in light intensity...
systems, which aim to improve contrast and visibility of transparent specimens.
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 vary in their numerical apertureNumerical aperture
In optics, the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one...
(NA). It is NA that determines optical resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
, in combination with the NA of the objective. Different condensers vary in their maximum and minimum numerical aperture, and the numerical aperture of a single condenser varies depending on the diameter setting of the condenser aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
. In order for the maximum numerical aperture (and therefore resolution) of an objective lens to be realized, the numerical aperture of the condenser must be matched to the numerical aperture of the used objective. 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
Köhler illumination
Köhler illumination is a method of specimen illumination used for transmitted and reflected light optical microscopy. Köhler illumination acts to generate an extremely even illumination of the sample and ensures that an image of the illumination source is not visible in the resulting image...
.
The maximum NA is limited by the refractive index of the medium between the lens and the sample. 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
Oil immersion
In light microscopy, oil immersion is a technique used to increase the resolution of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.Immersion oils are...
(or, more rarely, under water immersion), with a layer of immersion oil placed in contact with both the slide/coverslip and the lens of the condenser. An oil immersion condenser may typically have NA of up to 1.25. 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.
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.