Condenser (optics)

A condenser is an optical lens that renders a divergent light beam from a point light source into a parallel or converging beam to illuminate an object to be imaged.

Condensers are an essential part of any imaging device, such as microscopes, enlargers, slide projectors, and telescopes. The concept is applicable to all kinds of radiation undergoing optical transformation, such as electrons in electron microscopy, neutron radiation, and synchrotron radiation optics.

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.

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.

There are three main types of microscope condenser:

The Abbe condenser is named for its inventor Ernst Abbe, 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 difficult to use for magnifications of above 400X, as the aplanatic cone is only representative of a numerical aperture (NA) of 0.6.

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 still the basis for most modern light microscope condenser designs, even though its optical performance is poor.

An aplanatic condenser corrects for spherical aberration in the concentrated light path, while an achromatic compound condenser corrects for both spherical and chromatic aberration.