May 1 2014
A condenser is, essentially, an optical lens that is used to convert a divergent beam from a point source into a parallel or converging beam. The main purpose of using a condenser is to illuminate an object. Condensers are an important part of a number of imaging devices, like microscopes, slide projectors, enlargers, and telescopes.
Condensers not only provide illumination in imaging devices, but also help in minimizing aberrations. Using condensers is one of the most effective ways of correcting aberrations. Condensers are particularly useful in devices that observe optical transformation phenomenon, such as neutron radiation, synchrotron radiation optics, etc. Condenser optics are used in illumination systems, such as the Kohler illumination system.
Condensers in Microscopes
Condensers are located beneath the stage of the microscopes. They are placed in this location so that the wavefronts from the light source of the microscope are gathered in the form of a cone of light, in order to illuminate the specimen being observed. The condenser in inverted microscopes is mounted above the stage, and the specimen is placed on a frame pillar.
The illumination achieved from the condenser is of uniform intensity. The light from the cone and the aperture must be adjusted, using numerical apertures, for different objective lenses. Each time the objective is changed, the condenser must also be modified in order to provide the right light cone to match the numerical aperture of the objective.
The two important factors for achieving the full potential of the objective are: the focus of the condenser, and aperture adjustment. Getting the right illumination can be achieved by using the adjustable aperture iris diaphragm, which is located inside the condenser or below it. The position of the condenser is adjusted by a rack and pinion gear system. In order to get the best image the condenser must be positioned with respect to the cone of illumination.
Classification of Condenser
The two main factors for classifying condensers are: mode of imaging, and degree of optical correction. Based on the mode of imaging, condensers are classified as phase contrast, dark field, and bright field. Based on the optical correction, they are classified as Abbe condenser, aplanatic, achromatic, and aplanatic-achromatic condensers. Each of these types varies in the number of internal lenses used.
- Abbe Condenser – This is the simplest of all condensers and offers the least amount of aberration correction. The chromatic and spherical aberrations are not corrected in this condenser. This type of condenser can have an aperture of up to 1.4 in models that have three or more lends arrangements
- Aplanatic Condenser – This type of condenser is exclusively used for spherical aberration correction
- Achromatic Condenser - This type of condenser is exclusively used for chromatic aberration correction
- Achromatic-Aplanatic Condenser - This type of condenser is capable of correcting both spherical and chromatic aberrations
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