OPERATION
Axiolab 5
Illumination and contrast methods in transmitted light
ZEISS
05/2019
430037-7444-001
109
4.2.6
Configuring transmitted light polarization for conoscopic observation – determining
the optical character of crystals
For the classification (and thus identification) of crystalline material, the analysis of an interference image
in the objective pupil provides more valuable information than the examination of the specimen itself.
This image is visible in the eyepiece when an additional lens (known as a Bertrand lens) is switched on.
Alternatively, the auxiliary microscope or a diopter may be used to view the interference image.
In contrast to orthoscopy, this is referred to as conoscopy, because the illumination is ideally provided by
a wide open cone. In practice this means that the aperture diaphragm is fully open and the objective
should likewise have a large aperture.
4.2.6.1
Application
Crystal analysis is used to determine the optical character of transparent and weakly absorbent crystals.
This method is also referred to as conoscopy.
Its main application is classic mineral microscopy. However, synthetic crystals, industrial minerals and
plastics (e.g. films) can also be identified and characterized.
(1) Instrumentation
Conoscopic viewing is preferably carried out on the Axiolab 5 microscope for transmitted light
conoscopy.
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Strain-free objectives; recommended:
N-Achroplan 50x/0.8 Pol objective or
EC Plan-Neofluar 40x/0.9 Pol objective
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Pol rotary stage
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Polarizer D (rotatable or fixed)
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0.9 Pol condenser
(2) Configuring the microscope for conoscopy
In the case of uniaxial crystals, the most favorable orientation for conoscopic viewing is obtained with
those specimen features (e.g. of a thin section) that in orthoscopic viewing change the brightness as little
as possible. In this case, the direction of viewing and the optical axis are parallel. The same applies to
biaxial crystals if viewed in or approximately in the direction of one of the two optical axes.
