OPERATION
ZEISS
Illumination and contrast methods in transmitted light
Axiolab 5
98
430037-7444-001
05/2019
4.2.5
Configuring transmitted light polarization with the conoscopy stand
4.2.5.1
Determining the optical character of crystals
For the classification (and thus identification) of crystalline material – instead of the observation of the
specimen itself – the analysis of an interference image in the objective pupil provides the more valuable
information. This image is visible in the eyepiece when an additional lens (so-called 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 high aperture.
(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.
(2) Instrumentation
Conoscopic viewing is preferably carried out on the Axiolab 5 microscope for transmitted light
conoscopy.
−
Strain-free objectives; recommended:
N-Achroplan 50x/0.8 Pol objective or
EC Plan-Neofluar 40x/0.9 Pol objective
−
Pol rotary stage
−
Polarizer D (rotatable or fixed)
−
0.9 Pol condenser
(3) 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.
•
Configure the microscope as for transmitted
light brightfield microscopy using the KÖHLER
method (see section 4.2.1).
•
Place the specimen on the stage and focus on
it.
•
Swivel the analyzer into the beam path (
on
position) with rotary knob
A
2
).
Fig. 4-11
Axiolab 5 for transmitted light
conoscopy
