OPERATION
Axiolab 5
Illumination and contrast methods in transmitted light
ZEISS
05/2019
430037-7444-001
101
4.2.5.2
Detecting birefringence with the Axiolab for conoscopy
(1) Application
The transmitted light polarization method is used for samples which change the polarization of the light.
Such samples are called birefringent. Examples include crystals, minerals or polymers. If such birefringent
substances are observed between crossed polarizers, the birefringent portion of the sample appears
bright while it’s surrounding remains dark.
A birefringent substance can be recognized by rotating the sample by 360° between crossed polarizers.
The sample should show four bright and four dark appearances during the rotation. During the rotation
procedure, interference colors appear that range from gray (mostly for biological samples) through white,
yellow and red to blue, depending on the birefringence, thickness and orientation of the sample. The
interference colors may be of the first or a higher order.
(2) Instrumentation
On the Axiolab 5 microscope for transmitted light conoscopy:
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Strain-free objectives
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Pol rotary stage
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Polarizer D (rotatable or fixed)
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Analyzer slider D
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Lambda compensator or lambda/4 compensator
NOTE
The depolarizer is already incorporated in the Axiolab 5 stand for conoscopy.
A depolarizer (quartz depolarizer) should be installed in all microscopes used for examining
mineral/geological specimens.
A depolarizer extinguishes undesirable polarization effects (e.g. false or pseudo-pleochroism) that may
occur behind the analyzer (e.g. on prism surfaces in the tube), or shifts them to higher orders.