OPERATIONAxiolab 5 Illumination and contrast methods in transmitted light ZEISS05/2019 430037-7444-001 1014.2.5.2 Detecting birefringence with the Axiolab for conoscopy(1) ApplicationThe 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 birefringentsubstances are observed between crossed polarizers, the birefringent portion of the sample appearsbright 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 rotationprocedure, 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. Theinterference colors may be of the first or a higher order.(2) InstrumentationOn the Axiolab 5 microscope for transmitted light conoscopy:− Strain-free objectives− Pol rotary stage− Polarizer D (rotatable or fixed)− Analyzer slider D− Lambda compensator or lambda/4 compensatorNOTEThe depolarizer is already incorporated in the Axiolab 5 stand for conoscopy.A depolarizer (quartz depolarizer) should be installed in all microscopes used for examiningmineral/geological specimens.A depolarizer extinguishes undesirable polarization effects (e.g. false or pseudo-pleochroism) that mayoccur behind the analyzer (e.g. on prism surfaces in the tube), or shifts them to higher orders.
