OPERATIONZEISS Illumination and contrast methods in transmitted light Axiolab 598 430037-7444-001 05/20194.2.5 Configuring transmitted light polarization with the conoscopy stand4.2.5.1 Determining the optical character of crystalsFor the classification (and thus identification) of crystalline material – instead of the observation of thespecimen itself – the analysis of an interference image in the objective pupil provides the more valuableinformation. This image is visible in the eyepiece when an additional lens (so-called Bertrand lens) isswitched on. Alternatively, the auxiliary microscope or a diopter may be used to view the interferenceimage.In contrast to orthoscopy, this is referred to as conoscopy, because the illumination is ideally provided bya wide open cone. In practice this means that the aperture diaphragm is fully open and the objectiveshould likewise have a high aperture.(1) ApplicationCrystal 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 andplastics (e.g. films) can also be identified and characterized.(2) InstrumentationConoscopic viewing is preferably carried out on the Axiolab 5 microscope for transmitted lightconoscopy.− Strain-free objectives; recommended:N-Achroplan 50x/0.8 Pol objective orEC Plan-Neofluar 40x/0.9 Pol objective− Pol rotary stage− Polarizer D (rotatable or fixed)− 0.9 Pol condenser(3) Configuring the microscope for conoscopyIn the case of uniaxial crystals, the most favorableorientation for conoscopic viewing is obtained withthose specimen features (e.g. of a thin section)that in orthoscopic viewing change the brightnessas little as possible. In this case, the direction ofviewing and the optical axis are parallel. The sameapplies to biaxial crystals if viewed in orapproximately in the direction of one of the twooptical axes.• Configure the microscope as for transmittedlight brightfield microscopy using the KÖHLERmethod (see section 4.2.1).• Place the specimen on the stage and focus onit.• Swivel the analyzer into the beam path (on position) with rotary knob A (Fig. 4-11/2).Fig. 4-11 Axiolab 5 for transmitted lightconoscopy
