guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
1.4 Light paths in the microscope
1.4.1 Köhler illumination
The Köhler illumination is based on a specific geometry of light paths in the illuminating
substage part of the microscope, and is achieved through a special arrangement of the light
source, collector lens, field diaphragm, aperture diaphragm and condenser lens (Fig. 1-7).
This special illumination mode ensures an even illumination of the viewed object field (light
field) and further allows to independently adjust the illumination aperture and the size of the
light field.
1. The collector lens projects an enlarged image of the light source (filament of the halogen
lamp) onto the front focal plane of the condenser where the aperture diaphragm resides. As a
result, the illuminating light beam is leaving the condenser as a light cone consisting of
bundles of parallel rays (Fig. 1-7). The aperture of the illuminating cone can be modified by
varying the aperture of the iris diaphragm. As each point in the object field receives light rays
from each point of the filament of the halogen lamp, an even illumination of the entire object
field is achieved. Further images of the light source (filament) are generated in the upper
focal plane of the objective (resp. the tube lens) and the upper focal plane of the ocular.
2. The condenser lens projects an image of the field diaphragm onto the specimen plane, and
hence, superposed images of both object and field diaphragm are generated by the objective
in the intermediate image plane where they are jointly observed through the ocular. The size
of the illuminated field of the specimen (light field) can be adjusted by varying the aperture
of the field diaphragm without affecting the illumination aperture (Fig. 1-7).
The microscope alignment for Köhler illumination is described in Chapter 1.5.
The Köhler illumination allows to examine optically anisotropic minerals in two different
modes:
Raith, Raase & Reinhardt – February 2012
1.4.2 Orthoscopic mode
The divergent light rays emanating from each point of a specimen are focused in the
intermediate image plane, thereby creating the real image of the specimen (Fig. 1-7A).
In an optically anisotropic mineral, along each direction of the illuminating cone (Ch. 4.1)
light waves with different velocity (birefringence; Ch. 4.2.3) and in part also different
amplitude (absorption) pass through the grain. The light waves are superposed at each point
of the object image. Therefore, the image of an individual mineral grain, when viewed under
strongly convergent illumination, does not provide information on the optical behaviour in
different directions of the mineral.
However, when the aperture of the illumination cone is reduced by closing down the aperture
diaphragm, the optical phenomena observed in the intermediate image are dominated by the
properties of light waves that pass through the mineral grain at right angle to the viewing
plane: orthoscopic mode (Ch. 4). It follows that direction-dependent optical properties of an
anisotropic mineral in thin section must be deduced from examining several grains cut in
different crystallographic orientations.
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