Geology Reference
In-Depth Information
SLR
Camera
Analyzer
Specimen holder
Light
controller
Goniometer
Polarizer
Light box
Figure 6.10 National Research Council (NRC) of Canada polariscope for observations of circular thin sections of
ice with diameters up to the maximum dimension of 300 mm (photos by N. K. Sinha, unpublished).
Grains (most often aggregate) of snow may be small
individual crystals could be significantly smaller than
1 mm, but those in natural ice are usually larger than a
few millimeters and, therefore, significantly larger in
comparison to those in most minerals. Consequently, for
mechanical tests on ice, such as creep or strength tests, ice
specimens have to be large enough to contain a sufficient
number of grains. Use of rectangular specimens with
dimensions of 250 mm × 100 mm × 100 mm or cylindrical
specimens with diameters of 100 mm, made from ice
cores, are common. A special polarizing system that can
accommodate thin sections of ice with dimensions of
100 mm × 250 mm was designed by the second author
(Nirmal Sinha) and fabricated at the NRC of Canada in
Ottawa. This instrument was rugged enough to be taken
to the field. In point of fact, the NRC polariscope had
traveled to many field laboratories in the Arctic, such as
Resolute, Mould Bay, Hobson's choice, man‐made off-
shore ice island (drilling platform) floating on top of the
North Magnetic pole, on board the icebreaker MV Arctic ,
and air bases of Thule and Nord in Greenland. Actually
this polariscope was used for taking most of the polar-
ized‐light photographs illustrated in this topic.
Two different views of the NRC polariscope are shown
in Figure 6.10. It consists of a light source consisting of
an array of incandescent bulbs for better distribution of
radiation, a light diffusing plate, two linear polarizers, a
specimen holder, and a holder for the universal stage
(goniometer). The Rigsby‐style stage is shown in
Figure  6.11. All the components of the polariscope are
free to rotate 360° for the determination of fabric dia-
grams (see section 4.3.4). The camera mount is capable of
handling 35 mm as well as large‐format professional view
cameras and consists of a carriage that allows rotation
and adjustments of the position of the camera with
respect to the object to be photographed. A built‐in dim-
mer switch allows adjustment of the light output. To pre-
vent infrared radiation affecting the specimen (specially
sea ice thin sections), it is actually preferred to use a low
level of light during the adjusting periods for photogra-
phy or performing measurements for fabric diagram.
Narrowband or monochromatic filters can be used in the
camera for increasing distinctions between individual
crystals in different shades of the used color and better
resolution of the grain interfaces and inclusions inside
and between crystals. Figure 6.12a shows the NRC polar-
iscope inside the field laboratory of Mould Bay, and
an  image of a large, 100 mm × 250 mm thin section of
deformed and recrystallized sea ice is shown in Figure 12b.
Note the black cross on the left of the thin section in this
photograph. The cross gives an indication of the polar-
izer and the analyzer and that they are in cross position.
This polarizer indicator was developed by the author
(Sinha, unpublished) using the principle of hoops stress
in circular plates.
For 35 mm single‐lens reflex (SLR) or digital single-
lens reflex (DSLR) cameras, linearly polarizing filters
can  be purchased from most photographic stores. Since
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