guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Measuring lengths
2.2 Measurement of lengths
The measuring of distances is necessary for the determination of grain size, length-width
ratios etc. For such measurements oculars are used that have a graticule (ocular micrometer)
which must be calibrated. The graticule is commonly combined with the ocular crosshairs as
a horizontally or vertically oriented scale attached to the E-W or N-S threads of the crosshairs,
respectively (Fig. 2-2).
If objectives with increasing magnification are used, the image increases in size
correspondingly. Therefore, the calibration of the graduations on the ocular scale must be
carried out for each ocular-objective combination separately. The engraved magnification
numbers on the objectives are only approximate values which would make a calibration based
purely on calculations too imprecise.
For calibration, a specific scale is used, called an object micrometer, with a graduation of 10
μm per mark; 100 graduation marks add up to 1 mm. The object micrometer is put in a
central stage position and, after focusing, is placed parallel and adjacent to the ocular scale
(Fig. 2-2). In example (a) 100 graduation marks of the object micrometer, adding up to 1000
μm, correspond to 78 graduation marks on the ocular micrometer. At this magnification
(objective 6.3x; ocular 12.5x) the distance between two graduation marks in the ocular is
1000 μm divided by 78. The interval is therefore 12.8 μm. Example (b) is valid for another
combination (objective 63x; ocular 12.5x).
If, for example, a grain diameter needs to be determined, the number of graduation marks
representing the grain diameter are counted and then multiplied by the calibration value for
the particular objective-ocular combination (Fig. 2-2c).
Raith, Raase & Reinhardt – February 2012
The total error of the described procedure is a complex accumulated value. Both the object
micrometer used for calibration and the ocular micrometer have a certain tolerance range.
Errors occur with strongly magnifying lens systems because the image is not entirely planar,
but has distortions in the peripheral domains. The largest error is commonly made by the
human eye when comparing object and graduation. For increased accuracy it is important to
use an objective with the highest possible magnification, such that the mineral grain covers a
large part of the ocular scale.
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