guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
The total magnification M of a compound microscope is the product of objective
magnification (M O ) and ocular magnification (M L ):
M = M O * M L
Example: A microscope equipped with an objective M O = 50 and an ocular M L = 10 has a
final magnification of 50 x 10 = 500.
In modern compound microscopes (with infinity-corrected optical systems) the magnification
of the object is performed in a somewhat different way. The specimen is placed in the lower
focal plane of the objective, so that its image is projected at infinity. An auxiliary lens (tube
lens or telan) placed within the tube between the objective and the eyepiece brings the
parallel light rays into focus and produces the real image which then is viewed with the
ocular. The infinity-corrected imaging technique allows to insert accessory components such
as analyzer, compensators or beam splitters into the light path of parallel rays between the
objective and the tube lens with only minimal effects on the quality of the image. It further
provides a better correction of spherical and chromatic aberration.
1.2 Objective and ocular (eyepiece)
1.2.1 Objective
The quality of the observed image is largely determined by the objective. The objective is
thus a key component in the microscope, being responsible for the primary image, its
magnification and the resolution under which fine details of an object can be observed. The
ocular merely serves to further magnify the fine details that are resolved in the intermediate
image, so that they exceed the angular resolution limits of the human eye and can be viewed
at visual angles larger than 1' (Ch. 1.1.1, loupe).
The important properties of an objective are its magnification, its numerical aperture and the
degree of aberration correction, whereby the latter two determine the quality of the
intermediate image.
Raith, Raase & Reinhardt – February 2012
Aberration
Simple biconvex lenses produce imperfect, distorted images that show spherical and
chromatic aberrations. In modern objectives, such optical aberrations are compensated to a
large extent by a combination of converging and diverging lenses that are made of materials
with different refractive indices and dispersion. Remaining abberations are compensated by
oculars with complementary corrections.
At high magnification and large aperture, the cover glass of thin sections introduces
chromatic and spherical aberrations which have an adverse effect on image quality. This is
because light rays emerging from an object point P are refracted at the boundary cover
glass/air. As a consequence, the backward extensions of the light rays do not focus in a spot,
but form a blurry, defocused area (Fig. 1-4A, grey areas). With increasing thickness of the
cover glass the blurring effect becomes more pronounced. High-power objectives are
therefore corrected for this type of cover glass aberration, commonly for a standard glass
thickness of 0.17 mm. Hence, the cover glass forms part of the objective system! Any
thickness deviating from 0.17 mm affects the intermediate image. Furthermore, if the cover
glass is too thick, it may not be possible to focus the specimen using high-power objectives,
due to the short free working distances of such objectives (see Table 1).
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