Transmitted Light Darkfield - Leica DMLS Bedienungsanleitung

Possible errors
Specimen: too thick, too thin, too brightly
stained; refractive index of mounting medium
and specimen identical so that there is no phase
jump.
Specimen slide too thick, so Koehler illumination
not possible.
Wedge-shaped coverglass position, so centra-
tion of light and phase ring is no longer effective.
Wrong light ring, or light ring has been inserted
upside down (see assembly → p. 13). Aperture
diaphragm not open. Light ring not centered.
Transmitted light darkfield with CL/CLP and
UCL/UCLP condensers
!
Darkfield is possible with most objectives from
10x magnification; the image background may
be inhomogeneously illuminated at lower
magnifications. The highest possible objective
aperture is 0.75, although objectives with higher
apertures can be used if it is possible to reduce
the aperture with a built-in iris diaphragm.
These objectives can be identified by the
fact that the maximum and minimum apertures
are given in the objective engraving and in our
lists, e. g. 1.30 – 0.60 (Fig. 21).
Rotate the condenser disc to position BF or pull
out DF light ring slide out as far as the stop.
Focus the specimen (10x objective). If you have
trouble finding the specimen plane, temporarily
close the aperture diaphragm (23.7).
36
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Set Koehler illumination (p. 30) (sharply focus
the centered field diaphragm together with the
specimen, Fig. 24), does not apply for simple
configuration.
Open the aperture diaphragm as far as the stop
(= pos. PH) and turn the disc to pos. D
(= darkfield ring) or insert slide with DF light ring
into condenser CL/PH or CLP/PH (Fig. 7). If the
DF image is inhomogeneous with a homo-
geneous specimen, center the DF light ring as
follows (does not apply for CL/PH and CLP/PH
condensers, Fig. 7.4): Use an objective with a
higher magnification (40x – 100x), observe with-
out an eyepiece or insert auxiliary telescope
→ p. 35 and focus. Put the two centering keys
into the condenser (23.2) from the back at an
angle and adjust until the brighter circle
(objective pupil) is no longer asymetrically
illuminated. Optimize image homogeneity by
slightly adjusting the height of the condenser.
Possible errors
Darkfield illumination is very sensitive to the
slightest inhomogeneities in the specimen. As
dust particles and fingermarks on the upper or
lower surface of the specimen and the front lens
of the condenser also cause scattering and
diffraction of light, it is essential to keep
specimen surfaces and neighbouring lenses
absolutely clean.