Axioskop 2
plus
OPERATION
Axioskop 2
mot plus
Illumination and contrasting techniques
Carl Zeiss
B 40-075 e 02/01
3-27
3.3.2
Setting of transmitted-light darkfield
(1) General
principle
On account of their light transmission, unstained biological specimens, such as bacteria or living cell
cultures, are often barely or not at all visible in transmitted-light brightfield. This changes markedly if such
specimens are viewed in transmitted-light darkfield, where the specimen is always illuminated using an
aperture which is larger than that of the objective used.
In darkfield, only the diffracted and scattered light components, which are important for image
formation, reach the objective, while the direct unchanged light bundles are directed past the objective.
This is one of the reasons why even fine structures can be resolved, although they are below the resolving
power of the light microscope and appear very bright on a dark background.
(2)
Transmitted-light darkfield configuration for the manual Axioskop
2
plus
and the
Axioskop
2
mot plus
−
Universal condenser with darkfield stop in position D and 0.76 ... 0.90 aperture on the illumination
side
−
Use of ICS objectives up to the maximum aperture of 0.75. Objectives with a higher aperture can only
be used together with the above universal condenser if they feature an integrated aperture iris stop.
(3) Transmitted-light darkfield settings on the manual Axioskop
2
plus
and
Axioskop 2
mot plus
•
Setting of KÖHLER illumination is identical to transmitted-light brightfield, except that the highest
aperture objective must be used instead of the 10x objective.
•
Turn turret disk of the universal condenser to position D (for Axioskop 2
mot plus
using key (3-7/
4
or
5
)) and swing in condenser front lens (for Axioskop 2
mot plus
3
)).
•
Remove eyepiece from the tube (replace it with the centering telescope) and check the centering of
the darkfield stop in the objective exit pupil. If the central darkfield stop D in the universal condenser is
outside or out of the center of the objective exit pupil, and if the exit pupil is not homogeneously
dark, the darkfield stop must be recentered.
•
To center the darkfield stop, use the two SW 1.5 (3-19/
and
4
) Allen screwdrivers and set the two
and
3
) until the objective exit pupil is homogeneously dark. After centering,
remove both SW 2 screwdrivers from the condenser.
If the two Allen screwdrivers are left in the motorized universal condenser by mistake, this provides no
risk to the condenser drives, since these switch off automatically as soon as an obstruction is approached.
After removal of the obstruction, the condenser can be used as usual.
☞
Since the apertures of objectives with an integrated aperture iris stop are too high for
transmitted-light darkfield, the aperture iris stop must be closed to the limit aperture of 0.75.
As dark a field background as possible is always the performance criterion for the darkfield technique.
