OPERATION
Axiolab 5
Illumination and contrast methods in transmitted light
ZEISS
05/2019
430037-7444-001
111
The lines of this black cross remain closed when the stage is rotated. Depending on the section it may lie
within or outside the displayed objective pupil.
With
optically biaxial
crystals, the cross resolves into two
dark hyperbola branches
(the so-called
isogyres)
depending on stage rotation
, which are surrounded by colored interference patterns
depending on the amount of birefringence and specimen thickness (suggestive of the figure "8").
Inserting a lambda compensator (473704-0000-000) or a lambda/4 (473714-0000-000) or a 0-4 lambda
wedge compensator (000000-1140-663) in the compensator slot with the initial state of the axial figure
being as illustrated in Fig. 4-18 results in the following changes in color (shown schematically in the blue
and yellow areas) of the axial figure, thus allowing differentiation in "optically positive" and "optically
negative".
Optically uniaxial
Optically biaxial
Positive
Negative
Positive
Negative
Lambda plate
(white
→
blue
→
yellow)
+ = Blue
– = Yellow
Quartz wedge
(Direction of motion
at insertion)
Direction of
movement
Lambda/4 plate
(position of black
spots)
Fig. 4-18
Determining the optical character
In the case of less favorable sections in which the cross-hair center is optically uniaxial or the isogyres are
optically biaxial specimens outside the objective pupil, an assessment can be done as follows:
•
If the black isogyres are
straight
and they run parallel to the pupil (in relation to the cross-hairs), the
specimen is
optically uniaxial
.
•
If the black isogyres are
curved lines
which wander on a circular path through the pupil, the
specimen is
optically biaxial
.
NOTE
Axial figures can often be better displayed with circular polarization. In particular, the axis
angle of optically biaxial specimens (quasi the distance between the isogyres) can be better
determined. The optical character can also be determined. the lambda compensator
(6x20mm), arranged in the compensator slot, is used for this purpose.
NOTE
Two storage compartments for 6x20mm slides are located at the back of the conoscopy
stand.