3 Product and Functional Description | 3.4 Microscopy and Contrast Methods
ZEISS
3.4.6.2 Determination of the Polarization Direction
The determination of the polarization direction of n
γ
or n
γ'
respectively (polarization direction with
the absolute or relative largest index of refraction) and n
α
or n
α'
respectively (polarization direction
with the absolute or relative smallest index of refraction) relative to the morphological directions,
e.g. of crystal surfaces, crystal needles or fibers, provide an important signature of the material.
This method is also used in the diagnosis of bio-crystals (e.g. gout and pseudo-gout).
1
2
3
Fig. 34: Determining the polarization direction n
γ'
using a synthetic fiber as an example
When the lambda compensator is put in, the sample changes its color depending on its orienta-
tion (northeast-southwest or northwest-southeast). Like the sample, the lambda compensator is a
birefringent object, but it has a defined path difference of 550 nm and a maximum oscillation di-
rection n
γ
pointing strongly to northeast-southwest.
The changes in color are based on optical interference. It is necessary to compare the interference
colors (path differences) in both diagonal positions (northeast-southwest and northwest-south-
east).
The path difference results from the interference of the polarization of the sample and the polar-
ization of the lambda compensator.
The largest path difference occurs when the polarization direction of the sample or the absolute
or relative largest index of refraction (n
γ
or n
γ'
) is parallel to the largest polarization direction of the
lambda compensator. The sample appears then e.g. in blue-green
2
.
The smallest path difference occurs when the direction of polarization of the sample with the ab-
solute or relative smallest index of refraction (n
α
or n
α'
) is perpendicular to the polarization direc-
tion of the lambda compensator. The sample then appears e.g. yellow
3
.
The gray-white color appearing first in the bright position in the above example
1
corresponds
to a path difference of 150 nm according to the Michel-Lévy color chart).
56
Instruction Manual ZEISS Axioscope 5, Axioscope 5/7 MAT | en-US | Rev. 13 | 430035-7344-001
