Analysis of solid samples
Reflection measuring attachment 11° - 60°
SPECORD S600 accessories Edition 09/2014 83
Calculation of the path length and refractive index
For the calculation of path length an additional software module "Path length" is
offered for WinASPECT.
Assuming that the refractive index is the same for all wavelengths, the
parthlength and refractive index can be calculated using the method described
below.
The calculation of the path length of a sample with a known refractive index
can be carried out using the following formula:
….,
2
1
m: Number of maximum interference values
after zeroed maximum (see example)
n: Refractive index of the sample
: Reflection angle set
1
Wavelength of the zeroed interference
maximum
2
Wavelength of the m-th interference
maximum
Example: Measuring curve (recorded at a reflection angle of 60°) with 5
maximum interference values. The first maximum value is not counted.
Fig.19-5 Example for interference values at a reflection angle of
= 60°
The wavelengths λ
1
and λ
2
must be determined as accurately as possible. To
do so, use the peak table of the module "Data processing" of the WinASPECT
software.
To calculate the refractive index (with unknown sample path length) the
measurements must be carried out with at least two different reflection angles,
because the formula includes two variables (d, n). The calculation is then
carried out on the basis of the solution of an equation with two variables.
Note: For the most accurate determination of the refractive index it is
recommended to carry out measurements at several reflection angles and take
the arithmetic mean of the results.
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