3-12 Chapter 3: Module components SHB_U14
Optical filter
Location in module
Depending on the module layout and measurement task, the optical filters are
located at the following points:
• Between the sample cell and infrared detector
• Between two infrared detectors
• Between the calibration unit and infrared detector
Structure
The optical filter (interference filter) consists of two silicon disks which are
metallically loaded to achieve the desired property.
The filter is fastened in a metal housing.
The housings are different for the various installation locations mentioned above.
Function
By installing interference filters, emission ranges in overlapping sector of the
sample and interference bands are tuned out. This means that, depending on the
its type, the filter only allows the passage of emissions in a specific wavelength
range.
For reasons of symmetry the filter operates on the sample and reference sides of
the beam path.
Filter use depends on the following limiting conditions:
The combination of gas components in the sample gas
Component concentrations
Measurement ranges
Filters are primarily used for the SO
2
, NO, N
2
O, and C
n
H
m
sample components.
Figure 3-14
Optical filters
1 optical filter, style A Installed between sample cell/detector or
Detector/Detector
2 optical filter, style B Installed between calibration cell/detector
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