FLIR GFx320 User Manual page 122

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Why do some gases absorb infrared energy?
Their increased degrees of freedom allow multiple rotational and vibrational transitions.
Because they are built from multiple atoms, they can absorb and emit heat more effec-
tively than simple molecules. Depending on the frequency of the transitions, some of
them fall into energy ranges that are located in the infrared region where the infrared
camera is sensitive.
Transition type
Rotation of heavy molecules
Rotation of light molecules and
vibration of heavy molecules
Vibration of light molecules.
Rotation and vibration of the
structure
Electronic transitions
In order for a molecule to absorb or emit a photon via a transition from one state to anoth-
er, the molecule must have a dipole moment capable of briefly oscillating at the same fre-
quency as the interacting photon. This quantum mechanical interaction allows the
electromagnetic field energy of the photon to be captured or emitted by the molecule.
FLIR GFx3xx series cameras take advantage of the absorbing and emitting nature of cer-
tain molecules, to visualize them in black or white in their native environments. The gas
visualization contrast is a function of the gas concentration multiplied by the path length
(CL), the temperature difference between to background (e.g. a wall) and the gas plume
temperature.
FLIR GFx3xx series focal plane arrays and optical systems are specifically tuned to very
narrow spectral ranges, in the order of hundreds of nanometers, and are therefore selec-
tive. Only gases with sufficient signal strength active in the infrared region that is delim-
ited by a narrow band pass filter can be detected.
Since the energy from the gases is very weak, all camera components are optimized to
emit as little energy as possible. This is a very effective solution to provide a sufficient
signal-to noise ratio. Hence, the filter itself is maintained at a cryogenic temperature.
Below, are the measured transmittance spectra of two gases, source: Pacific Northwest
National Laboratory (PNNL):
• Benzene (C H ), concentration length: CL=5000 ppmxm—absorbent in the MW
6 6
region
• Sulfur hexafluoride (SF ), concentration length: CL=50 ppmxm—absorbent in the LW
6
region
Figure 38.7 Benzene (C H ). Strong absorption around 3.2 - 3.3 μm, CL=5000 ppmxm, Source: PNNL
6 6
#T810169; r. AN/42241/42268; en-US
Frequency
10 9 –10 11 Hz
10 11 –10 13 Hz
10 13 –10 14 Hz
10 14 –10 16 Hz
Spectral range
Microwaves, above 3 mm/0.118
in.
Far infrared, between 30 μm
and 3 mm/0.118 in.
Infrared, between 3 μm and 30
μm
UV–visible
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