FLIR G300 pt Series, User Manual, Page: 50 / 80

Why do some gases absorb infrared energy? 16
more effectively 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 in-
frared camera is sensitive.
Transition type Frequency Spectral range
Rotation of heavy molecules 10 9 –10 11 Hz Microwaves, above 3 mm/0.118
in.
Rotation of light molecules and
vibration of heavy molecules 10 11 –10 13 Hz Far infrared, between 30 μm
and 3 mm/0.118 in.
Vibration of light molecules.
Rotation and vibration of the
structure 10 13 –10 14 Hz Infrared, between 3 μm and 30
μm
Electronic transitions 10 14 –10 16 Hz UV–visible
In order for a molecule to absorb a photon via a transition from one state to another, the
molecule must have a dipole moment capable of briefly oscillating at the same frequency
as the incident photon. This quantum mechanical interaction allows the electromagnetic
field energy of the photon to be “transferred” or absorbed by the molecule.
FLIR GF3xx series cameras take advantage of the absorbing nature of certain mole-
cules, to visualize them in their native environments.
FLIR GF3xx 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 ultra
selective. Only gases absorbent in the infrared region that is delimited 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 the only solution to provide a sufficient signal-to-
noise ratio. Hence, the filter itself is maintained at a cryogenic temperature: down to 60 K
in the case of the FLIR GF3xx series LW camera that was released in the beginning of
2008.
Below, are the transmittance spectra of two gases:
• Benzene (C
6
H
6
)—absorbent in the MW region
• Sulfur hexafluoride (SF
6
)—absorbent in the LW region.
Figure 16.7 Benzene (C
6
H
6
). Strong absorption around 3.2/3.3 μm
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