Detectors
An optical gas imaging camera can be considered a very specialized version of a thermographic or infrared camera. It consists of a lens, a detector, electronics to process the detector signal, and a viewfinder or display for the user to view the image produced by the camera. The detectors used for OGI cameras are quantum detectors that require cooling to cryogenic temperatures (around 70 K or -203 °C). Medium-wave cameras that detect gases such as methane typically operate in the 3 to 5 μm range and use an indium antimonide (InSb) detector. Long-wave cameras that detect gases such as sulfur hexafluoride typically operate in the range of 8 to 12 μm and use a quantum well infrared photodetector (QWIP).
When the materials used for quantum detectors are at room temperature, they have electrons at different energy levels. Some electrons have enough thermal energy to be in the conduction band, which means that they are free to move and the material can conduct electric current. However, most electrons are in the valence band and do not carry current because they cannot move freely.
When the material is cooled to a sufficiently low temperature, which varies depending on the material chosen, the thermal energy of the electrons can be so low that none reach the conduction band. Consequently, the material cannot carry current. When these materials are exposed to incident photons and the photons have sufficient energy, the energy stimulates the electrons located in the valence band and causes them to ascend to the conduction band. The material (the detector) can then carry photocurrent that is proportional to the intensity of the incident radiation.
There is a very precise energy threshold of incident photons that allows an electron to pass from a valence band to the conduction band. This energy is related to a certain wavelength: the limiting wavelength. Since photon energy is inversely proportional to its wavelength, the energies of the short wave or medium wave band are higher than those of the long wave. Thus, as a general rule, the operating temperatures of longwave detectors are lower than those of shortwave and mediumwave detectors. For a medium-wave InSb detector, the required temperature must be below 173 K (-100 °C), although it can operate at a much lower temperature. In contrast, a long-wave QWIP detector typically needs a temperature of about 70 K (-203 °C) or less. The wavelength and energy of the incident photons must be sufficient to overcome the energy gap, ΔE.
Refrigeration method
The detectors in most OGI cameras are cooled by Stirling coolers. The Stirling process removes heat from the cold finger (Figure 1) and dissipates it on the hot side. The efficiency of this type of cooler is relatively low, but it is good enough to cool an infrared camera detector.
Figure 1. The integrated Stirling cooler, which runs on helium gas, can cool the detector to -196 °C or sometimes lower.
Image normalization
Another complex aspect is the fact that each individual detector in the focal plane array (FPA) has a slightly different gain and zero offset. To create a useful thermogram, the different gains and offsets must be corrected to a normalized value. This multi-step calibration process is performed by the camera software. The final step in the process is the non-uniformity correction (NUC). In the measuring cameras, this calibration is performed automatically by the camera itself. In the OGI camera, calibration is a manual process. This is because the camera does not have an internal shutter that presents a uniform temperature source to the detector.
The end result is a thermogram that accurately represents the relative temperatures at the target object or scene. No compensation is made for emissivity or radiation from other objects that is reflected from the target object to the camera (apparent reflected temperature). The image is a true picture of the intensity of the radiation, regardless of the origin of the thermal radiation.
Spectral matching
The OGI camera uses a spectral filter method that allows it to detect gaseous compounds. The filter is installed in front of the detector and is cooled together with the detector to prevent the exchange of radiation between the filter and the detector. The filter restricts the wavelengths of radiation that can pass through the detector to a very narrow band called the bandpass. This technique is known as spectral matching.
Figure 2. Internal design of an optical gas imaging camera core.
Infrared absorption spectrum by gases
For most gaseous compounds, the infrared absorption characteristics are wavelength dependent. In Figures 3A and 3B, the absorption peak of propane and methane is manifested in the sharp decrease of the transmittance lines in the graphs. The regions in yellow represent a sample spectral filter used in an OGI camera, which is designed to correspond to the wavelength range in which most background infrared energy would be absorbed by the particular gas of interest.
Figure 3A. Infrared absorption characteristics of propane.
Figure 3B. Infrared absorption characteristics of methane.
Most hydrocarbons absorb energy in the range close to 3.3 μm, so the sample filter in Figure 3 can be used to detect a wide variety of gases.
Ethylene has two strong absorption bands, but a long-wave sensor will detect this gas more sensitively than a medium-wave sensor based on the transmittance curve shown below.
Figure 4. Infrared absorption characteristics of ethylene
Selecting a filter that limits the chamber to operate only at a wavelength where a gas has a very high absorption peak (or minimum transmission point) will improve the visibility of the gas. The gas will effectively "block" a larger amount of absorption coming from objects in the background behind the column.
Why do some gases absorb infrared radiation?
From the mechanical point of view, the molecules of a gas could be compared to two weights (the balls in Figure 5) held together by springs. Depending on the number of atoms, their corresponding size and mass, and the elastic constant of the springs, the molecules can move in a given direction, vibrate along the axis, rotate, bend, stretch, sway, move up and down, etc.
The simplest gas molecules are single atoms such as helium (He), neon (Ne) or krypton (Kr). These molecules cannot vibrate or rotate, and can only move by a translational motion in one direction at a time.
Figure 5. A single atom
The next most complex category of molecules is homonuclear and is composed of two atoms such as hydrogen (H2), nitrogen (N2) and oxygen (O2). These molecules have the ability to roll around their axes, as well as perform translational motions.
Figure 6. Two atoms
Then there are complex diatomic molecules such as carbon dioxide (CO2), methane (CH4), sulfur hexafluoride (SF6) or styrene (C6H5CH=CH2) (these are just a few examples).
Figure 7. Carbon dioxide: 3 atoms per molecule
Figure 8. Methane: 5 atoms per molecule
This assumption is only valid for multiatomic molecules.
Figure 9. Sulfur hexafluoride: 6 or 7 atoms per molecule
Figure 10. Styrene: 16 atoms per molecule
The increased degree of mechanical freedom of these molecules allows them to perform multiple rotational and vibrational transitions. Since they are composed of several atoms, they can absorb and emit heat more effectively than single molecules. Depending on the frequency of the transitions, some are in energy ranges that are in the infrared region where the infrared camera is sensitive.
| TYPE OF TRANSITION | FREQUENCY | SPECTRAL RANGE |
|---|---|---|
| Rotation of heavy molecules | 109 to1011 Hz | Microwave, above 3 mm |
| Rotation of light molecules and vibration of heavy molecules | 1011 to1013 Hz | Far Infrared, between 30 μm and 3 mm |
| Vibration of heavy molecules Rotation and vibration of the structure | 1013 to1014 Hz | Infrared, between 3 μm and 30 mm |
| Electronic transitions | 1014 to1016 Hz | UV: visible |
Table 1. Frequency and wavelength ranges of molecular motions.
For a molecule to absorb a photon (of infrared energy) by transitioning 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 energy field of the photon to be "transferred" to or absorbed by the molecule.
OGI cameras take advantage of the absorbing nature of specific molecules to image them in their native environments. The cameras' focal plane arrays (FPA) and optical systems are specifically tuned to very narrow spectral ranges, on the order of hundreds of nanometers, and are therefore ultra-selective. Only absorbing gases in the infrared region that is bounded by a narrow bandpass filter can be detected (Figures 3 and 4).
Visualization of the gas stream
If the camera is directed at a scene where no gas leak is present, objects in the field of view will emit and reflect infrared radiation through the camera lens and filter. The filter will only allow certain wavelengths of radiation to pass through the detector and from the camera will generate an uncompensated image of the intensity of the radiation. If there is a gas cloud between the objects and the camera, and the gas absorbs radiation in the bandpass range of the filter, the amount of radiation passing through the cloud to the detector will be reduced (Figure 11).
Figure 11. Effect of a gas cloud
To see the cloud relative to the background, there must be a radiant contrast between the cloud and the background. That is, the amount of radiation leaving the cloud must not equal the amount of radiation entering the cloud (Figure 12). If the blue arrow in Figure 12 is the same size as the red arrow, the cloud will be invisible.
Figure 12. Cloud radiant contrast
In reality, the amount of radiation reflected by the molecules in the cloud is very small and can be ignored. Therefore, the key to the cloud being visible is the apparent temperature difference between the cloud and the background (Figure 13).
Figure 13. Apparent temperature difference
Key concepts for making gas clouds visible
- The gas must absorb infrared radiation in the waveband seen by the camera.
- The gas cloud should have a radiant contrast with respect to the background.
- The apparent temperature of the cloud must be different from that of the background.
- The movement makes the cloud easier to see.
Calibration of the OGI equipment to measure temperature is crucial to be able to evaluate the Delta T (apparent temperature between the gas and the bottom).
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