OPTICAL GAS IMAGING
DETECTORS
An optical gas imaging camera can be considered a highly specialized version of an infrared or thermal imaging camera. There is a lens, a detector, some electronics to process the signal from the detector, and a viewfi nder or screen for the user to see the image produced by the camera. The detectors used for OGI cameras are quantum detectors that require cooling to cryogenic temperatures (around 70K or -203°C). Midwave cameras that detect gases such as methane commonly operate in the 3-5 µm range and use an indium antimonide (InSb) detector. Longwave cameras that detect gases such as sulfur hexafl uoride tend to operate in the 8-12 µm range 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 suffi cient thermal energy to be in the conduction band, meaning the electrons there are free to move and the material can conduct an electrical current. Most of the electrons, however, are found in the valence band, where they do not carry any current because they cannot move freely.
When the material is cooled to a low enough temperature, which varies with the chosen material, the thermal energy of the electrons may be so low that none can reach the conduction band. Therefore, the material cannot carry any current. When these materials are exposed to incident photons, and the photons have suffi cient energy, the energy stimulates electrons in the valence band, causing them to move up into the conduction band. Now the material (the detector) can carry a photocurrent, which is proportional to the intensity of the incident radiation.
There is a very exact energy threshold of incident photons that will allow an electron to jump from the valence band into the conduction band. This energy is related to a certain wavelength: the cut-off wavelength. Since photon energy is inversely proportional to its wavelength, the energies are higher in the shortwave/midwave band than in the longwave band. Therefore, as a rule, the operating temperatures for longwave detectors are lower than for shortwave/midwave detectors. For an InSb midwave detector, the necessary temperature must be less than 173 K (-100°C), although it may be operated at a much lower temperature. Whereas, a QWIP longwave detector typically needs to operate at about 70 K (-203°C) or lower. The incident photon wavelength and energy must be suffi cient to overcome the band gap energy, ΔE.
COOLING METHOD
The detectors in most OGI cameras are cooled using Stirling Coolers. The Stirling process removes heat from the cold fi nger (Figure 1) and dissipates it at the warm side. The effi ciency of this type of cooler is relatively low, but good enough for cooling an IR camera detector.
front of the detector and cooled along with it to prevent any radiation exchange between the fi lter and the detector. The fi lter restricts the wavelengths of radiation allowed to pass through to the detector to a very narrow band called the band pass. This technique is called spectral adaptation.
Figure 2. Internal design of an optical gas imaging core
Figure 1. Integrated Stirling cooler, working with helium gas, can cool the detector to -196ºC or sometimes lower
IMAGE NORMALIZATION
Another complexity 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 thermographic image, the different gains and offsets must be corrected to a normalized value. This multi-step calibration process is performed by the camera software. The fi nal step in the process is the Non-Uniformity Correction (NUC). In measurement cameras, this calibration is performed automatically by the camera. In the OGI camera, the calibration is a manual process. This is because the camera does not have an internal shutter to present a uniform temperature source to the detector. The ultimate result is a thermographic image that accurately portrays relative temperatures across the target object or scene. No compensation is made for emissivity or the radiation from other objects that is refl ected from the target object back into the camera (refl ected apparent temperature). The image is a true image of radiation intensity regardless of the source of the thermal radiation.
SPECTRAL ADAPTATION
The OGI camera uses a unique spectral fi lter method that enables it to detect a gas compound. The fi lter is mounted in
IET Annual Buyers’ Guide 2020/21
WWW.ENVIROTECH-ONLINE.COM Figure 3A. Infrared absorption characteristics for propane GAS INFRARED ABSORPTION SPECTRA
For the majority of gas compounds, infrared absorption characteristics are wavelength dependent. In Figures 3A and 3B, the absorption peak for propane and methane are demonstrated by the severe drop in transmittance lines on the graphs. The yellow regions represent a sample spectral fi lter used in an OGI camera, which is designed to correspond to the wavelength range where most background infrared energy would be absorbed by the particular gas of interest.
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