An OGI camera can be considered a highly specialised version of an infrared camera—featuring a detector, signal processor and viewfinder. OGI detectors are quantum detectors, requiring cooling to cryogenic temperatures (around 70K or -203°C).
When cooled to a low enough temperature, dependent on the material, the thermal energy of the electrons may be so low that there is no current. When exposed to incident photons with photons have sufficient energy, the energy stimulates electrons, allowing the detector to carry a photocurrent. Since photon energy is inversely proportional to its cut-off wavelength, the energies are higher in a shortwave/midwave band than in the longwave band usually less than 173 K (-100°C).
The detectors in most OGI cameras are cooled using Stirling Coolers—a relatively low efficiency, but enough for cooling an IR camera detector.
Each individual detector in the focal plane array (FPA) has a different gain and zero offset. To create a useful thermographic image, the different gains and offsets must be processed using a Non-Uniformity Correction (NUC). This is a manual process in OGI cameras because the camera does not have an internal shutter to regulate temperature. No compensation is made for the radiation from other objects, resulting in a true image of radiation intensity regardless of the source of the thermal radiation.
The OGI camera uses a unique front-mounted spectral filter that enables it to detect gas compounds. Spectral adaptation is the cooling process that restricts the wavelengths of radiation between the filter and the detector to a very narrow band called the band pass.
Gas infrared absorption spectra
For the majority of gas compounds, infrared absorption characteristics are wavelength dependent. Absorption peak for propane and methane are demonstrated by a severe drop in transmittance lines. An OGI camera is designed to correspond to a wavelength range where the greatest amount of background infrared energy can be absorbed.
Why some gases absorb infrared radiation
From a mechanical point of view, molecules in a gas could be compared to weights connected to springs. Depending on the number of atoms, their respective size and mass, and the elastic constant of the springs, molecules may move in a variety of directions. The more atoms the more emit heat they emit. 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.
Visualising the gas stream
Scenes without a gas leak will have objects that emit and reflect infrared radiation through the lens and filter of the camera—only allowing certain wavelengths through to the detector. This generates an uncompensated image of radiation intensity. In order to see the cloud in relation to the background, there must be a radiant contrast between the cloud and the background. In reality, the amount of radiation reflected from the molecules in the cloud is very small, making temperature difference between the cloud and the background the key to visibility.
To learn more about FLIR’s OGI cameras, check out the article 10 Tips for Getting the Most Out of an Optical Gas Imaging Camera.