Gas Imaging Cameras - Making Invisible Gas Leaks Visible
Steve Beynon is EMEA Gas Imaging Business Development Manager for FLIR Commercial Systems 2 Kings Hill Avenue , Kings Hill, West Malling, Kent , United Kingdom Tel: +44 1732 220011 • email:
gasimaging@flir.com
Many chemical compounds and gases are invisible to the naked eye. Yet many companies work intensively with these substances before, during and after their production processes.
This article discusses how thermal imaging cameras can be adapted to visualise fugitive gas leaks. The GF-Series thermal imaging cameras from FLIR Systems (see figure 1) were developed to produce a full picture of the scanned area and visualise gas leaks as ‘smoke like images’ on the camera’s viewfinder or on an inbuilt LCD, thereby allowing the user to see fugitive gas emissions. The GF-Series imaging technology allows gas leaks to be viewed in real time and images recorded in the camera for easy archiving.
The Gas Imaging Camera – Why is it different?
The construction of a thermal imaging camera is similar to the construction of a digital video camera. There is a lens, a detector, some electronics to process the signal from the detector and a viewfinder or screen for the user to see the image produced by the camera. The detectors used for the Gas Imaging cameras are quantum detectors that require cooling to cryogenic temperatures (around 70K or -203˚C). Gas Imaging Cameras operating in the mid-wave infrared region (MWIR) use an Indium Antimonide (InSb) detector, those operating in the long wave infrared region (LWIR) use a quantum well infrared photodetector (QWIP) detector.
Image Normalization
Each individual detector in a 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 normalised value. This multistep calibration process is typically performed by the camera software. The final step in the process is the Non-Uniformity Correction (NUC). In measurement cameras, this calibration is performed automatically by the camera. In the GF Series Gas Imaging 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 of this normalisation process is to produce a thermographic image that accurately portrays relative temperature across a target object or scene. No compensation is made for emissivity or the radiation from other objects that is reflected from the target object back into the camera (reflected apparent temperature). The image is a true illustration of radiation intensity regardless of the source of thermal radiation
Spectral Adaption
The gas imaging camera differs from measurement cameras. In addition to the lens, detector, cooler and image processing electronics there is a filter mounted on the front of the detector. This filter is cooled with the detector to prevent any radiation exchange between the filter and the detector. The filter restricts the
Mid Wave Long Wave Figure 1: A Gas Imaging Camera (in action)
wavelengths of radiation allowed to pass through to the detector to a very narrow band called spectral adaption. The filter band wavelengths for the different gas detection cameras are shown below:
Camera Model
Detector Spectral Range
3-5 µm 10-11 µm Filter Waveband Approx. 3.3 µm Approx. 10.5 µm
IET May / June 2011
www.envirotech-online.com
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