Method and apparatus for the imaging of gases

Radiant energy – Infrared-to-visible imaging

Reexamination Certificate

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C250S339040, C250S339140, C250S342000

Reexamination Certificate

active

06297504

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for the imaging of gases, in particular to Fourier Transform Infra-red (FTIR) imaging of gases.
Remote sensing—in particular, chemically selective remote sensing—of gas clouds, gas plumes and the like is a field of obvious environmental significance. Such gas clouds may be due, to, for instance, unintentional gas leakage or to the expulsion of gaseous effluents.
Infra-red vibrational spectroscopy in the 8-14 and 3-5 &mgr;m spectral regions is an analytical technique well suited to such investigations, since most molecules possess an unique infra-red spectrum, and the atmosphere is relatively transparent at these wavelengths. Thus it is perhaps surprising that there appears to be a dearth of literature concerning passive infra-red spectroscopic monitoring of gas clouds and the like. Passive monitoring—wherein absorption or emission of background infra-red radiation is detected—has obvious attractions due to its unobtrusiveness and simplicity is not necessary, for example, to provide an interrogating infra-red radiation source and to position a reflector for such a source, or to position a source at a distance from the detector.
FTIR spectroscopy is a technique of high sensitivity which is well suited to passive measurements. Of particular relevance to the present application is European Patent Application EP-A-0 287 929, which describes a device in which passive FTIR monitoring of gas clouds is combined with a video camera providing a visual image of the monitored area. However, since a single infra-red detector is employed in a standard interferometer arrangement, an infra-red spectrum is obtained which represents a single measurement over the entire field of view of the interferometer. Furthermore, the device does not produce truly quantitative concentration data, since temperature effects are not accounted for.
An improvement upon such passive FTIR systems would be a system capable of producing an IR image of a gas cloud. In this way, the cloud becomes “visible”, since its size and location can be determined. The most straightforward practical implementation would be to employ some kind of array of IR detectors in conjunction with suitable imaging optics. In fact, the field of FTIR imaging appears to be a nascent one, a situation which is probably in large measure due to the fact that the computational requirements are quite severe: Fourier transforms must be performed upon a plurality of interferograms, corresponding to the plurality of detectors in the array, at a realistic duty cycle. It is only recently that suitably powerful data processing technologics have become routinely available.
To date, FTIR imaging appears to have been directed towards military applications such as the tracking of missile or jet vapour streams. It is more than arguable that the imaging of ‘typical’ gas clouds is a more exacting task, since jet omissions and the like are extremely hot typically, at temperatures of 500° C. or more, and therefore emit IR radiation strongly. Gas clouds produced for example, by accidental industrial gas leakage are likely to be at significantly lower temperatures, probably close to or at ambient temperature. Furthermore, such military directed imaging is not concerned with the derivation of quantitative data, i.e, gas column densities. Clearly, quantitative data is highly desirable in the context of gas cloud imaging: for example, such data indicates if a hazardous concentration threshold is being exceeded.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a passive FTIR gas cloud imaging system capable of generating quantitative gas concentration data.
It is a further object of the present invention to provide a composite gas cloud imaging device comprising a passive FTIR gas cloud imaging system combined with a camera system in which the results of the two interrogation techniques—an IR image and a visible image—are displayed simultaneously.
For the avoidance of doubt, the term “gas” is understood to encompass any species in the gas phase, including vapours.
According to a first aspect of the invention there is provided a method for imaging a quantity of gas present in the atmosphere of a selected area comprising the steps of:
directing background IR radiation from the selected area into an interferometer;
imaging the IR radiation emerging from the interferometer onto at least one IR detector;
obtaining a plurality of FTIR spectra in the 8 to 14 &mgr;m spectral region, each spectrum corresponding to IR radiation collected from a different portion of the selected area; and
displaying in a suitable form an IR image, said IR image comprising the plurality of IR spectra, or quantities derived therefrom;
in which the temperature of the quantity of gas or ambient temperature is measured, the temperature of the background is measured, and the difference between the two measured temperatures is used to derive gas column densities from said IR spectra.
The method comprises a quantitative, passive FTIR imaging process in which the absorption or emission of constituents of a gas cloud, plume or the like are measured with respect to a background ‘blackbody’ IR radiation source.
The IR radiation emerging from the interferometer may be imaged onto an array of IR detectors, and each spectrum in the plurality of IR spectra may correspond to the transformed output of a detector in the array.
The background temperature may be measured from the intensity or the intensity distribution of the IR spectra.
The IR image may comprise quantities related to the intensity of an absorption or emission feature in the plurality of IR spectra. In this way, chemically selective intensity data may be displayed.
The difference between the two measured temperatures may be less than 20° C. In other words, the invention is applicable not only to the measurement of “hot” gases, in which the temperature difference is large, but also to “cool” gases, at near ambient temperature.
According to a second aspect of the invention there is provided a method for imaging a quantity of gas present in the atmosphere of a selected area comprising the steps of:
directing background IR radiation from the selected area into an interferometer;
imaging the IR radiation emerging from the interferometer onto at least one TR detector;
obtaining a plurality of FTIR spectra, each spectrum corresponding to IR radiation collected from a different portion of the selected area;
obtaining a visible image of an area which includes the selected area; and
simultaneously displaying in a suitable form (i) an IR image, said IR image comprising the plurality of IR spectra, or quantities derived therefrom, and (ii) the visible image.
The IR radiation emerging from the interferometer may be imaged onto an array of IR detectors, each spectrum in the plurality of IR spectra corresponding to the transformed output of a detector in the array.
The IR image may comprise quantities related to the intensity of an absorption or emission feature in the plurality of IR spectra.
The display of the IR image man overlay the visible image in a portion of the visible image which substantially corresponds to the selected area.
The method according to the second aspect of the invention may also be in accordance with the first aspect of the invention.
According to the third aspect of the invention there is provided apparatus for imaging a quantity of gas present in the atmosphere of a selected area comprising:
an IR collection device capable of collecting IR radiation in the 8 to 14 &mgr;m spectral region;
an interferometer capable of analysing the collected IR radiation in the 8 to 14 &mgr;m spectral region comprising imaging means for imaging IR radiation emerging from the interferometer onto at least one IR detector, the interferometer producing a plurality of interferograms, each interferogram corresponding to a different portion of the selected area;
computing means for obtaining a plurality of IR spectra by performing Fourier transformations of the

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