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Hyperspectral Imaging Infrared Sensor Used for Enviromental Monitoring M Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 3 Optical and Acoustical Methods in Science and Technology 2013 Hyperspectral Imaging Infrared Sensor Used for Enviromental Monitoring M. Kasteka;∗, T. Piatkowskia, M. Zyczkowskia, M. Chamberlandb, P. Lagueuxb and V. Farleyb aInstitute of Optoelectronics, Military University of Technology, S. Kaliskiego 2, 00-908 Warsaw, Poland bTelops Inc., Quebec, Canada This paper presents detection, identication and quantication of gases using an infrared imaging Fourier- -transform spectrometer. The company Telops has developed an imaging Fourier-transform spectrometer instru- ment, Hyper-Cam sensor, which is oered as short or long wave infrared sensor. The principle of operation of the spectrometer and the methodology for gases detection, identication and quantication has been shown in the paper, as well as theoretical evaluation of gases detection possibility. The variation of a signal reaching the imaging Fourier-transform spectrometer caused by the presence of a gas has been calculated and compared with the reference signal obtained without the presence of a gas in the imaging Fourier-transform spectrometer eld of view. Some result of the detection of various types of gases has been also included in the paper. DOI: 10.12693/APhysPolA.124.463 PACS: 07.57.Ty, 78.20.Ci, 78.40.−q, 78.47.jg 1. Introduction A problem of remote detection of chemical substances appears in many, sometimes extremely dierent elds of human activities. Applications of detection devices in- clude monitoring of technological processes, diagnostics of industrial installations, monitoring of natural environ- ment, and military purposes. Such diversity has caused development of many detection methods employing vari- Fig. 1. The infrared IFTS Hyper-Cam. ous physical phenomena due to which detection and iden- tication of chemical compounds are possible. Among these methods, a signicant group constitutes the solu- tions using a phenomenon of selective absorption of elec- tral characteristics and resolution matched to the absorp- tromagnetic radiation by chemical compounds. Here, we tion bands of compounds to be detected. Two types can distinguish the solutions based on absorption of laser of such devices can be distinguished. One, it is a sys- radiation emitted by an illuminator, being an element of tem similar to typical thermal camera but additionally a measuring system, and thermovision methods record- equipped with the lter system ensuring the required ing thermal radiation. The methods of the rst group are spectral resolution and the signal analysis system. The the active methods employing absorption of laser radia- other type includes the device based on the principles tion by chemical compounds. Thermovision methods are of the Fourier spectroscopy (Fourier transform infrared based on using the absorption bands of chemical com- spectroscopy FTIR) that is expensive and so not read- pounds in IR. ily available. Thermovision methods, employing arrays detectors, Fourier-transform spectrometers (FTS) are renowned provide an image (in real time) of the observed scenery instruments, particularly well-suited to remotely pro- with the marked regions of searched chemical substances. vide excellent estimates of quantitative data. Many au- Usefulness of this method results from the fact that a lot thors have presented how they are using conventional of substances and chemical compounds have their absorp- (non-imaging) FTS to perform quantication of distant tion bands in IR. Figure 1 shows absorption bands of the gas emissions. Amongst others, we should mention the selected chemical compounds. important contributions made by Harig et al. [1, 2]. The devices operating in IR, used for detection of His group performed many measurement campaigns for chemical substances (gases) in the atmosphere have spec- which they obtained excellent results by ensuring proper modeling of the scene and by paying attention to under- stand (and to take into account) the instrument signa- ture. Other groups developed similar approaches how- ∗corresponding author; e-mail: [email protected] ever limiting their study to optically thin plumes [3, 4] to dedicated instruments performing optical subtraction (463) 464 M. Kastek et al. [5, 6], or introducing Bayesian algorithms to maximize image. The spectral distribution for an entire image is the use of a priori information [7]. On the other hand, obtained by performing the analysis according to (2) for detection and quantication activities with an imaging every pixel of an FPA array. The results can be treated as Fourier-transform spectrometer (IFTS) have been rst 3-dimensional data (two image coordinates and a wave- presented by Spisz et al. [8]. length) and can be analysed using the special software The present paper deals with the remote gas identica- for detection and identication of gases. tion and quantication from turbulent stack plumes with Imaging Fourier-transform spectroradiometer Hy- an IFTS. It presents rst the modeling that is required perCam uses the layout of the Michelson interfero- in order to get an appropriate understanding of both the meter. Its schematic diagram, showing the elements re- scene and the instrument. Next it covers the method- sponsible for the change of the length of optical path, is ology of the developed quantication approach. Finally, shown in Fig. 2. results are presented to demonstrate the capabilities and the performances of the remote gas quantication by us- ing hyperspectral data obtained from the Telops Hyper- Cam. The latter is described in much detail in prior Refs. [912]. 2. Imaging Fourier transform spectroradiometer The infrared IFTS Hyper-Cam LWIR used in experiment was built by Telops Inc. This IFTS use 320 × 256 pixels mercury cadmium telluride (MCT) fo- cal plane arrays (FPA) with a 6◦ × 5◦ FOV. The FPA has Stirling cooler to provide good noise gures in a eld ready package. Spectral information is obtained using Fig. 2. Block diagram of imaging Fourier-transform a technique called Fourier transform infrared radiometry spectroradiometer. (FTIR). FTIR is a classical interference based technique applied to gas spectroscopy that uses a Michelson inter- In the presented system the retro-reectors are used ferometer to mix an incoming signal with itself at several instead of at mirrors. As a result it is easier to move dierent discrete time delays. The resulting time do- M2 mirror keeping the reecting plane in constant po- main waveform, called an interferogram, is related to the sition with respect to the optical axis. It should be power spectrum of the scene through the Fourier trans- mentioned here that the interferometer works in the far form. An interferogram for each pixel in an image is cre- infrared range, thus the necessary dierence in optical ated by imaging the output of the interferometer onto a paths requires large mirror travel. For such long moves focal plane array and collecting data at each discrete time in a classical setup with at mirrors the skew of mirror delay. Advantages of using an FTIR sensor over lter or plane is dicult to avoid which may result in consid- grating based system include higher resolution for equal erable measurement errors. The solution presented in cost and the absence of misalignment of dierent color Fig. 2 minimizes such errors. images due to platform motion. However, FTIR does The main dierence between standard and imaging have the disadvantage of producing a slower frame rate interferometer is the application of FPA detector type. than lter based systems because twice as many points The imaging device can be described as classical inter- are taken for the same number of spectral points. For ferometer multiplied by the number of pixels in the ar- atmospheric tracking of gases however, the LWIR sensor ray. As a result the spectral analysis of interferograms has sucient frame rate [13]. at single pixels gives the spectral information of the en- Data for this collection can be taken from 0.25 cm−1 to tire image. The analysis of incident radiation at pixel 150 cm−1 spectral resolution between 830 cm−1 (12 µm) level is identical for every array element. The change in and 1290 cm−1 (7.75 µm) at a frame rate of 0.2 Hz. In amplitude as a result of optical path dierence x, for λi addition to the infrared data, visible imagery was taken wavelength is given by using a rewire camera that is boresighted to the IR sen- sor. Figure 1 shows a picture of a Hyper-Cam LWIR. The x A(x) = Ai cos 2π : (1) sensor is controlled by eld computer and data is stored λi on a RAID drive to guarantee integrity of data [13]. Detector response is proportional to the radiant intensity The main dierence between the standard and imaging of incident radiation interferometer is the application of FPA detector type. 2 x I(t) = Ii cos 2π : (2) The imaging device can be described as classical inter- λi ferometer multiplied by the number of pixels in the ar- Considering the trigonometric relations for the cosine of ray. As a result the spectral analysis of interferograms at double angle and by limiting the analysis to variable com- single pixels gives the spectral information of the entire ponent only, the radiant intensity as the function of op- Hyperspectral Imaging Infrared Sensor Used . 465 tical path dierence can be written as ment can be achieved by applying longer optical path x dierences and by increasing the number of measurement I(x) = Ii cos 4π : (3) points. Unfortunately both methods lead to longer mea- λi For the sources with continuous radiation characteristics surement times, which are still shortest in case of Fourier- in the range from to , whose spectral characteristics -transform spectrometers than in any other type. The λ1 λ2 advantage of imaging spectrometer over scanning ones is is described by a function S(λ), the radiant intensity can be described as that for a given optical path dierence the entire image is analyzed simultaneously.
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