Current stand-off hyperspectral imaging detection solutions that operate in the mid-wave infrared (MWIR), nominally 2.5 – 5 μm spectral region, are limited by the number of absorption bands that can be addressed. This issue is most apparent when evaluating a scene with multiple absorbers with overlapping spectral features making accurate material identification challenging. This limitation can be overcome by moving to the long wave IR (LWIR) region, which is rich in characteristic absorption features, which can provide ample molecular information in order to perform presumptive identification relative to a spectral library. This work utilises an instrument platform to perform negative contrast imaging using a novel LWIR optical parametric oscillator (OPO) as the source. The OPO offers continuous tuning in the region 5.5 – 9.5 μm, which includes a number of molecular vibrations associated with the target material compositions. Scanning the scene of interest whilst sweeping the wavelength of the OPO emission will highlight the presence of a suspect material and by analysing the resulting absorption spectrum, presumptive identification is possible. This work presents a selection of initial results using the LWIR hyperspectral imaging platform on a range of white powder materials to highlight the benefit operating in the LWIR region compared to the MWIR.
The ability of a stand-off chemical detector to distinguish two different chemical warfare agents is demonstrated in this paper. Using Negative Contrast Imaging, based upon IR absorption spectroscopy, we were able to detect 1 μl of VX, sulfur mustard and water on a subset of representative surfaces. These experiments were performed at a range of 1.3 metres and an angle of 45° to the surface. The technique employed utilises a Q-switched intracavity MgO:PPLN crystal that generated 1.4 – 1.8 μm (shortwave) and 2.6 – 3.6 μm (midwave) infrared radiation (SWIR and MWIR, respectively). The MgO:PPLN crystal has a fanned grating design which, via translation through a 1064 nm pump beam, enables tuning through the SWIR and MWIR wavelength ranges. The SWIR and MWIR beams are guided across a scene via a pair of raster scanned mirrors allowing detection of absorption features within these spectral regions. This investigation exploited MWIR signatures, as they provided sufficient molecular information to distinguish between toxic and benign chemicals in these proof-of-concept experiments.
The detection and identification of hazardous material is required in a wide range of application environments including military and civilian. Infrared (IR) absorption spectroscopy is a technique that can be used for material identification through comparing absorption spectra with reference data from a spectral library. The absorption spectrum of a compound is the result of light at certain wavelengths being absorbed through molecular vibrations of the compound. To build on this phenomenon, hyperspectral imaging can be used to add spatial information of the absorber. In this case, the IR source output, an optical parametric oscillator (OPO) operating at 1.5 to 1.7 μm in the short wave IR (SWIR) and 2.7 to 3.6 μm in the mid wave IR (MWIR), is raster scanned using a galvanometric mirror pair across a scene of interest. The resulting backscattered light is de-scanned through the same mirror pair and focussed onto point detectors and images in the IR are generated. This hyperspectral imaging instrument is a prototype that is currently being developed for a wide range of applications. If an absorber is present and the OPO wavelength is tuned to an absorption feature of this absorber, this interaction will appear as a dark area in the generated image. With the broad tunability of the OPO, a detailed absorption spectrum of the target compound can be recorded and used to aid material identification. This work presents a selection of results where explosive simulants and materials were investigated and analysed using the prototype instrument.
The ability to obtain accurate vapour parameter information from a compound’s absorption spectrum is an essential data
processing application in order to quantify the presence of an absorber. Concentration measurements can be required for
a variety of applications including environmental monitoring, pipeline leak detection, surface contamination and breath
analysis. This work demonstrates sensitive concentration measurements of complex mixtures of volatile organic
compounds (VOCs) using broadly tunable mid wave infrared (MWIR) laser spectroscopy. Due to the high absorption
cross-sections, the MWIR spectral region is ideal to carry out sensitive concentration measurements of VOCs by tunable
laser absorption spectroscopy (TLAS) methods. Absorption spectra of mixtures of VOCs were recorded using a MWIR
optical parametric oscillator (OPO), with a tuning range covering 2.5 μm to 3.7 μm. The output of the MWIR OPO was
coupled to a multi-pass astigmatic Herriott gas cell, maintained at atmospheric pressure that can provide up to 210 m of
absorption path length, with the transmission output from the cell being monitored by a detector. The resulting spectra
were processed by a concentration retrieval algorithm derived from the optimum estimation method, taking into account
both multiple broadband absorbers and interfering molecules that exhibit narrow multi-line absorption features. In order
to demonstrate the feasibility of the concentration measurements and assess the capability of the spectral processor,
experiments were conducted on calibrated VOCs vapour mixtures flowing through the spectroscopic cell with
concentrations ranging from parts per billion (ppb) to parts per million (ppm). This work represents as a first step in an
effort to develop and apply a similar concentration fitting algorithm to hyperspectral images in order to provide
concentration maps of the spatial distribution of multi-species vapours. The reported functionality of the novel fitting
algorithm makes it a valuable addition to the existing data processing tools for parameter information recovery from
recorded absorption data.
Active hyperspectral imaging is a valuable tool in a wide range of applications. A developing market is the detection and identification of energetic compounds through analysis of the resulting absorption spectrum. This work presents a selection of results from a prototype mid-infrared (MWIR) hyperspectral imaging instrument that has successfully been used for compound detection at a range of standoff distances. Active hyperspectral imaging utilises a broadly tunable laser source to illuminate the scene with light over a range of wavelengths. While there are a number of illumination methods, this work illuminates the scene by raster scanning the laser beam using a pair of galvanometric mirrors. The resulting backscattered light from the scene is collected by the same mirrors and directed and focussed onto a suitable single-point detector, where the image is constructed pixel by pixel. The imaging instrument that was developed in this work is based around a MWIR optical parametric oscillator (OPO) source with broad tunability, operating at 2.6 μm to 3.7 μm. Due to material handling procedures associated with explosive compounds, experimental work was undertaken initially using simulant compounds. A second set of compounds that was tested alongside the simulant compounds is a range of confusion compounds. By having the broad wavelength tunability of the OPO, extended absorption spectra of the compounds could be obtained to aid in compound identification. The prototype imager instrument has successfully been used to record the absorption spectra for a range of compounds from the simulant and confusion sets and current work is now investigating actual explosive compounds. The authors see a very promising outlook for the MWIR hyperspectral imager. From an applications point of view this format of imaging instrument could be used for a range of standoff, improvised explosive device (IED) detection applications and potential incident scene forensic investigation.
Active hyperspectral imaging is a valuable tool in a wide range of applications. One such area is the detection and
identification of chemicals, especially toxic chemical warfare agents, through analysis of the resulting absorption
spectrum. This work presents a selection of results from a prototype midwave infrared (MWIR) hyperspectral imaging
instrument that has successfully been used for compound detection at a range of standoff distances. Active hyperspectral imaging utilises a broadly tunable laser source to illuminate the scene with light at a range of wavelengths. While there are a number of illumination methods, the chosen configuration illuminates the scene by raster scanning the laser beam using a pair of galvanometric mirrors. The resulting backscattered light from the scene is collected by the same mirrors and focussed onto a suitable single-point detector, where the image is constructed pixel by pixel. The imaging instrument that was developed in this work is based around an IR optical parametric oscillator (OPO) source with broad tunability, operating in the 2.6 to 3.7 μm (MWIR) and 1.5 to 1.8 μm (shortwave IR, SWIR) spectral regions. The MWIR beam was primarily used as it addressed the fundamental absorption features of the target compounds compared to the overtone and combination bands in the SWIR region, which can be less intense by more than an order of magnitude. We show that a prototype NCI instrument was able to locate hydrocarbon materials at distances up to 15 metres.
A limiting factor of tuneable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) is the
presence of background residual amplitude modulation (RAM) on the recovered 1st harmonic signal. The presence of
this background term is due to direct modulation of the source laser power. This work presents a novel method to
optically remove the unwanted background, with the major benefit being that measurement sensitivity can be increased.
The recently developed phasor decomposition method1 (PDM), is a near IR (NIR) TDLS analysis technique that is used
with the addition of the new RAM nulling method to recover gas absorption line-shapes. The PDM is a calibration free
approach, which recovers the gas absorption line-shape and the isolated 1st derivative of the line-shape from the 1st
harmonic signal. The work presented illustrates and validates the new RAM nulling procedure with measurements
examining the 1650.96nm absorption line of methane (CH4) with comparisons to theory.