The progress in development of a dual-comb spectrometer for detection of traces of explosives at stand-off distances is reported. The spectral range of the spectrometer was extended to 1205-1305 cm-1, the stand-off distance was shortened to 0.5 m to access more potential use-cases, and the speckle contrast was decreased to 0.3%. Tests of the dual-comb spectrometer on RDX and PETN deposited on glass surfaces with a surface concentration of ~10 g/cm2 deposited using a sieving method will be presented and compared with the measurements carried out using a laboratory grade FTIR instrument.
Here we describe the performance of a recently developed infrared-based chemical sensor for detecting liquid chemical hazards on surfaces at stand-off distances. This sensor uses the signal acquired through three broadband infrared optical filters in the mid and long wave infrared (MWIR LWIR), as opposed to requiring a spectrally tuneable source (such as Quantum Cascade Laser systems) or an FTIR spectrometer, to discriminate between target chemicals and background interferents. Although this technique doesn’t interrogate the full MWIR/LWIR spectrum, it’s ideal for screening a pre determined list of hazards as well as lowering system cost and complexity. The technique is the IR analogue of how the human eye discriminates between different colors, and utilizes modified International Commission on Illumination (CIE) chromaticity charts to illustrate chemical detection performance. We demonstrate that the sensor can correctly discriminate between the chemical warfare agents VX and T-mustard, as well as DEET (N, N-Diethyl-meta-toluamide) insect repellent deposited upon surfaces at a distance of 1.9 meters.
The dual requirement for high spatial and substance specificity makes stand-off in-theatre biological detection of surface biological contaminants extremely challenging. We will describe a novel combined fluorescence multispectral imaging (MSI) and stand-off Raman approach which are united through their use of deep-UV (sub-250 nm excitation. This allows high-confidence location and classification of candidate contamination sites over the camera field of view, and subsequent resonance-Raman classification of these identified sites. Stand-off Raman is enabled through the use of a novel, extremely high-throughput Spatial Heterodyne spectrometer. The viability of this approach is confirmed through its use on application relevant biological simulant samples.
A key driver for the development of next-generation chemical sensors is the need to detect chemical hazards on various surfaces at a distance. By removing the operator from the immediate vicinity of the hazard, personnel safety is substantially improved. Such sensors need to correctly detect the presence of a hazardous target chemical, with minimal false alarms from common environmental interferents. These sensors ideally will be low cost, and able to provide rapid and reliable results with minimal processing. Here we describe recent advances in development of an infrared-based chemical sensor capable of detecting liquid chemical hazards on surfaces at proximal stand-off distances. This sensor, inspired by human color vision, uses only the response through three broadband infrared optical filters to discriminate between target chemicals and background interferents. We show computationally that a single set of three optical filters enable discrimination between several potential chemical nerve agent targets, simulants for these hazards, and common interferents. Finally, we demonstrate the capability of a bioinspired infrared optical sensor, incorporating a series of three infrared optical filters, to discriminate between chemicals for which the system was trained and those for which it was not. This bioinspired laboratory sensor utilizes only low-cost commercially available components, and can rapidly provide actionable detection results.
Ultrafast 2D-IR spectroscopy has proved to be a powerful analytical tool for the detection and differentiation of Bacillus spores as dry films on surfaces. Here, we expand on these findings by employing 2D-IR spectroscopy to study spores from B. atrophaeus (BG) in aqueous solution. Specific vibrational modes attributable to the calcium dipicolinate trihydrate biomarker for spore formation were observed alongside distinctive off-diagonal spectral features that can be used to differentiate spores from different Bacillus species, indicating that 2D-IR has potential for use as a sensing platform with both solid and liquid phase samples. The ability of 2D-IR to enhance the protein amide I band relative to the overlapping water bending vibration was exploited to compare the nature of the protein component of spores to that of solution phase protein molecules. The vibrational lifetime for the amide I band of the BG spore in H2O was 1.4 ± 0.1 ps, longer than those reported for the proteins in H2O solution. The nature of a band at 1710 cm-1 was also investigated. Collectively these results show the potential advantages of 2D-IR spectroscopy, with successful detection and classification of spores under different conditions being based on detailed molecular understanding of the spore state.
We report on a compact laser system for detection of hazardous biological agents by standoff coherent anti-Stokes Raman spectroscopy (CARS). The system is based on ytterbium-laser technology featuring broad spectral coverage and high sensitivity. High-quality CARS spectra have been obtained for NaDPA powder, a substitute for CaDPA, which is the Raman marker of bacterial spores. In addition, endospores of B. atrophaeus deposited over a glass substrate have been detected by their CARS signature at a standoff distance of 1 m and an integration time of 1 s. The system will be further developed for imaging of bacterial spores deposited over wide surface areas at standoff distances.
Significance: Spatial frequency domain imaging (SFDI) is an imaging modality that projects spatially modulated light patterns to determine optical property maps for absorption and reduced scattering of biological tissue via a pixel-by-pixel data acquisition and analysis procedure. Compressive sensing (CS) is a signal processing methodology which aims to reproduce the original signal with a reduced number of measurements, addressing the pixel-wise nature of SFDI. These methodologies have been combined for complex heterogenous data in both the image detection and data analysis stage in a compressive sensing SFDI (cs-SFDI) approach, showing reduction in both the data acquisition and overall computational time.
Aim: Application of CS in SFDI data acquisition and image reconstruction significantly improves data collection and image recovery time without loss of quantitative accuracy.
Approach: cs-SFDI has been applied to an increased heterogenic sample from the AppSFDI data set (back of the hand), highlighting the increased number of CS measurements required as compared to simple phantoms to accurately obtain optical property maps. A novel application of CS to the parameter recovery stage of image analysis has also been developed and validated.
Results: Dimensionality reduction has been demonstrated using the increased heterogenic sample at both the acquisition and analysis stages. A data reduction of 30% for the cs-SFDI and up to 80% for the parameter recover was achieved as compared to traditional SFDI, while maintaining an error of <10 % for the recovered optical property maps.
Conclusion: The application of data reduction through CS demonstrates additional capabilities for multi- and hyperspectral SFDI, providing advanced optical and physiological property maps.
Hybrid metal-semiconductor systems are promising substrates for field Raman analysis due to their ability to use both electromagnetic and chemical enhancement pathways for surface enhanced Raman spectroscopy (SERS). Photo-induced Raman spectroscopy (PIERS) has previously been shown to be a promising method utilizing an additional enhancement route through photo-inducing atomic surface oxygen vacancies in photocatalytic metal-oxide semiconductors. The photoinduced vacancies can form vibronic coupling resonances, known as charge transfers, with analyte molecules, enhancing the signal beyond conventional SERS enhancements. However, conventional UV sources most often used for excitation of the PIERS substrate are impractical in combination with portable Raman systems for field analysis. In this work we show how a small UVC LED, centered at 255 nm, can replicate the same results previously reported with the benefit of allowing greater in-situ real time measurements under constant UV exposure. The UV LED source can be controlled more easily and safely, making it a practical UV source for field PIERS analysis.
A stand-off photothermal sensor platform has been developed which combines two laser technologies: an external cavity Quantum Cascade Laser (EC-QCL), and a near-infrared laser Doppler vibrometer (NIR-LDV). The former is used as a 'pump' to induce vibrations/acoustic waves in the sample, whereas the latter is used to 'probe' these photothermal (PT) effects as the pump wavelength is varied; yielding spectral information on the target analyte. The EC-QCL uses an acousto-optic deflector (AOD) to obtain single-mode mid-infrared light of high output power with up to 1.6 µm of wavelength tuning. Using this AO based approach allows ultra-fast scanning across the full spectral bandwidth of the QCL gain chip at MHz rates, thus facilitating high speed identification of hazards. Initial validation of this pump-probe platform is demonstrated for the detection of 1,3-dinitrobenzene (DNB) and nitrobenzene (NB) on an aluminium substrate at a distance of several metres.
Infra-red (IR) spectroscopic imaging of live cells is greatly affected by the presence of water, which is a strong absorber of IR radiation. In order to overcome this, a variety of methods have been developed using complex microfluidic devices to reduce the liquid sample path length. However, these devices are often custom made needing both specialised equipment and detailed fabrication steps. Here we show the novel utilisation of a liquid-air interface configuration and a negative contrast imaging device (NCI) reflectance imaging system for the collection of spectral data from live cells within an in vitro environment. Spectral differences were observed between two different cell densities, both in the presence and absence of cell culture media. Additionally, differences were observed between control and test cultures exposed to dimethyl sulfoxide (DMSO) to induce cell apoptosis. The NCI system acquired data in the 2.5 – 3.5 μm spectral region, at a spectral sampling interval of 10 nm. This method will allow further investigation of spectral biomarkers within cell cultures to augment understanding of specific cell contributions to wound healing in vivo.
This article presents new hyperspectral imaging (HSI) results from a standoff chemical detection system that utilizes monolithic arrays of Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs) as a source, with each array element at a slightly different wavelength than its neighbor. In this rastering approach to HSI, analysis of analyte/substrate pairs benefits from a laser source with characteristics offered uniquely by a QCL Array. In addition to describing the HSI system developed, a description of experimental standoff detection results using the man-portable system from 1.4 meters are presented. We present HSI results on two very different chemical substrate pairs; trace solid PETN on aluminum and the liquid VX on polycarbonate.
Enhanced Raman relies heavily on finding ideal hot-spot regions which enable significant enhancement factors. In addition, the termed “chemical enhancement” aspect of SERS is often neglected due to its relatively low enhancement factors, in comparison to those of electromagnetic (EM) nature. Using a metal-semiconductor hybrid system, with the addition of induced surface oxygen vacancy defects, both EM and chemical enhancement pathways can be utilized on cheap reusable surfaces. Two metal-oxide semiconductor thin films, WO3 and TiO2, were used as a platform for investigating size dependent effects of Au nanoparticles (NPs) for SERS (surface enhanced Raman spectroscopy) and PIERS (photo-induced enhanced Raman spectroscopy – UV pre-irradiation for additional chemical enhancement) detection applications. A set concentration of spherical Au NPs (5, 50, 100 and 150 nm in diameter) was drop-cast on preirradiated metal-oxide substrates. Using 4-mercaptobenzoic acid (MBA) as a Raman reporter molecule, a significant dependence on the size of nanoparticle was found. The greatest surface coverage and ideal distribution of AuNPs was found for the 50 nm particles during SERS tests, resulting in a high probability of finding an ideal hot-spot region. However, more significantly a strong dependence on nanoparticle size was also found for PIERS measurements – completely independent of AuNP distribution and orientation affects – where 50 nm particles were also found to generate the largest PIERS enhancement. The position of the analyte molecule with respect to the metal-semiconductor interface and position of generated oxygen vacancies within the hot-spot regions was presented as an explanation for this result.
We report on a fiber-format laser system for Fourier-transform coherent anti-Stokes Raman spectroscopy (FT-CARS) of toxic chemical hazards, such as chemical warfare agents (CWAs). The system is based on ytterbium-fiber technology featuring ultra-broad spectral coverage and high-sensitivity. High-quality CARS spectra with maximum Raman shifts of 3000 cm-1 and signal-to-noise ratio >200 for observation times of 160 μs are measured; a detection limit of one part per thousand is demonstrated with a cyanide/water solution. The system was developed as the basis for a highly accurate, sensitive, reliable and portable device for the real time detection of water contaminants and deposited CWAs at trace levels.
Modern traumatic injuries, as encountered in battlefield conflicts, are often characterised by extensive soft tissue damage from blasts and high energy projectiles. This situation has created a challenge for wound stabilisation and repair, with surgical intervention common, via wound debridement procedures. These are often complex surgeries where necrotic and infected tissue is removed, usually with multiple remedial surgeries, designed to aid the natural healing process and to reduce the likelihood of infection. With extensive injuries, the preservation of viable tissue is paramount to functional recovery. Additionally, identifying wounds which are likely to heal without intervention, as well as those that exhibit precursors for impaired healing or infection, would assist in informing the appropriate medical care. Technologies that utilise concepts of non-contact imaging, such as optical imaging and spectroscopy can be used to obtain spatial and spectral maps of biomarkers, which provide valuable information on the wound (e.g. precursors to improper healing or delineate viable and necrotic tissue). A negative contrast imaging device (NCI) has been shown to characterise wound biopsies, through mid-IR (2.6 – 4 μm) non-invasive spectroscopic imaging. To better demonstrate the applicability of this technique, wound relevant cell cultures, subjected to induced trauma, are used to identify spectral changes between healthy and traumatised cells. This work highlights the available contrast in spectroscopic mid-IR signals and demonstrates the utility of spatially and spectrally derived maps as an assessment tool for wound diagnostics.
The global defense community requires new approaches for standoff detection of chemical, biological, radiological, nuclear and explosive (CBRNE) threats. Such standoff detection methods must be capable of discriminating the target hazardous materials from the environmental background. Therefore these sensors must exhibit high selectivity. High selectivity detection of CBRNE threats can be accomplished using infrared (IR) spectroscopy, which produces a unique spectral “fingerprint” of the target chemical, enabling discrimination of the target chemical from other chemicals in the background. Standoff detection using IR spectroscopy however requires that enough of the incident source light may be collected at the detector; therefore a high-power source is needed. Commercially available quantum cascade laser (QCL) sources are capable of projecting high power, coherent laser light at targets down range from the source. In order to collect complete IR spectra throughout the entire fingerprint region, the output of multiple QCL modules are combined into a single exit aperture. This is typically achieved using mirrors and other optics which are susceptible to vibrational and temperature misalignments in field systems. In order to provide a more ruggedized solution to combining the beam output of multiple QCL modules, we developed a unique chalcogenide optical fiber beam combiner which combines the output of four commercial QCL modules. This allows for scanning across a spectral range from 6.01 – 11.20 μm encompassing parts of both the IR functional groups and fingerprint regions. We demonstrate the ability of this QCL system to generate high quality IR spectra of hazardous materials.
The spectrum of mid-infrared light scattered from an actively illuminated aerosol was used to distinguish between different chemicals. Using spectrally broad illumination from an optical parametric oscillator covering 3.2 – 3.55 μm, characteristic absorption features of two different chemicals were detected, and two similar molecules were clearly distinguished using the spectra of backscattered light from each chemical aerosol.
The paper will review the feasibility of adapting the Modified Transient Plane Source (MTPS) method as a screening tool for early-detection of explosives and hazardous materials. Materials can be distinguished from others based on their inherent thermal properties (e.g. thermal effusivity) in testing through different types of barrier materials. A complimentary advantage to this technique relative to other traditional detection technologies is that it can penetrate reflective barrier materials, such as aluminum, easily. A strong proof-of-principle is presented on application of the MTPS transient thermal property measuring in the early-screening of liquid explosives. The work demonstrates a significant sensitivity to distinguishing a wide range of fluids based on their thermal properties through a barrier material. The work covers various complicating factors to the longer-term adoption of such a method including the impact of carbonization and viscosity. While some technical challenges remain, the technique offers significant advantages in complimenting existing detection methods in being able to penetrate reflective metal containers (e.g. aluminum soft drinkscans) with ease.
In recent conflicts, battlefield injuries consist largely of extensive soft injuries from blasts and high energy projectiles, including gunshot wounds. Repair of these large, traumatic wounds requires aggressive surgical treatment, including multiple surgical debridements to remove devitalised tissue and to reduce bacterial load. Identifying those patients with wound complications, such as infection and impaired healing, could greatly assist health care teams in providing the most appropriate and personalised care for combat casualties.
Candidate technologies to enable this benefit include the fusion of imaging and optical spectroscopy to enable rapid identification of key markers. Hence, a novel system based on IR negative contrast imaging (NCI) is presented that employs an optical parametric oscillator (OPO) source comprising a periodically-poled LiNbO3 (PPLN) crystal. The crystal operates in the shortwave and midwave IR spectral regions (ca. 1.5 – 1.9 μm and 2.4 – 3.8 μm, respectively). Wavelength tuning is achieved by translating the crystal within the pump beam. System size and complexity are minimised by the use of single element detectors and the intracavity OPO design. Images are composed by raster scanning the monochromatic beam over the scene of interest; the reflection and/or absorption of the incident radiation by target materials and their surrounding environment provide a method for spatial location. Initial results using the NCI system to characterise wound biopsies are presented here.
A hyperspectral imaging system was implemented using active illumination in the 3-4-μm band from an MgO:PPLN
ultrafast optical parametric oscillator. Using a staring configuration based on a high-resolution mid-IR camera it was
possible to distinguish between liquid chemicals based on their absorption characteristics, demonstrating the potential for
standoff detection of a wide range of liquids.
Laser-based stand-off sensing of threat agents (e.g. explosives, toxic industrial chemicals or chemical warfare agents), by detection of distinct infrared spectral absorption signature of these materials, has made significant advances recently. This is due in part to the availability of infrared and terahertz laser sources with significantly improved power and tunability. However, there is a pressing need for a versatile, high performance infrared sensor that can complement and enhance the recent advances achieved in laser technology. This work presents new, high performance infrared detectors based on III-V barrier diodes. Unipolar barrier diodes, such as the nBn, have been very successful in the MWIR using InAs(Sb)-based materials, and in the MWIR and LWIR using type-II InAsSb/InAs superlattice-based materials. This work addresses the extension of the barrier diode architecture into the SWIR region, using GaSb-based and InAs-based materials. The program has resulted in detectors with unmatched performance in the 2-3 μm spectral range. Temperature dependent characterization has shown dark currents to be diffusion limited and equal to, or within a factor of 5, of the Rule 07 expression for Auger-limited HgCdTe detectors. Furthermore, D* values are superior to those of existing detectors in the 2-3 μm band. Of particular significance to spectroscopic sensing systems is the ability to have near-background limited performance at operation temperatures compatible with robust and reliable solid state thermoelectric coolers.
We present the first demonstration of stand-off Fourier transform infrared spectroscopy using a broadband mid-infrared femtosecond optical parametric oscillator, with spectral coverage over 2700–3200 cm-1. Remote spectroscopy and chemical detection from 2700–3100 cm-1 is demonstrated for a thiodiglycol drop on concrete and anodized aluminum surfaces at a stand-off distance of 2 meters, as well as open-path spectroscopy of atmospheric water vapor from a concrete target at the same range. Comparison of the measured stand-off spectra with archived reference spectra for thiodiglycol and water vapor show good agreement. This technique provides greater spatial coherence and spectral brightness than a thermal source, and wider spectral coverage than a typical quantum-cascade laser, thereby presenting opportunities for application in the detection of industrial pollutants and the environmental identification of chemical warfare agents, explosives or other hazardous materials.
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 most desirable configuration for detection of toxic chemicals utilises the maximum distance between detector and
hazard. This approach minimises the contamination of equipment or personnel. Where the target chemical is an involatile liquid, indirect detection of the liquid contamination is made difficult by inherently low vapour pressure. In this instance, direct detection of the chemical hazard is the best approach. Recent technology developments have allowed spectroscopic systems to provide multiple options for the stand-off detection of involatile chemical warfare agents (CWAs). Two different stand-off spectroscopic systems, based upon IR absorption and Raman spectroscopic techniques are described here. The Negative Contrast Imager (NCI) is based upon an optical parametric oscillator (OPO) source comprising a Q-switched intracavity MgO:PPLN crystal. This crystal has a fanned grating design and wavelength tuning is achieved by translating the PPLN crystal within the 1064 nm pump beam. This approach enables the production of shortwave and midwave IR radiation (1.5 – 1.8 μm and 2.6 – 3.8 μm, respectively), which is scanned across the scene of interest. Target materials that have an absorption feature commensurate with the wavelength of incoming radiation reduce the intensity of returned signal, resulting in dark pixels in the acquired image. This method enables location and classification of the target material. Stand-off Raman spectroscopy allows target chemicals to be identified at range through comparison of the acquired signature relative to a spectral database. In this work, we used a Raman system based upon a 1047 nm Nd:YLF laser source and a proprietary InGaAsP camera system. Utilisation of a longer excitation wavelength than most conventional stand-off detection systems (e.g. 532 or 785 nm) enables reduction of fluorescence from both the surface and the deposited chemicals, thereby revealing the Raman spectrum. NCI and Raman spectroscopy are able to detect CWAs on surfaces at distances of 2 – 10 metres and have potential to detect over longer ranges. We report the successful identification of at least 60 μl of nitrogen mustard at a distance of a 2 m and 10 m using NCI and Raman spectroscopy.
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.
KEYWORDS: Raman spectroscopy, Signal to noise ratio, Signal detection, Luminescence, Urea, Explosives, Sensors, Glasses, Explosives detection, Chemical analysis
The capability to detect toxic chemicals and explosive materials through a wide range of container types has a variety of
applications, including liquid screening at airport entrance points. Conventional Raman spectroscopy is commonly used
for chemical detection, but can result in an intense spectral response due to scattering and/or fluorescence from the
container when used for through-barrier applications. Such a response can reduce the effectiveness of the technique for
analysis of the container contents by swamping the Raman signature of the target material.
By producing two spectra containing different contributions from the container and the contents, spatially offset Raman
spectroscopy (SORS) allows a spectrum of the contents to be obtained, even through fluorescing containers. This
innovative technique could provide a through-barrier detection capability for a wider range of containers than
conventional Raman spectroscopy, including containers made from coloured glass and opaque plastic. In this paper, the
use of SORS for through-barrier detection is introduced, and its ability to detect a range of analytes through a range of
container materials evaluated. The potential advantages of using a longer excitation wavelength (e.g. 1064 nm) to reduce
sample fluorescence are also explored, focussing on target analytes mixed with fluorescent materials.
Previous studies showed that imaging infrared spectrometry allows remote detection and identification of traces of
potentially hazardous liquid surface contaminants. Because the spectra depend on the liquid (composition and thickness),
the background material, and on the illumination of the surface, a comprehensive radiative transfer model has been
developed and applied to calculate synthetic spectra which are used to approximate measured spectra; these simulated
spectra are compared to experimentally acquired data. The model requires spectra of the complex refractive indices of
the liquids. These spectra were calculated by applying the Kramers-Kronig relations to spectra of the linear absorption
coefficient, which are contained in commonly available spectral libraries. As the agreement between measured and
modelled spectra of liquids on different surfaces is excellent, the results confirm the validity of the model.
The ability to remotely locate and classify potential liquid hazards is desirable in a variety of civilian and military
applications. Candidate technologies to satisfy these requirements include the fusion of imaging and optical
spectroscopy. Hence, a novel system based on IR Negative Contrast Imaging (NCI) is presented. The NCI system is
based on an OPO comprising a periodically-poled LiNbO3 (PPLN) crystal of fanned grating design that operates in both
the shortwave and midwave IR spectral regions (1.5 - 1.9 μm and 2.4 - 3.8 μm, respectively). Wavelength tuning is
achieved by translating the PPLN crystal within the 1064 nm pump beam. System size and complexity are minimised by
the use of InGaAs and Zn doped MCT single element detectors and the intracavity OPO design. Images are composed by
raster scanning the monochromatic beam over the scene of interest; the reflection and/or absorption of the incident
radiation by target chemicals and their surrounding environment provide a method for spatial location of the hazard. The
NCI has been employed to detect liquid chemicals on a variety of surfaces; initial results of laboratory investigations are
presented here.
The ability to remotely locate and identify liquid droplets coated upon surfaces is desirable in a variety of civilian and military applications. The fusion of imaging and optical spectroscopy is a promising route to produce technologies that fulfill this requirement. Hence, a novel system based on Raman line imaging is presented. The device utilises a frequency doubled Nd:YAG to produce 532 nm light pulses of energy ca. 400 mJ. This incident light is projected to form a 2 x 500 mm line, whereupon it interacts with a target scene and the resultant Raman shifted light is returned to an auto focus lens. A time-gated, intensified CCD detector is used to collect this light, which is synchronised to the probe laser pulse, thereby significantly suppressing ambient light and fluorescence effects. A library of characteristic spectra that are unique to each chemical species are used to identify the deposited substance. Results of initial experiments to characterise the instrument for remote detection are also reported, including the feasibility of single shot detection.
The in situ location and identification of discrete liquid droplets on surfaces is a technically challenging problem.
Successful solutions often combine real time imaging and optical spectroscopic techniques. To this end, we present
results of initial experiments using a dual-band mid- and shortwave IR (1.3 - 4.5 μm) imaging device to differentiate
between a selection of mineral and synthetic oils. The illumination source is an optical parametric oscillator comprising a
periodically-poled LiNbO3 crystal internally pumped by a Nd:YVO4 laser, which is pumped by a 3 W diode laser. The
source can produce output powers of ca. 0.3 and 0.1 W in the signal and idler fields, respectively. System size and
complexity are minimised by use of an MCT single element detector and images are acquired by raster scanning of the
target. The reflection, absorption and/or scatter of the incident radiation by the liquids and their surroundings provide a
method for spatial location, whereas the characteristic spectra obtained from each sample can be used to uniquely
identify the deposited substance. Both static and video images can be obtained at a range of < 10 metres by this
apparatus.
KEYWORDS: Liquids, Fourier transforms, Spectroscopy, Radiative transfer, Reflection, Temperature metrology, Contamination, Solids, Infrared spectroscopy, Signal to noise ratio
Imaging Fourier transform spectrometry (FTS) was applied to remotely identify liquids on various surfaces. The spectra
are dependent on the liquid film (composition and dimensions), the background surface and the illumination (artificial
source or radiation from the sky). A radiative transfer model was applied to calculate spectra of the liquid films. By
classifying the background materials by their optical properties, a reduced set of spectra was created as reference
signatures for automatic identification. Based on the radiative transfer model, an automatic identification algorithm was
implemented. Measurements were performed with an imaging Fourier transform spectrometer developed at TUHH. The
results of the analysis are displayed by a video image overlaid with an image of the identified liquid. Various liquids on
diverse surfaces were identified automatically. In addition to active measurements, passive measurements without an
artificial source of radiation were performed. The results presented show that by means of the radiative transfer model,
automatic remote identification of liquid contamination is possible.
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