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 <i>in situ</i> 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 LiNbO<sub>3</sub> crystal internally pumped by a Nd:YVO<sub>4</sub> 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
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.
Infrared spectrometry allows detection and identification of gases in the atmosphere as well as analysis of solids and
liquids from long distances. An application of the method which has received increased attention over the last years is
the detection of hazardous compounds. These may be present in the gas phase in the atmosphere but also as liquid
droplets on surfaces. In this study, imaging Fourier transform spectrometry (IFTS) was applied to detect both liquids
and gases. Measurements were performed with an imaging Fourier transform spectrometer developed at TUHH. The
imaging spectrometer and first results of measurements are presented.
In the case of chemical accidents, terrorist attacks, or war, hazardous compounds may be released into the atmosphere. Remote sensing by Fourier-transform infrared spectrometry allows identification and quantification of these hazardous clouds. The output of current standoff detection systems is a yes/no decision by an automatic identification algorithm that analyses the measured spectrum. The interpretation of the measured spectrum by the operator is complicated and thus this task requires an expert. Even if a scanning system is used for surveillance of a large area the operator is dependent on the decision of the algorithm. In contrast to that, imaging systems allow automatic identification but also simple interpretation of the result, the image of the cloud. Therefore, an imaging spectrometer, the scanning infrared gas imaging system (SIGIS) has been developed. The system is based on an interferometer with a single detector element (Bruker OPAG 22) in combination with a telescope and a synchronised scanning mirror. The results of the analyses of the spectra are displayed by an overlay of a false colour image, the "chemical cloud image", on a video image. In this work, the first application of the system as chemical warfare agent identification and imaging system is described. The system, the data analysis method, and results of measurements of chemical warfare agents are presented.
This is an overview of the work carried out in the UK on the stand-off detection of liquid contamination. The UK uses a two-stage concept employing LWIR (long wave infrared) reflectance imaging for location followed by Laser Induced Vapour Emission (LIVE) (patent pending) for identification of the material.
Research has been conducted into IR reflectance imaging, using a HgCdTe starring array and broad band source. 2-5mm diameter contaminant droplets were resolved at distances of 5.5 m on both painted plates and asphalt.
Short distances LIVE experiments using CW agents produced characteristic LWIR emission spectra. These spectra show clear differences between VX, GD and HD as well as backgrounds such as oil and water. Droplets were found to vaporise more efficiently from less absorbent surfaces such as metal and asphalt. A pyroprobe (a rapidly heating probe) was used to flash heat 20 μl droplets of HD, which was detected at 1 metre in a previous version of the experiment.
Longer distance experiments were successfully carried out using smaller amounts of simulant at distances of 18 m. This suggests identification of agent at 20 metres should be trivial providing the rapid heating and generation of hot vapour by remote means is successful. Further, the method is rapid: time resolved studies using a spectroradiometer capable of producing 20 spectra/second shows that 1 second data acquisition is sufficient.
A prototype optical system has been constructed that is chromatically-corrected from the deep ultraviolet (UV) to the far red (200-700nm), facilitating reliable and straightforward sample positioning, as required for quantitative resonance Raman spectroscopy (RRS). The collection side is fully achromatic, whilst the illumination side requires minimal user intervention. Results are presented that demonstrate the axial and spatial imaging performance of the instrument. Spectra illustrate the application of RRS for selective enhancement of analytes in low concentration. Variations in enhancement factor and spectral signature as a function of excitation wavelength are demonstrated. The results illustrate the need for well-characterized achromatic optics when carrying out quantitative investigation using a tuneable UV source laser. A UV-sensitive video-rate CCD is also incorporated into the optical scheme, enabling limited operation as a deep UV microscope. An imaging resolution of approximately 7 micrometers has been demonstrated.