Remote leak detection of gases such as the homonuclear molecules (N2, H2, etc.) and noble gases (He, Ar etc.) is still an issue for tunable laser spectroscopy (TLS) because these gases do not have infrared absorption bands. In order to detect a leak in air, the gas displacement of the ambient air is used as an indirect indication of the leak. So, the unique idea is to measure the reduced oxygen concentration by a standoff laser spectrometer at an emission wavelength of 761 nm. The advantage of oxygen as indicator gas is the stable concentration level with respect to low spatial and temporal fluctuations. The challenge of the standoff detection is to analyze the small relative transmission change for weak light intensity scattered by the background. Furthermore, a remote measurement technique for high-level oxygen concentration on ppm level resolution is demonstrated. Here the combination of a high performance distributed feedback laser at 761 nm and high end sophisticated electronics for driver and data acquisition is required and designed. With the direct absorption spectroscopy, the concentration change of 2000 ppm within a 1 cm plume size (10 ml/min flow, ambient room conditions) corresponds to a transmission change in order of 2E-4 has been resolved on a low absolute power level of few micro watts in 1m distance. The detection limit corresponds to a nitrogen leakage rate of 0.1mbar·l/s which is comparable to ordinary remote detection systems for methane leakages.
We present standoff detection of various explosives by backscattering spectroscopy, using a sensing system based on
mid-IR external-cavity quantum cascade lasers (EC-QCL) with a broad tunable range of about 300 cm-1. Traces of TNT
(trinitrotoluene), PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) as well as different nonhazardous
substances were investigated by illuminating them with the EC-QC laser and collecting the diffusely
backscattered light. Tuning the EC-QCL across the characteristic absorption spectra enables us to detect and identify the
explosives against a background of non-hazardous materials.
Infrared spectroscopy uses the characteristic absorption of the molecules in the mid infrared and allows the determination
of the gases and their concentration. Especially by the absorption at longer wavelengths between 8 μm and 12 μm, the so
called "fingerprint" region, the molecules can be measured with highest selectivity.
We present an infrared optical filter photometer for the analytical determination of trace gases in the air. The challenge in
developing the filter photometer was the construction of a multi-channel system using a novel filter wheel concept -
which acts as a chopper too- in order to measure simultaneously four gases: carbon monoxide, carbon dioxide, methane
and ammonia. The system consists of a broadband infrared emitter, a long path cell with 1.7m optical path length, a filter
wheel and analogue and digital signal processing.
Multi channel filter photometers normally need one filter and one detector per target gas. There are small detection units
with one, two or more detectors with integrated filters available on the market. One filter is normally used as reference at
a wavelength without any cross-sensitivities to possible interfering gases (e.g. at 3.95 μm is an "atmospheric window" -
a small spectral band without absorbing gases in the atmosphere). The advantage of a filter-wheel set-up is that a single
IR-detector can be used, which reduces the signal drift enormously. Pyroelectric and thermopile detectors are often
integrated in these kinds of spectrometers. For both detector types a modulation of the light is required and can be done -
without an additional chopper - with the filter wheel.
We present a system for the stand-off detection of solid explosive traces and precursors on surfaces. The system consists
of a widely tunable quantum cascade laser (QCL) and a thermal imaging camera. The external cavity quantum cascade
laser (EC-QCL) illuminates the surface of a distant object at different characteristic wavelengths. In synchronisation with
the camera a hyperspectral data cube of the backscattered radiation is generated allowing a multivariate analysis of the
scene. We demonstrate how multidimensional image processing is used in order to fast and sensitively detect traces of
hazardous substances such as trinitrotoluene (TNT) or pentaerythritol tetranitrate (PETN). The recognition algorithm is
developed to effectively suppress false alarms. Experiments are performed on real world like surfaces such as standard
car paint, synthetic cloth or jeans fabric.
Results on the detection of traces of trinitrotoluene (TNT) on different substrate-materials like Aluminum and
standard car paint are presented. We investigated different samples with a movable imaging standoff detection
system at angles of incidence far away from specular reflection. The samples were illuminated with a tunable
mid-infrared external-cavity quantum cascade laser. For collection of the diffusely backscattered light a highperformance
infrared imager was used. Trace concentrations of TNT corresponding to fingerprints on realworld-
substrates were detected, while false alarms of cross-contaminations were successfully suppressed.