Stand-off System Research Articles

Radiation heating tests to evaluate low-emissivity porous ceramics coatings for stand-off thermal protection systems

Diffuse reflection of thermal radiation transfer by Mie scattering can provide a thermal barrier without conversion leading to conduction heat transfer. Applying such a system to internal thermal insulating materials of a stand-off thermal protection systems (TPS) provides an efficient means of preventing thermal radiation transfer. Porous MgAl2O4 ceramics have high thermal resistance because of their low radiation absorption over a wide range from visible to near-infrared wavelengths. For this study, after porous MgAl2O4 ceramics was coated onto a fibrous insulator surface, we investigated its application to stand-off TPS, especially targeting reusable space transportation systems. Direct infrared heating tests were conducted in vacuum. Indirect infrared heating tests were done using graphite as a surface panel in an arc-heated wind tunnel with a high heat flux rate. The apparent thermal conductivity was obtained from the unsteady-state thermal history of the infrared heating experiment using an arc-heated wind tunnel to evaluate the radiation thermal barrier effects of the porous MgAl2O4 ceramics. The low emissivity of porous MgAl2O4 ceramics suppressed heat input and maintained a low surface temperature. Results suggest that thermal radiation transfer was suppressed and that the apparent thermal conductivity of the fibrous insulator was reduced.

Non-contact, portable, and stand-off infrared thermal imager for security scanning applications

In this article, we demonstrated the physical application of a portable infrared (IR) security scanning system for the non-contact and stand-off detection of target objects concealed underneath clothing. Such a system combines IR imaging and transfer learning with convolutional neural networks (CNNs) to enhance the detection of weak thermal signals and automate the classification of IR images. A mid-wavelength IR detector was used to record the real-time heat emitted from the clothing surface by human subjects. Concealed objects reduce the transmissivity of IR radiation from the body to the clothing surface, generally showing lower IR intensity compared to regions without objects. Due to limited resources for training data, the transfer learning approach was applied by fine-tuning a pre-trained CNN ResNet-50 model using the ImageNet database. Two image types were investigated here, i.e., raw thermal and Fuzzy-c clustered images. Receiver operating characteristic curves were built using a holdout set, showing an area-under-the-curve of 0.8934 and 0.9681 for the raw and Fuzzy-c clustered image models, respectively. The gradient-weighted class activation mapping visualization method was used to improve target identification, showing an accurate prediction of the object area. It was also found that complex clothing, such as those composed of materials of different transmissivity, could mislead the model in classification. The proposed IR-based detector has shown potential as a non-contact, stand-off security scanning system that can be deployed in diverse locations and ensure the safety of civilians.

Detection of hazardous chemical using dual-wavelength Raman spectroscopy in the ultraviolet region

This study proposes a stand-off Raman spectroscopy system using dual-wavelength in the ultraviolet (UV) region to detect hazardous chemicals. The Raman spectrum generated by the UV excitation source avoids solar background noise during daytime for chemical detection as the spectrum is in the solar blind range. Wavelengths of 213 and 266 nm by 5th and 4th harmonics are generated from Nd:YAG laser. However, Raman spectra of chemicals exhibit different signal-to-noise ratios for both the excitation wavelengths; therefore, to detect such chemicals, Raman spectra by two sources are required. Raman spectra were acquired using a dual-wavelength laser and spectrometer with a single grating and detector at the wavelengths of 213 and 266 nm simultaneously. The Raman spectra of sulfuric acid, 2-chloroethyl ethyl sulfide, and dimethyl methylphosphonate were acquired and analyzed, thus highlighting the application of dual-wavelength Raman spectroscopy. For efficient chemical detection in the field, we have ensured that the system developed in this study is robust.

Setup and Analysis of a Mid-Infrared Stand-Off System to Detect Traces of Explosives on Fabrics.

The increasing number of terrorist attacks within the last decade has demonstrated that taking preventive protective measures is highly important. In addition to existing measures, automated detection systems for fast and reliable explosive detection are required. A sensitive spectroscopic system based on mid-infrared spectroscopy has been developed and applied to explosive samples on different types of fabric under various geometric conditions. Using this system, traces of TNT, RDX, PETN and ammonium nitrate can be detected in less than a second. Various approaches for data pretreatment (wavelength calibration) and subsequent analysis (normalization, removal of atmospheric water absorption lines) are presented and the remaining challenges on the road to a fully automated system, including a robust classification algorithm, are discussed.

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1MHz) in quasi-continuous wave mode between 8.45 and 10.03μm (∼1182 to 1000 cm-1) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO2) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. The mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

Spectral Considerations for Standoff Infrared Detection of RDX on Reflective Aluminum.

This paper examines infrared spectroscopic effects for the standoff detection of an explosive material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), inkjet printed on an aluminum surface. Results of a spectroscopic study are described, using multiple optical setups. These setups were selected to explore how variations in the angles of incidence and collection from the surface of the material result in corresponding variations in the spectral signatures. The goal of these studies is to provide an understanding of these spectral changes since it affects standoff detection of hazardous materials on a reflective substrate. We demonstrate that variations in spectral effects are dependent on the relative surface concentration of the deposited RDX. We also show that it is reasonable to use spectroscopic data collected in a standard laboratory infrared spectrometer outfitted with a variable angle reflectometer set at 0° as reference spectra for data collected in a standoff configuration. These results are important to provide a systematic approach to understanding infrared (IR) spectra collection using standoff systems in the field, and to allow for comparison between such data, and data collected in the laboratory. Although the precise results are constrained to a specific material system (thin layers on a reflective substrate), the approach and general discussion provided are applicable to a broad range of IR standoff sensing techniques and applications.

Performance analysis and small signal identification of time-resolved stand-off Raman spectroscopy system

Over the last decade, the time-resolved stand-off Raman spectroscopy has attracted considerable attention due to its daytime measurement model and fluorescent suppression. Suffering from the small Raman scattering efficiency, this system perusing large detection distance requires both large power consumption and optical system, which blocks its applications. In order to seek for high signal collection efficient, in this work, we designed a 2-inch refractive telescope system, and obtained 20 m detection distance using a frequency-doubled 532 nm Nd: YAG laser with pulse energy of 30 mJ. By adjusting the excitation energy through a variable attenuator, we investigate the interrelations of input power, the signal and the noise consisting of the internal and signal-induced noises. For both main peak and sub peak, the signal-induced noises are subject to a power function of the signal intensity, and can be filtered and smoothed by the norm-distribution-function weighted moving average method (WMAM) without the loss of the signal information. After transforming the correlation spectrum, defined by the multiplication of the noise and signal spectrum, iteratively by WMAM, we can identify the small signal of about 1 count beyond the internal noise limitation. With help of the iteration method, the requirement of integration time as well as the excitation energy can be greatly reduced.

Effects of surface type in an uncovered stand-off pad system on comfort and welfare of non-lactating dairy cows during winter

The objective of this study was to investigate the effect of stand-off surface type on cow lying behaviour, cleanliness and lameness. One hundred and sixty Friesian × Jersey crossbreed, pregnant non-lactating dairy cows were blocked into five groups of 32 cows and randomly assigned to one of five treatments. Treatments consisted of cows grazing fodder beet in situ and remaining in the paddock with no stand-off (control), or grazing fodder beet in situ and spending 16 h on a stand-off pad with either a woodchip, round-stones (40–60 mm), sand, or geotextile ‘carpet’ surface. Lying duration, monitored by AfiAct electronic pedometers (Afimilk, Kibbutz Afikim, Israel), showed cows on stone spent significantly more (P < 0.001) time lying (9.5 h) than cows on woodchip (8.4 h), control (8.1 h), and sand (7.6 h), but similar to those on carpet (8.8 h). However, there were fewer (P < 0.001) lying bouts (4.6 bouts/d) and longer (P < 0.001) bout duration (137 min/bout) for cows on stone than other treatments (average 7 bouts/day and 85 min/bout), which may suggest that cows found standing and sitting on the stone surface uncomfortable. As measured by a cleanliness score, cows on the sand surface were less (P < 0.001) clean than cows on other surfaces. Lameness, which was determined by visual scoring, revealed only moderate cases of lameness which tended (P = 0.059) to be more prevalent for cows on stone (11.3%) or sand (12.5%), compared with lowest in woodchip (4.0%) or carpet (4.7%), and intermediate in the control treatment (8.8%). The data indicate that sand and the size of stone used are less suitable surfaces than woodchip or carpet for an uncovered winter stand-off pad to meet minimum requirements of animal comfort and welfare. While both the woodchip and carpet surfaces equally met animal welfare standards, the woodchip required less maintenance.

Applications of laser-induced breakdown spectroscopy for cultural heritage: A comparison with X-ray Fluorescence and Particle Induced X-ray Emission techniques

With the aim to establish advantages and limitations of techniques commonly employed for material characterization of Cultural Heritage objects, we performed comparative measurements by LIBS, X-ray Fluorescence (XRF) and Particle Induced X-ray Emission (PIXE) on four typologies of materials. The samples include: 1) egg tempera pigments on gypsum ground; 2) oil paints on gypsum ground with light or dark imprimitura; 3) fragments of decorated glazed ceramic, and 4) ancient Roman coins. The optimal choice of an analytical instrument depends also on the sample type, its dimensions and transportability, and for these reasons our measurements involved two types of instruments per technique. The LIBS probing was done by a table-top instrument (on coins and ceramics) and by a stand-off system at distance of 9.5 m (on pigments). The XRF measurements involved a laboratory micro-XRF system (on coins) and a portable instrument (on pigments and ceramics). The PIXE analyses were obtained by TOP-IMPLART accelerator at ENEA Frascati, using a low energy line that produces the proton beam with adjustable energy between 3 and 7 MeV (used for the pigments and ceramics), and by INFN-LABEC system with proton energy of 3 MeV and complemented by Elastic Backscattering Spectrometry (EBS), for coin samples. Results relevant to quantitative analysis of major sample constituents, identification of trace components, and stratigraphy are reported and discussed for the examined typologies of samples.

Mini-DIAL system measurements coupled with multivariate data analysis to identify TIC and TIM simulants: preliminary absorption database analysis.

Nowadays Toxic Industrial Components (TICs) and Toxic Industrial Materials (TIMs) are one of the most dangerous and diffuse vehicle of contamination in urban and industrial areas. The academic world together with the industrial and military one are working on innovative solutions to monitor the diffusion in atmosphere of such pollutants. In this phase the most common commercial sensors are based on “point detection” technology but it is clear that such instruments cannot satisfy the needs of the smart cities. The new challenge is developing stand-off systems to continuously monitor the atmosphere. Quantum Electronics and Plasma Physics (QEP) research group has a long experience in laser system development and has built two demonstrators based on DIAL (Differential Absorption of Light) technology could be able to identify chemical agents in atmosphere. In this work the authors will present one of those DIAL system, the miniaturized one, together with the preliminary results of an experimental campaign conducted on TICs and TIMs simulants in cell with aim of use the absorption database for the further atmospheric an analysis using the same DIAL system. The experimental results are analysed with standard multivariate data analysis technique as Principal Component Analysis (PCA) to develop a classification model aimed at identifying organic chemical compound in atmosphere. The preliminary results of absorption coefficients of some chemical compound are shown together pre PCA analysis.

In situ analysis of magnesium alloy using a standoff and double-pulse laser-induced breakdown spectroscopy system

To monitor the components of molten magnesium alloy during the smelting process in real time and online, we designed a standoff double-pulse laser-induced breakdown spectroscopy (LIBS) analysis system that can perform focusing, collecting and imaging of long-range samples. First, we tested the system on solid standard magnesium alloy samples in the laboratory to establish a basis for the online monitoring of the components of molten magnesium alloy in the future. The experimental results show that the diameters of the focus spots are approximately 1 mm at a range of 3 m, the ablation depth of the double-pulse mode is much deeper than that of the single-pulse mode, the optimum interpulse delay of the double pulse is inconsistent at different ranges, and the spectral intensity decays rapidly as the range increases. In addition, the enhancement effect of the double pulse at 1.89 m is greater than that at 2.97 m, the maximum enhancement is 7.1-fold for the Y(I)550.35-nm line at 1.89 m, and the calibration results at 1.89 m are better than those at 2.97 m. At 1.89 m, the determination coefficients (R2) of the calibration curves are approximately 99% for Y, Pr, and Zr; the relative standard deviations (RSDs) are less than 10% for Y, Pr, and Zr; the root mean square errors (RMSEs) are less than 0.037% for Pr and Zr; the limits of detection (LODs) are less than 1000 ppm for Y, Pr, and Zr; and the LODs of Y, Pr, and Zr at 2.97 m are higher than those at 1.89 m. Additionally, we tested the system on molten magnesium alloy in a magnesium alloy plant. The calibration results of the liquid magnesium alloy are not as favorable as those of the sampling solid magnesium alloys. In particular, the RSDs of the liquid magnesium alloy are approximately 20% for Pr and La. However, with future improvements in the experimental conditions, the developed system is promising for the in situ analysis of molten magnesium alloy.

Open-path measurement of ozone and methane gases using quartz-enhanced photoacoustic spectroscopy

ABSTRACTWe demonstrate a standoff system based on quartz-enhanced photoacoustic spectroscopy technique for the concentration measurements of atmospheric ozone and methane. The technique is a modified version of Photoacoustic spectroscopy. Two primary features of this technique are the employment of a tunable quantum cascade laser and a resonant quartz-crystal tuning fork detector. Both of these features facilitate simultaneous sensing of multiple molecular species. External-cavity quantum cascade laser having a spectral range from 7 to 10 micron is used. Diurnal concentration variations of methane and ozone are estimated for open-path up to 25 m. The ambient methane and ozone concentration maxima were observed to have values of 3.5 parts per million by volume and 140 parts per billion by volume, respectively. Finite-element mesh-based software is used to simulate the Eigen frequency of the tuning fork sensor. High-resolution transmission molecular spectroscopic database of atmospheric gases and the real-time gas concentration data from the Delhi Pollution Control Committee have been used as references.

Corrections for variable plasma parameters in laser induced breakdown spectroscopy: Application on archeological samples

The final scope of this work was to determine the elemental composition of different types of decorative layers present on ancient ceramic fragments through depth profiling by laser induced breakdown spectroscopy (LIBS). The measurements were performed by a stand-off LIBS system at distance of 10.5m, by employing ns laser pulses at 1064nm and an Echelle spectrometer. The detected plume intensity strongly differs from one sample/coating to another and changes importantly also in repeated measurements on the almost homogeneous bulk materials. Furthermore, the plasma intensity and its parameters widely change during the depth profiling, as evident from the ratio of here monitored Fe I and Fe II spectral lines. Averaging the line intensities over six repeated measurements, also on the bulk material and for a selected consecutive shot number, produces the errors up to 60% around the mean value and this makes impossible to compare composition of the ceramic body with its decorative layers. To overcome this problem, we developed a theoretically supported procedure for the spectral line corrections in presence of variable plasma parameters, which considers the relative changes among a sufficiently large data set. This method allowed improving the measurement precision up to five times, obtaining a flat response during the depth profiling, and measuring composition of the surface layers. The correction factors are specific for one analytical line of the considered element. The proposed procedure could be universally applied for increasing the LIBS precision in repeated samplings or during the depth profiling, without time consuming calculations of the plasma temperature and the electron density, which also suffer from large measurement errors.

Directed energy missions for planetary defense

Directed energy for planetary defense is now a viable option and is superior in many ways to other proposed technologies, being able to defend the Earth against all known threats. This paper presents basic ideas behind a directed energy planetary defense system that utilizes laser ablation of an asteroid to impart a deflecting force on the target. A conceptual philosophy called DE-STAR, which stands for Directed Energy System for Targeting of Asteroids and exploration, is an orbiting stand-off system, which has been described in other papers. This paper describes a smaller, stand-on system known as DE-STARLITE as a reduced-scale version of DE-STAR. Both share the same basic heritage of a directed energy array that heats the surface of the target to the point of high surface vapor pressure that causes significant mass ejection thus forming an ejection plume of material from the target that acts as a rocket to deflect the object. This is generally classified as laser ablation. DE-STARLITE uses conventional propellant for launch to LEO and then ion engines to propel the spacecraft from LEO to the near-Earth asteroid (NEA). During laser ablation, the asteroid itself provides the propellant source material; thus a very modest spacecraft can deflect an asteroid much larger than would be possible with a system of similar mission mass using ion beam deflection (IBD) or a gravity tractor. DE-STARLITE is capable of deflecting an Apophis-class (325m diameter) asteroid with a 1- to 15-year targeting time (laser on time) depending on the system design. The mission fits within the rough mission parameters of the Asteroid Redirect Mission (ARM) program in terms of mass and size. DE-STARLITE also has much greater capability for planetary defense than current proposals and is readily scalable to match the threat. It can deflect all known threats with sufficient warning.

Active standoff detection of CH4 and N2O leaks using hard-target backscattered light using an open-path quantum cascade laser sensor

Fugitive gas emissions from agricultural or industrial plants and gas pipelines are an important environmental concern as they contribute to the global increase of greenhouse gas concentrations. Moreover, they are also a security and safety concern because of possible risk of fire/explosion or toxicity. This study presents standoff detection of CH4 and N2O leaks using a quantum cascade laser open-path system that retrieves path-averaged concentrations by collecting the backscattered light from a remote hard target. It is a true standoff system and differs from other open-path systems that are deployed as point samplers or long-path transmission systems that use retroreflectors. The measured absorption spectra are obtained using a thermal intra-pulse frequency chirped DFB quantum cascade laser at ~7.7 µm wavelength range with ~200 ns pulse width. Making fast time resolved observations, the system simultaneously realizes high spectral resolution and range to the target, resulting in path-averaged concentration retrieval. The system performs measurements at high speed ~15 Hz and sufficient range (up to 45 m, ~148 feet) achieving an uncertainty of 3.1 % and normalized sensitivity of 3.3 ppm m Hz−1/2 for N2O and 9.3 % and normalized sensitivity of 30 ppm m Hz−1/2 for CH4 with a 0.31 mW average power QCL. Given these characteristics, this system is promising for mobile or multidirectional search and remote detection of gas leaks.

Application of Stand-off Double-Pulse Laser-Induced Breakdown Spectroscopy in Elemental Analysis of Magnesium Alloy**supported by National Natural Science Foundation of China (No. 61473279), the National High-Tech Research and Development Program of China (863 Program) (No. 2012AA040608) and Equipment Development Programs of the Chinese Academy of Sciences (No. YZ201247)

In this study, a stand-off and collinear double pulse laser-induced breakdown spectroscopy (DP LIBS) system was designed, and the magnesium alloy samples at a distance of 2.5 m away from the LIBS system were measured. The effect of inter-pulse delay on spectra was studied, and the signal enhancement was observed compared to the single pulse LIBS (SP LIBS). The morphology of the ablated crater on the sample indicated a higher efficiency of surface pretreatment in DP LIBS. The calibration curves of Ytterbium (Y) and Zirconium (Zr) were investigated. The square of the correlation coefficient of the calibration curve of element Y reached up to 0.9998.

Compact 405-nm random-modulation continuous wave lidar for standoff biological warfare detection

A compact bioaerosol standoff system was developed and tested for bio warfare agent stimulant aerosol detection. It used a continuous-wave semiconductor laser diode with an output power of 100 mW and a wavelength of 405 nm. It was modulated with a pseudorandom code at 100 MHz to provide the 1.5 m range resolution for both scattering and fluorescence detection. A 200-mm-diameter Cassegrain telescope and four photon-counting detection channels centered at 405 nm (two polarization detection channels), 450 nm, and 530 nm constituted the optical receiver. A field-programmable gate array chip was controlled to generate the laser driver signal and record four synchronized channel signals. Field tests were performed with stimulant aerosols of Bacillus subtilis, Staphylococcus aureus, and yeast. The detected biological warfare stimulant aerosol lidar was released at ranges of several hundred meters. The preliminary result showed that the fluorescence channel detected the stimulants at ranges up to 90 m and the elastic scattering channel up to 200 m.

Orbital Simulations for Directed Energy Deflection of Near-Earth Asteroids

Directed energy laser ablation at the surface of an asteroid or comet produces an ejection plume that will impart a thrust on the asteroid. This thrust can mitigate a threatened collision with the Earth. This technique uses the asteroid itself as the deflection propellant. The DE-STAR laser system is designed to produce a sufficiently intense spot on the surface of an asteroid to accomplish this in one of two operational modes. One is a complete “stand-off” mode where a large space based phased-array laser directed energy system can interdict asteroids at large distances allowing sufficient time to mitigate nearly all known threats. A much smaller version of the same system, called DE-STARLITE, can be used in a “stand-on” mode by taking a much smaller laser to the asteroid and slowly deflecting it over a sufficiently long period of time. Here we present orbital simulations for a range of near-Earth asteroid impact scenarios for both the stand-off and stand-on systems. Simulated orbital parameters include asteroid radius and composition, initial engagement time, total laser-on time and total energy delivered to target. The orbital simulations indicate that, for exposures that are less than an orbital time, the thrust required to divert an asteroid is generally inversely proportional to laser-on time, proportional to target mass and proportional to the desired miss distance. We present a detailed stand-on scenario, consistent with current dedicated mission capabilities, to show the potential for laser ablation to allow significant deflection of targets with small systems. As one example we analyze a DE-STARLITE mission scenario that is in the same mass and launch envelope as the proposed Asteroid Redirect Mission (ARM) but using a multi kilowatt class laser array capable of deflecting a 325 m diameter asteroid with 2N of thrust for 15 years in a small fraction of even the smallest SLS block 1 launch vehicle configuration.

Stand-off laser-induced breakdown spectroscopy of aluminum and geochemical reference materials at pressure below 1 torr

Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy that utilizes a highly irradiated pulse laser focused on the target surface to produce plasma. We obtain spectroscopic information from the microplasma and determine the chemical composition of the sample based on its elemental and molecular emission peaks. We develop a stand-off LIBS system to analyze the effect of the remote sensing of aluminum and various geochemical reference materials at pressures below 1torr. Using a commercial 4 inch refracting telescope, our stand-off LIBS system is configured at a distance of 7.2m from the four United States Geological Survey (USGS) geochemical samples that include granodiorite, quartz latite, shale-cody, and diabase, which are selected for planetary exploration. Prepared samples were mixed with a paraffin binder containing only hydrogen and carbon, and were pelletized for experimental convenience. The aluminum plate sample is considered as a reference prior to using the geochemical samples in order to understand the influence of a low pressure condition on the resulting LIBS signal. A Q-switched Nd:YAG laser operating at 1064nm and pulsed at 10Hz with 21.7 to 48.5mJ/pulse was used to obtain signals, which showed that the geochemical samples were successfully detected by the present stand-off detection scheme. A low pressure condition generally results in a decrease of the signal intensity, while the signal to noise ratio can vary according to the samples and elements of various types. We successfully identified the signals at below 1torr with stand-off detection by a tightly focused light detection and by using a relatively larger aperture telescope. The stand-off LIBS detection at low pressure is promising for potential detection of the minor elements at pressures below 1torr.

Sensitive Vibration Detection Using Ground-Penetrating Radar

A high-dynamic-range vibration detection system is presented based on analog cancellation. The prototype system operates at 900 MHz and is shown to detect vibrational displacement as low as 11 nm for a rectangular aluminum plate at a standoff distance of 2 m. By operating at a lower frequency than previous radar vibrometers, the system gains ground-penetrating capability and is shown to detect vibrations through 13 cm of sand. A fundamental relationship is introduced relating the minimum detectable vibration displacement to the standoff distance and system dynamic range.