Laser-induced Fluorescence Approach Research Articles

Instant plastic waste detection on shores using laser-induced fluorescence and associated hyperspectral imaging

Plastic pollution is a rising environmental issue, with millions of tons of plastic debris collecting in the world's seas and on its shores. Hyperspectral imaging (HSI) has become increasingly widely used as a more precise approach that can identify targets in remote sensing aquatic missions. The interference from other beach materials, and the need for proper identification of litter types can make identifying dumped plastics on sand-surrounded beaches challenging. This study lays the groundwork for a physical laboratory setting for images captured by a hyperspectral (HS) imager. The suggested testing setup included the development of a fluorescence signature for the target theater of operations (low-density polyethylene (LD-PE) and wood surrounded by sand) for detecting polymers in a simulated beach environment using the laser-induced fluorescence (LIF) approach. Initially using broadband-spectrum light, strong sample diffuse reflectance contrast is observed in the imaging at wavelengths between 400 and 460 nm. Next, a dedicated LIF system for plastic litter discovery was developed using an ultraviolet (UV) laser source. Initial findings show that there is a distinct fluorescence signal for plastics at 450 nm and at 750 nm for wood. Our pilot studies support current efforts to determine the optimum spectral signature that these polymers will appear with clarity on shorelines using an inexpensive imagery combined with our UV LIF approach, which may have an impact on applications for the detection of beach pollution. The knowledge gained from this study can be used to construct reliable aerial conventional cameras for plastic waste environmental monitoring and management.

A state-of-the-art review on laser-induced fluorescence (LIF) method with application in temperature measurement

Laser-induced fluorescence (LIF) is a robust and vigorous method for non-intrusive measurement of temperature, pressure, concentration, and pH in fluids. The application of LIF for measurement is also extendable to solid and gaseous environments. In this method, the fluorescent molecules are transferred to a higher energy state by absorbing the energy of a laser beam and subsequently undergo spontaneous emission of light in the form of fluorescence during the transition to the ground. The emitted fluorescence is commonly captured using a setup consisting of a solvent, fluorescent material, detecting cameras, lenses, and filters, which enables the determination of the thermophysical parameters of the measuring environment using pertinent mathematical formulae. This method can be used for precise measurements in a wide range of engineering and medical applications. Despite the numerous studies on the utilization of LIF for measurement, there is not a comprehensive review paper with a focus on various types of LIF techniques and their applications in temperature measurement. This paper seeks to address this gap by fully discussing these approaches, demonstrating temperature measurements in different environments and circumstances, and outlining their benefits and drawbacks. Accordingly, one-dye one-color, two-dye two-color, one-dye two-color, one-dye three-color, two-dye three-color, volumetric LIF, and encapsulation techniques are fully discussed as the main LIF approaches and their applications for the temperature measurement in fluid flows, droplets, microfluidics, and gas environments.

Examination of OH and H2O2 production by uniform and non-uniform modes of dielectric barrier discharge in He/air mixture

In this work we have carried out a parametrical study of hydroxyl radical (OH) generation in nanosecond dielectric barrier discharge (DBD) in He/air mixture using a laser-induced fluorescence approach. The foci of the study are the investigation of differences between uniform and non-uniform modes of the discharges and the difference in production of OH and H2O2 Nanosecond-time scale imaging of the discharge shows transition from streamer to diffuse mode when applied electric field to the discharge gap approaches ∼90 kV cm−1. The results show that both OH production in the gas phase and downstream H2O2 delivery rates to liquid depend on the discharge mode operation and are respectively 30% and 3 times higher for the non-uniform DBD compared to the diffuse discharge.

Laser-induced fluorescence detection of hot molecular oxygen in flames using an alexandrite laser.

The use of an alexandrite laser for laser-induced fluorescence (LIF) spectroscopy and imaging of molecular oxygen in thermally excited vibrational states is demonstrated. The laser radiation after the third harmonic generation was used to excite the B-X (0-7) band at 257 nm in the Schumann-Runge system of oxygen. LIF emission was detected between 270 and 380 nm, revealing distinct bands of the transitions from B(0) to highly excited vibrational states in the electronic ground state, X (v > 7). At higher spectral resolution, these bands reveal the common P- and R-branch line splitting. Eventually, the proposed LIF approach was used for single-shot imaging of the two-dimensional distribution of hot oxygen molecules in flames.

Direct Measurement of SO2 in Air by Laser Induced Fluorescence Spectrometry Using a Nontunable Laser Source

Studies have been performed to evaluate a direct laser induced fluorescence (LIF) technique for measurements of atmospheric sulfur dioxide (SO2). The technique is novel in that it uses a nontunable laser source that is spectrally coincident with absorption of the SO2 molecule near 223 nm that allows sensitive measurements at environmentally relevant concentrations. In this report, the spectral characteristics and analytical capabilities of the nontunable LIF approach have been evaluated and preliminary measurements of ambient SO2 are reported. The current limit of detection is 0.5 ppb and compares well to other analytical spectroscopy methods used for atmospheric measurements of SO2. The results indicate strong feasibility for the nontunable LIF approach for SO2 measurements and suggest ways for method improvement.

Dot-Projection Photogrammetry and Videogrammetry of Gossamer Space Structures

This paper documents the technique of using hundreds or thousands of projected dots of light as targets for photogrammetry and videogrammetry of gossamer space structures. Photogrammetry calculates the three-dimensional coordinates of each target on the structure, and videogrammetry tracks the coordinates versus time. Gossamer structures characteristically contain large areas of delicate, thin-film membranes. Examples include solar sails, large antennas, inflatable solar arrays, solar power concentrators and transmitters, sun shields, and planetary balloons and habitats. Using projected-dot targets avoids the unwanted mass, stiffness, and installation costs of traditional retroreflective adhesive targets. Four laboratory applications are covered that demonstrate the practical effectiveness of white-light dot projection for both static-shape and dynamic measurement of reflective and diffuse surfaces, respectively. Comparisons are made between dot-projection videogrammetry and traditional laser vibrometry for membrane vibration measurements. The paper closes by introducing a promising extension of existing techniques using a novel laser-induced fluorescence approach.

Proof-of-principle laser-induced fluorescence measurements of gas distributions from supersonic nozzles

We have applied the technique of acetone laser-induced fluorescence (LIF) to the measurement of gas distributions from axisymmetric supersonic nozzles used to produce loads for z-pinch plasma radiation sources. Typical peak particle densities are ∼1017 particles/cm3 for loads imploded on the Double-EAGLE facility. The experimental approach uses a pulsed laser (266 nm wavelength, 2.2 mJ per pulse, 5 ns pulse width, and 3×107 W/cm2 intensity) to obtain a snapshot along a chord through the center of the gas density distribution at an arbitrary axial distance, z, from the nozzle exit. We report measurements at 4.3 and 20.0 mm from the exit of the nozzle for comparison with previous measurements. We find acceptable agreement between LIF and laser interferometer measurements. Strengths of the LIF approach include simplicity of implementation and high radial spatial resolution.

Long-term effects of photodynamic therapy on fluorescence spectroscopy in the human esophagus.

Clinical interest in laser-induced fluorescence (LIF) spectroscopy and photodynamic therapy (PDT) is growing rapidly and may ultimately lead to close parallel use of these techniques. However, variations in LIF due to photosensitizer retention as well as tissue damage and healing processes may interfere with autofluorescence-based diagnostic methods. We have investigated the compatibility of these two techniques by quantifying PDT-induced changes in LIF in the human esophagus. Fluorescence spectra were collected endoscopically at excitation wavelengths (lambda ex) of 337, 400 and 410 nm in 32 patients. Measurements were performed immediately before and after PDT treatment with porfimer sodium and during follow-up procedures. In the months following PDT regions of reepithelialized squamous showed reduced autofluorescence in comparison with untreated squamous regions (P = 0.0007). Photosensitizer fluorescence was undetectable with lambda ex = 337 nm during follow-up procedures, whereas for lambda ex = 400 and 410 nm porfimer sodium fluorescence was noted for nearly a year after treatment. Therefore, residual photosensitizer fluorescence is likely to affect certain LIF-based diagnostic techniques during a period when patients are at high risk for tumor recurrence. Modification of LIF systems and/or the use of alternative photosensitizers may be required to optimize the detection of lesions in the post-PDT patient. Given the potential of LIF as a method for surveillance following cancer therapy, further investigation of the compatibility of specific LIF approaches with cancer pharmaceuticals may be warranted.

Fast LIF Approach to NO Rotational Temperature and Density Measurement: Application to a Gas-Dynamic Expansion

An excitation–detection laser-induced fluorescence (LIF) scheme is proposed for the measurement of the NO rotational temperature and density. It is based on the simultaneous excitation of three NO ro-vibronic transitions from only two—one low and one high—J-rotational levels, with the laser set at a suitable fixed wavelength. The procedure does not require the measurement of the excitation spectrum but only of a well-resolved small spectral region of the fluorescence spectrum. The temperature and the density measurements are based on a reference NO LIF spectrum obtained at a known pressure and temperature that is used as a standard. This method is easy and not time consuming. It has been applied to a gas-dynamic expansion.

Double resonance laser-induced fluorescence of Cadmium and Zinc in the inductively coupled plasma and electrothermal atomizer

Double resonance laser-induced fluorescence (LIF) spectrometry studies of Zn and Cd have been performed in the inductively coupled plasma (ICP) and electrothermal atomizer (ETA). Stepwise excitation of Zn is accomplished at 213.856 nm and 636.235 nm with fluorescence emissions observed near 334.5 nm. Excitation of Cd is accomplished at 228.802 nm and 643.847 nm with fluorescence emissions observed near 361.1 nm and 347.6 nm. Calibration studies demonstrate that the double resonance LIF approaches provide high sensitivity and linearity over several orders of magnitude in both atomizers. Limits of detection for Zn and Cd in the ICP are 1.7 ng/ml and 5 ng/ml, respectively. Excellent sensitivity is observed in the ETA resulting in limits of detection of 70 pg/ml (700 fg absolute mass) and 4 pg/ml (40 fg absolute mass) for Zn and Cd, respectively. The Zn content of a bovine serum standard reference material (NIST SRM #1598) has been determined by ETA-LIF and found to be 940±60 ng/g, which is in very good agreement with the certified value of 890±60 ng/g. Low-level ETA-LIF measurements of Zn in these studies are strongly limited by the high background level observed for this element.

Laser-induced fluorescence for esophageal cancer and dysplasia diagnosis.

A method using laser-induced fluorescence (LIF) for in vivo cancer diagnosis of the esophagus is described. Autofluorescence of normal and malignant tissues was measured directly using a fiberoptic probe inserted through an endoscope. The measurements were performed in vivo during routine endoscopy. Measurement of the fluorescence signal from the tissue was performed using laser excitation at 410 nm. The methodology was applied to differentiate normal and malignant tumors of the esophagus. The results of this LIF approach were compared with histopathology results of the biopsy samples and indicated excellent agreement in the classification of normal and malignant tumors for the samples investigated. The LIF procedure could lead to the development of a rapid and cost-effective technique for cancer diagnosis.

Laser-induced fluorescence of As, Se and Sb in the inductively coupled plasma

Stimulated Raman shifting (SRS) has been used to generate tunable UV radiation at 193.7 nm, 197.2 nm and 196.0 nm for the excitation of laser-induced fluorescence (LIF) of As and Se, respectively, in the ICP. An excited-state, stepwise LIF approach has also been explored for Se using laser excitation at 206.279 nm. LIF of Sb has been accomplished using laser excitation at 206.833 nm and 212.739 nm. Power dependence studies of the LIF signals for As, Se and Sb have been performed. Saturation behavior has been observed for each element and saturation spectral energy densities are reported at 193.696 nm and 197.197 nm for As, at 196.026 nm for Se and at 212.739 nm for Sb. Evaluation of the saturation spectral energy density provides insight into the relative magnitude of the fluorescence quantum efficiency of Se at 196.026 nm in the ICP. Limits of detection of the ICP LIF approaches are 4 ng ml −1, 2 ng ml −1 and 15 ng ml −1 for As, Se and Sb, respectively, and compare favorably with those reported previously by ICP emission and ICP atomic fluorescence spectroscopy approaches. Also reported are the first observations of high-lying excited-state transitions of Se in a flame by a LIF technique.

Two Dimensional Rotational Temperature Measurement by Multiline Laser Induced Fluorescence of Nitric Oxide in Combustion Flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A2∑+Π;X2II(0,0) Q1 and Q2 lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q1(31.5)/Q1(16.5) and Q1(18.5)/Q1(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q1(31.5)/Q1(16.5) excitation. However, for Q1(18.5)/Q1(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.

Two-Dimensional LIF Approaches for the Accurate Determination of Radical Concentrations and Temperature in Combustion

Laser-induced fluorescence (LIF) techniques for the quantitative, point-wise measurement of radical concentrations and temperature were developed and applied in laboratory-scale combustion systems. Currently, the potential of similar measurement strategies for accurate two-dimensional, single-pulse experiments is being investigated. The sensitivity of typical LIF approaches for the determination of temperature and concentration to several parameters, especially to collision processes, is examined. Instantaneous two-dimensional two-species fluorescence imaging is demonstrated in a turbulent H2 diffusion flame. Implications for the quantitative interpretation of these results are discussed in view of single-pulse multi-species spontaneous Raman measurements under similar conditions.

A 2‐λ laser‐induced fluorescence field instrument for ground‐based and airborne measurements of atmospheric OH

A modified laser‐induced fluorescence (LIF) technique for measuring tropospheric levels of OH is discussed. Although this system is still of the single‐photon laser‐induced fluorescence type (SP‐LIF), it has undergone major design changes. These changes have overcome several of the major problems encountered in first generation SP‐LIF sensors. Two of the more important of these are (1) the generation of high artificial levels of OH from the laser photolysis of atmospheric O3 and (2) the degradation in detection sensitivity resulting from temporal fluctuations in the nonresonant fluorescence background. In the 2‐λ LIF approach, two nearly identical laser systems are employed such that both “on” line OH signal monitoring and “off” line background levels are measured almost simultaneously (e.g., within 500 μs). This approach, in effect, freezes the atmosphere for purposes of comparing “on” versus “off” line signal measurements. Concerning the problem of laser‐generated OH, two approaches have been explored: the use of very short laser pulses and the use of reduced laser energies. The OH field measuring system reported on in this work used only the reduced energy scheme. Numerous tests have shown that the 2‐λ SP‐LIF system displays no measurable detection bias and, under typical operating conditions, displays shot‐noise‐limited extraction of weak signals. In addition, this in situ sampling system has lent itself to direct in‐flight OH calibrations. Tests also have been developed that quantitatively establish the magnitude of the O3/H2O/OH interference signal (which in nearly all cases was ≤20% of the total OH signal). Finally, the question of OH losses in the sampling manifold has been addressed, and the evidence strongly suggests that this loss is negligibly small. Null experiments, laboratory and in‐flight calibration exercises, and O3/H2O interference tests, as well as ground‐based OH measurements, are reported on. Collectively, these results indicate that the 2‐λ LIF OH system can provide reliable OH measurements down to the fundamental limits of its sensitivity, e.g., typically 1×106 molecules/cm3 for a 30‐min integration time at mid free‐tropospheric altitudes. The detection limit of this system at ground level as well as at mid‐latitudes should be adequate to address many interesting questions in tropospheric chemistry; however, further improvements will be required for routine high‐resolution OH measurements on aircraft sampling platforms.

Collisional deactivation of higher Nasand manifold states byN2

The cross sections for deactivation of the Na $5s\ensuremath{-}9s$ and $n=5\ensuremath{-}8$ manifold ($l\ensuremath{\ge}2$) states by ${\mathrm{N}}_{2}$ have been measured using a laser-induced fluorescence approach. The cross sections are: $6s$, 84(8) ${\mathrm{\AA{}}}^{2}$; $7s$, 82(9) ${\mathrm{\AA{}}}^{2}$; $8s$, 91(8) ${\mathrm{\AA{}}}^{2}$; $9s$, 104(10) ${\mathrm{\AA{}}}^{2}$; five manifold, 43(7) ${\mathrm{\AA{}}}^{2}$; six manifold, 36(8) ${\mathrm{\AA{}}}^{2}$; seven manifold, 32(7) ${\mathrm{\AA{}}}^{2}$; and eight manifold, 18(6) ${\mathrm{\AA{}}}^{2}$. The $s$-state cross sections are in qualitative agreement with a curve-crossing model. The decrease of the manifold cross sections with $n$ is attributed to the increasing multiplicity of the manifold with $n$.