KEYWORDS: Mid-IR, Sensors, Long wavelength infrared, Chemical analysis, Black bodies, Absorption, Atmospheric sensing, Chemical elements, Chemical detection, Industrial chemicals
Gaseous plume detection in the LWIR (thermal infrared) region of the
spectrum (7-14 μm) with the use of hyper-spectral imaging sensors is a rapidly advancing technology.1-2 There are many industrial pollutants
that have unique or strong absorption/emission signatures in the mid-wave infrared (MWIR) region of the spectrum. The C-H vibrational frequency
modes of hydrocarbons are clustered in the 3-4 micron region. Until recently
the use of the MWIR region has been hampered, in part, by a lack of
detailed and quantitative characterization of the phenomenology influencing the at-sensor radiance.3 The goal of this paper is to increase understanding of this phenomenology and thus the utility of the MWIR spectral region for industrial pollution monitoring applications.
We conducted experiments with side-by-side active and passive sensors in the 8-12 micron region in order to study similarities and differences in the spectral signatures detected by the two sensors. The active instrument was a frequency-agile CO2 lidar system operating on 44 wavelengths and at a total pulse repetition rate of 5 kHz. The passive system was an Aerospace Corp. dispersive imaging spectrometer with 128 spectral channels from 750-1250 cm-l. The sensors viewed both natural scenes and man-made objects typical of industrial scenes at ranges of 1-3 km along horizontal paths. Scenes were viewed under various ambient conditions in order to evaluate the effects of radiance contrast for the passive images at different times of a day. Both imaging and 'staring' experiments were conducted on the background scenes with a significant level of 'clutter'. Preliminary analysis shows that reflectance data (from an active sensor) does not necessarily have a simple relationship to passive data, which is influenced by ground emissivity, atmospheric radiance, and temperature differences.
The sensitivity of imaging, hyperspectral, passive remote sensors in the long-wavelength infrared (LWIR) spectral region is currently limited by the ability to achieve an accurate, time-invariant, pixel-to-pixel calibration of the elements composing the Focal Plane Array (FPA). Pursuing conventional techniques to improve the accuracy of the calibration will always be limited by the trade-off between the time required to collect calibration data of improved precision and the drift in the pixel response that occurs on a timescale comparable to the calibration time. This paper will present the results from a study of a method to circumvent these problems. Improvements in detection capability can be realized by applying a quick, repetitive dither of the field of view (FOV) of the imager (by a small angular amount), so that radiance/spectral differences between individual target areas can be measured by a single FPA pixel. By performing this difference measurement repetitively both residual differences in the pixel-to-pixel calibration and 1/f detector drift noise can effectively be eliminated. In addition, variations in the atmosphere and target scene caused by the motion of the sensor platform will cause signal drifts that this technique would not be able to remove. This method allows improvements in sensitivity that could potentially scale as the square root of the observation time.
Douglas Nelson, Roger Petrin, Charles Quick, L. John Jolin, Edward MacKerrow, Mark Schmitt, Bernard Foy, Aaron Koskelo, Brian McVey, William Porch, Joseph Tiee, Charles Fite, Frank Archuleta, Michael Whitehead, Donald Walters
The measurement sensitivity of CO2 differential absorption LIDAR (DIAL) can be affected by a number of different processes. Two of these processes are atmospheric optical turbulence and reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. The effects of this phenomenon include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of CO2 DIAL. We have previously developed a Huygens-Fresnel wave optics propagation code to separately simulate the effects of these two processes. However, in real DIAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. In this work, we briefly review a description of our model including the limitations along with a brief summary of previous simulations of individual effects. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements and show good agreement. In addition, simulation studies have been performed to demonstrate the utility and limitations of our model. Examples presented include assessing the effects for different array sizes on model limitations and effects of varying propagation step sizes on intensity enhancements and intensity probability distributions in the receiver plane.
Douglas Nelson, Roger Petrin, Edward MacKerrow, Mark Schmitt, Bernard Foy, Aaron Koskelo, Brian McVey, Charles Quick, William Porch, Joseph Tiee, Charles Fite, Frank Archuleta, Michael Whitehead, Donald Walters
The measurement sensitivity of CO2 differential absorption lidar (DIAL) can be affected by a number of different processes. We have previously developed a Huygens- Fresnel wave optics propagation code to simulate the effects of tow of these processes: effects caused by beam propagation through atmospheric optical turbulence and effects caused by reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. These effects include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of CO2 DiAL. However, in real DiAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements. The limitations of our model are also discussed. In addition, studies have been performed to determine the importance of key parameters in the simulation. The result of these studies and their impact on the overall results will be presented.
The measurement sensitivity of CO2 differential absorption LIDAR (DIAL) can be affected by a number of different processes. We will address the interaction of two of these processes: effects due to beam propagation through atmospheric turbulence and effects due to reflective speckle. Atmospheric turbulence affects the beam distribution of energy and phase on target. These effects include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has a major impact on the sensitivity of CO2 DIAL. The interaction of atmospheric turbulence and reflective speckle is of great importance in the performance of a DIAL system. A Huygens-Fresnel wave optics propagation code has previously been developed at the Naval Postgraduate School that models the effects of atmospheric turbulence as propagation through a series of phase screens with appropriate atmospheric statistical characteristics. This code has been modified to include the effects of reflective speckle. The performance of this modified code with respect to the combined effects of atmospheric turbulence and reflective speckle is examined. Results are compared with a combination of experimental data and analytical models.
Charles Quick, Charles Fite, Bernard Foy, L. John Jolin, Aaron Koskelo, Bryan Laubscher, Edward MacKerrow, Brian McVey, Donald Mietz, Douglas Nelson, Robert Nemzek, Roger Petrin, John Quagliano, Patrick Schafstall, Robert Sander, Joseph Tiee, Michael Whitehead
Issues related to the development of direct detection, long- range CO2 DIAL systems for chemical detection and identification are presented and discussed including: data handling and display techniques for large, multi-(lambda) data sets, turbulence effects, slant path propagation, and speckle averaging. Data examples from various field campaigns and CO2 lidar platforms are used to illustrate the issues.
Edward MacKerrow, Joseph Tiee, Charles Fite, Mark Schmitt, Michael Whitehead, Robert Nemzek, George Busch, Charles Quick, Dennis Remelius, Patrick Schafstall, David Thompson
Reflection of laser light from a diffuse surface exhibits a complex interference pattern known as laser speckle. Measurement of the reflected intensity from remote targets, common to `hard-target' differential absorption lidar, requires consideration of the statistical properties of the reflected light. We have explored the effects of laser speckle on the noise statistics for CO2 DIAL. For an ensemble of independent speckle patterns it is predicted that the variance for the measured intensity is inversely proportional to the number of speckle measured. We have used a rotating drum target to obtain a large number of independent speckle and have measured the predicted decrease in the variance after correlations due to system drifts were removed. Measurements have been made using both circular and linear polarized light. These measurements show a slight improvement in return signal statistics when circular polarization is used. We have conducted experiments at close range to isolate speckle phenomena from other phenomena, such as atmospheric turbulence and platform motion thus allowing us to gain a greater understanding of speckle issues. We have also studied how to remove correlation in the data caused by albedo inhomogenuties producing a more statistically independent ensemble of speckle patterns. We find that some types of correlation are difficult to remove from the data.
A combined experimental and computational approach utilizing CO2 infrared gas lasers and chemometric multivariate analysis was employed to detect chemicals and their concentrations in the open atmosphere under controlled release conditions. Absorption spectra of four organic gases were collected in the laboratory by lasing 40 lines of a Synrad 15 W CO2 laser in the 9.3 to 10.8 micron range. Several chemometric calibration models were constructed based on this IR data using the Partial Least Squares computational technique. The chemometric models were used to analyze in near real time the field DIAL data acquired over this exact wavelength range at round trip distances of 7 and 13 km. It will be shown that the ability to predict the chemicals and their respective concentrations depends on a variety of factors. In 39 of the 45 experiments, the identities of the released chemicals were correctly identified without predictions of false positives or false negatives. Under the best field conditions, we achieved predictions of absolute concentrations within 30% of the actual values.
Stephen Czuchlewski, Michael Brown, George Nickel, Charles Quick, John Schultz, Donald Casperson, Nigel Cockroft, Cheng Ho, Gerard Quigley, William Priedhorsky
Preliminary scoping exercises indicate that remote-sensing lidar can play a useful role in missions that involve determining regional weather patterns and atmospheric transport conditions. Both meteorological modeling and local atmospheric sensing should be employed. Satellite-based remote sensing systems, using an incoherent Doppler wind-sensor, seem feasible.
The ambient atmosphere between the laser transmitter and the target can affect CO2 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with CO2 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.
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