A combination of experimental lidar results and shape-dependent scatter amplitude matrix calculations are used to explore the utility of polarimetric lidars for aerosol characterization. Solutions are developed for the induced polarization vector response of skewed spheroidal particles and the scatter is then computed using an improved anomalous diffraction approximation method. Experimental data was collected on biological simulant, chemical simulant, and interferent aerosol clouds using a 1047 nm micropulse lidar designed to measure the simultaneous depolarization using a linearly polarized source. Depolarization signatures obtained during testing show a clear difference between wet and dry biological simulant aerosols, wet chemical simulant releases, and some interferents. Combining these measurements with the shape-dependent model calculations help us understand the unique polarimetric signatures that may be exploited for aerosol characterization using stand-off lidar techniques.
The MITA (Motion Imagery Task Analyzer) project was conceived by CBP OA (Customs and Border Protection - Office of Acquisition) and executed by JHU/APL (Johns Hopkins University Applied Physics Laboratory) and C5ISR Center NVESD (Command, Control, Computers, Communications, Cyber, Intelligence, Surveillance, and Reconnaissance Center Night Vision and Electronic Sensors Directorate). The intent is to develop an efficient methodology to characterize imaging system performance objectively, in a field setting, using a target resolution board and simultaneously measuring the turbulence along the camera line of sight. The initial design, development, and testing of MITA was previously reported (Hixson et al) and an additional set of field measurements and subsequent modeling results are reported here. The initial MITA design uses a transmitter at the imaging system location and a DIMM receiver in the field with the resolution target to measure the path turbulence, so a strong understanding of the path-averaged turbulence reciprocity is needed for proper implementation of the MITA system. To this end, a test series was conducted to explore the reciprocity of path averaged optical turbulence measurements using two scintillometers and the laser DIMM receiver in both bi-static and mono-static configurations. Finally, the path averaged measurements are compared with modeled turbulence along the path based off of the available meteorological data.
The MITA (Motion Imagery Task Analyzer) project was conceived by CBP OA (Customs and Border Protection - Office of Acquisition) and executed by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) and CERDEC NVESD MSD (Communications and Electronics Research Development Engineering Command Night Vision and Electronic Sensors Directorate Modeling and Simulation Division). The intent was to develop an efficient methodology whereby imaging system performance could be quickly and objectively characterized in a field setting. The initial design, development, and testing spanned a period of approximately 18 months with the initial project coming to a conclusion after testing of the MITA system in June 2017 with a fielded CBP system. The NVESD contribution to MITA was thermally heated target resolution boards deployed to support a range close to the sensor and, when possible, at range with the targets of interest. JHU/APL developed a laser DIMM (Differential Image Motion Monitor) system designed to measure the optical turbulence present along the line of sight of the imaging system during the time of image collection. The imagery collected of the target board was processed to calculate the in situ system resolution. This in situ imaging system resolution and the time-correlated turbulence measured by the DIMM system were used in NV-IPM (Night Vision Integrated Performance Model) to calculate the theoretical imaging system performance. Overall, this proves the MITA concept feasible. However, MITA is still in the initial phases of development and requires further verification and validation to ensure accuracy and reliability of both the instrument and the imaging system performance predictions.
In recent years, various terrestrial free-space optical (FSO) communications systems have been demonstrated to achieve high-bandwidth communications between mobile platforms. The terminal architectures fall into three general categories: (1) single aperture systems with tip/tilt control, (2) multi-aperture system with tip/tilt control, and (3) single aperture systems with tip/tilt control and higher order adaptive optics correction. Terrestrial modem approaches generally use direct detection receivers because they provide high bandwidth capability (0.1-10 Gbps) without the complexity of coherent detection. Modems are often augmented with a mix of forward error correction (FEC), interleaving, and/or retransmission for improved data transport. This paper will present a terminal and modem architecture for a low-SWAP FSO communications system that enables robust, high-bandwidth communications under highly scintillated links as found in terrestrial applications such as air-to-air, air-to-surface, and surface-to-surface links.
Yttrium Aluminum Garnet (YAG) is an important laser host material. Ideal host materials have low loss at the laser transition frequency. This becomes more important as the gain length increases or a low gain transition is of interest. Unfortunately, single crystal YAG suffers from relatively high scatter caused by strain induced index of refraction variations generated by the growth method. For this reason polycrystalline YAG has been developed with virtually no strain. Furthermore, this material can be doped with concentrations that vary spatially. This can provide a tremendous advantage in matching the gain volume to the mode volume in a laser. However, because of the grain boundaries and porosity, polycrystalline materials have scatter loss. Angle resolved, in-plane scatter measurements of polycrystalline YAG and Nd:YAG are reported from 405 to 1064 nm. This covers the range of interest for laser operation but also with enough bandwidth to derive a physical understanding of the scatter mechanisms. A model is also applied to provide physical insight and interpolation and meaningful extrapolation of the experimental results.
Applications involving space based instrumentation and aerodynamically heated surfaces often require knowledge of the
bi-directional reflectance distribution function (BRDF) of an exposed surface at high temperature. Addressing this need,
the Johns Hopkins University Applied Physics Laboratory (JHU/APL) developed a BRDF facility that features a
multiple–port vacuum chamber, multiple laser sources covering the spectral range from the longwave infrared to the
ultraviolet, imaging pyrometry and laser heated samples. Laser heating eliminates stray light that would otherwise be
seen from a furnace and requires minimal sample support structure, allowing low thermal conduction loss to be obtained,
which is especially important at high temperatures. The goal is to measure the BRDF of ceramic-coated surfaces at
temperatures in excess of 1000°C in a low background environment. Most ceramic samples are near blackbody in the
longwave infrared, thus pyrometry using a LWIR camera can be very effective and accurate.
The concentration of atmospheric oxygen is measured over a 540-m path using supercontinuum absorption spectroscopy. The absorption data compared favorably with MODTRAN™ 5 simulations of the spectra after adjusting for the differences of index of refraction of air and matching the instrument spectral resolution, as described by the effective slit width. Good agreement with the expected atmospheric oxygen concentration is obtained using a previously developed multiwavelength maximum likelihood estimation inversion algorithm. This study demonstrates the use of the SAS technique for measuring concentrations of chemical species with fine absorption structure on long-atmospheric paths.
Remotely sensed imagery of targets embedded in Earth’s atmosphere requires characterization of aerosols between the space-borne sensor and ground to accurately analyze observed target signatures. The impact of aerosol microphysical properties on retrieved atmospheric radiances has been shown to negatively affect the accuracy of remotely sensed data collects. Temporally and regionally specific meteorological conditions require exact site atmospheric characterization, involving extensive and timely observations. We present a novel methodology which fuses White Sands New Mexico regional aerosol micro pulse lidar (MPL) observations with sun photometer direct and diffuse products for broad-wavelength (visible – longwave infrared) input into the radiative transfer model MODTRAN5. Resulting radiances are compared with those retreived from the NASA Aqua MODIS instrument.
Elastic backscatter LIght Detection And Ranging (LIDAR) is a promising approach for stand-off detection of biological aerosol clouds. Comprehensive models that explain the scattering behavior from the aerosol cloud are needed to understand and predict the scattering signatures of biological aerosols under varying atmospheric conditions and against different aerosol backgrounds. Elastic signatures are dependent on many parameters of the aerosol cloud, with two major components being the size distribution and refractive index of the aerosols. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been in a unique position to measure the size distributions of released biological simulant clouds using a wide assortment of aerosol characterization systems that are available on the commercial market. In conjunction with the size distribution measurements, JHU/APL has also been making a dedicated effort to properly measure the refractive indices of the released materials using a thin-film absorption technique and laboratory characterization of the released materials. Intimate knowledge of the size distributions and refractive indices of the biological aerosols provides JHU/APL with powerful tools to build elastic scattering models, with the purpose of understanding, and ultimately, predicting the active signatures of biological clouds.
A multiwavelength, multistatic optical scattering instrument is being developed to characterize spherical
aerosols. This instrument uses 405 nm (blue), 532 nm (green) and 655 nm (red) diode lasers and two CCD imagers
to measure the angular distribution of light scattered from aerosols. The incident light is polarized parallel or
perpendicular to the scattering plane; the scattered intensity is measured at backscatter angles ranging from
120° to 170° by CCD imagers. The phase function for each polarization is used to form the polarization ratio,
which is used to characterize the aerosols. This method has proven to be a reliable way to characterize spherical
aerosols by matching the measured polarization ratio with the polarization ratio calculated by the Mie scattering
equations. This method is used to determine the number density, size distribution, and index of refraction of the
aerosols. The sensitivity of the polarization ratio to particle concentration is explored using a narrow distribution
of one micron polystyrene beads in a chamber. The aerosol concentration is found via an inversion technique
that is based on Mie calculations. This study provides the basis for transitioning this instrument to measure
multiple particle size ranges and concentrations for common aerosols in an outdoor environment.
Light detection and ranging (LIDAR) systems have demonstrated some capability to meet the needs of a fastresponse
standoff biological detection method for simulants in open air conditions. These systems are designed
to exploit various cloud signatures, such as differential elastic backscatter, fluorescence, and depolarization in
order to detect biological warfare agents (BWAs). However, because the release of BWAs in open air is forbidden,
methods must be developed to predict candidate system performance against real agents. In support of such
efforts, the Johns Hopkins University Applied Physics Lab (JHU/APL) has developed a modeling approach to
predict the optical properties of agent materials from relatively simple, Biosafety Level 3-compatible bench top
measurements. JHU/APL has fielded new ground truth instruments (in addition to standard particle sizers, such
as the Aerodynamic particle sizer (APS) or GRIMM aerosol monitor (GRIMM)) to more thoroughly characterize
the simulant aerosols released in recent field tests at Dugway Proving Ground (DPG). These instruments include
the Scanning Mobility Particle Sizer (SMPS), the Ultraviolet Aerodynamic Particle Sizer (UVAPS), and the
Aspect Aerosol Size and Shape Analyser (Aspect). The SMPS was employed as a means of measuring smallparticle
concentrations for more accurate Mie scattering simulations; the UVAPS, which measures size-resolved
fluorescence intensity, was employed as a path toward fluorescence cross section modeling; and the Aspect, which
measures particle shape, was employed as a path towards depolarization modeling.
A semi-empirical reflectance/scatterance model has evolved over the years to represent a
diverse set of materials from coated substrates to optical windows. This model separates the BRDF/BSDF
into four basic components, specular, near-specular, diffuse, and Lambertian (random diffuse) terms. The
specular and near-specular components employ a Gaussian phase function and the Fresnel power
reflection coefficient. The Lambertian component uses Kubelka-Munk theory for the total integrated
reflectance and transmittance. The model features wavelength, angle, and full hemispherical
dependencies. It is applied to a variety of samples, from painted surfaces to transparent windows, with
good success. This parameterized modeling approach is attractive because algorithms that use the model
can be computationally efficient. Previous work has only considered in-plane effects. The present paper
now explicitly takes into account the out-of-plane contribution and improves the total integrated factors.
The optical scattering signature and the absorbance of a material are of interest in a variety of engineering applications,
particularly for those pertaining to optical remote sensing. The John Hopkins University Applied Physics Laboratory
has developed an experimental capability to measure in-plane bidirectional scattering distribution functions to retrieve
optical properties of materials. These measurements are supported at high angular resolution with wavelengths that
span the ultra-violet to the long-wave infrared. Models have been developed to fit Lambertian, diffuse, near-specular,
and specular scattering at a range of incident angles. Useful material properties can then be determined through analysis
of the modeled BSDF. Optical characterization results are shown for a variety of materials, including paints, metals,
optical windows, and leaves.
There is an urgent need to develop standoff sensing of biological agents in aerosolized clouds. In
support of the Joint Biological Standoff Detection System (JBSDS) program, lidar systems have been a
dominant technology and have shown significant capability in field tests conducted in the Joint Ambient
Breeze Tunnel (JABT) at Dugway Proving Ground (DPG). The release of biological agents in the open air
is forbidden. Therefore, indirect methods must be developed to determine agent cross-sections in order to
validate sensor against biological agents. A method has been developed that begins with laboratory
measurements of thin films and liquid suspensions of biological material to obtain the complex index of
refraction from the ultraviolet (UV) to the long wave infrared (LWIR). Using that result and the aerosols'
particle size distribution as inputs to Mie calculations yields the backscatter and extinction cross-sections as
a function of wavelength. Recent efforts to model field measurements from the UV to the IR have been
successful. Measurements with aerodynamic and geometric particle sizers show evidence of particle
clustering. Backscatter simulations of these aerosols show these clustered particles dominate the aerosol
backscatter and depolarization signals. In addition, these large particles create spectral signatures in the
backscatter signal due to material absorption. Spectral signatures from the UV to the IR have been
observed in simulations of field releases. This method has been demonstrated for a variety of biological
simulant materials such as Ovalbumin (OV), Erwinia (EH), Bacillus atrophaeus (BG) and male specific
bacteriophage (MS2). These spectral signatures may offer new methods for biological discrimination for
both stand-off sensing and point detection systems.
Several laser remote sensing techniques are used to characterize the properties of aerosols. The various techniques
include: backscatter, optical extinction using Raman scatter, and bistatic/multistatic scattering using the polarization
ratio of the scattering phase function. The number density, size, and size distribution are obtained under the
assumption of spherical scatterers. Other measurements can be used to describe additional properties, such as
aerosol type based upon approximate refractive index and detected departure from spherical, when simultaneous
measurements at several wavelengths and several angles are analyzed. Examples are shown to demonstrate our
present capability to characterize aerosol particles using recently developed techniques.
A sensor for measuring scattering at multiple wavelengths and multiple angles has been designed and is being tested for
the characterization of atmospheric aerosols. Charge coupled device (CCD) imagers are used to record scattering
measurements at two polarizations and as a function of angle relative to the co-aligned laser beams. A diffraction grating
is used to spatially separate the wavelengths across the
field-of-view of the CCD array, allowing simultaneous
measurements at multiple wavelengths. Experiments are conducted to measure the scattering intensities for two
polarizations at discrete wavelengths that span the visible spectrum. The data from the CCD images are inverted using a
genetic algorithm and Mie scatter equations to determine aerosol properties of artificially generated fog. The results are
compared with in-situ measurements of the aerosol size distribution and concentration using an aerodynamic particle
sizer spectrometer and a condensation particle counter.
Extending our developments of a previously reported supercontinuum lidar system has increased the capability for
measuring long path atmospheric concentrations. The multi-wavelength capability of the supercontinuum laser source
has the advantage of obtaining multiple line differential absorption spectra measurements to determine the
concentrations of various atmospheric constituents. Simulation software such as MODTRANTM 5 has provided the
means to compare and evaluate the experimental measurements. Improvements to the nanosecond supercontinuum laser
fiber coupled transceiver system have allowed open atmospheric path lengths greater than 800 m. Analysis of
supercontinuum absorption spectroscopy and measurements utilizing the updated system are presented.
A multi-wavelength, multi-static lidar has been designed and is being tested for the characterization of atmospheric
aerosols. This design builds upon multi-static lidar, multiple scattering analyses, and supercontinuum DIAL experiments
that have previously been developed at Penn State University. Scattering measurements at two polarizations are
recorded over a range of angles using CCD imagers. Measurements are made using three discrete visible wavelength
lasers as the lidar sources, or using a supercontinuum source with a wavelength range spanning the visible and near-IR
wavelengths. The polarization ratios of the scattering phase functions are calculated for multiple wavelengths to analyze
and determine the aerosol properties of artificially generated fog.
Reflection, Scattering, and Diffraction from Surfaces IV
17 August 2014 | San Diego, California, United States
Reflection, Scattering, and Diffraction from Surfaces III
14 August 2012 | San Diego, California, United States
SC1107: IR Atmospheric Propagation for Sensor Systems
This course reviews the fundamental principles and applications concerning absorption, refraction and scattering phenomena in the atmosphere that impact infrared sensor performance. Topics include an introduction to atmospheric structure, a background reviewing the basic formulas concerning the complex index of refraction, a survey of molecular absorption bands and continuum absorption, the HITRAN database, atmospheric refractivity, molecular Rayleigh scattering, particle distribution functions, Mie scattering and anomalous diffraction approximation. This background is applied to atmospheric transmittance, path radiance, imaging systems and path fluctuations with examples from field measurements. These topics are further reinforced by practical examples on atmospheric optics whenever possible. A set of contemporary references is provided.