KEYWORDS: LIDAR, Receivers, Research management, Signal to noise ratio, Water, Signal attenuation, Calibration, Scattering, Pulsed laser operation, Ecosystems
The design of a compact, dual-polarization, nonscanning lidar system intended to fly in a small, single-engine aircraft for airborne study of freshwater marine ecosystems and mapping of fish schools in mountain lakes is discussed. Design trade-offs are presented with special attention paid to selecting the field of view and telescope aperture diameter. Example results and a comparison with a similar existing lidar system are presented.
Advances in microbolometer long-wave infrared (LWIR) detectors have led to the common use of infrared cameras that operate without active temperature stabilization, but the response of these cameras varies with their own temperature. Therefore, obtaining quantitative data requires a calibration that compensates for these errors. This paper describes a method for stabilizing the camera’s response through software processing of consecutive images of the scene and images of the camera’s internal shutter. An image of the shutter is processed so that it appears as if it were viewed through the lens. The differences between the scene and the image of the shutter treated as an external blackbody are then related to the radiance or temperature of the objects in the scene. This method has been applied to two commercial LWIR cameras over a focal plane array temperature range of ±7.2°C, changing at a rate of up to ±0.5°C/min. During these tests, the rms variability of the camera output was reduced from ±4.0°C to ±0.26°C.
A continuously operating all-sky polarization imager recorded the skylight polarization pattern as conditions transitioned
from clear and clean to extremely smoky. This transition included a period when a local wildfire plume filled part of the
sky with smoke, creating a highly asymmetric distribution of aerosols. Multiple scattering in the smoke plume strongly
reduced the degree of polarization in the smoky region of the sky. Once the smoke plume spread out to cover the entire
local sky, the degree of polarization was strongly reduced everywhere. However, this example differed from previously
observed smoke events because, even though the usual skylight polarization pattern generally persisted throughout the
event, this time the smoke-covered sky exhibited a spatially asymmetric profile along the band of maximum
polarization. This pattern of reduced polarization toward the horizon is hypothesized to be a result of an optically thick
but physically thin smoke layer. The skylight polarization observations are supplemented with optical depth
measurements and aerosol size distribution retrievals from a solar radiometer.
This article [Opt. Eng.. 52, (6 ), 061304 (2013)] was originally published online on 7 January 2013 with an error in the numerator of Eq. (13) that propagated into Eqs. (14), (15), and (17). The corrected Eq. (13) and subsequent equations are given here:
Because of the increasing variety of applications for polarization imaging and sensing, there is a growing need for information about polarization phenomenology in the natural environment, including the spectral distribution of polarization in the atmosphere. A computer model that has been validated in comparisons with measurements from our all-sky polarization imager has been used here to simulate the spectrum of clear-sky polarization at a many locations around the world, with a wide variety of underlying surface-reflectance and aerosol conditions. This study of the skylight polarization spectral variability shows that there is no simple spectrum that can be assumed or predicted without knowledge of the atmospheric aerosol properties and underlying surface reflectance.
Advances in microbolometer detectors have led to the development of infrared cameras that operate without active temperature stabilization. The response of these cameras varies with the temperature of the camera’s focal plane array (FPA). This paper describes a method for stabilizing the camera’s response through software processing. This stabilization is based on the difference between the camera’s response at a measured temperature and at a reference temperature. This paper presents the mathematical basis for such a correction and demonstrates the resulting accuracy when applied to a commercially available long-wave infrared camera. The stabilized camera was then radiometrically calibrated so that the digital response from the camera could be related to the radiance or temperature of objects in the scene. For FPA temperature deviations within ±7.2°C changing by 0.5°C/min , this method produced a camera calibration with spatial-temporal rms variability of 0.21°C, yielding a total calibration uncertainty of 0.38°C limited primarily by the 0.32°C uncertainty in the blackbody source emissivity and temperature.
A set of low-cost, compact multispectral imaging systems have been developed for deployment on tethered balloons for education and outreach based on basic principles of optical remote sensing. They have proven to be sufficiently capable, and they are now being used in research as well. The imagers use tiny complementary metal-oxide semiconductor cameras with low-cost optical filters to obtain images in red and near-infrared bands, and a more recent version includes a blue band. The red and near-infrared bands are used primarily for identifying and monitoring vegetation through the normalized difference vegetation index (NDVI), while the blue band can be used for studying water turbidity and so forth. The imagers are designed to be carried by tethered balloons to altitudes currently up to approximately 50 m. These undergraduate-student-built imaging systems are being used by university and college students for a broad range of applications in multispectral imaging, remote sensing, and environmental science.
Ground-based, low-cost, uncooled infrared imagers are specially calibrated and deployed for long-term measurements of spatial and temporal cloud statistics. Measurements of cloud optical depth are shown for thin clouds, and validated with a dual-polarization cloud lidar. Good comparisons are achieved for thin clouds having 550-nm optical depth of 3 or less.
Randal Larimer, Angela DesJardins, Joseph Shaw, Charles Kankelborg, Christopher Palmer, Larry Springer, Joey Key, W. Berk Knighton, Kevin Repasky, Nathan Pust, Mitchell Hobish, Edmond Wilson, Carrie Fitzgerald, Ryan Fitzgerald, Thomas Trickel, Clyde Jensen, Skye Dorsett, Matt Anderson, Jim Boger, Nate McCrady, Jaylene Naylor, Laurie Battle
The yearly National Student Solar Spectrograph Competition (NSSSC) is Montana Space Grant Consortium's Education
and Public Outreach (EP/O) Program for NASA's Interface Region Imaging Spectrograph (IRIS) mission. The NSSSC is
designed to give schools with less aerospace activity such as Minority Serving Institutions and Community Colleges an
opportunity for hands on real world research experience. The NSSSC provides students from across the country the
opportunity to work as part of an undergraduate interdisciplinary team to design, build and test a ground based solar
spectrograph. Over the course of nine months, teams come up with their own science goals and then build an instrument
to collect data in support of their goals. Teams then travel to Bozeman, MT to demonstrate their instruments and present
their results in a competitive science fair environment. This paper and poster will discuss the 2011-2012 competition
along with results as well as provide information on the 2012 -2013 competition opportunities.
A set of low-cost, compact multispectral imaging systems have been developed for deployment on tethered balloons for education and outreach based on basic principles of optical remote sensing. The imagers use tiny CMOS cameras with low-cost optical filters to obtain images in red and near-infrared bands, and a more recent version include a blue band. The red and near-infrared bands are used primarily for identifying and monitoring vegetation through the Normalized Difference Vegetation Index (NDVI), while the blue band is used for studying water turbidity, identifying water and ice, and so forth. The imagers are designed to be carried by tethered balloons at altitudes up to approximately 50 m. Engineering and physics students at Montana State University-Bozeman gained hands-on experience during the early stages of designing and building the imagers, and a wide variety of university and college students are using the imagers for a broad range of applications to learn about multispectral imaging, remote sensing, and applications typically involving some aspect of environmental science.
Observed polarization and radiance images from a ground-based full-sky polarimeter are compared against a Successive
Order of Scattering (SOS) radiative transfer model for July 2011 cloud-free days in Bozeman, Montana, USA. The
imaging polarimeter measures radiance and polarization in 10-nm bands centered at 450, 490, 530, 675, and 780 nm.
The AERONET-retrieved aerosol optical depth, size distribution, and refractive index are used as inputs to the SOS
model. MISR BRDF retrievals are used for the surface reflectance. As shown in a previous work1 that considered 2009
data, model results generally agree with observation, but the simulated degree of polarization is typically higher than
observed data. Potential sources of this difference are considered, especially errors in the AERONET-retrieved aerosol
real refractive index.
On 5 February 2009 a colorful corona was observed when the Moon passed behind a thin wave cloud in Bozeman,
Montana, USA. A dual-polarization lidar was used to determine that the diffracting particles were ice crystals, not the
usually assumed liquid water droplets. Fraunhofer diffraction theory was used to estimate that the ice crystal effective
diameter was 14.6 μm, much smaller than conventional ice crystals. The corona-producing cloud produced a lidar crosspolarization
ratio of 0.4, confirming the presence of ice. The corona-producing cloud was located 9.5 km above mean
sea level with air temperature near -60°C, thereby thermodynamically ruling out liquid water as well.
Increased use of polarization in optical remote sensing provides motivation for a study of instruments and methods that can be used to test and validate polarized atmospheric radiative transfer codes and simulation tools. An example comparison of measured skylight polarization and calculations from a preliminary version of the polarized MODTRAN radiative transfer code (MODTRAN-P) for cloud-free conditions is presented. The study combines data from an all-sky polarization imager at 452, 491, 532, 632, and 701 nm, a solar radiometer, a zenith-viewing aerosol and cloud lidar, a weather station, and radiosonde profiles of atmospheric temperature and pressure to compare measurements and model calculations of the maximum degree of linear polarization for cloud-free atmospheres. Comparisons for conditions ranging from extremely clear to thick forest fire smoke indicate that the additional data most needed for constraining calculations are aerosol size distributions. Nevertheless, comparisons made with standard aerosol models in version 2.1-alpha of MODTRAN-P with an unpolarized multiple-scattering algorithm illustrate the methodology and provide quantitative information about the range of conditions for which a single-scattering radiative transfer code is useful for predicting skylight polarization. This approach is also warranted because many users simulate atmospheres with the MODTRAN standard aerosol models. The agreement of model calculations with measurements is high for low aerosol optical depth and degrades with increasing optical depth. Agreement between measurements and model results is best for the longer wavelengths.
With the increasing use of polarization as an added dimension in imagery for a variety of scientific, defense, and civilian
applications comes a need for better understanding of how the natural environment affects polarization signatures. In the
visible and near-infrared spectral range, the most important environmental component is polarized skylight. To provide
data to help improve understanding of how atmospheric polarization varies with aerosols, clouds, and surface
reflectance, an all-sky polarization imager has been designed, built, calibrated, and operated in a variety of field
experiments. This paper describes modifications made to that instrument to enable continuous, unattended outdoor
operation. The primary modifications were development of a weather-proof housing and an automated sun occulter
incorporating an on-board microcontroller that continually calculates solar position and moves an occulting disk on a
thin metal band to prevent direct sunlight from falling on the polarimeter lens. This occulter is designed to not obstruct
the principal scattering plane, defined as the plane containing the zenith, the Sun, and the observer.
Measurements of visible-NIR skylight polarization are compared with radiative transfer calculations from a model that
employs polarized single scattering. The measurements are from a full-sky imaging polarimeter under conditions ranging
from very low aerosol content to thick forest fire smoke. Generally, the lack of multiple scattering in the model leads to
overestimated degree of polarization in all but very clear conditions at the longest wavelengths (> 600 nm).
An all-sky imaging polarimeter was deployed in summer 2008 to the Mauna Loa Observatory in Hawaii to study
atmospheric skylight polarization. This paper describes the Mauna Loa deployment and presents an initial comparison of
our data to those observed by Coulson with a zenith-slice polarimeter in the late 1970s and early 1980s. We show how
the all-sky imaging technique yields additional insight to the nature of skylight polarization beyond what is observed in a
single zenith scan.
Previous visible-band laboratory measurements have shown that polarization data can be used to determine optical
properties of materials such as the index of refraction with controlled illumination sources. For outdoor measurements,
the complex illumination formed by the polarized sky for visible wavelengths makes this process considerably more
difficult. This paper reports polarization measurements for horizontal painted-metal and PVC plates and the background
atmosphere from a quickly changeable dual-field imaging polarimeter which provides polarization of ground-based
objects nearly concurrently with full-sky polarization. A microfacet model has been developed which accounts for the
polarized sky illumination and solar-reflecting and flat-reflecting microfacets. Data from this model have been used to
explain the primary features of the polarization observed when viewing painted metal and PVC plates outdoors with
clear skies. Future work will attempt to use this model with polarimeter data to retrieve the index of refraction of the
observed plates.
KEYWORDS: Sensors, Auroras, Calibration, Signal detection, Radio optics, Analog electronics, Electronics, Interference filters, Microcontrollers, Prototyping
Natural optical phenomena enjoy a level of interest sufficiently high among a wide array of people to provide ideal education and outreach opportunities. The aurora promotes particularly high interest, perhaps because of its relative rarity in the areas of the world where most people live. A project is being conducted at Montana State University to use common interest and curiosity about auroras to motivate learning and outreach through the design and deployment of optical sensor systems that detect the presence of an auroral display and send cell phone messages to alert interested people. Project participants learn about the physics and optics of the aurora, basic principles of optical system design, radiometric calculations and calibrations, electro-optical detectors, electronics, embedded computer systems, and computer software. The project is moving into a stage where it will provide greatly expanded outreach and education opportunities as optical aurora detector kits are created and disbursed to colleges around our region.
An imaging spectro-polarimeter was used to measure polarization signatures in five visible and near infrared channels
(450, 490, 530, 630, 700 nm) from four painted steel plates throughout the day as the solar angle and sky conditions
changed. The primary objective of this study was to characterize the influence of variable sky conditions on the observed
polarization signatures. Smooth plates exhibited higher degree of polarization than rough plates and black plates
generally exhibited higher degree of polarization than tan plates. Changes in cloudiness caused large variations in the
polarization, usually reducing the observed degree of polarization.
We describe measurements of atmospheric polarization made with an all-sky imaging spectro-polarimeter in five 10-
nm-wide bands from 450 to 700 nm. The instrument uses two liquid crystal variable retarders and a fixed linear
polarizer to measure the Stokes vector in each pixel of a 1 Mpixel image that covers the entire sky dome. Degree of
polarization and angle of polarization images are shown for clear, partly cloudy, and smoke-filled conditions. Aerosols
and clouds generally reduce the degree of polarization observed throughout the image, even in clear portions of partly
cloudy skies. Comparisons of measurements and calculations show that the single-scattering algorithm in the early
polarized Modtran (Mod-P) radiative transfer code provide adequate prediction of sky polarization at red and near-infrared
wavelengths for low aerosol optical depths (~≤ 0.2), but significantly under-predict the degree of polarization
for short wavelengths, especially with higher optical depths and in the vicinity of clouds.
The polarization state of atmospheric radiance varies with cloudiness and cloud type. We have developed a dual-field-of-view imaging spectro-polarimeter for measuring atmospheric polarization in five spectral bands from 450 to 700 nm. This instrument improves the acquisition time of past full-sky digital camera designs to 400 ms using liquid crystal variable retarders (LCVRs). The system can be used to measure polarization with either fisheye or telephoto optics, allowing studies of all-sky and target polarization. We present and describe measurements of sky polarization with clear and variably cloudy sky conditions. In clear skies, we observe a slight upward trend of the degree of polarization with wavelength, in agreement with previous observations. Presence of clouds generally reduces both cloudy sky and surrounding clear sky degree of polarization. The polarization measured from a cloud often reflects only the Rayleigh scattering between the instrument and the cloud, but some of our recent data shows partially polarized cloud scattering.
Polarization signatures of ground-based objects depend upon both the physical characteristics of the object and the polarization of the incident light. Models of polarization can accurately represent the polarization of incident light for clear skies, but the effect of cloudy skies upon object signatures is more difficult to predict. In response to the need for measurements to characterize the effect of variable sky polarization on target polarimetry, a multispectral visible band polarimeter (440 - 710 nm) has been developed. This polarimeter quickly changes to measure both full sky polarization and narrow field-of-view target signatures. Sky polarization is measured with a fisheye lens. Target polarization is measured with a 300 mm telephoto lens. A Liquid Crystal Variable Retarder (LCVR) design has been selected that uses two Meadowlark LCVR-300 variable retarders at set rotation angles. Current calibration work involves the removal of the fisheye effects on the Stokes vector. Future measurements will be used for validation of Polarized MODTRAN.
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