Nathan J. Pust

Dual-field imaging polarimeter using liquid crystal variable retarders

An imaging Stokes-vector polarimeter using liquid crystal variable retarders (LCVRs) has been built and calibrated. Operating in five bands from 450 to 700 nm, the polarimeter can be changed quickly between narrow (12 degrees ) and wide (approximately 160 degrees) fields of view. The instrument is designed for studying the effects of differing sky polarization upon the measured polarization of ground-based objects. LCVRs exhibit variations in retardance with ray incidence angle and ray position in the aperture. Therefore LCVR-based Stokes polarimeters exhibit unique calibration challenges not found in other systems. Careful design and calibration of the instrument has achieved errors within +/-1.5%. Clear-sky measurements agree well with previously published data and cloudy data provide opportunities to explore spatial and spectral variations in sky polarization.

Radiometric cloud imaging with an uncooled microbolometer thermal infrared camera

An uncooled microbolometer-array thermal infrared camera has been incorporated into a remote sensing system for radiometric sky imaging. The radiometric calibration is validated and improved through direct comparison with spectrally integrated data from the Atmospheric Emitted Radiance Interferometer (AERI). With the improved calibration, the Infrared Cloud Imager (ICI) system routinely obtains sky images with radiometric uncertainty less than 0.5 W/(m(2 )sr) for extended deployments in challenging field environments. We demonstrate the infrared cloud imaging technique with still and time-lapse imagery of clear and cloudy skies, including stratus, cirrus, and wave clouds.

Comparison of full-sky polarization and radiance observations to radiative transfer simulations which employ AERONET products

Visible-band and near infrared polarization and radiance images measured with a ground-based full-sky polarimeter are compared against a successive orders of scattering (SOS) radiative transfer model for 2009 summer cloud-free days in Bozeman, Montana, USA. The polarimeter measures radiance and polarization in 10-nm bands centered at 450 nm, 490 nm, 530 nm, 630 nm, and 700 nm. AERONET products are used to represent aerosols in the SOS model, while MISR satellite BRF products are used for the surface reflectance. While model results generally agree well with observation, the simulated degree of polarization is typically higher than observed data. Potential sources of this difference may include cloud contamination and/or underestimation of the AERONET-retrieved aerosol real refractive index. Problems with the retrieved parameters are not unexpected given the low aerosol optical depth range (0.025 to 0.17 at 500 nm) during the study and the corresponding difficulties that these conditions pose to the AERONET inversion algorithm.

Effects of surface reflectance on skylight polarization measurements at the Mauna Loa Observatory

An all-sky imaging polarimeter was deployed in summer 2008 to the Mauna Loa Observatory in Hawaii to study clear-sky atmospheric skylight polarization. The imager operates in five wavebands in the visible and near infrared spectrum and has a fisheye lens for all-sky viewing. This paper describes the deployment and presents comparisons of the degree of skylight polarization observed to similar data observed by Coulson with a principal-plane scanning polarimeter in the late 1970s. In general, the results compared favorably to those of Coulson. In addition, we present quantitative results correlating a variation of the maximum degree of polarization over a range of 70-85% to fluctuation in underlying surface reflectance and upwelling radiance data from the GOES satellite.

Errata: Correcting for focal-plane-array temperature dependence in microbolometer infrared cameras lacking thermal stabilization

Advances in microbolometer detectors have led to the develop- ment of infrared cameras that operate without active temperature stabili- zation. The response of these cameras varies with the temperature of the cameras focal plane array (FPA). This paper describes a method for sta- bilizing the cameras response through software processing. This stabili- zation is based on the difference between the cameras 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 temper- ature deviations within � 7.2°C changing by 0.5°C∕min, this method pro- duced 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.

Wavelength dependence of the degree of polarization in cloud-free skies: simulations of real environments

The visible and NIR maximum degree of polarization (DoP) of cloud-free skylight depends on many factors, including wavelength, sun zenith angle, surface reflectance, and aerosol properties. For clear-sky environments, radiative transfer models accurately estimate the sky DoP when each of these properties is well constrained. (The model used here was recently compared with full-sky polarization measurements with excellent agreement.) Using coincident Hyperion satellite observations and AERONET retrievals to provide model inputs, we simulate the maximum sky DoP for a variety of locations. Results show large variations in the wavelength dependence of sky polarization across different Earth environments. Therefore, accurate modeling of the sky DoP depends largely upon proper representation of the surface and aerosols in the model. Simple models which do not incorporate accurate aerosol and surface information have limited utility for simulating cloud-free sky DoP.

Radiometric calibration of infrared imagers using an internal shutter as an equivalent external blackbody

Abstract. 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.

Icy wave-cloud lunar corona and cirrus iridescence

Dual-polarization lidar data and radiosonde data are used to determine that iridescence in cirrus and a lunar corona in a thin wave cloud were caused by tiny ice crystals, not droplets of liquid water. The size of the corona diffraction rings recorded in photographs is used to estimate the mean diameter of the diffracting particles to be 14.6 μm, much smaller than conventional ice crystals. The iridescent cloud was located at the tropopause [~11-13.6 km above mean sea level (ASL)] with temperature near -70 °C, while the more optically pure corona was located at approximately 9.5 km ASL with temperature nearing -60 °C. Lidar cross-polarization ratios of 0.5 and 0.4 confirm that ice formed both the iridescence and the corona, respectively.

All-sky polarization imaging

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.

Multispectral imaging systems on tethered balloons for optical remote sensing education and research

Abstract. 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.