Infrared Cloud Imager Research Articles

Digital all-sky polarization imaging of the total solar eclipse on 21 August 2017 in Rexburg, Idaho, USA

All-sky polarization images were measured from sunrise to sunset and during a cloud-free totality on 21 August 2017 in Rexburg, Idaho using two digital three-camera all-sky polarimeters and a time-sequential liquid-crystal-based all-sky polarimeter. Twenty-five polarimetric images were recorded during totality, revealing a highly dynamic evolution of the distribution of skylight polarization, with the degree of linear polarization becoming nearly zenith-symmetric by the end of totality. The surrounding environment was characterized with an infrared cloud imager that confirmed the complete absence of clouds during totality, an AERONET solar radiometer that measured aerosol properties, a portable weather station, and a hand-held spectrometer with satellite images that measured surface reflectance at and near the observation site. These observations confirm that previously observed totality patterns are general and not unique to those specific eclipses. The high temporal image resolution revealed a transition of a neutral point from the zenith in totality to the normal Babinet point just above the Sun after third contact, providing the first indication that the transition between totality and normal daytime polarization patterns occurs over of a time period of approximately 13 s.

Reflective all-sky thermal infrared cloud imager.

A reflective all-sky imaging system has been built using a long-wave infrared microbolometer camera and a reflective metal sphere. This compact system was developed for measuring spatial and temporal patterns of clouds and their optical depth in support of applications including Earth-space optical communications. The camera is mounted to the side of the reflective sphere to leave the zenith sky unobstructed. The resulting geometric distortion is removed through an angular map derived from a combination of checkerboard-target imaging, geometric ray tracing, and sun-location-based alignment. A tape of high-emissivity material on the side of the reflector acts as a reference that is used to estimate and remove thermal emission from the metal sphere. Once a bias that is under continuing study was removed, sky radiance measurements from the all-sky imager in the 8-14 μm wavelength range agreed to within 0.91 W/(m2 sr) of measurements from a previously calibrated, lens-based infrared cloud imager over its 110° field of view.

Correcting Calibrated Infrared Sky Imagery for the Effect of an Infrared Window

AbstractA method is demonstrated for deriving a correction for the effects of an infrared window when used to weatherproof a radiometrically calibrated thermal infrared imager. The technique relies on initial calibration of two identical imagers without windows and subsequently operating the imagers side by side: one with a window and one without. An equation is presented that expresses the scene radiance in terms of through-window radiance and the transmittance, reflectance, and emissivity of the window. The window’s optical properties are determined as a function of angle over the imager’s field of view through a matrix inversion using images observed simultaneously with and without a window. The technique is applied to calibrated sky images from infrared cloud imager systems. Application of this window correction algorithm to data obtained months before or after the algorithm was derived leads to an improvement from 0.46 to 0.91 for the correlation coefficient between data obtained simultaneously from imagers with and without a window. Once the window correction has been determined, the windowed imager can operate independently and provide accurate measurements of sky radiance.

Cloud statistics measured with the infrared cloud imager (ICI)

The Infrared Cloud Imager (ICI) is a ground-based thermal infrared imaging system that measures spatial cloud statistics with a 320/spl times/240-pixel uncooled microbolometer detector array. Clouds are identified from the residual radiance that remains after water vapor emission is removed from radiometrically calibrated sky images (the water vapor correction relies on measurements of precipitable water vapor and near-surface air temperature). Cloud amount, the percentage of an ICI image containing clouds, is presented for data from Atmospheric Radiation Measurement (ARM) sites at Barrow, AK in February-April 2002, Lamont, OK in February-April 2003, and Barrow, AK in March-April 2004. In Oklahoma, the percent cloud cover determined from full ICI images was slightly higher than that found from a single-pixel time series, suggesting that cloudiness may be under sampled by vertically viewing lidars or radars under highly variable conditions. Full-image and single-pixel statistics agreed more closely for Arctic clouds, which tend to be uniform for long periods of time. Good agreement is found in comparing cloud amount from ICI and active remote sensors during day and night, but much worse agreement is found between ICI and the ARM Whole Sky Imager during nighttime relative to daytime, indicating the importance of the diurnally consistent ICI measurements.

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.