Abstract
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.
Highlights
In addition to the most common visible and near-infrared (VNIR) cloud imagers, longwave infrared (LWIR) systems are being developed with the advantage of providing equal day and night sensitivity with the same cloud retrieval algorithm, as long as clear-air emission is properly estimated and removed
This paper describes a reflective all-sky Infrared Cloud Imager (ICI) system that uses a metal sphere to reflect the full sky into a low-cost, weather-proof microbolometer camera positioned off axis to allow for an unobstructed view of the zenith sky
These reflected angle maps were used to determine the instantaneous field of view (IFOV) solid angle for each pixel by calculating the fraction of the overlying 2π-sr hemisphere seen by each pixel
Summary
Spatial and temporal cloud statistics are important in studies of climate [1,2,3,4,5], solar radiation [6,7,8,9], astronomy [10,11,12,13], and earth-space optical communications [14,15,16,17,18,19] For these needs, autonomous imaging systems have been developed to make measurements of parameters such as the fraction of the sky that is cloudy [20], cloud height [21], cloud emissivity [22], and cloud optical depth [23,24]. We describe the reflective all-sky ICI system and data processing and compare data from it and a previously validated, lens-based ICI system
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