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Visibility Research

    Image Transmission

Overview of Current Work

History of the Atmospheric Optics Group

The Atmospheric Optics Group has been continually involved in research related to atmospheric optics for many decades. It began in the 1940's with studies in visibility and contrast transmission [link to Contrast Transmission] by Dr. Seibert Q. Duntley, as head of the Visibility Lab at MIT. Moving the group to SIO in the early 50's, Dr. Duntley headed studies in visibility and contrast transmission in the troposphere. Much of the Visibility Lab was involved at that time in underwater research and image processing. In the 60's and 70's, under Dr. Duntley and then under Richard W. Johnson, the Atmospheric Optics Group carried out extensive experimental programs in the airborne and ground-based measurement of sky and ground radiance and irradiance, scattering properties, and related meteorological and optical properties. These were tied together with related theoretical and modeling studies of the atmosphere and contrast transmission.

The image at the left shows a deployment near Mt. Ranier. This photograph was taken near the ground station as the instrumented C-130 aircraft flew over. Both the aircraft and the ground station were instrumented by the Atmospheric Optics Group, using instruments developed, built, and fielded by the group.

The Atmospheric Optics Group joined SIO's Marine Physical Lab in the mid 1980's, when the Visibility Lab closed. In the early 1980's, with the advent of solid state imagers, we developed a series of digital Whole Sky Imagers (WSI) for the measurement of the sky radiance and for measurement of cloud properties. These instruments combined the characteristics of the calibrated absolute radiance scanners with the airborne fisheye cameras used on the C-130. However they utilized modern solid state imaging technologies in order to acquire measurements of the full upper hemisphere, and utilized modern computer technologies to fully automate the data collection. Our first major deployment of a digital WSI was in 1984. Over the next few years, several WSI's were fielded in sites distributed throughout the USA, and a data base of cloud properties measured once a minute over approximately 2 years at several sites was acquired [link to CFLOS secn of Applications]. A cloud algorithm was developed to identify the presence of clouds in the image, and much of the data base was analyzed to extract cloud cover and cloud spatial characteristics.

Another interesting development at this time was the Horizon Scanning Imager. Using measurements of targets of opportunity and the measured horizon sky radiance, these instruments could determine the slant range visibility and contrast transmittance, and the variation in visibility around the horizon.

In the early 90's, a new generation of Day/Night Whole Sky Imagers was developed for use under all lighting conditions, including daylight, twilight, moonlight, and starlight. These instruments use a very low noise 16 bit digital camera, in conjunction full hemisphere fisheye lenses, and optical filter changers, electronic control, and computer control systems developed at MPL. The imagers operate automatically, with sufficient dynamic range to obtain high quality data under all natural outdoor lighting conditions. These were originally fielded at two DOD sites for use in Air Force, Army, and Navy applications.

In the mid 1990’s several improvements were made to the Day/Night WSIs for use at several sites by the DOE's Atmospheric Radiation Measurements (ARM) Program [link to ARM WSI section], in support of climate research. These developments included much stronger environmental housings and environmental hardening for use in such adverse environments as the Arctic and the Tropics. Instrument self-checking capability was added, so that the instrument could detect problems and either turn itself off or alert the user. Filters at 800 nm in the NIR were added for use in improved cloud algorithms. Using ideas developed initially at MPL, our sponsors at ARM further developed the cloud algorithms using this data and the night-time starlight data. Techniques for applying the absolute radiance calibration data were developed at MPL and fielded, to generate fully calibrated images of the sky.

The late 1990’s and early 2000’s have been spent on developing new instruments and new capabilities, as discussed in Overview of Current Work. New instruments include a new Daytime WSI and new miniaturized calibrated cameras for UAV's. A new field calibration device is in development. Current analysis work includes an analysis of Cloud Free Line of Sight statistics, improved cloud algorithms, and new algorithms for determining earth-to-space beam transmittance.


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