A new instrument has been developed for measurement of visibility and scattering characteristics in the visible spectrum over extended paths. The instrument, a Multispectral Scattering Imager, is designed to acquire calibrated radiance images in several wavelengths over extended paths. From measurements of the horizon radiance and the radiance of dark targets, combined with measurements of the inherent properties of the dark targets, visibility and effective scattering coefficient over the integrated path can be determined. The instrument has acquired several months of data over a 7.2 km path in San Diego, along with several transmissometers and nephelometers. Initial data comparisons show quite good comparisons with the other systems. The instrument has the advantage that measurements may be taken over extended paths in the wavelengths of choice, yet it is passive, and does not require an active two-ended transmitter/receiver system. We will show the results from the MSI in blue, green, red, and NIR wavebands, and show comparisons with other instruments.
A new system is being developed for measurement of visibility and scattering characteristics in the visible and NIR wavelengths over extended paths. This is being developed to better understand transmission properties over long horizontal near-surface propagation paths over the ocean. The instrument, a Multispectral Scattering Imager, is designed to acquire calibrated radiance images in several wavelengths over extended paths. From measurements of the radiance near the horizon and the radiance of dark targets, combined with measurements of the inherent properties of the dark targets, visibility and effective scattering coefficient over the integrated path can be determined. The technique uses the Koschmeider equations for radiative transfer, and it allows for correction of sensor characteristics such as non-linearity and of non-zero target reflectance. The technique is based in part on the Horizon Scanning Imager, a visibility system developed in the 1990's by the Marine Physical Lab. However, it utilizes better imaging systems than were available at that time, as well as improvements to the visibility and scattering algorithms. Initial experiments are designed to test the strengths and weaknesses of the system, and to provide multispectral visible band results. These results will be used for modeling and statistical studies with respect to simultaneous measurements by a suite of other instruments. This talk will present an overview of the system and algorithms, as well as initial experimental results.
Cloud Free Line of Sight (CFLOS) statistics can be important to a number of applications involving transmittance of light through the atmosphere, including laser propagation, light propagation, and detection of objects by humans and instruments. This paper will discuss Cloud Free Line of Sight (CFLOS) and cloud persistence statistics determined from cloud measurements taken with Whole Sky Imagers (WSI). The WSIs are ground-based digital imaging systems that image the full upper hemisphere down to the horizon in wavebands in the visible and NIR. Digital automated WSI systems were originally developed by the Marine Physical Lab in the 1980's to address the CFLOS application, and then further developed for 24-hour day and night capability. Approximately three million image sets have been acquired with the Day/Night WSI in conjunction with DOE's ARM program. Recent advances in the cloud decision algorithms at Marine Physical Lab have enabled the extraction of processed cloud images of sufficient quality to obtain reliable cloud statistics. A test sample of approximately 4500 image sets has been processed to yield CFLOS statistics down to the horizon, as well as statistics related to the persistence of clouds and cloud holes. This talk will provide a brief overview of the instruments and current algorithm developments. The CFLOS results and sample persistence results will be presented.
In order to measure cloud top radiances from Unmanned Aerial Vehicles (UAVs) or other light aircraft, two small calibrated fisheye imaging systems have recently been developed. One of these systems uses a visible-wavelength CCD and is optically filtered to measure cloud top and ground radiances near 645 nm. The other uses an InGaAs detector and is optically filtered to measure radiances near 1610 nm. These sensors are specifically designed for use with DOE's Atmospheric Radiation Measurement (ARM) Program UAV Project, and it is anticipated that they will be used for comparison with a variety of satellite-borne radiance measurements. Radiometric calibration of solid-state imagers is never trivial, as the effects of exposure time, system non-linearities, temperature, gain and other system characteristics must be adequately measured and characterized. Much experience has been gained with the ground-based Day/Night Whole Sky Imagers and the Daylight Visible/NIR Whole Sky Imagers developed and used by the group for many years. New techniques for the radiometric calibration of the two new airborne systems are being developed based on this experience and the characteristics of the sensors involved. In addition, new techniques for a more accurate angular calibration have been developed.
Measurements of UV radiation at the earth’s surface may be highly impacted by the presence of clouds. In order to provide support for UV research, a Daylight Visible/NIR Whole Sky Imager was developed to provide cloud fraction assessment over the whole sky, as well as measurements of the radiance distribution over the full sky in several spectral bands. Radiances are determined in approximately 700,000 directions simultaneously with a given optical filter. Data may be acquired in seven spectral bands that may be selected for the application. The current instrument uses filters near 450 and 650 nm, open-hole, filters in the blue-green broadband and NIR long-pass, and two polarizers. Opaque and thin cloud fraction is determined from images acquired in the blue and red wavelengths. A more sophisticated version of the algorithm to detect thinner clouds and enable aerosol assessment is in development, and will be based on use of the NIR data in conjunction with the blue and red data.
This paper will provide an overview of the instrument design and calibration, and sample sky radiance results. The cloud algorithms for determination of cloud fraction will be discussed, and the cloud imager results will also be presented.
Conference Committee Involvement (3)
Atmospheric Optics: Models, Measurements, and Target-in-the-Loop Propagation
27 August 2007 | San Diego, California, United States
Atmospheric Optical Modeling, Measurement, and Simulation II
15 August 2006 | San Diego, California, United States
Atmospheric Optical Modeling, Measurement, and Simulation
2 August 2005 | San Diego, California, United States