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.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.