WSI

Introduction
Day/Night WSI
Modified for Real-Time Processing
The Daylight Visible/NIR WSI
Earlier Day-Only WSI Versions
Airborne Visible and NIR Imagers
WSI Applications
Overview


Day/Night WSI

Primary Quantities Measured with System Typical
 

Whole Sky Imagers

Introduction

The Atmospheric Optics Group has specialized for many years in the development of Whole Sky Imagers and related systems designed to characterize the cloud and radiance field. These systems acquire images of the sky, from which the cloud fraction over the whole sky may be determined, as well as distributions of radiance. Some of the WSI systems are 24-hour systems capable of acquiring high quality data all day and all night, while others are more specialized systems designed for handling day only, or for looking down from an aircraft.

Most of these systems possess several common characteristic elements, regardless of their eventual field configuration. These characteristic elements are:
  • A super wide angle lens, usually covering a full hemisphere field of view. The Day/Night WSI systems use a Nikor, f/2.8 8 mm lens; other more recent systems that are used in higher ambient lighting use smaller fisheye lenses.
  • A digital camera chosen for the specific task. The Day/Night WSI systems use a Photometrics Series 300 camera made by Roper Scientific. This slow scan, 16-bit CCD camera with its thermo-electrically cooled imaging chip has proven a flexible and reliable detector in a broad variety of applications. For some applications, we have used a variety of cameras, including 12-bit digital cameras, video cameras, and a 12-bit InGaAs CMOS camera.
  • Those systems which are used to monitor the sky from the ground include an automatic solar or solar/lunar tracking device to shadow the lens at all times. This provides protection against stray light contamination in the image, and is a requirement for systems that will be calibrated to provide absolute radiance.
  • A temperature and humidity controlled, weather-proof housing to protect the camera sub-system from the rigors of continuous, remote operations. Some of the WSI units have been operating successfully in rigorous environments such as the arctic, desert, and tropics for a number of years.
  • A modern PC class computer with appropriate speed, capacity and display characteristics. It must run a selection of custom and third party computational display and IP software. Some current systems include real-time cloud algorithms for identifying the cloud cover in the imagery. Some of the older systems still run under Dos, but more modern systems run under various versions of Windows, according to the sponsor’s preferences.

Customized WSI Versions, past and present, discussed in this section, include the following:
The Day/Night WSI, this unit acquires images automatically throughout the 24-hour day, and is used at several sites.
The Day/Night WSI modified for real-time processing of cloud data.
The Daylight Visible/NIR WSI developed for the German government, this instrument uses more spectral filters in the visible and NIR.
A Mini-WSI: Airborne Visible and NIR Imagers calibrated to provide radiance distribution.
Earlier Day-only WSI, used extensively in the 80’s to acquire data once a minute at seven sites for a period of over two years.

The Applications Overview section discusses some of the current and potential uses for these devices.

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