A Multipurpose Scanning Spectral Polarimeter (SSP): Instrument Description and Sample Results

Abstract

This paper describes the design and characteristics of a scanning spectral polarimeter designed to measure spectral radiances and fluxes in the range between 0.4 and 4.0 ?m. The instrument characteristics are described, and the procedures to calibrate the unpolarized radiances and fluxes in the spectral range from 0.4 to 1.1 ?m are discussed along with detailed error analyses of this calibration. The accuracy of the radiance calibration was determined to be approximately 3%. The calibration of fluxes based on two different procedures is estimated to be accurate to 3%?6%. Detailed calibration of fluxes was performed using a standard lamp that is not an isotropic source. For this type of calibration the angular response of the flux channel deviates from a pure cosine function at wavelengths longer than 0.74 ?m, thus forming a frontal lobe. A less detailed calibration using a 40-in. integrated sphere was also performed. In that case the light source is isotropic and the frontal lobe does not appear. Calibration factors are derived by combining data from both calibration procedures. A comparison with spectral flux measurements obtained from different instruments with different angular response properties is presented with agreement that is within the quoted calibration accuracy. Measurements obtained from two different aircraft flights are presented to illustrate the types of application of the data. Data analyses shows that the reflected (unpolarized) fluxes measured above a cirrus cloud can be reasonably matched to modeled fluxes using optical properties retrieved from the measured (unpolarized) radiances.