On 21 August 2017 we measured skylight polarization during a total solar eclipse in Rexburg, Idaho, using two all-sky polarimetric imagers. The all-sky polarization images were recorded using three simultaneously operating digital singlelens-reflex (DSLR) cameras with good low-light sensitivity. Each camera was equipped with a 180° field-of-view fisheye lens to view the entire sky and each lens contained a fixed linear polarizer orientated at 0° , 60° , and 120° , respectively, to recover the first three Stokes parameters. Skylight polarization was measured from sunrise to sunset in the cameras’ blue, green, and red channels. Before and after totality, the maximum sky polarization occurred in its usual pattern with a band of maximum polarization positioned 90° from the sun. However, during totality skylight polarization became nominally symmetric about the zenith. This was observed clearly in the blue and green channels and less obviously in the red channel, which had a greatly diminished signal. At and near the observation site, we also operated an infrared cloud imager, a hand-held spectrometer to measure surface reflectance, and an AERONET solar radiometer to characterize the atmospheric aerosols. This ancillary data set provided a complete characterization of the conditions of the surrounding atmosphere and underlying surfaces.
This paper reviews the science of sun photometry, tracing its origins back to research by Newton. Modern sun photometry originated through the works of the Angstrom family in Sweden and the Smithsonian Astrophysical Observatory's program started by Langley and energized over many years by Abbott. In the case of the Smithsonian's program, the objective was to search for evidence between fluctuations in climate and solar radiation. Multiwavelength sun photometry as a science came about primarily by turning this nuisance factor of atmospheric corrections into useful scientific information. Modern sun photometry uses solar radiometers calibrated to accuracies of one part in a thousand and routinely assess atmospheric spectral optical depths to high accuracy. There is presently a global network of sun photometers providing information about the spectral variation of aerosol optical depth and information mapping atmospheric trace constituents.
In early studies of the sun’s spectrum it was found that the atmosphere diminishes the intensity of solar radiation. In general the effect is larger in the blue or violet than at longer wavelengths. This is why a scene viewed through a red filter, such as red cellophane, appears to be so “clear”. It was further discovered in the nineteenth century that the diminishing effect of the atmosphere can actually be more complicated than a smooth blue-to-red gradient and that certain wavelengths of sunlight are strongly absorbed by individual trace gases in specific wavelength bands and, moreover, to complicate things even further the diminution or “turbidity” was found to vary from one day to another. Assessing this “diminution” nuisance became especially important in quantifying incoming solar radiation reaching the earth. The investigation of solar radiation reaching the earth became a subject of great interest because it was suspected that the sun may undergoes changes in its luminosity that would modulate climate and weather and the growing of crops.