High resolution imaging spectroscopy of the sky

Diffuse sky radiance is a key variable for resolving the global energy budget, weather forecasting, radiative transfer modelling and estimating solar energy potential at the Earth's surface. This study is the first attempt to directly measure a full-sky view of the sky radiance using a pushbroom imaging spectrometer. We mounted the open-source hyperspectral imager (OpenHSI) on a tripod and rotation stage during a clear day in Rochester, NY, USA, and resampled the swaths to a sphere using the HEALPix projection. We achieved a 0.45 degrees angular resolution, which is an order of magnitude better than any previous experimental study. Our results showed agreement with analytic studies using radiative transfer codes, and with photometric studies, demonstrating a strong wavelength dependence for sky radiance distributions. Along the principal plane, we observed a spectral steepening that transitioned from predominantly Mie scattering in the circumsolar region to predominantly Rayleigh scattering. We also compared our results to a CIE standard clear sky. Although all the data were collected within one hour, the movement of the Sun during this time is noticeable in the results. Nevertheless, our study provides confidence in using existing models and introduces a new technique for high resolution mapping of the sky radiance distribution using pushbroom imaging spectrometers.