Throughout the Polar Regions the surface heat budget is dominated by the radiative terms; this is the case not only where the surface is frozen but also for summertime conditions over open water, such as polynyas and leads. The variability of the longwave (3 – 50µm) component is small in comparison to that in the shortwave (0.3 – 3µm), which in turn is dominated by uncertainties in the surface reflectivity, which can exhibit great variations on small spatial scales and short time intervals, especially during the melt season, and by uncertainties in specifying the influence of clouds. The influence of clouds in the global climate system can be couched in terms of feedback mechanisms that amplify or moderate the initial perturbations: the cooling of the Arctic in summer by clouds is a negative feedback as the increasing cloudiness attendant on a warmer, moister atmosphere leads to a reduction in the surface insolation. The reduced rate of ice melting also has a negative feedback on the surface absorption of the insolation by reducing the rate at which a low albedo surface replaces the high albedo snow and ice during the melt season. The high surface albedo over snow and ice leads to a positive cloud forcing, sometimes referred to as the radiation paradox. However, uncertainties in the surface reflectivity render even the sign of the net cloud radiative forcing unknown over much of the summertime Arctic. The High Efficiency HyperSpectral Imager (HEHSI) uses a simple and elegant method, based on a Sagnac interferometer, of providing measurements of the spectra of surface reflectivity in the pixels of an image. The Sagnac interferometer consists of a beam splitter and two mirrors forming a triangular light path, with the spatial displacement required to generate the interference patterns being provided by an offset of one of the mirrors from the position of symmetry. The interferometer is mounted in front of a camera with a single 2-D detector array. The instrument is lightweight and robust, and has been used on the research icebreaker Amundsen in the Arctic Ocean in the summer of 2004 to make measurements of the bidirectional reflectivity of sea ice and snow during the melt season. This type of instrument can make a significant contribution to studies of the surface reflectivity during the IPY.