Spectral reflectance measurements of snow and snow covered objects: experimental studies compared with mathematical models

Abstract

In this study, we have investigated reflectance spectra of different snow types under various conditions. Snow reflectance is interesting from a camouflage point of view as snow covers large land areas in many parts of the world during winter, at high altitudes or high (or low) latitudes. Snow reflectance of incident light differs from light reflected by other natural constituents such as soil and vegetation by the high reflectance, particularly in the visible wavelength range. It is therefore important to characterize various snow reflectance properties further. From a concealment point of view, it is also important to study differences – and similarities – in reflectance of snow and vegetation (including frost-covered vegetation) beyond the wavelengths visible to the naked eye. Especially because reflected light from vegetation is dominated more by water content as the wavelength increases. Finally, it is of interest to study the effects on snow coverage needed to mask signatures of underlying objects and to observe if snow reflectance, as a function of thin layers fits well to existing models of light reflected by thin, semi-transparent layers. We found that fresh powder snow, wet snow, coarse snow, and deep (and older) snow layers had similar reflectance spectra, albeit with important differences. Fresh powder snow reflected more light than wet, older or coarse snow that had lower reflectance values, yet distinctly different from vegetation for wavelengths below about 1000 nm. For longer wavelengths, however, the differences between pure snow and green vegetation were much less pronounced. Finally, the reflectance of frost-covered vegetation deviated from pure vegetation, but to a much less degree than pure snow. Layer thickness needed to mask underlying surfaces was studied for coarse snow distributed evenly onto a green reference object, and we found the characteristic thickness (corresponding to specific weights of snow per area) needed to effectively hide the spectral reflectance signatures.